Elucidating the role of Mn promotion in Co-based Fischer-Tropsch synthesis

Transcription

1 Elucidating the role of Mn promotion in Co-based Fischer-Tropsch synthesis Maarten Van Doorslaer Supervisor: Prof. dr. ir. Mark Saeys Counsellor: G. T. Kasun Kalhara Gunasooriya Master's dissertation submitted in order to obtain the academic degree of Master of Science in Chemical Engineering Department Of Materials, Textiles And Chemical Engineering Chair: Prof. dr. Paul Kiekens Faculty of Engineering and Architecture Academic year

2

3 FACULTY OF ENGINEERING AND ARCHITECTURE Laboratory for Chemical Technology Director: Prof. Dr. Ir. Guy B. Marin Laboratory for Chemical Technology Declaration concerning the accessibility of the master thesis Undersigned, Maarten Van Doorslaer Graduated from Ghent University, academic year and is author of the master thesis with title: Elucidating the role of Mn promotion in Co-based Fischer-Tropsch synthesis The author gives permission to make this master dissertation available for consultation and to copy parts of this master dissertation for personal use. In the case of any other use, the copyright terms have to be respected, in particular with regard to the obligation to state expressly the source when quoting results from this master dissertation. June 02, 2017 Laboratory for Chemical Technology Technologiepark 914, B-9052 Gent Secretariat : T +32 (0) F +32 (0) Petra.Vereecken@UGent.be

4 Elucidating the role of Mn promotion in Co-based Fischer-Tropsch synthesis Maarten Van Doorslaer Supervisor: Prof. dr. ir. Mark Saeys Counsellor: G. T. Kasun Kalhara Gunasooriya Master's dissertation submitted in order to obtain the academic degree of Master of Science in Chemical Engineering Department Of Materials, Textiles And Chemical Engineering Chair: Prof. dr. Paul Kiekens Faculty of Engineering and Architecture Academic year

5 Acknowledgment First of all I, would like to thank my coach Kasun, for his continuous guidance and support throughout the project. I would also like to thank professor Saeys, my promotor, who provided me with the opportunity to work on this project. His input during our meetings was paramount to overcoming some of the obstacles and provided guidance during the project. My colleague Sofie and her coach Jenoff have to be thanked for their input during our subgroup meetings where we were able to exchange ideas due to the similarity of our projects. I would also like to thank Lukas Buelens for his introduction into the usage of the Ekvicalc software. As a non-regular chemical engineering student who transferred over from industrial engineering, I have to thank my classmates at the Laboratory for Chemical Technology for accepting me into their group and helping and supporting me during our two years together. Special thanks goes to my fellow transfer students Dries, Thomas and Sander as well as the regular student Hannes. We cooperated extensively during the different projects and eventually developed a friendship outside of the school. Their support and friendship helped me through the heavy workload of the last two years. I would like to thank my parents and sister for supporting me at home and for encouraging me to continue my studies, which in hindsight is one of the most important (and best) decisions in my life. Special thanks to my grandmother Aline for her delicious meals. Lastly, I would like to thank my friends back home, especially Jean and Jakke, who provided the necessary relaxation during the weekends and who listened to my problems.

6 Elucidating the role of Mn promotion in Cobased Fischer-Tropsch synthesis Maarten Van Doorslaer Supervisor: Prof. dr. ir. Mark Saeys Counsellor: ir. G.T. Kasun Kalhara Gunasooriya Master s dissertation submitted in order to obtain the academic degree of Master of Science in Chemical Engineering Abstract Department of Materials, Textiles and Chemical Engineering Chair: Prof. dr. Paul Kiekens Faculty of Engineering and Architecture Academic year Mn promotion in cobalt based Fischer-Tropsch synthesis (FTS) has been shown to increase selectivity towards C5+ hydrocarbons as well as olefin to paraffin ratio for the C2 C4 fraction. However, the specific role of Mn in achieving these promotion effects remains highly debated. The nature, location and kinetic role of Mn promoters under FTS conditions was studied using Density functional theory (DFT). First, using an experimental and DFT based Mn-oxide phase diagrams, the nature of the promoter under reaction conditions was determined to be MnO. Next, the location of MnO under reaction conditions on a step and terrace sites were examined. It was determined that, on a terrace site the formation of a six MnO ring is most favorable while on a step site the substitution of one MnO into the edge is favored. Lastly, the effect of Mn promotion on the kinetics of CO and H2O dissociation was examined by studying the adsorption energies of CO, OH, H and H2O. Keywords Catalysis, cobalt, Fischer-Tropsch synthesis, promoters, MnO, density functional theory

7 Elucidating the role of Mn promotion in Co-based Fischer-Tropsch synthesis Maarten Van Doorslaer Supervisor(s): prof. dr. ir. Mark Saeys, counsellor: ir. G.T. Kasun Kalhara Gunasooriya Abstract Mn promotion in cobalt based Fischer-Tropsch synthesis (FTS) has been shown to increase selectivity towards C5+ hydrocarbons as well as olefin to paraffin ratio for the C2 C4 fraction. However, the specific role of Mn in achieving these promotion effects remains highly debated. The nature, location and kinetic role of Mn promoters under FTS conditions was studied using Density functional theory (DFT). First, using an experimental and DFT based Mn-oxide phase diagrams, the nature of the promoter under reaction conditions was determined to be MnO. Next, the location of MnO under reaction conditions on a step and terrace sites were examined. It was determined that, on a terrace site the formation of a six MnO ring is most favorable while on a step site the substitution of one MnO into the edge is favored. Lastly, the effect of Mn promotion on the kinetics of CO and H2O dissociation was examined by studying the adsorption energies of CO, OH, H and H2O. Keywords MnO, cobalt, Fischer-Tropsch synthesis, promotion, density functional theory I. INTRODUCTION Fischer-Tropsch synthesis (FTS) is an attractive process to convert natural gas, CO 2 and biomass to clean transportation fuels and chemicals. FTS transforms the syngas, a mixture of CO and H 2, is converted to long chain alkanes, alkenes, oxygenates and water. Cobalt based catalysts are commercially interesting due to their high activity and selectivity towards higher hydrocarbons as well as its low water gas shift activity. The FTS mechanism is highly debated with two main reaction mechanisms under consideration. [1] These are the carbide mechanism and the CO insertion mechanism. The carbide mechanism, which was originally proposed by Fischer and Tropsch, considers C-O activation to occur via direct CO dissociation. Carbide species are then hydrogenated to CH x species which are used as monomers for chain growth. Note that C-O scission occurs before C-C coupling. This mechanism requires a high rate of CO dissociation however, the direct dissociation of CO on Co terraces is a difficult step with high energy barriers. [2] This prompted the proposal of the CO insertion mechanism which considers C-C coupling to precede CO scission. Meaning that adsorbed RCH x groups first couple with the adsorbed CO before the CO bond is cleaved. [3] The primary path consist of RC* groups being hydrogenated to RCH* and coupled with CO yielding RCHCO* followed by hydrogenation to RCH 2CO* which will then undergo CO scission. Promotion in FTS is used for different purposes. Structural promotion is used for support stabilization, Co glueing and to increase Co dispersion. Electronic promotion can take effect by decoration of the Co surface or by Co alloying. Synergistic promotion is the addition of a promoter to catalyze another reaction, such as the water gas shift reaction, hydrogenation/dehydrogenation reaction, coke burning and H 2S adsorption. Several studies [4-7] have examined the oxidation state of Mn under reaction conditions and determined it to be MnO, corresponding to a Mn oxidation state of +2. These studies have also examined the effect of Mn promotion on the product distribution showing that Mn promotion increases C 5+ selectivity as well as increasing the olefin to paraffin ratio of the C 2 to C 4 hydrocarbons. Furthermore, examination of the turnover frequency of a Mn promoted Co FTS catalyst has shown that low Mn loading (<0.1 Mn/Co ratio) increases the turn over frequency while increased loading decreases the turn over frequency. It has been proposed by Johnson et al. [4] that this is due to increased blockage of the active cobalt surface by the manganese promoter with increased Mn loading. Bell et al. [8] further proposed that lewis-acid interactions between Mn and the O of adsorbed CO are the cause of the promotion effect in Mn promotion of Co catalyst. Density functional theory (DFT) is an excellent tool to investigate the role of Mn promotion in Co-based FTS. No studies have been found which have examined Mn promotion for Co-based FTS using DFT, as such this study could provide insight into the location and the kinetic effects of Mn promotion on the FTS mechanism. II. METHODS AND MODELS A. Catalyst and promoter bulk models Several cobalt catalyst models were used in this work. The cobalt terrace was modelled as a five layer fcc Co(111) slab, using a p(3x3) unit cell with 9 atoms per layer. The bottom two layers were constrained at the bulk positions with an optimized lattice constant of 3.56 Å, in good agreement with the experimental lattice constant of 3.54 Å. [9] The cobalt step site was modelled as a three layer Co(211) slab, using a p(4x4) unit cell where the bottom layer is constrained. Promoter bulk models are required for the construction of a phase diagram, as such Mn(gamma)[10], MnO[11], Mn 3O 4[12], Mn 2O 3[13] and MnO 2[14] are considered. Typically, the most stable metallic Mn phases consist of Mn(alpha). However, calculating accurate electronic energies for Mn(alpha) is computationally expensive due to its complex magnetic and geometrical structure. Therefore, Mn(gamma) phase is used in this study as a replacement. The energy difference between the Mn(gamma) and Mn(alpha) is reported to be kj/mol [15]. M. Van Doorslaer is with the Chemical Engineering Department, Ghent University (UGent), Gent, Belgium. Maarten.vandoorslaer@UGent.be

8 B. Experimental phase diagram An experimental phase diagram was constructed using Ekvicalc software. ratio and temperature was varied to determine the most stable phase at each set of conditions. The software determines the most stable phase by minimizing the Gibbs free energy of the system. The species taken into consideration are Mn(s), MnO(s), Mn 3O 4(s), Mn 2O 3(s), MnO 2(s), H 2O(g), H 2(g) where Mn(s) was considered as alpha Mn, beta Mn, gamma Mn and delta Mn. The amount of Mn used as input was taken at 10 mol and the amount of H 2O was fixed at mol while the amount of H 2 is varied. A low amount of Mn and high amount of H 2O are used to minimize the effect of reaction on the partial pressures of H 2O and H 2. The total pressure is fixed at 20 bar, which is a typical pressure under reaction conditions. The output of the software provides activities of the solid phases for each set of conditions, this data is processed using excel to construct equilibrium lines which are combined into a phase diagram. C. VASP calculations DFT calculations were performed using the VASP software and its implementation of the vdw-df functional. [16] The cutoff kinetic energy is set at 450 ev for the plane-wave basis set and an inter-slab distance of 15 Å is used to minimized the interactions between repeated slabs. A K-points mesh is generated, sampling the Brillouin zone with a (3 3 1) Monkhorst-Pack grid for all catalyst models. Adsorption energies are calculated as: = + (1) Where is the electronic adsorption energy of the adsorbate in kj/mol, the total electronic energy of the adsorbate on the slab, the electronic energy of the slab and the electronic energy of the adsorbate in a vacuum. For the construction of a DFT based phase diagram, it is required to construct equilibrium lines. Therefore, the Gibbs free reaction energy for the reduction reactions from MnO 2(s) have to be calculated. is used to perform the reductions. The following formula is used to calculated the Gibbs free reaction energy: Δ,! = Δ Δ" + # ln & ' ' ( (2) Where the partial pressure of H 2O was taken as 6.67 bar and the partial pressure of H 2 as 8.89 bar which are conditions for an average CO conversion of 60% at 20 bar, a conversion level and pressure often seen in commercial applications. [17] The Δ and Δ" values are calculated from the vibrational, rotational and translational partition functions. Note that the entropy and enthalpy changes for solid phases are assumed negligible. The Δ,! is then converted to ln & ( using the equation: ln & ( = )*+ = -. / 0, (3), 1 0, This yields the equilibrium lines used to construct the phased diagram. Bader charge calculations are performed in order to better understand the impact of the promoter on the cobalt surface and how it may affect the absorption of key species. The Bader charge method divides atoms using zero flux surfaces, these are 2D surfaces with a minimal charge density. In molecular systems charge density tends to minimize between atoms, providing a good separation between the region of influence of the different atoms. The Bader charge calculation yields a charge for each atom in the system. These charges can be compared against the charge of the atom at the ground level, yielding an increase or decrease in charge. This is especially important due to the presence of a promoting element, as the effect of the element on the charge of the surrounding atoms may e.g. facilitate adsorption of certain species. A. Nature of the promoter III. RESULTS AND DISCUSSION To determine the role of manganese promoters in Fischer- Tropsch Synthesis, a detailed understanding of the nature of the manganese promoter under reaction conditions is required. some studies in literature [18] studied the role of manganese promoters by substituting manganese atoms in the slab without determining the oxidation state under reaction conditions. This leads to inaccurate results and conclusions due to stronger/weaker adsorption of oxygen containing species since manganese is undersaturated/oversaturated. Therefore, the most prevalent oxidation state of the promoter has to be determined before any further investigation into the role of promoters could be performed. The oxidation state of the promoter under reaction conditions affects electronic behavior and also determines the most favorable location on/in the catalyst surface. Thus, constructing a phase diagram enables one to determine the oxidation state of the manganese promoters under different reaction conditions. 1) Experimental phase diagram The Ekvicalc software was used to construct a Mn oxide phase diagram which is shown in Figure 1. This figure shows that the most prevalent Mn phase under FTS reaction conditions is MnO, corresponding to an oxidation state of +2. Reaction conditions are taken as 500 K and 60% conversion or an H2 and H2O partial pressures of 8.89 bar and 6.67 bar respectively. [17]The phase diagram also shows that the MnO phase is stable under a wide range of ln 2 3 at the 500 K point indicating a high stability of the MnO phase under operating conditions. Figure 1 The Experimental phase diagram for Mn oxides constructed using Ekvicalc software. The FTS operating point is indicated at 500 K and which corresponds with 60% conversion. [17] The phase diagram shows that the MnO 2 phase is preferable at high ln 2 3 ratios. The high ln 2 3 requirement can be explained due to MnO 2 requiring a strong oxidizing

9 atmosphere or high ratio to form. The reason the phase diagram only shows MnO 2 at high temperature is because an increase in temperature decreases the ln 2 3 requirement ratio) which is due to the equation (3). (requiring a lower Metallic Mn phases are show on the bottom of the diagram as they require a strongly reducing atmosphere or low ratio. Here, the phases are formed at higher ln 2 3 ratios as increasing the temperature will decrease the ln 2 3 requirement. It is counter intuitive that an increase in temperature can work for both Mn and MnO 2. This is due to taking the logarithm of the ratio as in the case of metallic Mn ln 2 3 is negative and for MnO 2 it is positive, as such increasing the temperature will make ln 2 3 respectively more negative and more positive decreasing the requirement for both cases. 2) DFT based phase diagram Before the phase diagram was constructed a benchmark of different DFT functionals was performed for the formation enthalpy of the different Mn oxides. Reaction (4) is used to examine the formation energies, as based on [15] , : (4) The results are shown in Figure 2. As expected, PBE functional shows a strong deviation from the experimental results which is further validated by our calculations. The difference between the results reported by the VASP group and in this study stems from the usage of different settings. The hybrid functionals PBE0, PBEU4 and HSE perform much better, since these functionals include both local and non-local interactions whereas non-local interactions are not taken into account in non-hybrid functionals. However, these extra nonlocal interactions increase the computational cost. The vdw- DF functional approaches the experimental results quite well, though not as well as the hybrid functionals. In this study, hybrid functionals are not pursued due to the extensive computational time and therefore the vdw-df functional is used. Figure 2 A comparison between the experimental formation energy and the calculated formation energy, the values for the PBE, PBEU4, PBE0 and HSE functionals are taken from [15] Although the error is limited through the selection of an appropriate functional, it still remains quite large for some oxides. This causes inversion of transitions meaning that e.g. the transition of MnO to Mn occurs before the transition of Mn 3O 4 to MnO. This problem was mediated through selection of an oxidation state, which prevents this issue, as a basis for each transition. It has been determined that using MnO 2 as a basis solves the problem. The DFT based phase diagram is show in Figure 3. The most prevalent phase under reaction conditions was determined to be MnO, with an oxidation state of +2. It is however noted that the most prevalent phase at 0 K, which is the temperature at which regular VASP calculations evaluate the models, is Mn 3O 4. This means that regular VASP calculations will tend to over-oxidze Mn instead of preferring the MnO phase. However, if frequency calculations are employed for the addition of extra oxygen through the system, the overoxidzation should be averted. Frequency calculations are not performed for solid phases such as Mn substituted in the Co slab as their contribution was assumed to be negligible. Figure 3 The DFT based phase diagram for Mn oxides. The FTS operating point is indicated at 500 K and which corresponds with 60% conversion. [17] Comparing the experimental phase diagram ( Figure 1) with the DFT based phase diagram (Figure 3) shows that the MnO 2, Mn 2O 3 and Mn 3O 4 phases are shifted downwards in the DFT based phase diagram. This indicates that these higher oxides are predicted more stable than they actually are. Therefore, DFT prefers to overoxidize Mn and could hence lead to the wrong phase being predicted by the calculations. Secondly, the upward movement of the Mn phase which is caused by an overestimation of the stability of Mn, amplified by the error on the MnO 2 reference. The last difference between the two diagrams is the change in slope seen in the transition from Mn 3O 4 to MnO. As the phase diagram plots a function of the form: 9 = <, where in both cases (DFT & Ekvicalc) + = 10 1 the = < will be positive but the will be positive for the DFT 1 10 case while it should be negative. Since a plot 9 = > will 0 will show a show an increase with T and a plot 9 = + > 0 decrease with T, the reversed sign of the is to blame for the change in shape. This sign reversal is caused by the error on the value meaning that the slope change is due to the error in the DFT calculations. Despite the differences between the two phase diagrams, both still predict MnO as the most prevalent phase under reaction conditions. As such the DFT results predict the correct phase and the location of the promoter can be determined in the next step. Note that during the search for the location care must be taken to use MnO as a basis for

10 forming more complex Mn structures as it is the most prevalent phase and any extra oxygen has to be added using a system that is evaluated at 500 K. B. Location of the promoter The location of Mn promoters need to be determined using DFT calculations as the existing experimental evidence [4, 6, 7] does not reveal the exact location of the promoter. Two Cosurfaces are considered in this work. A terrace surface modelled as a fcc Co(111) slab and a step surface modelled as a Co(211) slab. A terrace site needs to be taken into consideration because this surface is kinetically most important under FTS conditions, as there is a high possibility that the under-coordinated edge sites become poised by binding carbon too strongly.[19] A step surface needs to be examined as surface reconstruction under reaction conditions has been shown to form step surfaces. [20] 1) Terrace surface Different adsorption and substitution positions were examined, starting with Mn addition into the slab using the following reaction:?@ AB C C D?@ AA 56 > EC + 1?@C + (5) Where?@ AB C is the energy of the Co(111) slab, D?@ AA 56 > EC is the energy of the Co(111) slab with one Co atom substituted by Mn. 1Co(s) refers to the Co atom replaced by Mn, its energy is calculated using the equation: > G H = I J KLMN/ OKLPQI R KLMN/ OKLP (6) S Where B : is the energy of a 5 layer 3x3 Co(111) slab and A : is the energy of a 4 layer 3x3 Co(111) slab, the division by 9 is required as each layer contains 9 atoms. This equation yields -345 kj/mol. Additions into the surface, subsurface and bulk were examined with Gibbs free formation energies of respectively 33 kj/mol, 46 kj/mol and 37 kj/mol indicating that substitution of Mn into the slab is not favorable. Next, MnO addition into the slab was examined using equation (7), where oxygen on top of Mn, in HCP and in FCC was examined yielding Gibbs free formation energies of respectively 105 kj/mol, 2 kj/mol and 0 kj/mol. The MnO substitution will not be the most prevalent state under reaction conditions as it is likely that more favorable structures exist.?@ AB C + 56C D?@ AA 56 > EC + 1?@C (7) Next, MnO adsorption was examined. Firstly, single MnO adsorption was extensively examined in order to determine the most favorable positions of the Mn and O atom. Equation (8) was used to examine these adsorptions where the most favorable position was determined to be Mn in a bridge-hcp position and O in an FCC position with a Gibbs free formation energy of 70 kj/mol.?@ AB C C D?@ AB 56, EC (8) This optimal position was then used to construct structures consisting of two to six MnO by first focusing on stoichiometric structures. The most stable configuration was determined to be a six MnO ring with Mn in bridge position and O in shifted top positions, as shown in Table 1-A. Monolayers were also taken under consideration but were found to be unfavorable with the most favorable formation energy being 13 kj/mol/mno. structure on a p(6x6) Co(111) surface. d) One MnO substitution in the edge of a p(4x4) Co(211) surface. A top and side view are shown as well as the Gibbs free formation energy at 500K. A) B) Δ T 500 W = -18 kj/mol/mno C) D) Δ T 500 W = 5 kj/mol/mno Δ T 500 W = Δ T 500 W = -13 kj/mol/mno -31 kj/mol/mno To further optimize the ring structure extra oxygen atoms were added inside of the ring using equation (9) which was determined with a formation energy of 19 kj/mol/mno. + (9) > (10) Liu et al.[21] recently examined the formation of triangular ZnO patches on a copper catalyst for CO 2 hydrogenation to methanol. Inspired by the work, similar structures were examined for MnO in this work. To add the extra O and H required to form these structure, reactions (9) and (10) are used. The structure is shown in Table 1-B, however its Gibbs free formation energy of 5 kj/mno indicates that it is unfavorable. Hydrogen termination of the oxygen atoms was also examined, however the increased in stability is not enough to compensate the entropic cost of reaction (10). The sites indicated in orange are thought to be under-coordinated, as such OH termination of these sites was examined as well. Still the entropic cost of O and H addition could not be compensated by the increased stability. Another attempt to optimize the six MnO ring was made by constructing hexagonal patches based on the triangular patches. The structure is displayed in Table 1-C and its formation enthalpy is -22 kj/mol/mno, indicating it is more stable than the six MnO ring. However, the loss in entropy due to the addition of the extra oxygen causes the Gibbs free formation energy to be only -13 kj/mol/mno making it less stable than the six MnO ring. Table 1: a)the 6 MnO ring on a p(6x6) Co(111) surface. b) The triangle structure on a p(6x6) Co(111) surface. c) The hexagonal

11 2) Step surface First, the substitution of Mn into the edge of a step surface using equation (5) was examined. The Gibbs free formation energy of 236 kj/mol/mno indicated that this structure is unfavorable. Afterwards the substitution of MnO into the edge was examined. The most optimal configuration is shown in Table 1-D and has a Gibbs free formation energy of -31 kj/mol/mno. The coverage of this structure was examined however, additional substitutions were determined to be unfavorable. 3) Bader charge analysis In order to understand the driving forces behind the formation of the most favorable structures in Table 1, Bader charge analysis was performed. For the terrace surface this analysis revealed that manganese interferes with the Co-O bonds decreasing their strength but at the same time preventing oxygen from pulling its bonded cobalt atom too much out of the slab which limits the instability generated by the deformation of the cobalt slab. This interference is optimal when it occurs equally on both sides of the manganese, otherwise there is a decrease in stability due to decreased/increased strength of the cobalt oxygen bond. Manganese further donates charge to its bonded cobalt atoms, in the most optimal ring configuration. This stabilizes the cobalt slab by decreasing the Co-Co bond strength between these cobalt atoms and the cobalt atom bonded to oxygen. This is because a higher charge of a cobalt atom means it has a weaker bond with the surrounding atoms, one of which is the cobalt atom bonded to oxygen. The shape and structure of the six MnO ring allows this last effect to influence most cobalt atom bonded to oxygen and also allows the destabilization of the Co-O bonds to occur on both sides of the manganese atom equally. This is why the six MnO ring is most favorable. On a step surface the analysis showed that the substitution of extra MnO into the edge is unfavorable because there is not enough room to accommodate extra Mn in the edge without the Mn atoms shifting out of the edge. Because there is enough room to accommodate Mn in the edge for the single MnO substitution, O can bind strongly to all its bonded atoms without stretching the cobalt manganese bonds limiting the impact on the deformation of the cobalt slab making the structure more favorable. C. Kinetic role of the promoter In order to study the impact of Mn promotion on FTS, the adsorption energies of CO, H, OH and H 2O are examined on a terrace surface and a step surface. 1) Terrace surface Table 2 shows that the six MnO ring only lightly affects the adsorption of CO but has a larger effect on the adsorption of the other three species. Where H and OH adsorption are less favored and H 2O adsorption is more favored on the promoted surface. This indicates that H 2 dissociation to H will become less favorable decreasing the amount of hydrogenating species on the surface. Therefore, an increased selectivity towards higher hydrocarbons is expected as there is less probability that a propagating chain is terminated by hydrogenation. Furthermore, an increase in olefin to paraffin ratio is expected because there is lower probability for olefins to be hydrogenated to paraffins. This is in line with experimental data [4, 6, 7] which shows an increased selectivity to C 5+ and an increase in olefin to paraffin ratio for the C 2 to C 4 fraction. The H and OH adsorption structures show a deformation of the six MnO ring due to the adsorption of the respective species. For H adsorption, the manganese and oxygen closest to the adsorbate are shifted towards the inside of the ring. In the case of OH, only the closest manganese atom is shifted. This destabilize the six MnO ring causing the difference in adsorption energies between the promoted and unpromoted surface. H 2O adsorption shows that H 2O adsorbs onto the six MnO ring which is expected to be the reason for the increase in its adsorption energy. 2) Step surface Table 2 shows that MnO substitution into the edge, with O in an FCC position on top of the edge increases the adsorption strength of CO, H and OH while the adsorption strength of H 2O remains almost the same. The increase in H and OH adsorption with the H 2O adsorption remaining almost the same indicates that the rate of H 2O dissociation will increase. The increased adsorption strength for CO could increase CO dissociation rate. However, the dissociation should be examined through transition state calculation to further gain insights. Table 2 The adsorption energies of key intermediates (CO, H, OH, H2O) on an unpromoted Co(111) and Co(211) surface and a promoted Co(111) and Co(211) surface. Promoted surface is promoted with a six MnO ring for a Co(111) surface and with one MnO substituted Co(211) surface. Electronic adsorption energy [kj/mol] Adsorbed Unpromoted Promoted species Co(111) Co(211) Co(111) Co(211) CO H OH H 2O IV. CONCLUSION A benchmark of different functionals indicates that the usage of hybrid functionals is required to accurately predict the formation enthalpy of the Mn-oxides. However, the usage of hybrid fucntionals is computationally expensive prompting the usage of the vdw-df functional. This functional offers adequate accuracy with manageable calculation time. The experimental and DFT based phase diagrams both confirm that the most prevalent Mn phase is MnO which corresponds with a Mn oxidation state of +2. The experimental phase diagram indicates that this phase is highly stable, the DFT based phase diagram does not show a high stability of this phase which is due to the error on the DFT results. However, as the DFT calculations still predict the correct Mn phase further investigation into location and kinetics is reliable. The most stable MnO structure formed under reaction conditions (500K, 60% conversion) on a terrace surface is the 6 MnO ring configuration, as shown in Table 1-A. On a step surface, the most stable structure is one MnO substitution into the edge, as shown in Table 1-D. Bader charge analysis showed that the six MnO ring is most favorable due to Mn interference with the Co-O bond limiting the extent to which this bond can deform the Co slab. The Mn interference occurs equally on both sides allowing a strong Co- O bond but still limiting the extent of slab deformation. Lastly, Mn donates charge to the Co atoms surrounding the Co atom bonded to oxygen, decreasing the Co-Co bond strength which limits the destabilization due to slab deformation. Single MnO substitution into the edge is most favorable on a step surface as this configuration allows oxygen to bind strongly to all its bonded atoms, allowing it to keep the Mn atom in the edge

12 without stretching the cobalt manganese bonds limiting the impact on the deformation of the cobalt slab. Mn promotion on a terrace surface has been shown to destabilize H and OH surface intermediates and hence affect H 2 dissociation to be less favorable and H 2O adsorption to be more favorable. H and OH adsorption have been shown to destabilize the six MnO ring while H 2O adsorbs onto the six MnO ring instead of onto the surface. These changes are proposed to be the cause for the change in adsorption energy. Mn promotion on a step surface has been shown to affect H 2O dissociation as H and OH adsorption are more favorable while H 2O adsorption is slightly less favorable. CO adsorption is also improved. Further investigation into possible locations and especially the kinetics, through transition state calculations and further adsorption calculations, would provide more insight into the role of Mn promotion in FTS. 19. den Breejen, J.P., et al., On the Origin of the Cobalt Particle Size Effects in Fischer Tropsch Catalysis. Journal of the American Chemical Society, (20): p Bartholomew, C.H. and R.C. Reuel, Cobalt-support interactions: their effects on adsorption and carbon monoxide hydrogenation activity and selectivity properties. Industrial & Engineering Chemistry Product Research and Development, (1): p Kattel, S., et al., Active sites for CO₂ hydrogenation to methanol on Cu/ZnO catalysts. Science, (6331): p REFERENCES 1. Zhuo, M., et al., Density Functional Theory Study of the CO Insertion Mechanism for Fischer Tropsch Synthesis over Co Catalysts. The Journal of Physical Chemistry C, (19): p Ojeda, M., et al., CO activation pathways and the mechanism of Fischer Tropsch synthesis. Journal of Catalysis, (2): p Gunasooriya, G.T.K.K., et al., Key Role of Surface Hydroxyl Groups in C O Activation during Fischer Tropsch Synthesis. ACS Catalysis, (6): p Johnson, G.R., S. Werner, and A.T. Bell, An Investigation into the Effects of Mn Promotion on the Activity and Selectivity of Co/SiO2 for Fischer Tropsch Synthesis: Evidence for Enhanced CO Adsorption and Dissociation. ACS Catalysis, (10): p Morales, F., et al., Mn promotion effects in Co/TiO2 Fischer Tropsch catalysts as investigated by XPS and STEM-EELS. Journal of Catalysis, (2): p den Breejen, J.P., et al., A Highly Active and Selective Manganese Oxide Promoted Cobalt-on-Silica Fischer Tropsch Catalyst. Topics in Catalysis, (13): p Morales, F., et al., In Situ X-ray Absorption of Mn/Co/TiO2 Catalysts for Fischer Tropsch Synthesis. The Journal of Physical Chemistry B, (41): p Johnson, G.R. and A.T. Bell, Effects of Lewis acidity of metal oxide promoters on the activity and selectivity of Co-based Fischer Tropsch synthesis catalysts. Journal of Catalysis, : p Cerda, J.R., et al., Epitaxial growth of cobalt films on Cu(100): a crystallographic LEED determination. Journal of Physics: Condensed Matter, (14): p Häglund, J., et al., Theory of bonding in transition-metal carbides and nitrides. Physical Review B, (16): p Pacalo, R.E. and E.K. Graham, Pressure and temperature dependence of the elastic properties of synthetic MnO. Physics and Chemistry of Minerals, (1): p Baron, V., et al., The influence of iron substitution on the magnetic properties of hausmannite, Mn2+(Fe,Mn)3+2O4, in American Mineralogist p Klein, H., Structure solution of oxides from zone axes precession electron diffraction data, in Zeitschrift für Kristallographie - Crystalline Materials p Bolzan, A., et al., Powder Neutron Diffraction Study of Pyrolusite, β-mno₂. Australian Journal of Chemistry, (6): p Hafner, J. and D. Hobbs, Understanding the complex metallic element Mn. II. Geometric frustration in \ensuremath{\beta}-mn, phase stability, and phase transitions. Physical Review B, (1): p Dion, M., et al., Van der Waals Density Functional for General Geometries. Physical Review Letters, (24): p Tan, K.F., et al., Effect of boron promotion on the stability of cobalt Fischer Tropsch catalysts. Journal of Catalysis, (1): p Ma, X., et al., Carbon monoxide adsorption and dissociation on Mn-decorated Rh(1 1 1) and Rh(5 5 3) surfaces: A first-principles study. Catalysis Today, (1): p

13 Table of Contents Chapter 1Introduction Justification Structure of this work Bibliography Fischer-Tropsch synthesis Fischer-Tropsch synthesis Process Fischer-Tropsch Catalysts Fischer-Tropsch mechanism Promoters General introduction to promoters and their role Promoters in Co-based Fischer-Tropsch synthesis Manganese as a promoter for Co-based Fischer Tropsch synthesis Oxidation state Location Preparation method Effect on activity and selectivity Bibliography Chapter 3 :Methods and Materials Density Functional Theory Catalyst and promoter bulk models Cobalt catalyst models Promoter bulk models Experimental phase diagram VASP calculations Gas phase calculations i

14 3.4.2 Adsorption calculations Bulk calculations Frequency calculations Bader charge Transition state calculations DFT Phase diagram construction Bibliography Chapter4:Nature of the promoter Thermodynamics Experimental phase diagram DFT based phase diagram Bulk structures Benchmark of different functionals Challenges in constructing the phase diagram Phase diagram Comparison DFT and Ekvicalc phase diagram Bibliography Chapter5:Location Terrace surface Substitution into the terrace Increasing MnO coverage Saturation MnO coverage Non-stoichiometric patches Conclusion Step surface Single MnO substitution Increasing MnO coverage ii

15 5.2.3 Conclusion Bader charge analysis Terrace surface Step Surface Bibliography Chapter6:Kinetics Terrace surface Step surface Bibliography Chapter7:Conclusion Nature of the promoter Location of the promoter Kinetics Future work Appendix A: Ekvicalc memo Appendix B: List of calculations iii

16 List of Figures Figure 1-1 The bending of the C-O bond by Mn promotion in Fischer-Tropsch synthesis illustrated. Where O interacts with the metal-oxide and C with the active metal. This interaction is thought to destabilize the C-O bond facilitating C-O activation, a crucial step if Fischer-Tropsch synthesis. As reproduced from [10]... 3 Figure 2-1 A schematic representation of the FTS process, first syngas production takes place by autothermal reforming or steam reforming of methane. Secondly the syngas is converted to hydrocarbons and syncrude in the Fischer-Tropsch reactor. Lastly the reactor product is hydrocracked and separated into commercial products. As reproduced form [3] Figure 2-2 The main reaction path for the CO insertion mechanism. R signifies hydrogen or an alkyl group.(-o*= CO scission, +CO*=CC coupling, +H*= hydrogenation), as adapted from [5] Figure 2-3 The direct CO dissociation pathway and the hydroxyl assisted CO activation pathway Figure 2-4 Mn K-edge XANES spectra for a Mn/Co =0.1 catalyst, comparing the passivated, reduced and during reaction state. The reduced state data was acquired after reducing the catalysts for 2 h at 673K in H2. The FTS reaction data was acquired after exposing the reduced catalyst to syngas at 493 K for 6 h. As reproduced from [61] Figure 2-5 Elemental map of a Co Mn catalyst (directly reduced, Mn/Co = 0.5) obtained by STEM EDS, the Co channel is displayed, on the right is the Mn channel. Images were acquired at 200 kv accelerating voltage and 0.6 na beam current. As reproduced from [66] Figure 2-6 (a) Methane (b) C5+ selectivity (molar carbon basis) of the Co and Co-Mn (Mn/Co =0.5) catalysts under FTS. Experimental conditions: 493 K, 1 bar H2/CO = 2 ; data was extrapolated to 0% CO conversion. As reproduced from [66] Figure 2-7 Effect of Mn promotion on (a) product selectivity and (b) O/P ratio for the C2-C4 product fraction. Measured at 493 K and 1 atm with a feed ratio H2/CO/Ar = 2/1/0.2, flows = ml/min, catalyst mass = 100 mg, CO conversion = 2%. As reproduced from [71] iv

17 Figure 2-8 (a) An illustration of the formation of promoted active sites by Mn promotion of Co, note the decrease in total (promoted and unpromoted) available surface (or active sites) with increased loading. (b) A comparison of the rates per gram of Co for several Mn/Co ratios at 493K. Data is based on Co consumption at 0% conversion. The curves are created by fitting the data to the rate law seen in eq1. As reproduced from [61] Figure 2-9 (a) Selectivity towards CH4, C2 C4, and C5+ as a function of the promoter Lewis acidity. Here, NM-2δM is used as a proxy for the relative Lewis acidity of the promoter oxide, and the unpromoted catalyst was assigned a value of 0. (b) The selectivity towards C5+ as a function of pressure for the different metal oxide promoters. The selectivity correspond to those where the promoter loading is high enough such that the product selectivity were insensitive to promoter loading. The data were collected at 493 K at atmospheric pressure with a feed composition of 7% Ar, 31% CO, and 62% H2 ( ml/min) and were extrapolated to 0% CO conversion. As reproduced from [67] Figure 2-10 (a) FTS turnover frequencies (b) Rates of CO consumption per gram Co (right) as a function of pressure for the unpromoted and metal oxide-promoted Co/SiO2 catalysts. The catalyst are: La/Co = 0.1; Ce/Co = 2.0; Mn/Co = 0.1; Gd/Co = 1.0; Zr/Co = 1.0. The data were collected at 493 K with a reactor inlet feed of 7% Ar, 31% CO, and 62% H2. All data points were extrapolated to 0% CO conversion. The curves in each plot are fits to the data using the rate law given by eq. 1. For the left figure it is also assumed that the Co nanoparticles are covered by half a monolayer of promoter. As reproduced from [67] Figure 3-1 The user interface of the Ekvicalc software package: window 1 is used to input commands; window 2 displays the selected species to enter amounts; window 3 shows the total amount of mol of each reactant; window 4 shows the conditions temperature, pressure and volume (as well as how it is varied); window 5 shows the total amounts of mol of each atom; window 6 shows the solid (and liquid) phases under consideration as well as their activities at the end of each calculations; window 7 shows the gas phases under consideration as well as their partial pressures at the end of each calculation Figure 3-2 The adsorption positions considered in this thesis: B = bridge, T = top, F = fcc, H= hcp or hollow[17] Figure 4-1 The phase diagram for Co as reproduced from [2] v

18 Figure 4-2 The experimental phase diagram, constructed using Ekvicalc software, for the Mnoxides Figure 4-3 A comparison between the experimental formation energy and the calculated formation energy, the values for the PBE, PBEU4, PBE0 and HSE functionals are taken from [5] Figure 4-4 An illustration of how the errors on two-phase transition can cause the inversion of these transitions Figure 4-5 The DFT based phase diagram Figure 4-6 A comparison between the DFT phase diagram (b) and the Ekvicalc phase diagram (a) Figure 5-1 A) The Co(211) slab without MnO addition. B) The 4 MnO structure with the Mn atoms at full scale vi

19 List of Tables Table 2-1 Product properties comparison of high temperature and low temperature Fischer- Tropsch synthesis, based on typical industrial operation, as reproduced from [7]. 9 Table 2-2 A comparison the of Ni-,Fe-,Co- and Ru-based FTS catalysts reproduced from [3] Table 2-3 Examples of promoters in specific process and their effects, as reproduced from [27] Table 2-4 An overview of the promotion effects of different elements used in literature on Co based Fischer-Tropsch catalysts, as reproduced form [27] Table 3-1 The four cobalt catalyst models used in this thesis, respectively the p(3x3) Co(111) model, the p(4x4) Co(111) model, the p(6x6) Co(111) model and the p(4x4) Co(211) model Table 3-2 The five promoter bulk models used in this thesis, respectively the Mn(gamma) model, the MnO model, Mn3O4 model, the Mn2O3 model and the MnO2 model. 44 Table 3-3The source of the model and Kpoints used for the different oxidations states of considered Mn in this study Table 4-1 The oxidation state and energy of the different Mn species considered using the vdw- DF functional Table 4-2 The enthalpy and entropy of the gas phase H2O and H2 system as obtained from the frequency calculations using the vdw-df functional Table 4-3 Reaction enthalpy, entropy and Gibbs free energy of the reduction reactions of the Mn-oxides based on the vdw-df calculations and experimental data [6, 7] Table 4-4 The error on the calculated reaction enthalpy, entropy and free Gibbs energy for the individual transitions. The error is calculated as by subtracting the experimental value from the calculated one. T is taken at 500 K Table 4-5 The final reactions used in the construction of the phase diagram and the reaction enthalpy, entropy and free Gibbs energy at 500 K (based on vdw-df calculations and expertimental data [6, 7]) Table 4-6 The error on the calculated reaction enthalpy, entropy and free Gibbs energy for the transitions based on MnO2. The error is calculated as by subtracting the experimental value from the calculated one. T is taken at 500 K vii

20 Table 5-1 The catalyst models with a top and side view, a description, the formation enthalpy at 500 K and Gibbs free formation energy at 500 K for the Co(111) slab and for Mn substituted at surface, subsurface and bulk positions in the Co(111) slab Table 5-2 The different configurations for the substitution of MnO into the surface of the Co(111) slab. A top view, side view, description and the Gibbs free energy of formation at 500 K are shown Table 5-3 The single MnO configurations examined on a Co(111) slab with a top and side view as well as a description of the configuration Table 5-4 An overview of the 6 optimized configurations for the addition of 1 MnO to a Co(111) slab. A top and side view are given as well as a description of the configuration and the Gibbs free formation energy at 500 K Table 5-5 An overview of the optimized two MnO structures on a Co(111) slab. A top and side view are given as well as the Gibbs free formation energy at 500 K Table 5-6 An overview of the optimized three MnO structures on a Co(111) slab. A top and side view are given as well as the Gibbs free formation energy at 500 K Table 5-7 An overview of the optimized four MnO structures on a Co(111) slab. A top and side view are given as well as the formation enthalpy at 500 K and the free Gibbs formation energy at 500 K Table 5-8 An overview of the optimized five MnO structures on a Co(111) slab. A top and side view are given as well the Gibbs free formation energy at 500 K Table 5-9 An overview of the optimized six MnO structures on a Co(111) slab. A top and side view are given as well as the Gibbs free formation energy at 500 K Table 5-10 The optimized eight MnO structure on a Co(111) slab. A top and side view are given as well as the formation enthalpy at 500 K and the free Gibbs formation energy at 500 K Table 5-11 An overview of the optimized monolayer structures on a Co(111) slab. A top and side view are given as well as the Gibbs free formation energy at 500 K Table 5-12 The optimized ring with additional oxygen inside structure on a Co(111) slab. A top and side view are given as well as the formation enthalpy at 500 K and the Gibbs free formation energy at 500 K. Additional oxygen is added using H2O H2+ O* and additional hydrogen using 12H2 H* Table 5-13 An overview of the optimized triangle structures on a Co(111) slab. A top and side view are given as well as the formation enthalpy at 500 K and the Gibbs free viii

21 formation energy at 500 K. Additional oxygen is added using H2O H2+O* and additional hydrogen using 12H2 H* Table 5-14 An overview of the optimized hexagon structures on a Co(111) slab. A top and side view are given as well as the formation enthalpy at 500 K and the Gibbs free formation energy at 500 K. Additional oxygen is added using H2O H2+O* and additional hydrogen using 12H2 H* Table 5-15 The optimized Mn substitution into the Co(211) slab. A top and side view are given as well as the Gibbs free formation energy at 500 K Table 5-16 An overview of the optimized MnO substitutions into a Co(211) slab. A top and side view are given as well as the Gibbs free formation energy at 500 K Table 5-17 An overview of the MnO structures substituted in a Co(211) slab used to determine coverage. A top and side view are given as well as the Gibbs free formation energy at 500 K and the Gibbs free addition energy at 500 K Table 5-18 Bader charge results for the 6 MnO ring on a p(6x6) slab, the 6 MnO ring on a p(4x4) slab and the 5 MnO ring on a p(4x4) slab. Only the values for the surface layer of the cobalt slab are reported as the lower layers are less relevant. Charge results are calculated using equation 5-8. Formation energies of the structures are calculated as discussed in the corresponding chapters. For the 6 MnO ring p(6x6) only the Co charges in the highlighted (in orange) section are displayed as the charges outside are of no importance. The numbers indicated on the second figure for the 5 MnO ring p(4x4) are the manganese atom bonded to these cobalt atoms Table 5-19 The results of a Bader charge analysis on the addition of MnO to the Co(211) surface, additions from one to four MnO are considered. Only the charges of the top layer of the slab are reported as the lower layers are less relevant. Charges indicated in purple are the charge of the manganese atoms while charges indicated in bold are at the edge. The values indicated in red are the cobalt atoms bonded to oxygen Table 6-1 The adsorption energies of key intermediates (CO, H, OH, H2O) on a unpromoted Co(111) surface and a six MnO ring promoted Co(111) surface Table 6-2 The structure of the most favorable CO, H, OH and H2O adsorption on a six MnO ring promoted Co(111) surface. Note the deformation of the six MnO ring for the H and OH adsorption and the H2O adsorbing onto the six MnO ring ix

22 Table 6-3 The adsorption energies of key intermediates (CO, H, OH, H2O) on a unpromoted Co(111) surface and a one MnO substituted (or promoted) Co(211) surface Table 6-4 The structure of the most favorable CO, H, OH and H2O adsorption on a one MnO substitution promoted Co(211) surface Table B-1: Calculations performed during this thesis along with their location on the CD-ROM Table 2-1 Product properties comparison of high temperature and low temperature Fischer- Tropsch synthesis, based on typical industrial operation, as reproduced from [7]. 9 Table 2-2 A comparison the of Ni-,Fe-,Co- and Ru-based FTS catalysts reproduced from [3] Table 2-3 Examples of promoters in specific process and their effects, as reproduced from [27] Table 2-4 An overview of the promotion effects of different elements used in literature on Co based Fischer-Tropsch catalysts, as reproduced form [27] Table 3-1 The four cobalt catalyst models used in this thesis, respectively the p(3x3) Co(111) model, the p(4x4) Co(111) model, the p(6x6) Co(111) model and the p(4x4) Co(211) model Table 3-2 The five promoter bulk models used in this thesis, respectively the Mn(gamma) model, the MnO model, Mn3O4 model, the Mn2O3 model and the MnO2 model. 44 Table 3-3The source of the model and Kpoints used for the different oxidations states of considered Mn in this study Table 4-1 The oxidation state and energy of the different Mn species considered using the vdw- DF functional Table 4-2 The enthalpy and entropy of the gas phase H2O and H2 system as obtained from the frequency calculations using the vdw-df functional Table 4-3 Reaction enthalpy, entropy and Gibbs free energy of the reduction reactions of the Mn-oxides based on the vdw-df calculations and experimental data [6, 7] Table 4-4 The error on the calculated reaction enthalpy, entropy and free Gibbs energy for the individual transitions. The error is calculated as by subtracting the experimental value from the calculated one. T is taken at 500 K Table 4-5 The final reactions used in the construction of the phase diagram and the reaction enthalpy, entropy and free Gibbs energy at 500 K (based on vdw-df calculations and expertimental data [6, 7]) x

23 Table 4-6 The error on the calculated reaction enthalpy, entropy and free Gibbs energy for the transitions based on MnO2. The error is calculated as by subtracting the experimental value from the calculated one. T is taken at 500 K Table 5-1 The catalyst models with a top and side view, a description, the formation enthalpy at 500 K and Gibbs free formation energy at 500 K for the Co(111) slab and for Mn substituted at surface, subsurface and bulk positions in the Co(111) slab Table 5-2 The different configurations for the substitution of MnO into the surface of the Co(111) slab. A top view, side view, description and the Gibbs free energy of formation at 500 K are shown Table 5-3 The single MnO configurations examined on a Co(111) slab with a top and side view as well as a description of the configuration Table 5-4 An overview of the 6 optimized configurations for the addition of 1 MnO to a Co(111) slab. A top and side view are given as well as a description of the configuration and the Gibbs free formation energy at 500 K Table 5-5 An overview of the optimized two MnO structures on a Co(111) slab. A top and side view are given as well as the Gibbs free formation energy at 500 K Table 5-6 An overview of the optimized three MnO structures on a Co(111) slab. A top and side view are given as well as the Gibbs free formation energy at 500 K Table 5-7 An overview of the optimized four MnO structures on a Co(111) slab. A top and side view are given as well as the formation enthalpy at 500 K and the free Gibbs formation energy at 500 K Table 5-8 An overview of the optimized five MnO structures on a Co(111) slab. A top and side view are given as well the Gibbs free formation energy at 500 K Table 5-9 An overview of the optimized six MnO structures on a Co(111) slab. A top and side view are given as well as the Gibbs free formation energy at 500 K Table 5-10 The optimized eight MnO structure on a Co(111) slab. A top and side view are given as well as the formation enthalpy at 500 K and the free Gibbs formation energy at 500 K Table 5-11 An overview of the optimized monolayer structures on a Co(111) slab. A top and side view are given as well as the Gibbs free formation energy at 500 K Table 5-12 The optimized ring with additional oxygen inside structure on a Co(111) slab. A top and side view are given as well as the formation enthalpy at 500 K and the Gibbs free formation energy at 500 K. Additional oxygen is added using H2O H2+ O* and additional hydrogen using 12H2 H* xi

24 Table 5-13 An overview of the optimized triangle structures on a Co(111) slab. A top and side view are given as well as the formation enthalpy at 500 K and the Gibbs free formation energy at 500 K. Additional oxygen is added using H2O H2+O* and additional hydrogen using 12H2 H* Table 5-14 An overview of the optimized hexagon structures on a Co(111) slab. A top and side view are given as well as the formation enthalpy at 500 K and the Gibbs free formation energy at 500 K. Additional oxygen is added using H2O H2+O* and additional hydrogen using 12H2 H* Table 5-15 The optimized Mn substitution into the Co(211) slab. A top and side view are given as well as the Gibbs free formation energy at 500 K Table 5-16 An overview of the optimized MnO substitutions into a Co(211) slab. A top and side view are given as well as the Gibbs free formation energy at 500 K Table 5-17 An overview of the MnO structures substituted in a Co(211) slab used to determine coverage. A top and side view are given as well as the Gibbs free formation energy at 500 K and the Gibbs free addition energy at 500 K Table 5-18 Bader charge results for the 6 MnO ring on a p(6x6) slab, the 6 MnO ring on a p(4x4) slab and the 5 MnO ring on a p(4x4) slab. Only the values for the surface layer of the cobalt slab are reported as the lower layers are less relevant. Charge results are calculated using equation 5-8. Formation energies of the structures are calculated as discussed in the corresponding chapters. For the 6 MnO ring p(6x6) only the Co charges in the highlighted (in orange) section are displayed as the charges outside are of no importance. The numbers indicated on the second figure for the 5 MnO ring p(4x4) are the manganese atom bonded to these cobalt atoms Table 5-19 The results of a Bader charge analysis on the addition of MnO to the Co(211) surface, additions from one to four MnO are considered. Only the charges of the top layer of the slab are reported as the lower layers are less relevant. Charges indicated in purple are the charge of the manganese atoms while charges indicated in bold are at the edge. The values indicated in red are the cobalt atoms bonded to oxygen Table 6-1 The adsorption energies of key intermediates (CO, H, OH, H2O) on a unpromoted Co(111) surface and a six MnO ring promoted Co(111) surface xii

25 Table 6-2 The structure of the most favorable CO, H, OH and H2O adsorption on a six MnO ring promoted Co(111) surface. Note the deformation of the six MnO ring for the H and OH adsorption and the H2O adsorbing onto the six MnO ring Table 6-3 The adsorption energies of key intermediates (CO, H, OH, H2O) on a unpromoted Co(111) surface and a one MnO substituted (or promoted) Co(211) surface Table 6-4 The structure of the most favorable CO, H, OH and H2O adsorption on a one MnO substitution promoted Co(211) surface Table B-1: Calculations performed during this thesis along with their location on the CD-ROM xiii

26 List of symbols Acronyms and abbreviations (C)NEB DFT FCC FT FTS GTL HCP HTFTS LTFTS ML PBE revpbe RWGS SMSI STEM EDS TOF VASP vdw-df WGS XANES ZPE (Climbing image) nudged elastic band Density functional theory face-centered cubic Fischer-Tropsch Fischer-Tropsch synthesis Gas to liquid hexagonal close-packed high temperature Fischer-Tropsch synthesis low temperature Fischer-Tropsch synthesis monolayer Perdew-Burke-Ernzerhof functional Revised Perdew-Burke-Ernzerhof functional Reverse Water Gas Shift strong metal-support interactions Scanning Transmission Electron Microscopes energy dispersive X-ray spectroscopy Turnover Frequency Vienna Ab-initio Simulation Package Van der Waals Density Functional Water Gas Shift X-ray absorption near edge structure Zero Point energy Symbols [ 0] [ 0] [ 0] Rotational enthalpy correction Translational enthalpy correction Vibrational enthalpy correction xiv

27 Δ Δ 500 Δ /Δ Δ Δ 500 Δ /Δ Δ Δ /Δ Gibbs free adsorption energy Gibbs free formation enthalpy Gibbs free reaction energy Adsorption enthalpy Formation enthalpy Reaction enthalpy Adsorption entropy Reaction entropy total electronic energy without entropy of the adsorbate in a vacuum electronic adsorption energy total electronic energy without entropy of the slab total electronic energy without entropy of the slab with the adsorbate adsorbed Rotational entropy contribution Rotational entropy contribution (for a linear molecule) Translational entropy contribution Vibrational entropy contribution Boltzmann constant ( 1.38*10 23 J/K)!" Partial pressure of hydrogen!"# Partial pressure of water $ Vibrational frequencies of the system h K T & ' Planck constant (6.626*10-34 Js) Equilibrium constant Temperature Universal gas constant (8.314 J/(mol*K)) Symmetry number xv

28 xvi

29 Chapter 1: Introduction Increasing legislative and social incentives to decrease CO2 emissions drive the development and improvement of CO2 conversion processes. Fischer-Tropsch synthesis is one such process and has been extensively studied for over 50 years. Improving the Fischer-Tropsch product distribution by decreasing methane selectivity is especially important to increase its potential as a CO2 conversion process. Computational catalysis can aid in understanding the role of a promoter in Fischer-Tropsch synthesis, which allows optimization of promoters making the process economically more interesting. 1

30 1.1 Justification Fischer-Tropsch synthesis (FTS) is an attractive technology to convert natural gas, CO2 and waste biomass to clean transportation fuels and chemicals. FTS converts synthesis gas, a mixture of CO and H2, to long-chain alkanes, alkenes, oxygenates and water. [1] However, recent research has led to the development of modified-fts which is able to convert highly concentrated CO2 to hydrocarbons and fuels. This technology can potentially allow production of CO2 neutral fuels. [2] Regular FTS can also be used in decreasing CO2 emissions by using gas to liquid (GTL) technology to convert the typically flared or dumped associated natural gas (gas produced at oil wells) to more easily transportable liquid products. [3, 4] Furthermore, GTL technology can also be used to monetize stranded gas increasing the available world gas supply, stranded gas is natural gas that is difficult to exploit by conventional transport technologies (e.g.: pipelines). [4] Supported Cobalt (Co) catalysts are often preferred for FTS due to their high activity, selectivity towards long chain hydrocarbons, low CO2 selectivity and low water-gas shift activity. [5] The use of non-catalytically active elements to promote catalyst activity, selectivity and stability is a well-explored topic on the context of Co-based FTS catalysts. Most studies have focused on the addition of noble metals to increase reduction of Co to its active metallic state. [6-9] However, many of the studied elements have yielded limited or even negative results. Manganese (Mn) is one of the elements which has been demonstrated to consistently decrease the selectivity toward methane while increasing the selectivity towards long chain hydrocarbons as well as increasing the olefin to paraffin ratio for the C2 to C4 fraction in Co-based FTS. [10] Moreover, it is found that Mn forms a variety of oxide structures associated with the active Co phase. While the effects of Mn promotion on FTS over Co are well documented, the role by which Mn promotion effects catalyst activity and selectivity remains speculative. [10-12] The most frequently cited explanation for the effects of metal oxide promoters for CO hydrogenation over transition metals is that CO adsorbed at metal-metal oxide interface can interact with both with metal through the C atom and with a cation of the oxide through the O atom, as is illustrated in Figure 1-1. Where the interaction between Mn and oxygen is thought to destabilize the C-O bond, facilitating CO hydrogenation and thereby CO dissociation (hydrogen assisted CO dissociation). [10] The Mn promoter is hypothesized to serve as a Lewis acid which is confirmed by another study, which examines different Lewis acids as Co-based Fischer 2 Introduction

31 Tropsch promoters and shows a correlation between lewis acidity and promotion effect. [13] Figure 1-1 The bending of the C-O bond by Mn promotion in Fischer-Tropsch synthesis illustrated. Where O interacts with the metal-oxide and C with the active metal. This interaction is thought to destabilize the C-O bond facilitating C-O activation, a crucial step if Fischer- Tropsch synthesis. As reproduced from [10] Introduction 3

32 1.2 Structure of this work In this work, computational catalysis is used to firstly determine the most prevalent oxidation state of Mn under typical Fischer-Tropsch reaction conditions (500 K, 20 bar and 60% conversion). This is required to confirm that Density functional theory calculations predict the correct oxidation state, which has been experimentally determined to be MnO. [12] Furthermore, the most prevalent oxidation state is required in order to use the correct basis for location investigation. Secondly, the location of the Mn promoter is examined. To this end substitution and adsorption of MnO structures are considers on a Co terrace (Co(111)) surface and a Co step (Co(211)) surface. Addition of extra O and H is also examined. Lastly, the kinetics are examined. Allowing insight into the impact of Mn promotion on the Fischer-Tropsch process which could be compared to experimental observations. 4 Introduction

33 1.3 Bibliography 1. Gunasooriya, G.T.K.K., et al., Key Role of Surface Hydroxyl Groups in C O Activation during Fischer Tropsch Synthesis. ACS Catalysis, (6): p Rodemerck, U., et al., Catalyst Development for CO2 Hydrogenation to Fuels. ChemCatChem, (7): p Gas Flaring in Nigeria: Some Aspects for Accelerated Development of SasolChevron GTL Plant at Escravos. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, (15): p Wood, D.A., C. Nwaoha, and B.F. Towler, Gas-to-liquids (GTL): A review of an industry offering several routes for monetizing natural gas. Journal of Natural Gas Science and Engineering, : p Iglesia, E., Design, synthesis, and use of cobalt-based Fischer-Tropsch synthesis catalysts. Applied Catalysis A: General, (1): p Tsubaki, N., S. Sun, and K. Fujimoto, Different Functions of the Noble Metals Added to Cobalt Catalysts for Fischer Tropsch Synthesis. Journal of Catalysis, (2): p Jacobs, G., et al., Fischer Tropsch synthesis: deactivation of noble metal-promoted Co/Al2O3 catalysts. Applied Catalysis A: General, (1 2): p Jacobs, G., et al., Fischer Tropsch synthesis: support, loading, and promoter effects on the reducibility of cobalt catalysts. Applied Catalysis A: General, (1 2): p Jacobs, G., et al., Fischer Tropsch synthesis: Temperature programmed EXAFS/XANES investigation of the influence of support type, cobalt loading, and noble metal promoter addition to the reduction behavior of cobalt oxide particles. Applied Catalysis A: General, (2): p Johnson, G.R., S. Werner, and A.T. Bell, An Investigation into the Effects of Mn Promotion on the Activity and Selectivity of Co/SiO2 for Fischer Tropsch Synthesis: Evidence for Enhanced CO Adsorption and Dissociation. ACS Catalysis, (10): p den Breejen, J.P., et al., A Highly Active and Selective Manganese Oxide Promoted Cobalt-on-Silica Fischer Tropsch Catalyst. Topics in Catalysis, (13): p Morales, F., et al., In Situ X-ray Absorption of Mn/Co/TiO2 Catalysts for Fischer Tropsch Synthesis. The Journal of Physical Chemistry B, (41): p Johnson, G.R. and A.T. Bell, Effects of Lewis acidity of metal oxide promoters on the activity and selectivity of Co-based Fischer Tropsch synthesis catalysts. Journal of Catalysis, : p Introduction 5

34 6 Introduction

35 Literature survey The literature survey begins with a discussion on Fischer-Tropsch process, catalysts used in the process and brief summary on two dominant reaction mechanisms. i.e. carbide and CO insertion mechanisms. In the second section, a general introduction to promoters are provided and their role as structural and electronic promoters is discussed. Next, promoters commonly used in Cobased Fischer-Tropsch synthesis are introduced. In the final section, manganese as a promoter for Fischer-Tropsch synthesis is discussed and existing literature on the oxidation state and location of manganese under Fischer-Tropsch reaction conditions are explored. Moreover, the impact of the preparation method on location and oxidation state is discussed. The effect of manganese promotion on selectivity and activity under Fischer-Tropsch reaction conditions is examined and the effect of the Lewis acidity of the promoting element on activity and selectivity is discussed. 7

36 Fischer-Tropsch synthesis Fischer-Tropsch synthesis Process Fischer-Tropsch synthesis (FTS) is a process developed in the 1920 s by Frans Fischer and Hans Tropsch [1]. FTS allows conversion of coal into liquid fuels, a necessity for oil lacking countries such as Germany during world war 2 or South-Africa during the apartheid regime. [2] Recent years Fischer-Tropsch synthesis is used in the PEARL GTL project in Qatar where it is used to convert natural gas to liquids such as kerosene, gasoline and naphtha. [3] The general reaction in the FTS process is the conversion of CO and H2 to hydrocarbons and water (eq 2-1), however the water gas shift (WGS) reaction must also be considered for iron catalysts (eq 2-2). CO2 H CH H O (eq 2-1) COH O H CO (eq 2-2) The CO/H2 mixture is usually called synthesis gas or syngas in short, it is generally produced from coal, biomass or methane, by either partial oxidation or steam reforming. The H2/CO ratio of the feed is an important aspect of the Fischer-Tropsch process and a ratio of 2 would be stoichiometric based on eq 2-1. A lower H2/CO ratio will decrease the hydrogenation activity and increasing the amount of surface CHx yielding a higher chain growth probability. An increase in chain growth probability signifies a move towards longer hydrocarbon products, an increase in olefins will also be detected as there hydrogenation rate to paraffin is decreased. [4] Coal and biomass yield a low H2/CO ratio while methane yields a much higher ratio as it contains more hydrogen. It is possible to increase the H2/CO ratio using the WGS reaction (eq 2-2) by adding an additional reactor containing a WGS catalyst before the FTS reactor or by using an iron catalyst in the FTS reactor (which innately promotes WGS). Note that a too low H2/CO ratio will promote carbon formation and reduction in catalysts lifetime. An advantage of FTS is its ability to produce very clean fuels and therefore, there is less need for additional treatments such as desulphurization. Synthetic diesel fuels also tend to have a high cetane number and low NOx emission. Furthermore, when using natural gas or biomass as a feedstock it is possible to monetize difficult to market gas and to create renewable fuels. Natural gas produced from crude oil wells provides a good opportunity for FTS as it is possible to convert this gas into a more easily transportable liquid product. Current practice of flaring or 8 Literature survey

37 even dumping methane increases greenhouse gases and therefore converting it to a useable liquid provides a reduction in greenhouse gas emissions. [3] In Figure 2-1 the FTS process is illustrated. Firstly, the syngas is produced form natural gas, coal or biomass by steam reforming or partial oxidation. Secondly, the syngas is converted to hydrocarbons in the FTS reactor. In this case a slurry reactor is illustrated. Generally, the reaction is performed at low temperature of 500 K and high pressure of 20 bar. [5] Lastly, the hydrocarbon product is upgraded through hydrocracking yielding a high value product fraction such as diesel and naphtha. It has to be noted that FTS can also be more focused on the production of olefins, like some of the SASOL plants in South-Africa. [6] The FTS process focused on production of wax is called low temperature Fischer-Tropsch synthesis (LTFTS), whereas when the focus is olefins it is called high temperature Fischer-Tropsch synthesis (HTFTS) [7]. A general product distribution for both is shown in Table 2-1. Table 2-1 Product properties comparison of high temperature and low temperature Fischer- Tropsch synthesis, based on typical industrial operation, as reproduced from [7]. FTS product property HTFTS LTFTS Carbon number range C1-C30 C1-C120 Main product C2-C10 alkenes waxes Normal product phases* -Gases (C1-C4) 20-25% 5-10% -Oil 20-25% 15-20% -Waxes 0% 20-25% -Aqueous organics ± 5% 1-2% -Water 45-50% ** 50-55% ** Organic compound classes* -Alkanes (paraffins) 20-30% Major product (>70%) -Cycloalkanes (naphtenes) <1% <1% -Alkenes (olefins) Major product (>50%) 15-20% -Aromatics 1-5% <1% -oxygenates 10-15% ± 5% *Percentages are mass based **Closed gas loop, i.e., no net water gas shift conversion Literature survey 9

38 Operating the reactor at higher temperatures than 500K would increase CO conversion. However, an unwanted decrease in selectivity towards olefins and higher hydrocarbons is to be expected. The increased hydrogenation activity due to the higher temperature is responsible as it promotes the saturation of olefins and the formation of methane. [4, 8] Increasing the reaction pressure further would also increases the selectivity towards higher hydrocarbons but also increases the cost of the reactor vessel. [8] Figure 2-1 A schematic representation of the FTS process, first syngas production takes place by autothermal reforming or steam reforming of methane. Secondly the syngas is converted to hydrocarbons and syncrude in the Fischer-Tropsch reactor. Lastly the reactor product is hydrocracked and separated into commercial products. As reproduced form [3] Fischer-Tropsch Catalysts Most of the Group VIII transition metals are active for the FTS reaction. The ones which provide sufficient activity to be used commercially are Ni, Co, Fe and Ru. The choice of catalyst primarily depends on the syngas source, catalysts price and the desired products. When using coal or biomass, the low H2/CO ratio requires the use of iron based catalyst as they have a high water gas shift activity (see R2). [3] This increases the amount of hydrogen while rejecting carbon in the form of CO2. Using a low H2/CO2 ratio is detrimental as it promotes the formation of coke on the catalyst. [9] This is especially undesirable on the more expensive catalysts, such as those based on Co and Ru, as it would decrease their lifetime resulting in an increased operating cost. For a natural gas feedstock the usage of a Co or Ru based catalyst is more appropriate due to their lower water gas shift activity and higher Fischer-Tropsch activity compared to iron based catalyst. Lastly, Ni based catalysts have a very high hydrogenation activity which causes a high yield of methane.[3] In Table 2-2 several key characteristics of the above catalyst material are provided. From this table it can be derived that Co and ruthenium based catalysts require a long lifetime due to their high price. Fe based catalysts will produce 10 Literature survey

39 more oxygenates when compared to the other materials due to its low hydrogenation activity. However a higher hydrogenation activity will also reduce the amount of olefins produced which is unfavorable if the aim is the production of olefins. Co based catalysts tend to be commercially the most interesting due their high selectivity towards long chain paraffins and long catalyst lifetime. [10] Table 2-2 A comparison the of Ni-,Fe-,Co- and Ru-based FTS catalysts reproduced from [3] Active metal Price FTS activity WGS activity Hydrogenation activity Ni / Fe Co /- +++ Ru /- +++ Often the catalyst are supported on a high surface area support such as alumnia, titania or silica. This support material primarily provides mechanical strength and thermal resistance for the catalyst nanoparticles. The most important function of a support is, of course, the dispersion of the active metal phase over a large surface area, minimizing the required amount by increasing contact area to maintain catalyst activity. When considering supported Co catalysts for FTS it is important that the nanoparticles are not smaller than 8-10 nm as the catalysts will then show an increase in methane selectivity and a decrease in turn over frequency. [11-13] It is also important to note that FTS catalysts are sensitive to sulfur poisoning, especially Co based catalysts. Sulfur blocks active sites on the catalyst surface decreasing the activity and through decreasing conversion and associated steam generation also affects selectivity, pushing it towards lower hydrocarbons. [4] It is known that Iron catalyst have a higher resistance towards sulfur poisoning. [3] Fischer-Tropsch mechanism The exact mechanism of Fischer-Tropsch synthesis is highly debated and two main mechanisms are proposed: the CO insertion mechanism and the carbide mechanism. [14, 15] The carbide mechanism, which was originally proposed by Fischer and Tropsch, considers C- O activation to occur via direct CO dissociation. Carbide species are then hydrogenated to CHx species which are used as monomers for chain growth. Note that C-O scission occurs before C- Literature survey 11

40 C coupling. In order to favor chain growth over termination by hydrogenation, the CHx surface concentration must be sufficiently high. [16] Therefore, a high CHx concentration requires a high hydrogenation rate of the carbide species as well as a high rate of CO dissociation to be able to form these carbide species. [17] However, the direct dissociation of CO on Co terraces is a difficult step with high energy barriers.[18, 19] Since the turn over frequency is not affected by Co particle size above 10 nm [11, 20], terrace sites appear to be dominant for the kinetically important steps. [17] This is further supported by the high possibility of the under coordinated sites (edge sites) to become poised by binding carbon too strongly.[11] Meaning that the above proposed CO activation mechanism is kinetically unfavorable on the dominant terrace sites. The CO insertion mechanism considers C-C coupling to precede CO scission. Meaning that adsorbed RCHx groups first couple with the adsorbed CO before the CO bond is cleaved. [17] The primary path consist of RC* groups being hydrogenated to RCH* and coupled with CO yielding RCHCO* followed by hydrogenation to RCH2CO* which will then undergo CO scission, this reaction path is displayed in Figure 2-2 [14, 21]. Note that either the second hydrogenation or the CO scission step is rate limiting, as both reactions have a transition state with a large barrier and only a small difference exists between the two. [5, 17] However, also in the CO insertion mechanism, the formation of the chain-initiating CH* species requires sufficiently fast CO activation. It is important to recognize that CO activation is an important step in both the carbide and the CO insertion mechanism. The product distribution is dependent on the reaction pathway followed by the RCH2CO* species. The dominant pathway as seen in Figure 2-2, where the RCH2CO* species is indicated with red, is a CO scission (indicated as O*) to yield RCH2C species. Other possibilities are C hydrogenation and O hydrogenation. O hydrogenation is not dominant as it has a high barrier, C hydrogenation however is kinetically most favorable but still not dominant. The reason CO scission is dominant over C hydrogenation is due to the effect of CO coverage and the reaction conditions. C hydrogenation is responsible for the formation of oxygenate products. [5] 12 Literature survey

41 Figure 2-2 The main reaction path for the CO insertion mechanism. R signifies hydrogen or an alkyl group.(-o*= CO scission, +CO*=CC coupling, +H*= hydrogenation), as adapted from [5] As mentioned before, CO activation is an important step in both the carbide and the CO insertion mechanism. CO could undergo direct CO dissociation as shown in Figure 2-3. This pathway is unfavorable on the kinetically dominant terrace sites. To account for observed CO activation rates on terraces other mechanisms were proposed. It has to be noted that edge sites can dissociate CO more easily however, they tend to be blocked under reaction conditions due to the strong adsorption of carbon. As CO activation by direct CO dissociation is unfavorable at terrace sites, the hydrogen-assisted CO activation mechanism has been proposed. [14, 18, 21, 22] Here, adsorbed CO species are hydrogenated to form adsorbed formyl (HCO*), formaldehyde (H2CO*) and hydroxyl methylene (HCOH*) before CO scission. The hydrogenation step weakens the CO bond, substantially decreasing the barrier for CO scission. [5] Note that the hydrogenation of the O atom of CO has a lower barrier but is not preferred due to not being in equilibrium, as there is a high barrier for the reverse reaction. [18] A positive reaction order of hydrogen further supports the participation of hydrogen in the rate limiting CO activation step. [18] Further support for this pathway can be found in the predicted formation of OH* which reacts with H* forming H2O, this is in agreement with the almost exclusive oxygen rejection through water observed for FTS catalysts. [23] However the hydrogenation of adsorbed CO species is endothermic [14, 22] making the entire CO activation process unfavorable due to a high barrier. [14] Adding to this the kinetically more favorable desorption of formaldehyde means that formaldehyde is wrongly anticipated as one of the main products.[17] The presence of water has been show to increase C5+ selectivity as well as having a kinetic role [6] and high coverage of OHx species on the catalyst [24-26] lead to the study of the role of surface hydroxyls. It has been shown that the activation of CO using OH* groups uses a different pathway as indicated in Figure 2-3. First the C atom of the CO is hydrogenated using Literature survey 13

42 a H*, followed by the hydrogenation of the oxygen using a OH*. Lastly, CO scission yields a CH* species. [17] + H* + OH* Hydrogenatio Hydrogenatio CO scission n of C by H* n of O by OH* Figure 2-3 The direct CO dissociation pathway and the hydroxyl assisted CO activation pathway. 14 Literature survey

43 Promoters General introduction to promoters and their role Promoters, also called modifiers, are substances which are added in small quantities to the catalyst material to improve their selectivity, activity or stability of the catalyst. Note that these promoters are not active for the considered reaction by themselves. It is possible for a promoter to influence the active component of the catalyst by changing its electronic or crystal structure, by extending its available surface area or by altering the ligand sphere of the catalyst. Promoters can be beneficial by increasing a desired effect, which usually comes at a cost. Most often rate or selectivity of the catalyst is lost as a cost for increasing e.g. stability. Naturally for promotion to be industrially relevant the cost and advantage of the promotion have to be optimized so that a maximum of advantage is achieved with a minimum of cost. Most often the amount of promoter is a key factor. An example could be maximizing selectivity while minimizing loss in activity by only adding promoter up to the point at which the increase in selectivity is too expensive in activity loss. Promoters can also be essential in which case they are called cocatalysts. Instead of altering the active phase these promoters are required to form the active phase. An example of this is alkali additives to Vanadium oxide used for sulphuric acid synthesis, as the additives are required to form the active phase (ternary alkali vanadate). [27, 28] Two classes of promoters are generally used, structural and electronic promoters: Structural promoters: Promoters in this category affect the formation and stability of the active phase of a catalyst. Typically its function is the fixation of a given metastable defective structure of the catalyst by segregating into this site and decreasing its mobility. [27, 28] Electronic promoters: Promoters in this category directly affect the elementary steps on the catalyst. They tend to affect the local electronic structure of the active metal by adding or withdrawing electron density near the Fermi level in the valence band of the metal. As such they affect the chemisorption properties and the surface coverage of the reactants. [27, 28] Note that sometimes these effects blend into each other as geometric structural changes can cause electronic structural changes. Some examples of promoters for several systems are given in Table 2-3. [27] Literature survey 15

44 Table 2-3 Examples of promoters in specific process and their effects, as reproduced from [27] Catalyst used/application Promoter Effect Pt/alumina (reforming) Re Lowers hydrogenolysis and catalyst sintering Ni/ceramic support (steam reforming) K Improves cokes removal Cu (hydrogenation) Ni Increases activity and selectivity Promoters in Co-based Fischer-Tropsch synthesis As promoters can give rise to improved activity and selectivity of the catalyst, many different types of promotion have been found in Co FTS literature: structural, electronic and textural promoters, stabilizers, catalyst-poison-resistant promoters. [3] As many of the above mentioned effects tend to overlap, the classification used in [3] is used here as well resulting in 2 classes, namely structural and electronic promoters (for definitions see 2.2.1) Structural promoters Structural promoters are most often added to increase the activity or stability of the catalyst. An increase in selectivity is more difficult as structural promoters do not tend to affect the active centers of the catalyst. Logically, an increase in activity coincides with an increased stability of the active phase as the increase in activity is created by an increase of the active phase dispersion. An increase in dispersion meaning more active surface area for the same amount of catalyst material. This can be achieved by preventing the formation of spinal phases containing the active metal in an inactive state, e.g. with the support. Another possibility is to prevent the Co nanoparticles from agglomerating and sintering preventing the formation of larger particles with a lower surface area to mass ratio.[27, 28] A first way is to stabilize the oxide support using promoter elements. This will prevent the formation of a Co-compound with the support. Since the most used supports are titania, alumina and silica the formation of Co titanate, Co silicate and Co aluminate are to be avoided. Examples of promoter elements which could aid in this process are Zirconium or Lanthanum.[29-31] The second possibility is to glue the Co particles on the supporting oxide, here a promoter is used as an oxidic interface to shield the Co particles form the support. Increasing the stability of these nanoparticles which also decreasing sintering.[32] Finally, following the general description it is logical to mention the use of promoters to increase the 16 Literature survey

45 Co dispersion directly. These promoter elements will prevent the formation of large Co nanoparticles. Note that it is important to mention that some of these promoter elements can form small nanoparticles of themselves. These nanoparticles can then dissociate hydrogen which will spill over to the Co particles, increasing the extent of the Co reduction and thereby the catalyst activity. Examples of promoter elements which provide such an effect are noble metals such as Re, Pt and Ru. [33-35] Electronic promoters Electronic promoters work by affecting the electronic nature of an active Co site. Therefore, by removing or donating electrons form/to the active site, resulting in a change in selectivity or an increased intrinsic turn over frequency. Several more specific ways on how to achieve these effects are discussed below.[27, 28] A first possibility is for the promoter element, a metal oxide in this case, to decorate the active Co surface. The presence of these metal oxides alters the surface properties and improves activity or alters the selectivity of the catalyst. Note that it is important that the promoter does not block of the active sites as this would decrease activity.[36, 37] It is also possible that the supporting oxide is able to decorate the catalyst and provide the same affect, known as the strong metal-support interactions (SMSI) effect.[27, 38] Another possibility is for a promoting metal to form an alloy with Co, this alloy may than show increased activity, stability or a more favorable selectivity.[39, 40] Inherent promoter activity In general, promoter elements are not catalytically active for the considered reaction. But the added promoter element can be active for another reaction than the one under consideration. Therefore, the promoter can change the overall product distribution. Several more specific examples of this effect are discussed below.[27] The promoter element may catalyze the water gas shift reaction and affect the CO/H2 ratio causing changes in the surface coverage of the catalysts, affecting its selectivity and/or activity. [41] A second possibility is hydrogenation/dehydrogenation reactions. The promoter element may catalyze these reactions, having an impact on the amount of olefins produced. Since the hydrogenation rate of the catalyst will also affect the polymerization probability this will also affect the average chain length of the product. [35] Another possibility is that the promoter aids in the regeneration of the catalyst by catalyzing the oxidation of coke. Coke is typically removed by an oxidative treatment and the promoter element may decrease the required temperature for Literature survey 17

46 this treatment, reducing the amount of sintering and increasing catalyst lifetime. [42] Lastly, the incorporation of promoter elements which make the catalyst less prone to poisoning is considered. As Co based catalysts are sensitive to H2S poisoning, it is interesting to add promoters which can increase the H2S tolerance of the catalyst. [43, 44] Promoter elements used in Co-based Fischer Tropsch synthesis A summary of promoter elements used in Co-based Fischer-Tropsch synthesis are displayed in Table 2-4. Note that one element can have multiple promotion modes, e.g. Boron which has 3 modes (Co gluing, decoration of Co surface and watergas shift promotion). [27] The detection of different modes for the same promoting element is caused by a difference in concentration of the promotion element, the catalyst preparation method and reaction conditions. 18 Literature survey

47 Table 2-4 An overview of the promotion effects of different elements used in literature on Co based Fischer-Tropsch catalysts, as reproduced form [27] Promotion type Structural Electronic Promotion mode Activity Selectivity Stability Elements Support stabilization + + Mg, Si, Zr, Nb, Rh, La, Ta, Re, Pt Co glueing + + B, Mg, Zr Co dispersion increase Decorating Co surface + + Ti, Cr, Mn, Zr,Mo, Ru, Rh, Pd, Ce, Re, Ir, Pt, Th B, Mg, K, Ti, V, Cr, Mn, Zr, Mo, La, Ce, Gd, Th Co alloying Ni, Cu, Ru, Pd, Ir, Pt, Re Synergistic Watergas shift + + B, Mn, Cu, Ce Hydrogenation/ dehydrogenation + Pt* Coke burning + Ni, Zr, Gd H2S adsorption + B, Mn, Zn, Zr, * Note that only Pt is given but (de-)hydrogenation reactions can also be catalyzed by other metals and metal oxides known to be active for this reaction. The most studied and used promoters are noble metals as they increase Co reducibility and providing more active sites for reaction. Most notable are ruthenium, rhenium and platinum. The first two increase the reducibility by hydrogen spillover [45, 46] while the last prevent the formation of Co support species. [47] These noble metals also tend to increase the dispersion of the Co nanoparticles [45-47]. The extent of reduction is dependent on the strength of the interaction between the Co oxide species and the support indicating that the extent at which the reducibility is enhanced is also support dependent. [48] Transition metal oxides are mostly regarded as electronic promoters, even though some also possess a synergistic or structural role. [27] These promoters tend to spread over the surface of Mo Literature survey 19

48 the Co metal, change the active sites of the catalyst impacting the adsorption behavior of the catalyst. If the concentration of these oxides becomes too high (greater than one monolayer), they cover up many active sites and will hence decrease the activity of the catalysts. 20 Literature survey

49 Manganese as a promoter for Co-based Fischer Tropsch synthesis Mn has been used, as a promoter, in Fe, Ru and Co based catalysis, commercially a patent exists for the usage of Mn as a promoter in Co FTS [49]. Its usage in Fe based catalysts is mainly to increase the production of C2-C4 olefins. It has been reported that the addition of Mn to Fe catalysts increases the CO capacity of the catalyst. [50] Mn also caused the particle size of the iron oxide precursor to decrease which promotes the formation of alkenes. [51] The application of Mn in ruthenium based catalysts yields increased amounts of unsaturated species and shifts the product distribution towards longer chains.[52] It was suggested that Mn formed a layer on top of the Ru nanoparticles as increased amount of Mn lead to a decrease in CO adsorption capacity while Mn itself was found to have no CO adsorption capacity. Indicating that the active sites of the ruthenium catalyst are being covered by Mn. Mn has also been shown to act as a structural promoter, increasing the Ru dispersion. [27] Unsupported Co catalysts promoted with Mn have been reported to increase C3 selectivity whilst suppressing CH4 formation. [53] Analysis of the catalyst showed the catalyst contains metallic Co supported on MnO. [54] It has also been reported that MnO has water gas shift activity, due to an increase in the production of CO2 when introducing it in Co-based FTS. [55-57] Mn promotion of a Co/Al2O3 catalyst showed an increase in activity as well as a higher C5+ selectivity. The promoter improved the dispersion of Co nanoparticles and, when added in small quantities, increased the hydrogen uptake. [58] And it has been shown that the addition of Mn also provided an electronic promotion effect. Some studies have indicated that MnO, when highly dispersed over the support, could be bound to the support with the metallic Co located on top. [59, 60] Oxidation state The Nature of the manganese promoter under Fischer-Tropsch reaction conditions provides insights into how it influences the catalytic process. The oxidation state of the promoting element determines the electron donor/withdrawer properties of Mn. The oxidation state of the catalyst provides information about the extent of reduction to the active phase (metallic Co) and thereby how the promoter affects this. Literature survey 21

50 Several studies [61-64] have examined the oxidation state of Co for Fischer-Tropsch synthesis and determined that before reduction of the catalyst, in the passivated state, Co is present as a mixture of CoO and Co3O4. After reduction XANES data shows a shift of the Co edge towards the metallic Co standard. [63] The addition of Mn decreases the size of the shift, indicating that Mn limits the reduction of Co. After reduction Co is a mixture of metallic Co and CoO. [61-64] Note that the extent of Co reduction will affect the activity of the catalyst, see section More important is the oxidation state of the promoter as this will be key in understanding its promoting effects as well as its location. XANES analysis, as seen Figure 2-4, determined that the passivated catalyst has an oxidation state between MnO2 and Mn2O3. After reduction the edge energy is close to that of the MnO standard. To examine a possible change of the oxidation state during reaction, the catalyst was exposed to syngas, this only caused a small change in the spectrum. Concluding that even under FTS conditions, the promoter remains in the MnO oxidation state. [61-63] A more detailed indicates that the oxidation state of Mn is around 2.5 after reduction and decreases to 2.0 for Mn/Co ratios lower than 0.1, after exposure to syngas. For a Mn/Co ratio of 0.1 or larger the oxidation state remains around 2.5. [61] The formation of MnO (Mn oxidation state 2+) and Mn2O3 (Mn oxidation state 3+) after reduction is confirmed by other studies. [59, 65] Figure 2-4 Mn K-edge XANES spectra for a Mn/Co =0.1 catalyst, comparing the passivated, reduced and during reaction state. The reduced state data was acquired after reducing the catalysts for 2 h at 673K in H2. The FTS reaction data was acquired after exposing the reduced catalyst to syngas at 493 K for 6 h. As reproduced from [61] 22 Literature survey

51 2.3.2 Location The location of the Mn promoter during reaction is of the upmost importance to determine the promoting effect. Bezemer et al. [64] performed an analysis of spatial correlation between Mn and Co by using elemental maps. As shown in Figure 2-5 there is a spatial correlation between Mn and Co, as regions of high intensity for both elements coincide. However, the figures are not identical and it appears that Mn is more dispersed over the silica surface than the Co metal. [64, 66] There is evidence for a strong effect of the amount of promoter on the spatial association of Mn and Co. Mn seems to prefer associating with Co below Mn/Co atomic ratios of 0.1. This point is called the critical loading point and is approximately the amount of promoter required to form a half monolayer on Co nanoparticles. At a Mn/Co of 0.5 the average nanoparticle composition was 0.38 indicating that the rest of the Mn is associated with the support, indicating a high affinity of Mn for the support at high loadings. [67] This to the extent that when using a TiO2 support and high Mn loading, spinal phases such as Ti2MnO4 have been detected. [63] The lack of large MnO particles for Mn/Co ratios lower than 0.1 and the similar bulk and nanoparticles composition indicate that Mn preferably associates with the Co for loadings lower than 0.1. [61] Another study [62] confirmed that at higher loadings Mn is less prone to associate with Co, results indicated that the promoter will disperse over the support but will be more concentrated closer to the Co nanoparticles. Since a weak Mn signal is still detected along the Co nanoparticles, some Mn could still be on top of the Co surface. It is however noted that this study used a support with higher affinity for the Mn promoter than the above. An increase in Mn/Co and Mn/support atomic ratios with reduction time indicates the enrichment of MnO on the catalyst surface during reduction. [62] This could be due to migration of MnO particles towards the support during the reduction of Co3O4 to Co metal. [63, 68] Confirming that MnO has a higher affinity for the support than for metallic Co in the case of a TiO2 support. [62] However the affinity of MnO with the support is dependent on the support in question meaning that the migration direction of the MnO particles might change. Literature survey 23

52 Figure 2-5 Elemental map of a Co Mn catalyst (directly reduced, Mn/Co = 0.5) obtained by STEM EDS, the Co channel is displayed, on the right is the Mn channel. Images were acquired at 200 kv accelerating voltage and 0.6 na beam current. As reproduced from [66] Preparation method As expected, preparation method of the catalysts affects the promotion effect of Mn. Different preparation methods might cause Mn to be initially more prevalent on the support or cause Mn to migrate more to the support. Therefore, the type of support and the chosen precursor material for both Co and Mn has an influence in the promotion effect. Johnson et al. [66] analyzed the spatial correlation between Mn and Co using elemental maps generated using STEM-EDS of a Mn promoted (Mn/Co =0.5) Co/SiO2 catalyst. The spatial correlation between Mn and Co was found to be dependent on the preparation method of the catalyst as a directly reduced catalyst shows a higher special correlation between the two elements than an initially calcined catalyst. As shown in Figure 2-6, the directly reduced catalyst has a greater effect on the catalyst selectivity compared to the rest. These two points indicate that a higher spatial association between Mn and Co increases the extent of Mn promotion suggesting that a close proximity of Mn an Co is required for the promotion effect to manifest. Particle size effect are unlikely the cause of the observed difference in selectivity as the surface mean diameters of the Co nanoparticle were similar for the unpromoted and promoted catalysts. [66] It has furthermore been shown that the optimal Mn/Co ratio for promotion is dependent on the preparation method, more specifically on the source of Mn used and the procedure of impregnation. [61] 24 Literature survey

53 a) b) Figure 2-6 (a) Methane (b) C5+ selectivity (molar carbon basis) of the Co and Co-Mn (Mn/Co =0.5) catalysts under FTS. Experimental conditions: 493 K, 1 bar H2/CO = 2 ; data was extrapolated to 0% CO conversion. As reproduced from [66] A comparison of the two preparation methods, direct impregnation and strong electrostatic adsorption, was performed by Feltes et al [69]. This study showed a strong migration of Mn oxide species during the reduction process. For the direct impregnation method, the Mn-oxides moved towards the Co particles and enrich the Co-nanoparticles with the promoter element. For the strong electrostatic adsorption technique, the reverse was observed. The direct impregnation technique has a much lower initial concentration of Mn-oxides on the Co nanoparticles compared to the other method. These observations indicate the existence of a common thermodynamic optimum, suggesting that sufficient reduction can possibly offset the original difference in concentration created by a difference in method. [69] The thermodynamic optimum has been shown to vary depending on the support used. This indicate that a catalyst prepared with equal amounts of Mn will display different actual Mn/Co ratios in the nanoparticles due to the usage of a support with a different affinity for the promoter. [62] For example, there is evidence for strong interactions between MnO-TiO2 [70], showing that a catalyst using a titania support will have more migration of the promoter to the support. Further observations showed that the promoter never completely covers the reduced Co surface, preferring the interface.[69] The extent to which Mn impacts the Co reducibility has been shown to be dependent on the preparation method. A catalysts created using simultaneous deposition of Co and Mn with the homogeneous deposition precipitation method even showed the incorporation of Mn in the Co3O4 structure, strongly inhibiting its reducibility. [63] Literature survey 25

54 2.3.4 Effect on activity and selectivity To achieve a positive effect from the Mn promotion, either an increase in activity or improvement in the product selectivity is required. An improvement in activity is rather straight forward but in case of product selectivity it is important to know that the main products of Fischer-Tropsch synthesis. The main products are methane (C1), petroleum gas (C2-C4), gasoline (C5-C11), diesel (C12-C20), and wax (C21+)[7] Maximization of the more valuable C5+ products is desired, combined with a minimization of methane production Selectivity As shown in Figure 2-7a, Dinse et al showed that Mn promotion favorably affects the FTS product selectivity by increasing C5+ selectivity and decreasing C1 selectivity. Interestingly, increasing the Mn/Co ratio beyond 0.1 causes plateaus in the selectivity. The study further confirmed that this behavior also occurs at higher total pressure. Although the promotion becomes less pronounced as the difference between the promoted and unpromoted catalyst decreases with increasing total pressure. [61] The C2-C4 selectivity appears independent of the Mn/Co ratio. The Figure 2-7b shows an increase in the olefin to paraffin ratio when increasing the Mn/Co ratio.[71] It has been hypothesized that the changes in selectivity are due to a lower availability of adsorbed hydrogen compared to CO, increasing the chain growth probability. [62, 71] The observation of a plateau in selectivity with increasing promoter loading cannot be explained away by stating that this point coincides with the critical promoter loading (see 2.4.2) of the catalyst as the average Mn/Co ratio of the nanoparticles has been shown to increase beyond critical loading. To account for this a theory has been proposed by Johnson et al. [67] where two kinds of sites are assumed, a promoted site and an unpromoted site. Where the promoted site has decreased CH4 selectivity and increased C5+ selectivity when compared to the unpromoted site. The catalyst will then show a selectivity which is the average of the two sites weighted with turn over frequency multiplied by number of sites. This theory explains the observed changes in selectivity with increasing promoter loading. Beyond the critical point the plateauing is due to the low amount of unpromoted sites which remain at such high promoter loading. [67] The two site assumption is illustrated in Figure 2-9a. 26 Literature survey

55 a) b) Figure 2-7 Effect of Mn promotion on (a) product selectivity and (b) O/P ratio for the C2-C4 product fraction. Measured at 493 K and 1 atm with a feed ratio H2/CO/Ar = 2/1/0.2, flows = ml/min, catalyst mass = 100 mg, CO conversion = 2%. As reproduced from [71] Morales et al. [62] used CO TPD data and linked two CO desorption peaks with CO chemisorption onto the catalyst. The second peak (desorbing at the highest temperature) is attributed to CO adsorbed in contact with the Mn promoter and the first peak to CO chemisorption on Co. It was determined that the second peak maximum shifts to higher temperatures with increasing Mn loading. However, the peak remains the same when reaching a Mn/Co ratio of 0.1. Such a profile is similar to the trend seen in the C5+ selectivity data, indicating that a link between the two exists. Furthermore, the area ratios of the higher to the lower temperature desorption peaks increases linearly with increasing Mn loading implying that a larger amount of CO is bound in contact with Mn, when more Mn is present. [61, 72] In situ infrared (IR) spectra of the catalyst when exposed to CO show the development of a secondary peak (1588 cm -1 ) next to the primary peak (2077 cm -1 ) which corresponds to linearly adsorbed CO, as reported by Fredriksen et al. [61, 73] The secondary peak was proposed to be cause by CO interacting with the Mn promoter as this peak was not observed on the unpromoted catalyst or on the MnO control sample. It is noted that spillover of CO onto the MnO is likely a contribution to this second peak as it scales linearly with the amount of Mn promoter. [61] The peak has a frequency similar to that of carbonyl complexes where separate metal atoms are bound to both the C and O atoms of the CO. [74] It is further proposed that in this case adsorbed CO interacts with the catalyst metal through its carbon atom and to the oxophilic part of the promoter through its oxygen atom. [75] This could explain the difference in selectivity observed as these interactions with the promoter would cause the C-O bond to be weakened and making C-O dissociation easier.[61] Literature survey 27

56 Another study by den Breejen et al [76] attributed the increasing C5+ selectivity to the increased surface coverage of CHx with promoter loading. A higher CHx coverage will lead to an increased C-C coupling probability which will increase the chain growth probability and as such shift the product distribution towards higher hydrocarbons Activity As shown in Figure 2-8a, Dinse et al. showed that at low total pressure, there is an optimal Mn/Co ratio of 0.05 which maximizes the CO consumption rate. This optimum was found to be same for the three conversion levels examined. Figure 2-8b shows that an increasing CO conversion level causes a stronger decrease in CO consumption rate for the promoted catalyst than for the unpromoted catalysts. However, the catalysts with a Mn/Co ratio of 0.05 and retain a higher rate than the unpromoted catalyst until a respective CO conversion of 40% and 35% is reached.[71] The same study reported decrease in H2 uptake and a decrease in Co nanoparticle size with increasing Mn/Co ratio. The decrease in H2 uptake is linked to a decrease in available Co active sites. Despite a decrease in Co active sites an increase in rate is observed due to promotion for loadings lower than This leads to the conclusion that Mn promotion enhances the rate enough to offset the loss due to the reduction of available sites. The optimal point between loss of Co sites and enhancing of rate due to increased Mn/Co ratio is seen as the peak of the curves Figure 2-a. When increasing the Mn/Co ratio further it is clear that the loss of active sites dominates by decreasing the rate.[67, 71] A possible explanation for the improved activity is that MnOx partially covers the Co nanoparticles and that Mn n+ acts as a Lewis-acid which improves CO adsorption and facilitates its dissociation. [77, 78] Which is based on a more general postulation, that the presence of metal cations located at the metal-metal oxide interface will act as Lewis-acids and interacts with the O of surface CO species, while the C interacts with the metal surface. This weakens the CO bond facilitating the dissociation directly or following partial hydrogenation. [74, 79, 80] The dissociation after partial hydrogenation is thought to be favored as the rate of C deposition via CO disproportionation in the absence of H2 is much lower than the rate of FTS at the same temperature. [61] Change in Co nanoparticle size with increasing Mn/Co ratio can also contribute to a change in activity. However, the nanoparticle sizes in this study are outside of the range in which these effects are relevant. [71] Using O2 titration combined with XANES measurement, Bell et al. [61] concluded that the fraction of Co in the metallic state after reduction decreases with increasing Mn loading. 28 Literature survey

57 Indicating that Mn impedes the reduction of Co. Since a decrease in extent of reduction will decrease the overall catalysts activity, this will negatively affect the catalyst activity with increasing Mn loading. [61, 81] Dinse et al. [71] showed that, at high pressure, the rate per gram of Co for the unpromoted catalyst will be higher than that of the promoted catalyst. Logically this phenomena is due the higher coverage of the catalyst at a higher pressure, allowing both the promoted and unpromoted catalyst to use more active sites. But as the promoted catalysts has a lower amount of active sites available, due to partial coverage by the promoter, it will show a lower activity per gram of Co (especially at higher pressure). [67, 71] a) b) Figure 2-8 (a) Influence of the Mn/Co ratio on the CO consumption rate for different CO conversion levels (b) Influence of the conversion level on the CO consumption rate for different Mn/Co ratios. Measured at 493 K and 1 atm. Feed ratio H 2/CO/Ar = 2/1/0.2, flows = ml/min, catalyst mass = 100 mg. As reproduced from [71] Concluding that Mn-promotion, in the range examined above, increases the turn over frequency but decreases the activity per gram under operating conditions due to blockage of active sites by the promoter. The observation of a decrease in H2 uptake with increasing Mn loading further supports the blockage of active sites by the promoter as the promoter and support did not show signs of H2 chemisorption. The blockage of active sites with increasing promoter loading is illustrated in Figure 2-8a, note that this figure also shows the two sites mentioned in the selectivity section. [61, 76] Literature survey 29

58 a) b) Figure 2-8 (a) An illustration of the formation of promoted active sites by Mn promotion of Co, note the decrease in total (promoted and unpromoted) available surface (or active sites) with increased loading. (b) A comparison of the rates per gram of Co for several Mn/Co ratios at 493K. Data is based on Co consumption at 0% conversion. The curves are created by fitting the data to the rate law seen in eq1. As reproduced from [61]. A more recent study by Johnson et al. [61], examined the 0.01 to 0.1 Mn/Co ratio range in more detail, as this regions shows the strongest changes in activity and selectivity. This study confirmed that increasing Mn loading increases TOF up to a Mn/Co ratio of 0.1 but decreases when increased further. They did however discover that a catalyst with a Mn/Co ratio of 0.01 shows a high increase in the rate per gram of Co, as can be seen Figure 2-8b, where an increase of about 50% is observed. The results from this figure do differ from those reported above where higher Mn loadings are examined. This is thought to be due to a difference in preparation method. Despite this these findings do confirm that at low loadings, depending on the preparation method, it is possible for Mn promotion to increase the rate per gram of Co whilst still providing an increased C5+ selectivity. The good agreement of the kinetic data obtained from the promoted catalyst with the rate equation used commonly in Fischer-Tropsch synthesis also indicates that the promoted catalyst has a reaction mechanism that yields the same rate law (eq1). [61] r = (eq 2-3) 30 Literature survey

59 Lewis acidity of promoting element As discussed in the previous section, it is speculated that Mn promotes the Fischer-Tropsch reaction through Lewis-acid interactions. To confirm this hypothesis, Bell et. al. [67] performed a series of experiments on other possible promoter elements that could function as a Lewis-acid such as La, Ce, Gd and Zr. As shown in Figure 2-9a, Bell et. Al. [67] first examined the effect on selectivity by varying the promoter elements. With increasing the relative Lewis acidity, the positive effect on selectivity is enhanced indicating that the Lewis acidity of the promoter element is indeed responsible for the changes in selectivity. Note that the selectivity values plotted in Figure 2-9a are for promoter loadings at their critical loading indicating that the selectivity plateau is reached. Figure 2-9b shows that the relative promotional effect decreases with increasing pressure and that the selectivity converge with increasing pressure. [67] Authors argued that, a possible explanation of the change in selectivity is through facilitation of C-O bond cleavage which will increase the amount of hydrogen scavenging C1 monomers. This cause a decrease in adsorbed hydrogen and lower termination rate thus increasing the selectivity towards higher hydrocarbons. [61] a) b) La Ce Mn Gd Zr Figure 2-9 (a) Selectivity towards CH4, C2 C4, and C5+ as a function of the promoter Lewis acidity. Here, NM-2δM is used as a proxy for the relative Lewis acidity of the promoter oxide, and the unpromoted catalyst was assigned a value of 0. (b) The selectivity towards C5+ as a function of pressure for the different metal oxide promoters. The selectivity correspond to those where the promoter loading is high enough such that the product selectivity were insensitive to promoter loading. The data were collected at 493 K at atmospheric pressure with a feed composition of 7% Ar, 31% CO, and 62% H2 ( ml/min) and were extrapolated to 0% CO conversion. As reproduced from [67]. Secondly, the effect of Lewis acidity on the activity of the catalyst was examined by assuming the rate equation (eq 2-3). This equation was derived assuming that the hydrogen assisted CO dissociation step is rate determining [18]. Where a is the apparent rate constant, b is the CO Literature survey 31

60 adsorption constant while -rco is the CO consumption rate and the parameters a and b were fitted based on kinetic experiments. According to equation 2-3, both the apparent rate constant and the CO adsorption constant increase with increasing relative Lewis acidity. Increase in apparent rate constant indeed increases the CO consumption rate indicating Mn facilitates the CO dissociation step, which is assumed to be rate-determining in Fischer-Tropsch synthesis. However, the effect in increasing CO adsorption constant is more complicated. At lower total pressures, a high adsorption constant will give higher rates as the CO surface coverage is increased. Higher pressures mean that the surface is almost saturated by CO and here an increase in the CO adsorption constant will decrease the CO consumption rate. This indicates that an optimum point must exits where a promoter element has a Lewis acidity corresponding to a combination of apparent rate constant and CO adsorption constant that maximizes the CO consumption rate at a given temperature and total pressure. This optimum has been reported to be Gd2O3 and MnO for reaction at 493K and 10 bar. [67] a) b) Figure 2-10 (a) FTS turnover frequencies (b) Rates of CO consumption per gram Co (right) as a function of pressure for the unpromoted and metal oxide-promoted Co/SiO2 catalysts. The catalyst are: La/Co = 0.1; Ce/Co = 2.0; Mn/Co = 0.1; Gd/Co = 1.0; Zr/Co = 1.0. The data were collected at 493 K with a reactor inlet feed of 7% Ar, 31% CO, and 62% H2. All data points were extrapolated to 0% CO conversion. The curves in each plot are fits to the data using the rate law given by eq. 1. For the left figure it is also assumed that the Co nanoparticles are covered by half a monolayer of promoter. As reproduced from [67]. It is important to note that, all the considered metal oxides in this study increases the FTS TOF. As shown in Figure 2-12a, Zr, Gd and Mn exhibits increase in FTS TOF for the entire examined pressure range while Ce and La exhibits increase in FTS TOF only at low pressures. Both Mn and Gd are the most optimal promoters and shows the highest boost in TOF especially at high pressures. However when examining the rate per gram of Co, as shown in Figure 2-12b, the promotion can no longer be considered beneficial. This is naturally due to blockage of active 32 Literature survey

61 sites by the promoter element, which has been covered in section [67] Noting that promotion with Mn at much lower loadings that the critical point can provide an increase in rate per gram of Co, as mentioned before. Altogether these findings support the proposition that the metal oxide promoter facilitates the dissociation and adsorption of CO through Lewis acid base interactions. Literature survey 33

62 Bibliography 1. Fischer, F. and H. Tropsch, Über die Synthese höherer Glieder der aliphatischen Reihe aus Kohlenoxyd. Berichte der deutschen chemischen Gesellschaft (A and B Series), (11): p Schulz, H., Short history and present trends of Fischer Tropsch synthesis. Applied Catalysis A: General, (1 2): p Promotion Effects in Co-based Fischer-Tropsch Catalysis, in Catalysis: Volume 19, J.J. Spivey and K.M. Dooley, Editors. 2006, The Royal Society of Chemistry. p Rytter, E., N.E. Tsakoumis, and A. Holmen, On the selectivity to higher hydrocarbons in Co-based Fischer Tropsch synthesis. Catalysis Today, : p Zhuo, M., A. Borgna, and M. Saeys, Effect of the CO coverage on the Fischer Tropsch synthesis mechanism on cobalt catalysts. Journal of Catalysis, : p Schulz, H., E. vein Steen, and M. Claeys, Selectivity and mechanism of Fischer-Tropsch synthesis with iron and cobalt catalysts, in Studies in Surface Science and Catalysis, H.E. Curry-Hyde and R.F. Howe, Editors. 1994, Elsevier. p de Klerk, A., Fischer Tropsch Process, in Kirk-Othmer Encyclopedia of Chemical Technology. 2000, John Wiley & Sons, Inc. 8. Bian, G., et al., Activation and Catalytic Behavior of Several Co/SiO2 Catalysts for Fischer Tropsch Synthesis. Energy & Fuels, (4): p Tsakoumis, N.E., et al., Deactivation of cobalt based Fischer Tropsch catalysts: A review. Catalysis Today, (3 4): p Iglesia, E., Design, synthesis, and use of cobalt-based Fischer-Tropsch synthesis catalysts. Applied Catalysis A: General, (1): p den Breejen, J.P., et al., On the Origin of the Cobalt Particle Size Effects in Fischer Tropsch Catalysis. Journal of the American Chemical Society, (20): p Barbier, A., et al., Characterization and Catalytic Behavior of Co/SiO2 Catalysts: Influence of Dispersion in the Fischer Tropsch Reaction. Journal of Catalysis, (1): p Bartholomew, C.H. and R.C. Reuel, Cobalt-support interactions: their effects on adsorption and carbon monoxide hydrogenation activity and selectivity properties. Industrial & Engineering Chemistry Product Research and Development, (1): p Zhuo, M., et al., Density Functional Theory Study of the CO Insertion Mechanism for Fischer Tropsch Synthesis over Co Catalysts. The Journal of Physical Chemistry C, (19): p Davis, B.H., Fischer Tropsch Synthesis: Reaction mechanisms for iron catalysts. Catalysis Today, (1 2): p van Santen, R.A., et al., Structure sensitivity of the Fischer-Tropsch reaction; molecular kinetics simulations. Catalysis Science & Technology, (6): p Gunasooriya, G.T.K.K., et al., Key Role of Surface Hydroxyl Groups in C O Activation during Fischer Tropsch Synthesis. ACS Catalysis, (6): p Ojeda, M., et al., CO activation pathways and the mechanism of Fischer Tropsch synthesis. Journal of Catalysis, (2): p Ge, Q. and M. Neurock, Adsorption and Activation of CO over Flat and Stepped Co Surfaces: A First Principles Analysis. The Journal of Physical Chemistry B, (31): p Literature survey

63 20. Herranz, T., et al., Influence of the Cobalt Particle Size in the CO Hydrogenation Reaction Studied by In Situ X-Ray Absorption Spectroscopy. The Journal of Physical Chemistry B, (31): p Inderwildi, O.R., D.A. King, and S.J. Jenkins, Fischer-Tropsch synthesis of liquid fuels: learning lessons from homogeneous catalysis. Physical Chemistry Chemical Physics, (47): p Cheng, J., et al., A First-Principles Study of Oxygenates on Co Surfaces in Fischer Tropsch Synthesis. The Journal of Physical Chemistry C, (25): p Keyser, M.J., R.C. Everson, and R.L. Espinoza, Fischer Tropsch Kinetic Studies with Cobalt Manganese Oxide Catalysts. Industrial & Engineering Chemistry Research, (1): p Weststrate, C.J., et al., Ethanol Decomposition on Co(0001): C O Bond Scission on a Close-Packed Cobalt Surface. The Journal of Physical Chemistry Letters, (12): p Kizilkaya, A.C., J.W. Niemantsverdriet, and C.J. Weststrate, Oxygen Adsorption and Water Formation on Co(0001). The Journal of Physical Chemistry C, (9): p Weststrate, C.J., et al., Elementary steps in Fischer Tropsch synthesis: CO bond scission, CO oxidation and surface carbiding on Co(0001). Surface Science, : p Front Matter, in Catalysis from A to Z, W.A.H. Boy Cornils, Horst-Werner Zanthoff, Chi-Huey Wong, Editor. 2013, Wiley-VCH Verlag GmbH & Co. KGaA. p. I-XXXVII. 28. Deutschmann, O., et al., Heterogeneous Catalysis and Solid Catalysts, in Ullmann's Encyclopedia of Industrial Chemistry. 2000, Wiley-VCH Verlag GmbH & Co. KGaA. 29. All, S., B. Chen, and J.G. Goodwin, Zr Promotion of Co/SiO2 for Fischer-Tropsch Synthesis. Journal of Catalysis, (1): p Vada, S., et al., La Promotion of Co Fischer-Tropsch Catalysts. Studies in Surface Science and Catalysis, : p Hosseini, S.A., et al., Fischer Tropsch synthesis over Ru promoted Co/γ-Al2O3 catalysts in a CSTR. Catalysis Communications, (3): p Moradi, G.R., et al., Promotion of Co/SiO2 Fischer Tropsch catalysts with zirconium. Catalysis Communications, (1): p Price, J.G., et al., Fischer-Tropsch synthesis: DRIFTS and SIMS surface investigation of Co and Co/Ru on titania supports. Studies in Surface Science and Catalysis, : p Rygh, L.E.S., et al., Diffuse reflectance IR studies of bimetallic Fischer-Tropsch catalysts. Journal of Molecular Structure, : p Lu, G., T. Hoffer, and L. Guczi, Reducibility and CO hydrogenation over Pt and Pt-Co bimetallic catalysts encaged in NaY-zeolite. Catalysis Letters, (2): p Bäurle, G., et al., Conversion of Syngas to Aromatic Hydrocarbons on Cobalt- Manganese-Zeolite Catalysts. Studies in Surface Science and Catalysis, : p Li, J. and N.J. Coville, The effect of boron on the catalyst reducibility and activity of Co/TiO2 Fischer Tropsch catalysts. Applied Catalysis A: General, (1): p Voß, M., D. Borgmann, and G. Wedler, Characterization of Alumina, Silica, and Titania Supported Cobalt Catalysts. Journal of Catalysis, (1): p Sun, S., et al., Fischer Tropsch synthesis using Co/SiO2 catalysts prepared from mixed precursors and addition effect of noble metals. Fuel, (11 12): p Literature survey 35

64 40. Rønning, M., D.G. Nicholson, and A. Holmen, In situ EXAFS study of the bimetallic interaction in a rhenium-promoted alumina-supported cobalt Fischer Tropsch catalyst. Catalysis Letters, (3): p Bahia, A. and J.M. Winterbottom, CATALYSIS BY INTERMETALLIC COMPOUNDS.1. A COMPARISON OF THE CATALYTIC BEHAVIOR OF CEPD3, ZRPD3 AND PD SPONGE FOR BUTA-1,3-DIENE HYDROGENATION AND BUT-1-ENE HYDROISOMERIZATION. Journal of Chemical Technology and Biotechnology, (3): p Nagaoka, K., K. Takanabe, and K.-i. Aika, Modification of Co/TiO2 for dry reforming of methane at 2 MPa by Pt, Ru or Ni. Applied Catalysis A: General, (1 2): p Li, J. and N.J. Coville, Effect of boron on the sulfur poisoning of Co/TiO2 Fischer Tropsch catalysts. Applied Catalysis A: General, (1 2): p Koizumi, N., et al., Development of sulfur tolerant catalysts for the synthesis of high quality transportation fuels. Catalysis Today, (4): p Jacobs, G., et al., Fischer Tropsch synthesis: study of the promotion of Re on the reduction property of Co/Al2O3 catalysts by in situ EXAFS/XANES of Co K and Re LIII edges and XPS. Applied Catalysis A: General, (2): p Bruce, L.A., et al., Ruthenium promotion of fischer-tropsch synthesis over coprecipitated cobalt/ceria catalysts. Applied Catalysis A: General, (1): p Zsoldos, Z., T. Hoffer, and L. Guczi, Structure and catalytic activity of aluminasupported platinum-cobalt bimetallic catalysts. 1. Characterization by x-ray photoelectron spectroscopy. The Journal of Physical Chemistry, (2): p Jacobs, G., et al., Fischer Tropsch synthesis: Temperature programmed EXAFS/XANES investigation of the influence of support type, cobalt loading, and noble metal promoter addition to the reduction behavior of cobalt oxide particles. Applied Catalysis A: General, (2): p Chettouf, A., R.J. Dogterom, and C.M.A.M. Mesters, Process for preparing a catalyst. 2008, Google Patents. 50. Xu, L., et al., The promotions of MnO and K2O to Fe/silicalite-2 catalyst for the production of light alkenes from CO2 hydrogenation. Applied Catalysis A: General, (1): p Das, C.K., et al., Hydrogenation of carbon monoxide on unsupported Fe-Mn-K catalysts for the synthesis of lower alkenes: promoter effect of manganese. Applied Catalysis A: General, (2): p Hussain, S.T., TEMPERATURE-PROGRAMMED DESORPTION (TPD) AND TEMPERATURE-PROGRAMMED REDUCTION (TPR) STUDIES OF ALUMINA- SUPPORTED PLOYCRYSTALLINE RU/MN BIMETALLIC SYSTEM. Journal of the Chemical Society of Pakistan, (2): p van der Riet, M., G.J. Hutchings, and R.G. Copperthwaite, Selective formation of C3 hydrocarbons from co + H2 using cobalt-manganese oxide catalysts. Journal of the Chemical Society, Chemical Communications, 1986(10): p Colley, S.E., et al., Identification of body-centred cubic cobalt and its importance in CO hydrogenation. Nature, (6220): p Jiang, M., et al., Adsorption properties of cobalt and cobalt-manganese catalysts studied by in situ diffuse reflectance FTIR using CO and CO+H2 as probes. Applied Catalysis A: General, (1 2): p Literature survey

65 56. Riedel, T., et al., Comparative study of Fischer Tropsch synthesis with H2/CO and H2/CO2 syngas using Fe- and Co-based catalysts. Applied Catalysis A: General, (1 2): p Keyser, M.J., R.C. Everson, and R.L. Espinoza, Fischer Tropsch studies with cobalt manganese oxide catalysts: Synthesis performance in a fixed bed reactor. Applied Catalysis A: General, (1): p Jun-Ling, Z., et al., Effect of Manganese Promoter on the Performance of Co/Al₂O₃ Catalysts for Fischer Tropsch Synthesis Acta Phys. -Chim. Sin., (03): p Tan, B.J., et al., An EXAFS study of cobalt-manganese/silica bimetallic solvated metal atom dispersed (SMAD) catalysts. Journal of the American Chemical Society, (18): p Klabunde, K.J. and Y. Imizu, Bimetallic solvated metal atom dispersed catalysts. New materials with low-temperature catalytic properties. Journal of the American Chemical Society, (9): p Johnson, G.R., S. Werner, and A.T. Bell, An Investigation into the Effects of Mn Promotion on the Activity and Selectivity of Co/SiO2 for Fischer Tropsch Synthesis: Evidence for Enhanced CO Adsorption and Dissociation. ACS Catalysis, (10): p Morales, F., et al., Mn promotion effects in Co/TiO2 Fischer Tropsch catalysts as investigated by XPS and STEM-EELS. Journal of Catalysis, (2): p Morales, F., et al., In Situ X-ray Absorption of Mn/Co/TiO2 Catalysts for Fischer Tropsch Synthesis. The Journal of Physical Chemistry B, (41): p Bezemer, G.L., et al., Investigation of promoter effects of manganese oxide on carbon nanofiber-supported cobalt catalysts for Fischer Tropsch synthesis. Journal of Catalysis, (1): p Tan, B.J., K.J. Klabunde, and P.M.A. Sherwood, XPS studies of solvated metal atom dispersed (SMAD) catalysts. Evidence for layered cobalt-manganese particles on alumina and silica. Journal of the American Chemical Society, (3): p Johnson, G.R., et al., Investigations of element spatial correlation in Mn-promoted Cobased Fischer Tropsch synthesis catalysts. Journal of Catalysis, : p Johnson, G.R. and A.T. Bell, Effects of Lewis acidity of metal oxide promoters on the activity and selectivity of Co-based Fischer Tropsch synthesis catalysts. Journal of Catalysis, : p Boot, L.A., et al., Preparation, characterization and catalytic testing of cobalt oxide and manganese oxide catalysts supported on zirconia. Applied Catalysis A: General, (1): p Feltes, T.E., et al., The Influence of Preparation Method on Mn Co Interactions in Mn/Co/TiO2 Fischer Tropsch Catalysts. ChemCatChem, (9): p Kantcheva, M., M.U. Kucukkal, and S. Suzer, Spectroscopic Investigation of Species Arising from CO Chemisorption on Titania-Supported Manganese. Journal of Catalysis, (1): p Dinse, A., et al., Effects of Mn promotion on the activity and selectivity of Co/SiO2 for Fischer Tropsch Synthesis. Journal of Catalysis, : p Morales, F., et al., Effects of manganese oxide promoter on the CO and H2 adsorption properties of titania-supported cobalt Fischer Tropsch catalysts. Journal of Catalysis, (1): p Literature survey 37

66 73. Fredriksen, G.R., et al., CO hydrogenation over supported cobalt catalysts: FTIR and gravimetric studies. Chemical Engineering & Technology, (2): p Sachtler, W.M.H., et al., Promoter action in Fischer-Tropsch catalysis. Journal of Catalysis, (2): p Nonneman, L.E.Y. and V. Ponec, Promotion and support effects in syngas reactions. Catalysis Letters, (1): p den Breejen, J.P., et al., A Highly Active and Selective Manganese Oxide Promoted Cobalt-on-Silica Fischer Tropsch Catalyst. Topics in Catalysis, (13): p Boffa, A.B., et al., Lewis acidity as an explanation for oxide promotion of metals: implications of its importance and limits for catalytic reactions. Catalysis Letters, (3): p Johnson, G.R. and A.T. Bell, Role of ZrO2 in Promoting the Activity and Selectivity of Co-Based Fischer Tropsch Synthesis Catalysts. ACS Catalysis, (1): p Na, K., et al., Effect of Acidic Properties of Mesoporous Zeolites Supporting Pt Nanoparticles on Hydrogenative Conversion of Methylcyclopentane. Journal of the American Chemical Society, (49): p Sachtler, W.M.H. and M. Ichikawa, Catalytic site requirements for elementary steps in syngas conversion to oxygenates over promoted rhodium. The Journal of Physical Chemistry, (20): p Morales, F., et al., X-ray Absorption Spectroscopy of Mn/Co/TiO2 Fischer Tropsch Catalysts: Relationships between Preparation Method, Molecular Structure, and Catalyst Performance. The Journal of Physical Chemistry B, (17): p Literature survey

67 Chapter 3 : Methods and Materials This section details the computational models and tools utilized in this work. First, Density Functional Theory (DFT) and its applications are briefly introduced. Secondly, the catalysts and promoter bulk models are discussed. Next, the method to construct the experimental phase diagram using Ekvicalc software is discussed. Lastly, the different types of VASP calculations that were performed during this thesis are discussed, starting with gas phase calculations, adsorption calculations and bulk calculations. Then, frequency calculations are discussed together with the methodology to calculate Gibbs free energies. Bader charge calculations, transition state calculations and the construction of the DFT phase diagram are discussed. 39

68 3.1 Density Functional Theory Density Functional Theory (DFT) is a quantum-mechanical modelling approach that is widely used to approximate the Schrödinger equation for different systems. Since, Schrödinger equation that accurately describes a system is unsolvable for systems larger than a hydrogen atom, needs to be approximated. Most commonly the electronic structure of gas molecules, bulk phases, adsorbed species and transition states are examined. The range of application and the accuracy of the modeling functionals has increased much over the years, this is primarily possible due to development of more powerful computing capacity. [1] The purpose of this thesis is not to go into the detail of the principles behind DFT calculations, however, a short explanation can be found in [1] and a more detailed at [2]. 40 Methods and Materials

69 3.2 Catalyst and promoter bulk models Cobalt catalyst models Several cobalt catalyst models were used in this work as shown in Table 3-1. The cobalt terrace site was modelled as a five layer fcc Co(111) slab, using a p(3x3) unit cell with 9 atoms per layer. The bottom two layers were constrained at the bulk positions with an optimized lattice constant of 3.56 Å, which is in good agreement with the experimental lattice constant of 3.54 Å. [3] The cobalt step site was modelled as a three layer Co(211) slab, using a p(4x4) unit cell, where the bottom layer is fixed. Later in this work, a larger terrace surfaces was used to test MnO structures. Having a large unit cell avoid the interactions between neighboring unit cells. Therefore, a p(4x4) and a p(6x6) Co(111) was used. To minimize the computational time, only three layers were used and the bottom layer was constrained at the bulk positions. Methods and Materials 41

70 Table 3-1 The four cobalt catalyst models used in this thesis, respectively the p(3x3) Co(111) model, the p(4x4) Co(111) model, the p(6x6) Co(111) model and the p(4x4) Co(211) model. p(3x3) Co(111) model p(4x4) Co(111) model p(6x6) Co(111) model p(4x4) Co(211) model Promoter bulk models The construction of a DFT phase diagram for manganese promoters, which is discussed in section requires examination of different possible oxidation states. In this study, following manganese oxidation states are considered: Mn(gamma), MnO, Mn3O4 (=MnO.Mn2O3), Mn2O3, MnO2. Note that different Kpoints are used in these calculations to account for differences in unit cell size. Typically, the most stable manganese metallic phase consists of Mn(alpha). However, calculating accurate electronic energies for Mn(alpha) is computationally expensive due to its complex magnetic and geometrical structure. Therefore, Mn(gamma) phase is used in this study as a replacement. The energy difference between the Mn(gamma) and Mn(alpha) is reported to 42 Methods and Materials

71 be kj/mol. [4] using standard DFT calculations. The Kpoints mesh of (Monkhorst-Pack) was used to optimize the Mn(gamma) phase and, the Kpoints for the other manganese oxides were scaled accordingly based on their lattice parameters. The model sources and the Kpoints used for the different oxidation states of Mn are given in Table 3-3 and a representation of the models can be found in Table 3-2. Methods and Materials 43

72 Table 3-2 The five promoter bulk models used in this thesis, respectively the Mn(gamma) model, the MnO model, Mn3O4 model, the Mn2O3 model and the MnO2 model. Mn(gamma) model MnO model Mn3O4 model Mn2O3 model MnO2 model 44 Methods and Materials

73 Table 3-3The source of the model and Kpoints used for the different oxidations states of considered Mn in this study. Oxidation state Model source K-points mesh used Mn(gamma) [5] MnO [6] Mn3O4 [7] Mn2O3 [8] MnO2 [9] Methods and Materials 45

74 3.3 Experimental phase diagram Using Ekvicalc software an experimental phase diagram was constructed. To construct a phase diagram, it is require varying the and the temperature of the system. The user interface of the software is displayed in Figure 3-1. The first step is to input all considered species, in the correct aggregation state, the species required here are Mn(s), MnO(s), Mn3O4(s), Mn2O3(s), MnO2(s), H2O(g), H2(g). Note that for the different phases of the promoter element only the solid phase is considered. This prevents any liquid forming at higher temperatures and disturbing the equilibrium. Similarly, only gas phase is considered for H2O and H2 as the condensation of these species may have a large effect on the ratio. As there are multiple phases of solid Mn, all of them were takin into consideration to increase the accuracy of the phase diagram. The second step is to input the amount of reactants into the interface. A corresponding amount has to be specified for H2O, H2 and a selected phase of the promoter, here the Mn(alpha) phase is used. Very small amounts of Mn (10 mol) were specified in order to avoid any changes in the partial pressure of the gasses. On the other hand, large quantities of the gasses are used ( mol). To be able to vary the, it is required to change the amount of either substance and keep the other fixed. In this study, the program changes the amount of H2 by setting a minimum and maximum amount, as well as a factor. The factor is used to vary the amount of H2 using the following formula: new amount=old amount Lastly, the reaction conditions are implemented where the pressure is taken as a constant at 20 bar. To vary the temperature, a minimum and maximum temperature, as well as a temperature step are required. Running the program will then yield equilibrium quantities of all reactants for each condition, as well as activities for the solid phases. The point at which equilibrium between two phases occurs is determined using the activities of the solid phases. At equilibrium, two phases have an activity of 1. Using excel to plot all equilibrium points will yield equilibrium lines of ln ( ) versus temperature. Combining all equilibrium lines for each transition gives rise to a phase diagram. 46 Methods and Materials

75 It is important to note that the Ekvicalc software uses total energy minimization to determine the composition at each point. [10] Moreover, Ekvicalc software uses approximations when the implemented conditions are beyond the database limits for that substance Figure 3-1 The user interface of the Ekvicalc software package: window 1 is used to input commands; window 2 displays the selected species to enter amounts; window 3 shows the total amount of mol of each reactant; window 4 shows the conditions temperature, pressure and volume (as well as how it is varied); window 5 shows the total amounts of mol of each atom; window 6 shows the solid (and liquid) phases under consideration as well as their activities at the end of each calculations; window 7 shows the gas phases under consideration as well as their partial pressures at the end of each calculation. Methods and Materials 47

76 3.4 VASP calculations The Vienna Ab-initio Simulation Package (VASP) is a software package able to simulate the molecular dynamics of a system, at finite temperature, using an ab-initio quantum-mechanical approach. A more detailed account on the usage and its working can be found in the VASP guide available at the university of Wien website [11]. The INCAR file containing the settings for different calculations was provided by the coach of this thesis. This to assure that consistent settings are used, allowing comparison of results. This further assures the usage of more optimal settings for Co based systems. As this thesis is more focused on interpretation of the results generated than on the optimization of the VASP calculations, the description of the calculations below will not contain a detailed explanation of each INCAR file. To guarantee possible continuation of this work and to allow insight into the results, a short explanation of some settings will be provided Gas phase calculations The gas phase calculations are required to determine the energy of the adsorbate molecules in the gas phase. The calculations are performed in a [15x15x15] box under vacuum. The K-points grid is gamma-point only with one K-point in units of the reciprocal lattice vector as Adsorption calculations Adsorption calculations were executed on the catalyst models mentioned in section 3.2.1, sometimes even on the promoter oxidation state models. All calculations performed needed to be spin-polarized due to the presence of either Co or Mn, as both have an uneven number of valance electrons. The revised Perdew-Burke-Ernzerhof (revpbe) functional [12] was used, including non-local vdw-df correlations [13]. The cut-off kinetic energy is set at 450 ev, and 15 Å is used as the inter-slab distance to minimize the interactions between the consecutive slabs. A K-points mesh is generated, sampling the Brillouin zone with a (3 3 1) Monkhorst- Pack grid for all catalyst models. The actual K-point mesh is generated using the Monkhost- Pack procedure [14]. The adsorption energy is then calculated using the following equation: E =E! (E! +E ) (eq 3-1) Where E! is the total electronic energy without entropy of the slab with the adsorbate adsorbed. E! is the total electronic energy without entropy of the adsorbate in 48 Methods and Materials

77 a vacuum and E is the total electronic energy without entropy of the slab (bulk calculation). [15] Note that DFT calculations have an average error of 5 kj/mol.[16] The adsorption sites that are considered can be seen in Figure 3-2. Figure 3-2 The adsorption positions considered in this thesis: B = bridge, T = top, F = fcc, H= hcp or hollow[17] Bulk calculations Bulk calculations were used to determine the energy of a catalyst slab or for the different promoter oxidations state models. These calculations used the same settings as the adsorption calculations. However, the ISIF = 3 tag is added to ensure calculation of the stress tensor Frequency calculations VASP electronic energies are calculated for a temperature of 0 K and a pressure of 0 pa. Therefore, in order to calculate thermodynamics properties of the reactions, frequency calculations are required. Vibrational, rotational and translational partition functions allow calculation of the vibrational, rotational and translational enthalpy and entropy contributions. These values are used to calculate of Gibbs free adsorption energies ΔG as a function of temperature and pressure. [18] The expression used to determine the Gibbs free adsorption energy is given below: ΔG (T,p)=ΔH (T) T ΔS +R T ln-. / 0 (eq 3-2) Where, p 1 is the partial pressure of species x. First, the zero-point energy is determined using the vibrational frequencies from the frequency calculation. The zero-point energy (ZPE) is the correction used to account for the vibrational motions at 0 K. Adding ZPE to the VASP Methods and Materials 49

78 calculated electronic energy yields the corrected energy of the system. ZPE is calculated as below: 789: ZPE=. v 4 ;. (eq 3-3) Where, v are the vibrational frequencies of the system and N is the number of atoms in the adsorbate. The calculation of the enthalpy and entropy are, as mentioned before, split in three parts, the vibrational, rotational and translational partition functions. The entropy calculations can be calculated as: S < = R ln=1 e A B C D E+R F < A G C H! A B C D J.9! A B C D K (eq 3-4) S =R ln-4 L M 0+ N ln(k F 4 T) ln(p)+ N 0 (eq 3-5) 4 S =R -ln- P L Q ln-4 L G C H 0+. ln(i F 4 S I T I U )+ 7 0 (eq 3-6) 4 The enthalpy values are calculated as: S =R -ln- P L V G C H 0+10 (eq 3-7) Q F [H(T) H(0)] < =R T F < A G C H! A B C D J.9! A B C D K (eq 3-8) [H(T) H(0)] = N R T (eq 3-9) 4 [H(T) H(0)] =R T (eq 3-10) Where, R is the universal gas constant at J/(mol*K), h is the Planck constant at 6.626*10-34 Js, k T is the Boltzmann constant evaluated at 1.38*10 23 J/K, v the vibrational frequencies of the adsorbed species in s -1, Ii is the principal moments of inertia in gcm² and σ is the external symmetry number.[18] Bader charge Bader charge calculations are performed in order to better understands the impact of the promoter on the cobalt surface and how it affects the absorption of key species. The Bader 50 Methods and Materials

79 charge method divides atoms using zero flux surfaces; these are 2D surfaces with a minimal charge density. In molecular systems charge density tends to minimize between atoms, providing a good separation between the regions of influence of the different atoms. [17] The Bader charge calculation yields a charge for each atom in the system. These charges can be compared against the charge of the atom at the ground level, yielding an increase or decrease in charge. This is especially important due to the presence of a promoting element, as the effect of the element on the charge of the surrounding atoms may e.g. facilitate adsorption of certain species Transition state calculations Transition state calculations are used to determine the activation energy of a reaction. This requires determination of the transition state structure by finding the saddle point of the potential energy surface. This was achieved by combining two methods: the climbing image nudged elastic band (cneb) method [19] and the DIMER method [20] Climbing Image Nudged Elastic Band method Climbing image nudged elastic band method is used to determine an approximate transition state based on the reactants and products. The method firstly uses a script to generate several images, 8 images are used in this thesis, between the initial state (reactants) and the final state (products). Afterwards, these images are optimized with constraints. These constraints are required to keep the images from moving too much up/down the potential energy surface and keeping them at equal distances from each other. The constraints are implemented by adding spring forces between each of the images. An improved version of the NEB method includes a climbing image. In this approach, the image closest to the actual transition state is pushed even closer to the actual transition state. As the transition state is the highest energy state during the process, the image closest to it is the highest energy image and by further maximizing its energy it gets closer to the actual transition state. [21] After the NEB calculation a frequency calculation is performed on the highest energy image to determine the frequency closest to the transition state. The information of this frequency is then used for the DIMER calculation along with the optimized structure from the cneb calculation DIMER method DIMER method is used to optimize the transition state obtained from the cneb calculation. This method can normally be used to determine the transition state independently, by Methods and Materials 51

80 maximizing the energy of the system to get closer and closer to the actual transition state. However, this is computationally expensive and therefore, the cneb method is used to provide a good initial guess DFT Phase diagram construction In order to determine the oxidation state of the promoting element, a DFT based phase diagram was constructed. To construct a DFT based phase diagram several steps need to be considered. First, bulk calculations on the promoter oxidation state models as well as gas phase calculations for H2O and H2 are performed Secondly; frequency calculations are required for H2O and H2 gas species. The entropy changes in the different oxidation states are considered negligible and as such no frequency calculations is required for these models. The bulk calculations yield energies for the oxidation state models and the frequency calculations yield entropy and enthalpy in function of temperature for H2O and H2. To determine the most stable phase at each condition, the Gibbs free reaction energy for the reactions converting one oxidation state to another have to be determined. These reactions are of the form (the inverse reaction can also be used): higher oxidation state+x H 4 (g) lower oxidation state+x H 4 O(g) (eq 3-11) A more specific example is: MnO 4 (s)+2 H 4 (g) Mn(s)+2 H 4 O (g) (eq 3-12) The H 4 (g) H 4 O (g) system is used to perform the reduction (the reverse reaction is used for oxidation) because H2 and H2O are major constituents of the reaction mixture during FTS. Since CO is also a major constituent of the FTS reaction it is noted that the CO(g) CO 4 (g) system can be read in the same graph with only a shift in chemical potential. Calculating the delta G reaction uses an equivalent equation as the one used in section : ΔG! (T,p)=ΔH! (T) T ΔS! +RTln- / 0 (eq 3-13) Where, the partial pressure of H2O and H2 are taken as 6.67 bar and 8.89 bar respectively. This partial pressure are obtained for an average CO conversion of 60% at 20 bar, a typical conversion level and pressure often seen in commercial applications. [22] The exponent of the partial pressures is the stoichiometric coefficient of H2O and H2, taken from the reaction under consideration. ΔH! is determined by the enthalpies of the reactant and products, where 52 Methods and Materials /

81 the VASP energy from the bulk calculation is used for the oxidation state models. As mentioned before the ΔS! reaction is based only on the H 4 (g) H 4 O (g) system as the entropy contribution of the two solids is negligible. Before plotting these values, the ΔG! is converted to ln- 0 using the following equations: ln- 0= (f) = gh ijklmano (H,) 1 p H 1 (eq3-14) All the values are normalized per Mn atom to be consistent with the different stoichiometry used in different reactions. Plotting these values with the temperature gives a line that represent the transition between the two phases of the promoter examined. Combining multiple of these lines than gives rise to the phase diagram. Methods and Materials 53

82 3.5 Bibliography 1. Kohn, W., Nobel Lecture: Electronic structure of matter\char22{}wave functions and density functionals. Reviews of Modern Physics, (5): p Calais, J.-L., Density-functional theory of atoms and molecules. R.G. Parr and W. Yang, Oxford University Press, New York, Oxford, IX pp. Price International Journal of Quantum Chemistry, (1): p Cerda, J.R., et al., Epitaxial growth of cobalt films on Cu(100): a crystallographic LEED determination. Journal of Physics: Condensed Matter, (14): p Hafner, J. and D. Hobbs, Understanding the complex metallic element Mn. II. Geometric frustration in \ensuremath{\beta}-mn, phase stability, and phase transitions. Physical Review B, (1): p Häglund, J., et al., Theory of bonding in transition-metal carbides and nitrides. Physical Review B, (16): p Pacalo, R.E. and E.K. Graham, Pressure and temperature dependence of the elastic properties of synthetic MnO. Physics and Chemistry of Minerals, (1): p Baron, V., et al., The influence of iron substitution on the magnetic properties of hausmannite, Mn2+(Fe,Mn)3+2O4, in American Mineralogist p Klein, H., Structure solution of oxides from zone axes precession electron diffraction data, in Zeitschrift für Kristallographie - Crystalline Materials p Bolzan, A., et al., Powder Neutron Diffraction Study of Pyrolusite, β- MnO₂. Australian Journal of Chemistry, (6): p Buelens, L.S., S., Chemical Equilibrum Calculations Kresse, G., M. Marsman, and J. Furhtmüller VASP the GUIDE Zhang, Y. and W. Yang, Comment on ``Generalized Gradient Approximation Made Simple''. Physical Review Letters, (4): p Dion, M., et al., Van der Waals Density Functional for General Geometries. Physical Review Letters, (24): p Monkhorst, H.J. and J.D. Pack, Special points for Brillouin-zone integrations. Physical Review B, (12): p Zhao, Y.-F., et al., Insight into methanol synthesis from CO2 hydrogenation on Cu(1 1 1): Complex reaction network and the effects of H2O. Journal of Catalysis, (2): p Sholl, D.a.S., J. A., Density functional theory: a practical introduction. 2011: John Wiley & Sons. 17. Gunasooriya, G.T.K.K., Combined Theoretical and Experimental Study of CO Adsorption and Reactivity Over Platinum and Cobalt Irikura, K.K., Essential Statistical Thermodynamics, in Computational Thermochemistry: prediction and estimation of molecular thermodynamics, K.K. Irikura and D.J. Frurip, Editors. 1998, American Chemical Society: Washington. p Nudged Elastic Band. 2017; Available from: The Dimer method. 2017; Available from: Henkelman, G., B.P. Uberuaga, and H. Jónsson, A climbing image nudged elastic band method for finding saddle points and minimum energy paths. The Journal of Chemical Physics, (22): p Methods and Materials

83 22. Tan, K.F., et al., Effect of boron promotion on the stability of cobalt Fischer Tropsch catalysts. Journal of Catalysis, (1): p Methods and Materials 55

84 56 Methods and Materials

85 Chapter4: Nature of the promoter To determine the role of manganese promoters in Fischer-Tropsch Synthesis, a detailed understanding of the nature of the manganese promoter under reaction conditions is required. Many studies in the literature explored the role of manganese promoters by substituting manganese atoms in the slab without determining the oxidation state under reaction conditions. [1] This leads to inaccurate results and conclusions due to stronger/weaker adsorption of oxygen containing species since manganese is under saturated/oversaturated. Therefore, the most prevalent oxidation state of the promoter has to be determined before any further investigation into the role of promoters could be performed. The oxidation state of the promoter under reaction conditions affects electronic behavior and also determines the most favorable location on/in the catalyst surface. Thus, constructing a phase diagram enables one to determine the oxidation state of the manganese promoters under different reaction conditions. In this chapter, first, a brief introduction to the essential thermodynamics used in this study is presented. Next, steps required to construct an experimental phase diagram using Ekvicalc software is discussed and such experimental phase diagram for manganese oxides is constructed. Next, a DFT based phase diagram is constructed. Under this section, a benchmark of different functionals to optimize the accuracy/computational time ratio, the challenges encountered during construction of a DFT phase diagram and how to overcome such challenges are discussed. Finally, the experimental and DFT based phase diagrams are compared and the oxidation state of the manganese promoters under FTS conditions was determined. 57

86 4.1 Thermodynamics Phase diagrams are constructed at conditions where thermodynamically distinct phases occur and coexist at equilibrium. At equilibrium, there is no further driving force for net change. For example, thermal equilibrium is when two systems have the same temperature because at that point there is no longer any driving force to transfer energy from one system to another. Note that equilibrium is often dynamic, indicating that there is still transfer from one system to another and back but due to the lack of driving force there is no net transfer. More specifically for a phase diagram, equilibrium is a point at which the reaction rate of one species (e.g. MnO) converting to another species (e.g. Mn3O4) is the same as its inverse reaction, hence, no net reaction. This shows that this is still a dynamic equilibrium as reaction is still taking place, but there is no longer any driving force to create a net conversion from one phase to another. It is possible that more than two phases are in equilibrium at the same time; e.g. triple point of water. Equilibrium forms due to a minimization of total system energy or Gibbs free energy. In this thesis, an experimental phase diagram is constructed using Ekvicalc software and a DFT based phase diagram constructed. To determine the equilibrium for experimental phase diagram, the Ekvicalc software uses a database of Gibbs free energies for different species in a limited temperature range. Using Gibbs free energies, the software determines the composition of the system that minimizes the total Gibbs free energy of the system under the given conditions and constrained with the initial amount of each atom. To determine the equilibrium for DFT phase diagram, Gibbs free reaction energies for each conversion reactions (e.g.: + + ) was used to calculate the equilibrium coefficient (K): = (eq 4-1) This equilibrium coefficient can also be calculated using the following equation (for the following reaction: + + ): = (eq 4-2) The activity of the solid phases is taken as 1 and therefore, left out of the equation. H2O and H2 are only considered in the gas phase as such partial pressures are used for those species. Plotting the equation below will than yield the equilibrium line for the considered reaction. Note that the free Gibbs reaction energy or Δ is also temperature dependent. 58 Nature of the promoter

87 ln =! (eq 4-3) A phase diagram is a collection of these equilibrium lines where these lines form the separation between the areas in which each phase is most favorable. Most often a phase diagram is plotted as ln in function of " and is only valid at a specific pressure as a change in pressure will alter the ratio. For example the phase diagram of Co is given in Figure 4-1, where the equilibrium lines between two phases are clearly separating the alpha-co phase from the CoO phase and the CoO phase form the Co3O4 phase. Figure 4-1 The phase diagram for Co as reproduced from [2]. Nature of the promoter 59

88 4.2 Experimental phase diagram As mentioned in section 3.4, the Ekvicalc software minimizes the total free Gibbs energy of the system to determine the equilibrium composition for each set of conditions. After processing the data using Excel, a phase diagram was obtained. The Ekvicalc database uses experimental data to calculate the equilibrium and hence, can be compared with the DFT based phase diagram. The Experimental phase diagram obtained from Ekvicalc software is shown in Figure 4-2. The different oxidation states of manganese and different possible solid metallic manganese phases are indicated. The axes of the diagram are chosen as ln for the y-axis and temperature for the x-axis. The phase diagram shows that the MnO2 phase is preferable at high ln ratios. The high ln requirement can be explained due to MnO2 requiring a strong oxidizing atmosphere or high ratio to form. The reason the phase diagram only shows MnO2 at high temperature is because an increase in temperature decreases the ln requirement (requiring a lower ratio) which is due to the equation: ln =! (eq 4-4) Metallic Mn phases are on the bottom of the diagram as they require a strongly reducing atmosphere or low ratio. Here, the phases are formed at higher ln ratios as increasing the temperature will decrease the ln requirement. It is counter intuitive that an increase in temperature can work for both Mn and MnO2. This is due to taking the logarithm of the ratio as in the case of metallic Mn ln is negative and for MnO2 it is positive, as such increase the temperature will make ln respectively more negative and more positive decreasing the requirement for both cases. To determine the oxidation state of the Mn promoter under Fischer-Tropsch synthesis conditions, a temperature of 500 K and conversion of 60% are used, resulting in an H2 and H2O 60 Nature of the promoter

89 partial pressures of 8.89 bar and 6.67 bar respectively. [3] This leads to ln equal to Under these conditions, the most prevalent phase was determined to be MnO that corresponds to an oxidation state of 2+ for Mn. The phase diagram also shows that the MnO phase is stable under a wide range of ln at the 500 K point indicating a high stability of the MnO phase under operating conditions. Mn 2 O 3 MnO 2 Mn 3 O FTS MnO beta delta Mn Figure 4-2 The experimental phase diagram, constructed using Ekvicalc software, for the Mnoxides. Nature of the promoter 61

90 4.3 DFT based phase diagram Bulk structures Bulk calculations for the different promoter oxidation state models were performed (see more in section 3.3). The results of these calculations are shown in Table 4-1. Do note that Mn3O4 is a meta-stable state [4] and therefore not used in the phase diagram. Instead of the lowest energy alpha phase, gamma manganese phase was used in this study to decrease the calculation time and to increase the accuracy. Gamma phase is less magnetically and geometrically complex. [5] Table 4-1 The oxidation state and energy of the different Mn species considered using the vdw- DF functional. Mn species Oxidation state System energy [kj/mol per molecule] Mn(gamma) MnO Mn3O4 (= MnO.Mn2O3) +2 & Mn2O MnO The entropy impact of the solid Mn species is neglected for construction of the phase diagram. As such no frequency calculations are performed for these species and the values in the table above are used for the calculation of the reaction enthalpy. A reservoir of H2/H2O was used to reduce/oxidize the different Mn species. Gas phase calculation for both H2 and H2O was performed, followed by a frequency calculation to include the temperature effects. The entropy and enthalpy value for H2 and H2O at 500 K can be found in Table 4-2. Note that only the value for 500 K is displayed here but for the purpose of constructing the phase diagram, enthalpy and entropy values over the entire considered temperature range are required. Table 4-2 The enthalpy and entropy of the gas phase H2O and H2 system as obtained from the frequency calculations using the vdw-df functional. H2O (g) H2 (g) Enthalpy [kj/mol] Entropy [J/(mol K)] Nature of the promoter

91 4.3.2 Benchmark of different functionals Calculating accurate formation energies for manganese oxides using existing DFT-based functionals remains an important challenge in DFT. The VASP group examined this challenge intensively. [5] In their study, authors examined the formation energy of different manganese oxides using the following reaction: # $%&''&(+) * + (eq 4-5) The formation energies for manganese oxides were compared in Figure 4-3 using commonly used DFT functionals. As expected, PBE functional shows a strong deviation from the experimental results that are further validated by our calculations. The difference between the results reported by the VASP group and in the study stems from the usage of different settings. The hybrid functionals PBE0, PBEU4 and HSE perform much better, since these functionals include both local and non-local interactions whereas non-local interactions are not taken into account in non-hybrid functionals. However, these extra non-local interactions increase the computational cost of the functionals. Figure 4-3 shows that the vdw-df functional approaches the experimental results quite well, though not as well as the hybrid functionals. In this study, hybrid functionals are not pursued due to the extensive computational time and therefore the vdw-df functional is used. Nature of the promoter 63

92 0 Calculated formation energy (ev) Mn 3 O 4 Mn 2 O 3 MnO 2 MnO Experimental formation energy (ev) Figure 4-3 A comparison between the experimental formation energy and the calculated formation energy, the values for the PBE, PBEU4, PBE0 and HSE functionals are taken from [5]. Although the error on the bulk calculations is limited as much as possible by the selection of an appropriate functional the question remains as to why the above deviations occur. Since a thorough understanding of the functionals is required to correctly assess this problem and such a detailed investigation is well beyond the scope of this thesis. The VASP group [5] indicated that these errors are most likely due to either a shortcoming in the description of O2 or of unaccounted correlation effects in the Mn-oxides Challenges in constructing the phase diagram When constructing a DFT based phase diagram several challenges were faced. For example, when the reactions displayed in Table 4-3 were used to plot a phase diagram, the transition of Mn3O4(s) to MnO(s) is predicted incorrectly to occur after the transition of MnO(s) to Mn(s). Moreover, the generated DFT phase diagram does not predict the same phase as the experimental one. 64 Nature of the promoter

93 Table 4-3 Reaction enthalpy, entropy and Gibbs free energy of the reduction reactions of the Mn-oxides based on the vdw-df calculations and experimental data [6, 7]. Reaction vdw-df Experimental * - * * #* - #* #* $/(+ 1 2 $%( $/(+ 1 2 $%( $/(+ 1 6 $%( $/(+ 1 6 $%( $/(+ 1 3 $%( $/(+ 1 3 $%( $/(+ $%( $/(+ $%( *500 K; H: kj/mol; G:kJ/mol; S: J/(mol K); includes pressure correction; # values H & G for 298K The first problem shows behavior that is counter intuitive, not seen in the Ekvicalc phase diagram (see section 4.2) and not consistent with the experimental data shown in Table 4-3 where the experimental reaction Gibbs free energy values do show the correct order of transitions. To determine the cause of the problem the errors of reaction enthalpy, entropy and free Gibbs energy are examined as shown in Table 4-4. From these values it was concluded that the issue is caused by the free Gibbs energy errors on the transition from Mn3O4(s) to MnO(s) and MnO(s) to Mn(s). These errors cause the first transition to occur later than expected (positive error) while the second transition comes sooner than expected (negative error). To clarify further the issue is illustrated on Figure 4-4, which shows how the transitions can be inverted due to a different sign in the errors on both. Nature of the promoter 65

94 Table 4-4 The error on the calculated reaction enthalpy, entropy and free Gibbs energy for the individual transitions. The error is calculated as by subtracting the experimental value from the calculated one. T is taken at 500 K. Reaction ; 7 < 7 $/(+ 1 2 $%( $/(+ 1 2 $%( $/(+ 1 6 $%( $/(+ 1 6 $%( $/(+ 1 3 $%( $/(+ 1 3 $%( $/(+ $%( $/(+ $%( Correct transition 1 Positive error on transition 1 Inversion of transitions Negative error on transition 2 Correct transition 2 Figure 4-4 An illustration of how the errors on two-phase transition can cause the inversion of these transitions. Differences in experimental and DFT based phase diagram could be attributed to general trend towards overoxidation of Mn (compared to experimental data) observed in the DFT results. This trend was also identified by the VASP group. [5] The trend means that the DFT calculations prefer more oxidized forms of Mn at all temperatures than is experimentally correct. It is possible to correct the first problem by including entropy contributions for the solid phases. However, this corrected the MnO(s) to Mn(s) entropy error by only 5 kj/mol that is within the standard DFT error range. This combined with the difficulty of converging the frequency calculations for the manganese oxides meant another solution is required. The second possibility is to use the same reactant Mn-oxide phase for each reaction to make sure all errors are either positive or negative. It was determined that using MnO2 as a basis is the most optimal. 66 Nature of the promoter

95 This was anticipated from Figure 4-3, which shows MnO2 as the Mn-oxide with the smallest error on formation energy. This correction also solved the second problem as the correct phase is now predicted under FTS conditions. The reason that this correction solved the first problem is thought to be the incorrectly high stability of the MnO2 phase as predicted by the DFT calculation due to its tendency to overoxdize Mn Phase diagram After overcoming the challenges the finalized values for 500 K were determined, as seen in Table 4-5. The calculated reaction free Gibbs energy shows an increase for each consecutive transition meaning there is no inversion of two equilibrium lines. The small error on the calculated reaction entropy when compared with the experimental reaction entropy supports the previously made assumption of neglecting the entropy contributions of the solid phases. There does however remain a large error on the reaction free Gibbs energies when compared to the experimental values, which is for the most part caused by the error on the reaction enthalpy. The errors between experimental and calculate values are shown in Table 4-6. These values still show large errors in reaction free Gibbs energy and confirm that it is mainly due to an error on reaction enthalpy. As mentioned in section 4.3.2, this error is due to the usage of less time consuming functionals. Nature of the promoter 67

96 Table 4-5 The final reactions used in the construction of the phase diagram and the reaction enthalpy, entropy and free Gibbs energy at 500 K (based on vdw-df calculations and expertimental data [6, 7]). Reaction vdw-df Experimental * - * * #* - #* #* $/(+ 1 2 $%( $/(+ 1 2 $%( $/(+ 2 3 $%( $/( $%( $/(+ $%( $/(+ $%( $/(+2 $%( $/(+2 $%( *500 K; H: kj/mol; G:kJ/mol; S: J/(mol K) includes pressure correction # values H & G for 298K Table 4-6 The error on the calculated reaction enthalpy, entropy and free Gibbs energy for the transitions based on MnO2. The error is calculated as by subtracting the experimental value from the calculated one. T is taken at 500 K. Reaction ; 7 < 7 $/(+ 1 2 $%( $/(+ 1 2 $%( $/(+ 2 3 $%( $/( $%( $/(+ $%( $/(+ $%( $/(+2 $%( $/(+2 $%( Nature of the promoter

97 Plotting the DFT based phase diagram requires the reaction Gibbs free energy of the different transitions over the examined temperature interval. As stated in section 4.1, the equation below can be used to link the reaction free Gibbs energy to ln : ln =ln$(=! (eq 4-5) Using this equations combined with reaction free Gibbs energies for the examined temperature interval allows construction of the phase diagram, see Figure 4-5. To use this diagram to determine the oxidation state of the Mn promoter under reaction conditions, typical Fischer- Tropsch synthesis conditions were used. The temperature was taken at 500 K while the H2 and H2O partial pressures used were respectively 8.89 bar 6.67 bar [3] which results in a ln equal to Mn 2 O 3 MnO 2 Mn 3 O FTS MnO Gamma-Mn Figure 4-5 The DFT based phase diagram After the Fischer-Tropsch operating point was indicated on the phase diagram, the most prevalent phase under reaction conditions could be determined as MnO, with an oxidation state of +2. It is however noted that the most prevalent phase at 0 K, which is the temperature at which regular VASP calculations evaluate the models, is Mn3O4. This means that regular VASP calculations will tend to overoxidze Mn instead of preferring the MnO phase. However if Nature of the promoter 69

98 frequency calculations are employed for the addition of extra oxygen through the system, the overoxidzation should be averted. Frequency calculations are not performed for solid phases such as Mn substituted in the Co slab as their contribution was assumed to be negligible, much like for the Mn-oxide bulk phases. 70 Nature of the promoter

99 4.3.5 Comparison DFT and Ekvicalc phase diagram The DFT based phase diagram and the experimental phase diagram were compared as shown Figure 4-6. a) b) Figure 4-6 A comparison between the DFT phase diagram (b) and the Ekvicalc phase diagram (a) The first observation is the downward movement of the MnO2, Mn2O3 and Mn3O4 phases in the DFT based phase diagram. This indicates that these higher oxides are predicted more stable than they actually are. This indicates, DFT prefers to overoxides the Mn and hence this could lead to the wrong phase being predicted by the calculations. Secondly, the upward movement of the Mn phase which is caused by an overestimation of the stability of Mn, amplified by the error on the MnO2 reference. The last difference between the two diagrams is the change in slope seen in the transition from Mn3O4 to MnO. As the phase diagram plots a function of the form: )=8 =! + >, where in both cases (DFT & Ekvicalc) the > will be positive but the =! will be positive for the DFT case while it should be negative (see Table 4-5). Since a plot )= 8? will show an increase with T and a plot )=! +? will show a decrease with T, the reversed! sign of the is to blame for the change in shape. This sign reversal is caused by the error on the value meaning that the slope change is due to the error in the DFT calculations. Despite the differences between the two phase diagrams, both still predict MnO as the most prevalent phase under reaction conditions. As such the DFT results predict the correct phase and the location of the promoter can be determined in the next step. Note that during the search for the location care must be taken to use MnO as a basis for forming more complex Mn structures as it is the most prevalent phase and any extra oxygen has to be added using a system that is evaluated at 500 K (see section 4.3.4). Nature of the promoter 71

100 4.4 Bibliography 1. Ma, X., et al., Carbon monoxide adsorption and dissociation on Mn-decorated Rh(1 1 1) and Rh(5 5 3) surfaces: A first-principles study. Catalysis Today, (1): p Jahangiri, H., et al., A review of advanced catalyst development for Fischer-Tropsch synthesis of hydrocarbons from biomass derived syn-gas. Catalysis Science & Technology, (8): p Tan, K.F., et al., Effect of boron promotion on the stability of cobalt Fischer Tropsch catalysts. Journal of Catalysis, (1): p Azzoni, C.B., et al., Thermal stability and structural transition of metastable Mn5O8: in situ micro-raman study. Solid State Communications, (7): p Franchini, C., et al., Ground-state properties of multivalent manganese oxides: Density functional and hybrid density functional calculations. Physical Review B, (19): p Jacob, K.T., et al., Thermodynamic Data for Mn3O4, Mn2O3 and MnO2, in High Temperature Materials and Processes p Binnewies, M. and E. Milke, Data Section M, in Thermochemical Data of Elements and Compounds. 2008, Wiley-VCH Verlag GmbH. p Nature of the promoter

101 Chapter5: Location After determining the oxidation state of the promoter under reaction conditions, the next step is to determine the location of the MnO promoter in/on the slab. Prior to a discussion of the results, the reason why the location needs to be determined and the reason why two Co slabs, terrace and step, need to be considered is explained. Interestingly, existing experimental evidence [1-3] does not reveal the exact location of the promoter. These studies indicate that the Mn promoter covers most of the Co surface at higher loadings but the exact structures that the Mn promoter forms are unknown. However, no existing DFT literature was found that studies the location of the Mn-promoter in Co-based Fischer-Tropsch catalysis. One study [4] focused on examining Mn-promotion in Ru-based Fischer-Tropsch catalysis. In this study metallic Mn was used as a basis for the formation of Mn-Ru alloys. Typically, Mn is usually [1-3] added to the catalysts as MnO and in chapter four of this work it has been shown that MnO does not reduce to Mn under Fischer-Tropsch conditions. Due to the lack of previous work in determining the location of the Mn-promoter, an extensive investigation into possible locations has been performed in this work. Two Co-surfaces are considered in this work. A terrace surface modeled as a fcc Co(111) slab and a step surface modeled as a Co(211) slab. A terrace surface need to be taken into consideration is because this surface is kinetically most important under FTS conditions, as there is a high possibility that the under-coordinated edge sites become poisoned by binding carbon too strongly. [5] A step surface needs to be examined as surface reconstruction under reaction conditions has been shown to form step surfaces. [6] This chapter starts with examining different Mn promoter structures on a terrace surface. Secondly, Mn promoter structures on a step surface are examined. After examining the location of the promoter on a terrace and an edge surface, the driving force behind the formation of the most favorable structures was investigated using Bader charge calculations. 73

102 5.1 Terrace surface The study of MnO structures on/in the Co terrace surface has been split up in substitution into the terrace, increasing coverage, saturation coverage, ring optimization and MnO patches. The first section considers the substitution of the Mn promoter into the Co slab. The second section considers increasing coverage from 1 to 6 MnO, allowing a more efficient design of higher coverage structures through a wide screening of the 1 MnO structures. The saturation coverage section examines monolayers formation. While the ring optimization section tries to optimize the previously found optimal configuration. The MnO patches section considers nonstoichiometric MnO patches as well as MnO patches with hydrogen termination. Due to the size of these patches large Co slabs were required that increased the calculation time substantially. The limited timeframe of this thesis required a more efficient screening of these computationally demanding structures that was accomplished by initially stabilizing the structure on a fixed first layer of the Co slab. Afterwards the bottom layers were added and the calculation was performed with the first layer not fixed Substitution into the terrace As to verify the assumptions made in [4] the first possibility that has been examined is the substitution of metallic Mn into the slab. Three models were examined with a Co atom substituted in the surface, subsurface and bulk. A 5 layer Co(111) slab was used and all these models are shown in Table 5-1. Note that metallic Mn substituted in the bulk is modeled as Mn substituted in the third layer of the slab that, together with the 4 th and 5 th layer, represents the infinite bulk. The reaction used to represent these substitutions is: (eq 5-1) Where, Coslab is the energy of the Co(111) slab and is the energy of the Co(111) slab with one Co atom substituted by metallic Mn. 1 Co refers to the replaced Co atom and calculated as the bulk energy of Co. Co bulk energy of -345 kj/mol was calculated using the following equation: =!" #$% &!" #$ ' (eq 5-2) Where, ()*+,(- is the energy of a 5 layer 3x3 Co(111) slab and ()*+,(- is the energy of a 4 layer 3x3 Co(111) slab. Dividing by 9 is required to calculate the bulk energy per Co atom. 74 Location

103 The formation energies of different metallic Mn substituted structures can be found in Table 5-1. These values indicated that the substitution of metallic Mn into the Co slab is unfavorable for all considered positions with the surface position being the most favorable. Note that the reduction from MnO to Mn using the reaction below (eq 5-3) has a Gibbs free reaction energy of +38 kj/mol. This means that the substitution of Mn into the surface of the slab has a 5 kj/mol stabilization effect as the total free Gibbs formation energy of the substitution is 33 kj/mol. + + (eq 5-3) Table 5-1 The catalyst models with a top and side view, a description, the formation enthalpy at 500 K and Gibbs free formation energy at 500 K for the Co(111) slab and for Mn substituted at surface, subsurface and bulk positions in the Co(111) slab. Structure 1 Structure 2 Clean Co slab Mn at the surface Δ / = 65 kj/mol Δ3 / = 33 kj/mol Location 75

104 Structure 3 Structure 4 Mn at the subsurface Δ / = 77 kj/mol Δ3 / = 46 kj/mol Mn in the bulk (third layer) Δ / = 69 kj/mol Δ3 / = 37 kj/mol Note that the reason the Gibbs free energy of formation is lower than the formation enthalpy in Table 5-1 is an gain in entropy due to the production of H2O from H2. In an attempt to stabilize these structures, the substitution of MnO into the slab was considered. Substitutions into the subsurface and bulk position are not examined, as this would strongly deform the Co slab making it more unfavorable. The substitution of MnO into the surface is examined with Mn substituted into the slab and oxygen atom on top of the surface, the reverse is thought to be too unstable and was therefore not examined. Different positions of the oxygen atom were examined to optimize the configuration. The reaction used to calculate the formation energies of these structures is: (with Co bulk energy taken as -345 kj/mol),(- +,(- +1 (eq 5-4) The different configurations along with their formation energies are shown in Table 5-2. The high formation energy of the configuration with oxygen on top of manganese makes this 76 Location

105 structure unfavorable. When oxygen occupies a HCP position, the structure becomes more favorable compared to other oxygen positions but is still unstable. Interestingly, when oxygen is located in an FCC position the formation is equilibrated with Gibbs free formation energy of zero. Meaning that under reaction conditions the substitution of MnO into the slab with oxygen in an FCC position will occur but it will most likely not be the most prevalent structure, as other more favorable structures could exist. Only the Gibbs free energy of formation is shown in Table 5-2, this is because the entropy contributions of bulk structures and slabs are assumed negligible and therefore, there is no energy difference with the formation enthalpy. When comparing the Gibbs free formation energy for substitution of metallic Mn into the surface in Table 5-1 with the formation energy for the substitution of MnO with oxygen in an FCC position in Table 5-2, a difference of 34 kj/mol is observed. This indicates that the oxidation of manganese in the slab increases stability by 38 kj/mol, negates the 5 kj/mol stabilization of the slab due to manganese substitution. Table 5-2 The different configurations for the substitution of MnO into the surface of the Co(111) slab. A top view, side view, description and the Gibbs free energy of formation at 500 K are shown. Structure 1 Structure 2 Structure 3 Δ3 / 500 2= 105 kj/mol Δ3 / 500 2= 2 kj/mol Δ3 / 500 2= 0 kj/mol Mn: substituted O: on top of Mn Mn: substituted O: HCP Mn: substituted O: FCC In conclusion, the substitution of metallic Mn in the Co(111) slab is unfavorable. The substitution of MnO into the Co slab on the other hand is possible when oxygen is in FCC Location 77

106 position. This structure has a Gibbs free energy of formation of 0 kj/mol and unlikely that it will be the most prevalent structure under reaction conditions Increasing MnO coverage The results in section indicated that substitution of the metallic Mn promoter into the Co(111) slab is not highly favorable prompting examination into adsorption of MnO structures. First a wide screening of single MnO configurations were performed; afterwards the information obtained from this screening was used to construct structures with 2 to 6 MnO. The structures under consideration in this section are all stoichiometric, as no oxygen or hydrogen is removed or added in Single MnO adsorption At first, ten configurations for MnO adsorption were optimized. Two special cases where oxygen was below the manganese atom were also considered for completeness. All the considered configurations are displayed in Table 5-3. In order to discover potentially unconsidered configurations a second optimization was performed for the first ten configurations in Table 5-3, with MnO located at approximately 3 Å from the Co surface. This allowed more freedom during the optimization process revealing more optimal configurations. The last two structures were not re-examined as these configurations with manganese on top of the oxygen atom were determined to be unfavorable. 78 Location

107 Table 5-3 The single MnO configurations examined on a Co(111) slab with a top and side view as well as a description of the configuration Structure 1 Structure 2 Structure 3 Mn: FCC; O: HCP Mn: FCC; O: top Mn: FCC; O: on top of Mn Structure 4 Structure 5 Structure 6 Mn: HCP; O: FCC Mn: HCP; O: on top of Mn Mn: HCP; O: top Location 79

108 Structure 7 Structure 8 Structure 9 Mn: top; O: bridge Mn: top; O: FCC Mn: top; O: HPC Structure 10 Structure 11 Structure 12 Mn: top; O: on top of Mn Mn: on top of O; O: HCP Mn: on top of O; O: bridge After performing the optimization of the structures in Table 5-3 and a secondary optimization where MnO was positioned three Å from the Co surface, 6 different configurations remained. These configurations, together with their Gibbs free energy of formation as well as a description can be found in Table 5-4. The formation energies of these structures were calculated using the reaction: + (eq 5-5) 80 Location

109 Table 5-4 An overview of the 6 optimized configurations for the addition of 1 MnO to a Co(111) slab. A top and side view are given as well as a description of the configuration and the Gibbs free formation energy at 500 K Structure 1 Structure 2 Structure 3 Mn: FCC; O: HCP Mn: bridge-hcp; O: top Mn: FCC; O: on top of Mn 72 kj/mol 108 kj/mol 163 kj/mol Structure 4 Structure 5 Mn: bridge-hcp O: FCC Mn: HCP O: on top of Mn 70 kj/mol 159 kj/mol Where is the Co slab with the MnO structure adsorbed,,(- is the Co slab without the MnO and MnO represents the MnO bulk. All values in Table 5-4 are positive and higher than 70 kj/mol indicating none of the optimized structures is stable under reaction Location 81

110 conditions. The most favorable configuration is structure four in Table 5-4 and has manganese in a bridge-hcp position and oxygen in an FCC position with Gibbs free formation energy of 70 kj/mol. As none of the single MnO configurations are stable, higher coverage structures need to be examined. The most favorable single MnO configuration shows that Mn prefers bridge-hcp positions and oxygen FCC positions; this information was used as a basis to construct higher coverage structures Two-six MnO structures adsorption To construct higher coverage structures, the most stable single MnO configuration was used as a basis. To allow more freedom during the optimization process the calculations were performed with the MnO structure placed three Å above the surface of the Co slab, this increases the possibility of finding the most optimal configuration. Another method used to increase the possibility of finding the most optimal configuration is to execute additional optimizations where the examined structures are shifted relative to the Co slab. The structures examined in this section are stoichiometric structures of MnO and no oxygen has been extracted or added in. Non-stoichiometric structures are examined in section The reaction used to calculated the formation energy of the structures in this section is of the same form as the one used in section and is displayed below: +4 5 (eq 5-6) The formation energies reported in this section are normalized to the adsorption of one MnO molecule. This allows comparison of structures containing different amounts of MnO. Note that the structure displayed in this section are the optimized forms and that if multiple initial states had converged towards a similar optimized structure with the energy difference between them smaller than 1 kj/mol only the most favorable structure was reported. In Table 5-5, the MnO structures containing two MnO are displayed. The formation energies show that the first structure that forms a curving line on the surface is the most favorable as it has a Gibbs free formation energy of -3 kj/mol/mno. The second structure has a ring like form while the third has oxygen on top of the manganese; one manganese occupies an FCC position while the other an HCP with the oxygen atoms tilted towards the outside. 82 Location

111 Table 5-5 An overview of the optimized two MnO structures on a Co(111) slab. A top and side view are given as well as the Gibbs free formation energy at 500 K. Structure 1 Structure 2 Structure 3-3 kj/mol/mno 69 kj/mol/mno 137 kj/mol/mno Table 5-6 contains the optimized MnO structures consisting of three MnO molecules, The Gibbs free formation energies indicate that none of the these structures are stable under reaction conditions. The three first structures all have a ring shape, the first one is the most stable with manganese in the optimal bridge position and oxygen in a top, shifting to bridge, position. The second is less stable with manganese in an FCC position and oxygen in a top, shifted to FCC, position. The third has a configuration much like the first one and there is only a 3 kj/mol/mno energy difference with the first ring configuration, indicating that it had reached a local minimum. The fourth structure is a triangle of manganese atoms on top of the surface with oxygen on top of the manganese but tilted outwards. The lack of oxygen adsorption onto the surface is likely the cause of its high formation energy. Location 83

112 Table 5-6 An overview of the optimized three MnO structures on a Co(111) slab. A top and side view are given as well as the Gibbs free formation energy at 500 K. Structure 1 Structure 2 5 kj/mol/mno 33 kj/mol/mno Structure 3 Structure 4 8 kj/mol/mno 120 kj/mol/mno The optimized MnO structures for structures containing four MnO molecules can be found in Table 5-7. Structure one and structure eight are both stable with Gibbs free energy of formations 84 Location

113 -12 kj/mol/mno and -13 kj/mol/mno respectively. This indicates that structure eight is slightly more favorable. Structure one shows a double curved line on the surface, with two bridging Mn-O bonds in between. The high stability of this structure is likely due to the adsorption of most of the oxygen onto the Co surface which appears to be favorable. The second structure has a ring shape and could exist under reaction conditions due to its free Gibbs formation energy of 0 kj/mol/mno, but as more favorable structures exist it is not the most prevalent. Structure three has a square shape and is less optimal than structure two due to oxygen atoms being in less favorable position to adsorb to the Co surface and due to the several manganese atoms being in the unfavorable top position. Structure four is closer to a ring shape and is more favorable than structure three due to the manganese being closer to the favorable bridge sites and the two of the oxygen atoms being in top positions facilitating their adsorption to the surface. The structure four is less favorable than structure two as two of its oxygen atoms are at bridge sites instead leading to them not adsorbing onto the surface. Structure five has all manganese in bridge positions but because three of its four oxygen atoms cannot adsorb onto the surface it is unstable. The sixth structure has four manganese atoms packed together, all in HCP sites with three of the oxygen atoms on top of the manganese pointing outwards and one that is connected to the surface. Structure seven is similar to structure six but with all manganese in bridge sites but none of the oxygen atoms adsorbed onto to the surface. Structure eight shows a crisscross pattern over the surface with manganese at bridge sites and two of the four oxygen atoms adsorbed in top sites, the last two oxygen atoms are not adsorbed. The previous observations showed that adsorption of oxygen onto the surface increase the stability and that it more readily bonds to the surface if it occupies a top position or a shifted top position. Location 85

114 Table 5-7 An overview of the optimized four MnO structures on a Co(111) slab. A top and side view are given as well as the formation enthalpy at 500 K and the free Gibbs formation energy at 500 K Structure 1 Structure 2-12 kj/mol/mno 0 kj/mol/mno Structure 3 Structure 4 5 kj/mol/mno 2 kj/mol/mno 86 Location

115 Structure 5 Structure 6 10 kj/mol/mno 99 kj/mol/mno Structure 7 Structure kj/mol/mno -13 kj/mol/mno Table 5-8 shows the optimized MnO structures containing five MnO. The free Gibbs formation energies show that structure one is stable. Structure one has a pentagonal shape with all manganese in bridge or shifted bridge positions and tree of the five oxygen atoms are connected to the surface, while the last two oxygen atoms are not connected to the surface. Structure two also shows a pentagonal shape but here only two of the oxygen atoms are adsorbed onto the Location 87

116 surface. Again the adsorption of oxygen onto the surface shows its importance in stabilizing the structure on the slab. Table 5-8 An overview of the optimized five MnO structures on a Co(111) slab. A top and side view are given as well the Gibbs free formation energy at 500 K. Structure 1 Structure 2-4 kj/mol/mno 5 kj/mol/mno The optimized MnO structures containing six MnO are shown in Table 5-9. Structures one to five all contain rings with structure two being the most stable. Structure two is the most stable, with all manganese atoms in bridge position and all oxygen atoms in a shifted top position allowing all oxygen atoms to adsorb unto the surface. The structures one, three, four and five all have less oxygen adsorbed onto the surface and manganese in less favorable positions making them less favorable than structure two. The top view of structure two shows that the p(4x4) Co slab is very packed with the six MnO ring, leading to the proposition that the close proximity of the rings would lead to increased deformation of the Co surface. To increase the distance between two successive rings a larger Co slab was needed, as such the six MnO ring was examined on a p(6x6) Co slab. Structure six shows this configuration and is 10 kj/mol/mno more stable than structure two, confirming the previous proposition. The symmetry of the ring is thought to increase the stability of the configuration as it spreads the deformation of the Co slab more evenly instead of focusing it in one point. 88 Location

117 Table 5-9 An overview of the optimized six MnO structures on a Co(111) slab. A top and side view are given as well as the Gibbs free formation energy at 500 K. Structure 1 Structure 2-1 kj/mol/mno -8 kj/mol/mno Structure 3 Structure 4-3 kj/mol/mno 1 kj/mol/mno Location 89

118 Structure 5 Structure 6 3 kj/mol/mno -18 kj/mol/mno Lastly, one optimized structure with eight MnO was found, as shown in Table This structure has a complex build up with two manganese in bridge position, six in FCC/HCP positions and only one of the oxygen atoms adsorbed onto the surface. The free Gibbs energy of formation of this structure is -6 kj/mol/mno. 90 Location

119 Table 5-10 The optimized eight MnO structure on a Co(111) slab. A top and side view are given as well as the formation enthalpy at 500 K and the free Gibbs formation energy at 500 K Structure 1-6 kj/mol/mno In conclusion, six MnO ring (Table 5-9, structure six) is the most stable structure with a Gibbs free formation energy of -18 kj/mol/mno. This indicates that this structure will be the most prevalent of all the MnO structures considered in this section Saturation MnO coverage Under saturation coverage, MnO is likely to form monolayers. Therefore, this section examines different monolayer configurations of MnO on the Co(111) surface. Two saturation MnO structures are shown in Table Structure one considers manganese in an HCP position and oxygen in an FCC position while structure two considers manganese in a FCC position and oxygen in a top position. Both structures were found to be unstable; with structure two being 15 kj/mol/mno more favorable indicating that oxygen in a top position is more favorable. Other monolayers configurations that were examined became chaotic and were not reported. These results indicated that monolayers would not form under FTS reaction conditions. Location 91

120 Table 5-11 An overview of the optimized monolayer structures on a Co(111) slab. A top and side view are given as well as the Gibbs free formation energy at 500 K. Structure 1 Structure 2 28 kj/mol/mno 13 kj/mol/mno Non-stoichiometric patches This section considers the formation of non-stoichiometric MnO structures by adding oxygen and hydrogen are considered. In order to add extra oxygen and hydrogen the reaction used in section needs to be altered. The reaction used in this section then becomes: ) 9 +8 (eq 5-7) Where is used to add oxygen atoms and where is used to add hydrogen atoms to the adsorbed patches. As H2O and H2 are gaseous species, enthalpy, entropy and pressure corrections are required for these species as such, from this point on both formation enthalpy and free Gibbs formation energy at 500 K are reported. In chapter four of this work, the oxidation state of the manganese under reaction conditions was determined to be MnO, note that this does not mean that MnO structures formed on the catalyst surface are stoichiometric. The interaction of the adsorbed MnO with the catalysts surface could favor the addition or removal of oxygen atoms forms the MnO structures. Experimental studies 92 Location

121 [1, 3, 5] also indicated MnO as most prevalent phase but these measurements primarily measure the MnO nanoparticles, not the small amount of MnO that migrated to the Co surface. Firstly an attempt is made to further optimize the most stable structure, the six MnO ring, from section 5.1.2, afterwards triangle and hexagonal structures are examined Ring patches In section the six MnO ring structure was found to be most stable MnO structure. As such an attempt was made to stabilize this structure further through the addition of extra oxygen in the center of the ring. Therefore, several configurations were examined with the addition of three and six extra oxygen into the ring. Configurations with six extra oxygen atoms disintegrated and did not provide an increase in stability when compared to the original 6 MnO ring. The addition of three oxygen atoms gave rise to structure one in Table Different locations for the oxygen atoms were examined but the energy difference between the structures was smaller than 1 kj/mol/mno, as such only the most optimal structure is shown. However, all of these structures were unstable as they have Gibbs free formation energy of 19 kj/mol/mno. The difference between the formation enthalpy and the free Gibbs formation energy comes from the loss of entropy from converting H2O to H2. Location 93

122 Table 5-12 The optimized ring with additional oxygen inside structure on a Co(111) slab. A top and side view are given as well as the formation enthalpy at 500 K and the Gibbs free formation energy at 500 K. Additional oxygen is added using + and additional hydrogen using. Structure 1 Δ / 500 2= 3 kj/mol/mno Δ3 / 500 2= 19 kj/mol/mno Triangular patches Liu et al. [7] recently examines the formation of triangular ZnO patches on a copper catalyst for CO2 to methanol. Inspired by the work, we conducted similar study for MnO. As shown in Table 5-13, structure one was evaluated with one additional oxygen added to the MnO patch. This structure was found to be 5 kj/mol/mno unstable. An attempt to mediate this instability was thought to be hydrogen termination of the oxygen atoms, as shown in structure two. The tips of the triangle were also terminated with OH, indicated in orange, as it was theorized that OH species, which occupy the catalysts surface under reaction conditions, bind to the tips of the triangle as these position are under coordinated. Interestingly, the structure is stable in terms of formation enthalpy is exothermic but the stability is unfavorable due to entropy cost of the hydrogen addition. 94 Location

123 Table 5-13 An overview of the optimized triangle structures on a Co(111) slab. A top and side view are given as well as the formation enthalpy at 500 K and the Gibbs free formation energy at 500 K. Additional oxygen is added using + and additional hydrogen using. Structure 1 Structure 2 Δ / 500 2= 0 kj/mol/mno Δ3 / 500 2= 5 kj/mol/mno Δ / 500 2= -23 kj/mol/mno Δ3 / 500 2= 54 kj/mol/mno Hexagonal patches Based on the triangular patches but combined with the information from section were a six MnO rings was found to be the most stable structure. A hexagonal shape was designed, this shape makes it easier for the external oxygen atoms to adsorb onto the catalyst surface but with the under coordinated center of the ring filled in. The first two structures were based primarily on the triangular shapes, as seen in Table Structure one shows that hexagonal shapes allow easier adsorption of the external oxygen atoms than triangles, this optimized form is stable but the shape indicates the optimization is stuck at a local minimum. The second structure is an attempt to look into hydrogen termination. The shape of the patch appears more optimal than the first structure and the hydrogen addition provides a strong increase in stability when examining the formation enthalpy. However, much like with the triangular structure the high entropy cost required for the addition of hydrogen cannot be compensate. Together with structure one, structure three was examined. It was thought that the high oxygen density in structure one might be unfavorable but the results show that the opposite is true, with structure three being 10 kj/mol/mno less stable than structure one. Likewise structure four was Location 95

124 examined to decrease the hydroxyl density in structure two, here the removal of three OHgroups increased the formation enthalpy by 25 kj/mol/mno. But, the decrease in the amount of OH-group, making the structure even under stoichiometric oxygen, decreases the entropy cost enough for the structure to be stable. Structure five and six are an attempt at terminating each manganese with respectively oxygen and hydroxyl-groups. Terminating each manganese with oxygen increases the amount of oxygen bonded to the surface which was shown to be favorable. Contrary to expectations, the structure is enthalpically less favorable than structure one and adding the high entropy cost for the addition of extra oxygen to the surface makes it unfavorable. The hydroxyl termination on the other hand makes the structure more stable than structure two based on formation enthalpy but the entropy cost for the addition of extra oxygen and hydrogen is too high resulting in an unfavorable structure. Lastly based on the visually more optimal shape of structure two, combined with the high stability of structure one, structure seven was examined. This structure proved to be the most optimal, closely resembling the six MnO ring with the center filled in with manganese and three oxygen. Structure eight was an attempt at further stabilizing structure seven by terminating the center oxygen atoms with hydrogen. The hydrogen addition increased the formation enthalpy of structure seven by 6 kj/mol/mno. But, on a Gibbs free energy basis structure eight is 8 kj/mol/mno less stable than structure seven due to the high entropy cost for hydrogen addition. Nevertheless, structure eight is stable based on Gibbs free formation energy but less so then structure seven. 96 Location

125 Table 5-14 An overview of the optimized hexagon structures on a Co(111) slab. A top and side view are given as well as the formation enthalpy at 500 K and the Gibbs free formation energy at 500 K. Additional oxygen is added using + and additional hydrogen using. Structure 1 Structure 2 Δ / 500 2= -22 kj/mol/mno Δ3 / 500 2= -13 kj/mol/mno Δ / 500 2= -50 kj/mol/mno Δ3 / 500 2= 2 kj/mol/mno Structure 3 Structure 4 Δ / 500 2= 1 kj/mol/mno Δ3 / 500 2= -3 kj/mol/mno Δ / 500 2= -28 kj/mol/mno Δ3 / 500 2= -4 kj/mol/mno Location 97

126 Structure 5 Structure 6 Δ / 500 2= -1 kj/mol/mno Δ3 / 500 2= 21 kj/mol/mno Δ / 500 2= -59 kj/mol/mno Δ3 / 500 2= 21 kj/mol/mno Structure 7 Structure 8 Δ / 500 2= -24 kj/mol/mno Δ3 / 500 2= -15 kj/mol/mno Δ / 500 2= -30 kj/mol/mno Δ3 / 500 2= -7 kj/mol/mno In this section the most optimal structure has been determined to be hexagon structure seven, which closely resembles the six MnO ring of section Its Gibbs free formation energy is -15 kj/mol/mno. In the analysis above the entropy of the patches on the slab has been neglected. It is thought that the hydrogen and oxygen addition to the patches might become more favorable if the enthalpy and entropy corrections are included. However, the high computational time requirements for the optimization calculations of these patches (let alone 98 Location

127 for the frequency calculations) combined with the time limitations of this thesis made examining the effect of these correction not possible Conclusion The most optimal structure on a terrace site has been determined to be a six MnO ring, which an free Gibbs formation energy of -15 kj/mol/mno. Location 99

128 5.2 Step surface To determine the possible location of manganese in a step surface, an initial screening of one MnO substitution configurations was performed. After determining the most optimal configuration, the coverage of this configuration was further examined Single MnO substitution In correspondence with section 5.1.1, at first the substitution of metallic Mn into the slab was examined. As shown in Table 5-15, the high formation enthalpy and Gibbs free formation energy of this structure makes the addition of metallic Mn into the step site unfavorable. To calculate the formation energies equation 5-3 is used. Table 5-15 The optimized Mn substitution into the Co(211) slab. A top and side view are given as well as the Gibbs free formation energy at 500 K. Structure 1 Δ / 500 2= 268 kj/mol/mno Δ3 / 500 2= 236 kj/mol/mno Section showed that the substitution of MnO into the terrace is more favorable than the substitution of Mn. Therefore; MnO substitution into the slab was examined. As shown in Table 100 Location

129 5-16, five optimized structures were obtained. The first structure considers oxygen in a bridge position on the edge and is stable with Gibbs free formation energy of -13 kj/mol/mno. Structure two shows that oxygen adsorption in a B5 position is unfavorable. Next, oxygen was considered in an HCP position at the edge as seen in structure three, this is the most optimal position for oxygen as this structure has the lowest Gibbs free formation energy. Structure four examines oxygen in a bridge position behind the edge while structure five considered oxygen in a bridge position on the edge. Both configurations are stable and only differ 1 kj/mol/mno. As structure three is the most stable, it will be the most prevalent under reaction conditions (500 K, 20 bar and 60% conversion). Table 5-16 An overview of the optimized MnO substitutions into a Co(211) slab. A top and side view are given as well as the Gibbs free formation energy at 500 K. Structure 1 Structure 2 Δ3 / 500 2= -13 kj/mol/mno Δ3 / 500 2= 8 kj/mol/mno Location 101

130 Structure 3 Structure 4 Δ3 / 500 2= -31 kj/mol/mno Δ3 / 500 2= -10 kj/mol/mno Structure 5 Δ3 / 500 2= -11 kj/mol/mno 102 Location

131 5.2.2 Increasing MnO coverage After determining the most optimal configuration for a single MnO substitution into a step site, the question of the MnO coverage on the edge remains. Therefore, substitution of additional MnO into the step site was examined and shown in Table The Gibbs free formation energy given here is the average formation energy required to substitute the MnO, more important is the Gibbs free addition energy which is the energy required to add the additional MnO into the slab. Table 5-17 shows that the addition of 1 MnO has the highest addition Gibbs free energy, meaning that this addition is always the most favorable and any will be preferred. A possible reason for the second MnO addition to be less favorable is the difference in Co-O and Mn-O bond length. As can be seen in the two MnO structure, both manganese atoms are pushed out of the edge while this is not the case in the one MnO structure. It is proposed that the Co edge can accommodate one manganese per three Co atoms due to a compression of the Co-Co bond as seen in the one MnO structure. This compression is however no longer possible after the second MnO has been added making it less favorable. The third addition is more favorable than the second, most likely because the adsorption of oxygen is favorable and the pushing out of the manganese atom seen for both manganese atoms in the two MnO structure is only applicable on one of the three manganese atoms in the three MnO structure. The last addition forms an edge entirely of MnO and is proposed to be unfavorable because the Mn-Mn bond length is higher than the Co-Co bond length meaning the Mn are compressed into the edge. This compression can be observed visually when comparing Figure 5-1-B, where the manganese atoms are not scaled, with Figure 5-1-A. Location 103

132 Table 5-17 An overview of the MnO structures substituted in a Co(211) slab used to determine coverage. A top and side view are given as well as the Gibbs free formation energy at 500 K and the Gibbs free addition energy at 500 K. 1 MnO structure 2 MnO structure Δ3 / 500 2= -31kJ/mol/MnO Δ3 ;;<<= 500 2= -31kJ/mol/MnO Δ3 / 500 2= -20 kj/mol/mno Δ3 ;;<<= 500 2= -8 kj/mol/mno 104 Location

133 3 MnO structure 4 MnO structure Δ3 / 500 2= -17 kj/mol/mno Δ3 ;;<<= 500 2= -11 kj/mol/mno Δ3 / 500 2= 4 kj/mol/mno Δ3 ;;<<= 500 2= 68 kj/mol/mno A) B) Figure 5-1 A) The Co(211) slab without MnO addition. B) The 4 MnO structure with the Mn atoms at full scale Conclusion In conclusion, the substitution of Mn into the edge of a step site is unfavorable but the substitution of MnO is favorable. The most stable configuration is structure three (Table 5-16) where oxygen occupies an HCP position on top of the edge. The coverage of MnO in the edge has been determined to be 1 MnO in a p(4x4) slab. Location 105

134 5.3 Bader charge analysis In an attempt to further understand the driving forces behind the formation of the structures examined before in this chapter, several selected structures are examined through Bader charge analysis. A point of reference is required for the charge of cobalt, manganese and oxygen. To this end a Bader charge analysis of a Co(111) p(4x4) slab was performed. The charges of the atoms at the surface layers are averaged out yielding For manganese and oxygen, the reference is based on their number of valence electrons which is respectively seven and six. The charges reported in this chapter are calculated using the equation: >h@ab=>@c>dc@ebf >h@ab ABHBAB>B (eq 5-8) A distinction is made between a terrace site and an edge site as section 5.1 and 5.2 have shown that MnO will from structures on top of a terrace site but will substitute into a step site Terrace surface Table 5-18 shows the result of Bader charge analysis on three selected MnO structures. The six MnO ring p(6x6) has very similar charges for all manganese atoms as well as for all oxygen atoms. This is due to the symmetry of the structure and the almost identical location of each atom on the slab. This symmetry is also reflected in the charges of the Co slab where the charge of the atoms bonded to oxygen, which are indicated in red, are similar. The charges of the cobalt atoms in contact with manganese, which are indicated in blue, also show similar values. The cobalt atoms in contact with manganese outside of the ring have a charge of around while those inside the ring have charges around The difference between the cobalt on the outside and inside is thought to be due to the difference in distance to the manganese, the outside cobalt is further away and receives less charge. The values indicate that oxygen gains charge from manganese and the bound cobalt atoms while manganese loses charge to both oxygen and its bound cobalt atoms. The small deviations in the values are most likely due to the innate error of DFT calculations. It has to be noted that manganese also binds to the cobalt atoms to which the adjacent oxygen s are bound. The six MnO ring on a p(4x4) slab shows similar results for the charges of manganese and oxygen, with only an average 0.02 decrease in the charge of oxygen. Meaning that the smaller unit cell does not appear to have a large impact on the charge of these atoms. The charge of the cobalt atoms in contact with oxygen, indicted in red, is also not affected by the decrease in the slab size, even when two oxygen atoms share one cobalt atoms as is the case for oxygen 3 and 106 Location

135 6. Cobalt atoms in contact with manganese and positioned on the inside of the ring are also unaffected. But the ones outside of the ring are affected as they are all shared between two manganese atoms each. The charge of these atoms, indicated in green, has nearly doubled when compared to the p(6x6) slab which is in line with them being shared by two manganese atoms. It is thought that this increase in charge and thereby the sharing of a cobalt atoms by the manganese atoms of the circle is unfavorable and contributes to the energy difference between the ring on a p(6x6) surface and a p(4x4) surface. The five MnO ring on a p(4x4) surface has charges similar to the six MnO ring for manganese one, two and three and for oxygen one, two and three. All these atoms occupy positions similar to the ones observed in the six MnO ring. The slight increase in charge gained by oxygen one and two is due to decreased interference from the nearby manganese in the Co-O bond when compared to the six MnO ring as these cobalt atoms do not bond with manganese. Oxygen four can draw less charge as manganese four is nearly in an HCP position with the Co atom the oxygen is bonded to, increasing its interference in the Co-O bond. This also explains the decrease in the loss of charge of this cobalt atom. The fifth oxygen atom is not bonded to the surface, decreasing the amount of charge it can draw from its surroundings. This is partially compensated by drawing more charge from manganese four and five, which is reflected in the increased loss of charge in these atoms. Examining the cobalt surface leads to additional observations. Firstly, oxygen three is bonded to a cobalt atom which loses less charge than a similar one in the six MnO ring structure, this is due to manganese two and three interfering with the Co-O bond. This interference also occurs in the six MnO ring but there it happens on both sides of the manganese atom, here both manganese two and three only interfere with oxygen three which increases the amount of interference. Oxygen three partially compensates the decrease in its gained charge by draining more form manganese two and three. Secondly oxygen two is strongly bonded to the cobalt atom it is above which is reflected in the large charge loss of this cobalt atom. This is facilitate by the positive charge of several of its surrounding cobalt atoms due to their bond with manganese, decreasing the Co-Co bond with this particular atom. Lastly, the two cobalt charges in bold are examined. The first has a charge of which is higher than expected because manganese one is bonded to no other cobalt atoms meaning it only donates charge to this cobalt atom. The cobalt atom has an increased charge for similar reasons but manganese five is also bonded to another cobalt atom hence the lower charge value. Other differences in the charge of cobalt atoms are thought to be Location 107

136 due to proximity of manganese, much like with the six MnO ring, and due to a difference in available charge from manganese (e.g. if more is sent to oxygen). Table 5-18 Bader charge results for the 6 MnO ring on a p(6x6) slab, the 6 MnO ring on a p(4x4) slab and the 5 MnO ring on a p(4x4) slab. Only the values for the surface layer of the cobalt slab are reported as the lower layers are less relevant. Charge results are calculated using equation 5-8. Formation energies of the structures are calculated as discussed in the corresponding chapters. For the 6 MnO ring p(6x6) only the Co charges in the highlighted (in orange) section are displayed as the charges outside are of no importance. The numbers indicated on the second figure for the 5 MnO ring p(4x4) are the manganese atom bonded to these cobalt atoms. 6 MnO ring p(6x6) Charge Mn charge 1= = = = = = O charge 1= = = = = = Co charge Δ3 / 500 2= -18 kj/mol/mno 108 Location

137 6 MnO ring p(4x4) Charge Mn charge 1= = = = = = O charge 1= = = = = = Co charge Δ3 / 500 2= -8 kj/mol/mno 5 MnO ring p(4x4) Charge Mn charge 1= = = = = O charge 1= = = = = Co charge Δ3 / 500 2= -4 kj/mol/mno Location 109

138 In conclusion, manganese interferes with the oxygen cobalt bonds decreasing its strength but at the same time prevents oxygen from pulling the cobalt too much out of the slab which limits the instability generated by the deformation of the cobalt slab. This interference is optimal when it happens equally on both sides of the manganese, otherwise there is a decrease in stability due to decreased/increased strength of the cobalt oxygen bond. Lastly manganese donates charge to its bonded cobalt atoms, in the most optimal ring configuration this stabilizes the cobalt slab by decreasing the cobalt cobalt bond strength with the cobalt atom bonded to oxygen. This is because a higher charge of a cobalt atom means it has a weaker bond with the surrounding atoms, one of which is the cobalt atom bonded to oxygen. The shape and structure of the six MnO ring allows this last effect to influence most cobalt atom bonded to oxygen and also allows the destabilization of the cobalt oxygen bonds to happen on both sides of the manganese atom equally. This is why the six MnO ring is most favorable Step Surface In Table 5-19 the results of Bader charge analysis on the addition of MnO into a step site are shown. A first observation can be made from the charges of the oxygen atoms were a decrease in gained charge is observed with increasing MnO molecules. The charge of the oxygen atoms in one structure is also very similar, with the exception of the three MnO structure. The deviations in the three MnO structure occur due to a difference in bonding partners, oxygen one can draw more charge from the leftmost manganese as it is not bonded to another oxygen. The second oxygen is bonded to two manganese while oxygen three is bonded to one manganese and one cobalt atom. This indicates that oxygen can draw more charge from manganese than from cobalt. The decrease in average oxygen charge is due to an increase in interactions between manganese and the cobalt atoms bound to the oxygen, decreasing the amount of charge taken form the cobalt atoms. Secondly, the charge of manganese remains the same in the structure with one MnO and two MnO as in both cases the manganese is bonded to one oxygen and there is only a small shift in its position. The structure with three MnO does show change as the first manganese atom has a charge closer to the one of the previous two structures which is because it is bonded to only one oxygen atom. The other two manganese lose more charge as they are bonded to two oxygen with the third manganese losing the most because it has been pushed out the least making its bond with the oxygen stronger, thereby losing more charge to it. Adding four MnO makes all manganese charges equal because of the symmetry and its value is in between the first and second manganese of the three MnO structure as its position is also between the two. 110 Location

139 Changes in the charge of the different cobalt atoms are also observed, with the cobalt atoms bonded to manganese gaining charge (when they are not bonded to oxygen as well), the charge transferred appears to increase with a decreasing distance between both atoms. The cobalt atoms (not in bold) bonded to oxygen staty the same whether there is one or two MnO in the slab but when three MnO are substituted a decrease is observed. This decrease is caused by the bonded oxygen atoms binding stronger to their bonded manganese atoms, the reason the cobalt atoms bonded to oxygen one and two lose even less charge is because their connected manganese atoms move more outwards meaning the bond with their cobalt atom is less strong. The increase in the charge loss of these cobalt atoms in the four MnO structure is due to their bonded oxygen atoms not being forced to one side due to a difference in manganese oxygen and cobalt oxygen bond length (as is the case for one MnO and two MnO structures) or due to the manganese atoms being pushed out of the slab (three MnO case). This allows the oxygen atoms to from a stronger bond with their respective bonded cobalt atoms. Table 5-19 The results of a Bader charge analysis on the addition of MnO to the Co(211) surface, additions from one to four MnO are considered. Only the charges of the top layer of the slab are reported as the lower layers are less relevant. Charges indicated in purple are the charge of the manganese atoms while charges indicated in bold are at the edge. The values indicated in red are the cobalt atoms bonded to oxygen. 1 MnO structure Charge 1 Slab charges Oxygen charge: 1= Δ3 / 500 2= -31kJ/mol/MnO Δ3 ;;<<= 500 2= -31kJ/mol/MnO Location 111

140 2 MnO structure Charge 1 2 Slab charges Oxygen charge: 1= = Δ3 / 500 2= -20 kj/mol/mno Δ3 ;;<<= 500 2= -8 kj/mol/mno 3 MnO structure Charge Δ3 / 500 2= -17 kj/mol/mno Δ3 ;;<<= 500 2= -11 kj/mol/mno Slab charges Oxygen charge: 1= = = MnO structure Charge Slab charges Δ3 / 500 2= 4 kj/mol/mno Δ3 ;;<<= 500 2= 68 kj/mol/mno Oxygen charge: 1= = = = Location

141 In conclusion, the fourth addition is unfavorable because it causes the manganese atoms to be too close together. These atoms are forced into the edge because oxygen cannot fully reach the +1 charge, as observed in the section 5.3.1, it requires and in an attempt to attain this charge it bonds manganese and cobalt more strongly. The third addition will not occur for a similar reason. Each oxygen tries to pull manganese closer to gain more charge. But with oxygen three also bonded to a cobalt atom it shifts in position and pulls the rightmost manganese upwards quite strongly. Oxygen two also pulls its bonded manganese into the edge but as it is not shifted it doesn t pull so strongly, which is why the center manganese can move a bit more out of the edge. Oxygen atom number one acts much like oxygen two on the center manganese atom but as it is the only oxygen bonded to the leftmost manganese this manganese can move even further out of the edge. Because this last manganese can move out of the edge, the manganese atoms aren t forced into the edge like in the four MnO structure which makes it more favorable. The second addition is also less favorable than the first because the addition of two manganese causes the manganese to move out of the edge decreasing the bond strength between oxygen and the cobalt atom in the edge. In the first addition there is enough room to accommodate the manganese by compressing the cobalt atoms in the edge. This allows oxygen to bind strongly to all its connected species allowing it to secure manganese into the slab without stretching the cobalt manganese bonds limiting the impact on the deformation of the cobalt slab making the structure more favorable. Location 113

142 5.4 Bibliography 1. Morales, F., et al., In Situ X-ray Absorption of Mn/Co/TiO2 Catalysts for Fischer Tropsch Synthesis. The Journal of Physical Chemistry B, (41): p den Breejen, J.P., et al., A Highly Active and Selective Manganese Oxide Promoted Cobalt-on-Silica Fischer Tropsch Catalyst. Topics in Catalysis, (13): p Johnson, G.R., S. Werner, and A.T. Bell, An Investigation into the Effects of Mn Promotion on the Activity and Selectivity of Co/SiO2 for Fischer Tropsch Synthesis: Evidence for Enhanced CO Adsorption and Dissociation. ACS Catalysis, (10): p Ma, X., et al., Atomic and molecular adsorption on RhMn alloy surface: A first principles study. The Journal of Chemical Physics, (24): p den Breejen, J.P., et al., On the Origin of the Cobalt Particle Size Effects in Fischer Tropsch Catalysis. Journal of the American Chemical Society, (20): p Bartholomew, C.H. and R.C. Reuel, Cobalt-support interactions: their effects on adsorption and carbon monoxide hydrogenation activity and selectivity properties. Industrial & Engineering Chemistry Product Research and Development, (1): p Kattel, S., et al., Active sites for CO₂ hydrogenation to methanol on Cu/ZnO catalysts. Science, (6331): p Location

143 Chapter6: Kinetics In this section, the effect on adsorption of the most stable MnO structure on both a step and terrace site is discussed. Adsorption of CO, H, OH and H2O is examined on a terrace surface with the six MnO ring and on a step surface with one MnO. Adsorption on a clean terrace and step surface is also examined in order to compare the promoted to the unpromoted surface. This comparison will allow more insight into how manganese promotion affect the Fischer-Tropsch reaction. The reason CO, H, OH and H2O are examined is the importance of CO dissociation and H2O dissociation (to H and OH) in Fischer-Tropsch synthesis. Studying these reactions will give basic insight into the effect of manganese promotion, more in depth insight would require studying a large amount of reactions/species which is not feasible within the timeframe of this thesis. The adsorption energies reported in this section are all calculated using the following equation: = (eq 6-1) Where is the electronic energy of the slab with the adsorbate adsorbed. is the electronic energy of the adsorbate in the gas phase and is the electronic energy of the slab without the adsorbated. Due to the limited timeframe of this thesis, no frequency calculations for the adsorbates (when adsorbed onto the slab) and therefore only the electronic adsorption energy values are reported. This section is divided into two subsections, terrace and step, as both sites need to be considered as in chapter five. 115

144 6.1 Terrace surface Adsorption on a Co(111) surface with and without the six MnO ring is considered. Only the most favorable adsorption energies for the clean and promoted surface are reported as these are the most prevalent states. Table 6-1 The adsorption energies of key intermediates (CO, H, OH, H2O) on a unpromoted Co(111) surface and a six MnO ring promoted Co(111) surface. Adsorbed species Electronic adsorption energy [kj/mol] Unpromoted Co(111) Promoted Co(111) CO H OH H2O Table 6-1 shows that the six MnO ring only has a slight effect on the adsorption of CO but has a larger effect on the adsorption of the other three species. H and OH adsorptions found to be less favored while H2O adsorption is more favored on the promoted surface. This indicates that H2 dissociation to H and possibly H2O dissociation to OH will become less favorable decreasing the surface coverage of hydrogenating species. Therefore, an increased selectivity towards higher hydrocarbons is expected as there is less chance that a propagating chain is terminated by hydrogenation. Furthermore, an increase in olefin to paraffin ratio is expected because there is lower probability for olefins to be hydrogenated to paraffins. This is in line with experimental data [1-3] which shows an increased selectivity to C5+ and an increase in olefin to paraffin ratio for the C2 to C4 fraction. In an attempt to look into the cause of the changes in adsorption energies observed in Table 6-1, the structures of the adsorption were examined and are shown in Table 6-2. The H and OH adsorption structures show a deformation of the six MnO ring due to the adsorption of the respective species. For H adsorption, the manganese and oxygen indicated in orange are shifted towards the inside of the ring. In the case of OH, only the indicated manganese atom is shifted. This is thought to destabilize the six MnO ring causing the difference in adsorption energies between the promoted and unpromoted surface. The H adsorption structure also shows a shift in the position of the hydrogen atom where the hydrogen atom is not entirely in a HCP position but between a HCP and bridge position. A possible reason is the low charge of the cobalt atom 116 Kinetics

145 bonded to oxygen, which was observed in section 5.3.1, this would cause this cobalt atom to have less electrons left to interact with the hydrogen atom thereby not attracting it into the HCP position. Note that the deformation of the ring could change the charge of this cobalt atom rendering this hypothesis invalid. Further research into this adsorption is required to fully explain the observed behavior. The CO adsorption does not show any change in the six MnO ring even when it adsorbs at positons where it is in contact with the ring or with the cobalt atoms bonded to the ring. Which explains why there was little change in the CO adsorption energy. Lastly H2O adsorption on the promoted surface shows that H2O preferably binds to the ring which explains the increase in the adsorption energy. Table 6-2 The structure of the most favorable CO, H, OH and H2O adsorption on a six MnO ring promoted Co(111) surface. Note the deformation of the six MnO ring for the H and OH adsorption and the H2O adsorbing onto the six MnO ring. CO-adsorption promoted Co(111) H-adsorption promoted Co(111) OH-adsorption promoted Co(111) H2O-adsorption promoted Co(111) Kinetics 117

146 In conclusion, the observed changes indicate that the effect of manganese promotion on a terrace site is caused by changes in the rate of H2O dissociation rather than CO dissociation which was proposed by Johnson et al. [3]. Do note that the effect on C and O adsorption has not been investigated and that thereby the effect of manganese promotion on CO dissociation is examined solely based on CO adsorption. 118 Kinetics

147 6.2 Step surface Adsorption of CO, H, OH and H2O on a promoted and unpromoted step surface is compared in order to evaluate the impact of the promotion on the Fischer-Tropsch mechanism. Only the most favorable adsorption energies for the clean and promoted surface are reported as these are the most prevalent states. Table 6-3 The adsorption energies of key intermediates (CO, H, OH, H2O) on a unpromoted Co(111) surface and a one MnO substituted (or promoted) Co(211) surface. Adsorbed species Electronic adsorption energy [kj/mol] Unpromoted Co(211) Promoted Co(211) CO H OH H2O Table 6-3 shows that MnO substitution into the edge, with O in an FCC position on top of the edge increases the adsorption strength of CO, H and OH while the adsorption strength of H2O remains almost unchanged. The increase in H and OH adsorption with the H2O adsorption remaining almost the same indicates that the rate of H2O dissociation will increase. The increased adsorption strength for CO could increase CO dissociation rate. However, the C and O adsorption should also be examined in order to reach a conclusion. In Table 6-4, the adsorption structures on a Co(211) surface are shown. The CO-adsorption structure shows that CO prefers adsorbing onto the Co atom next to Mn, which is not bonded to oxygen. The reason CO binds stronger on the promoted surface is due to change in surface charge of the Co and Mn atoms. Hydrogen prefers adsorbing, on a Co(211) surface, in a shifted bridge position between Mn and the Co atom next to Mn which is not bonded to oxygen. It is proposed that H binds stronger due to the same reason as CO, namely the charge transfer from Mn to its neighboring Co atoms. OH prefers to adsorb in a bridge position between the Co atom in the edge bonded to the O of MnO and the Co atom not bonded to either the O or Mn of MnO. It is thought that OH binds stronger to the surface because its bond with the Co atom bonded to the O of MnO causes that bond to weaken, causing Mn to move slightly out of the edge, stabilizing it. H2O prefers to adsorb onto Mn because it is more electron rich than cobalt, allowing for a stronger bond. Kinetics 119

148 Table 6-4 The structure of the most favorable CO, H, OH and H2O adsorption on a one MnO substitution promoted Co(211) surface. CO-adsorption promoted Co(211) H-adsorption promoted Co(211) OH-adsorption promoted Co(211) H2O-adsorption promoted Co(211) In conclusion, MnO promotion in a step surface is thought to promote H2O dissociation by stabilizing the adsorption of H and OH. The effect on CO dissociation is expected to be positive, however, further analysis is required to make a conclusion. 120 Kinetics

149 6.3 Bibliography 1. den Breejen, J.P., et al., A Highly Active and Selective Manganese Oxide Promoted Cobalt-on-Silica Fischer Tropsch Catalyst. Topics in Catalysis, (13): p Morales, F., et al., In Situ X-ray Absorption of Mn/Co/TiO2 Catalysts for Fischer Tropsch Synthesis. The Journal of Physical Chemistry B, (41): p Johnson, G.R., S. Werner, and A.T. Bell, An Investigation into the Effects of Mn Promotion on the Activity and Selectivity of Co/SiO2 for Fischer Tropsch Synthesis: Evidence for Enhanced CO Adsorption and Dissociation. ACS Catalysis, (10): p Kinetics 121

150 122 Kinetics

151 Chapter7: Conclusion In this work, first, the nature of the Mn promoter under reaction conditions was examined. Next, the location of the Mn promoter on a Co(111) and Co(211) surface under FTS reaction conditions was investigated. Lastly, the kinetic impact of the Mn promoter on the Fischer- Tropsch reaction was investigated by examining the adsorption of key reaction intermediates such as CO, OH, H and H2O at the most stable MnO structures. 123

152 7.1 Nature of the promoter To investigate on the location and kinetical impact of the promoter, first the nature of the promoter was determined. Experimental evidence from literature showed that MnO is the most prevalent phase under reaction conditions, however, the relative stability of the MnO phase was unknown. First, an experimental phase diagram using the Ekvicalc software was constructed. This phase diagram was further used to verify the DFT based phase diagram and confirm the most prevalent Mn-oxide phase under reaction conditions. Experimental Mn-oxide phase diagram showed that MnO is the most prevalent phase under reaction conditions, where reaction conditions are taken as typical FTS process conditions (500 K, 20 bar and 60% conversion). Furthermore, the MnO phase was shown to be highly stable at temperatures lower than 500 K, indicating that no reduction of the MnO bulk takes place under reaction conditions. Prior to constructing a DFT based phase diagram, an appropriate DFT functional had to be selected. In this work, a benchmark was performed by calculating the Mn-oxides formation enthalpy using following functionals, HSE, PBE0, PBEU4, PBE, vdw-df. This showed that hydbrid functionals (HSE, PBE0, PBEU4) showed high accuracy. In this study, hybrid functionals are not pursued due to the extensive computational time and therefore the vdw-df functional is used. The DFT based phase diagram accurately predicted the most prevalent Mn-oxide phase under reaction conditions (500 K, 20 bar and 60% conversion) to be MnO. However, the stability of the MnO phase was predicted to be much smaller than in the experimental phase diagram which could be attributed to the errors of the chosen functional. Furthermore, DFT results has a general tendency to over-oxidize Mn. Therefore, transition to the higher oxidation state phases occurs under less oxidizing conditions and similarly, transition to the metallic Mn phase occurs under less reducing conditions. 124 Conclusion

153 7.2 Location of the promoter The location of the promoter was separately examined for a Co(111) and Co(211) surface. On a terrace surface, substitution of Mn and of MnO was found to be unfavorable. Afterwards, the adsorption of a single MnO was examined, where it was determined that the most favorable configuration for Mn to occupy a bridge position and O to a FCC position with a Gibbs free energy of formation of 70 kj/mol/mno. Further examination of stoichiometric MnO structures was performed for structures ranging from one to six MnO. This showed that a six MnO ring with all Mn in a bridge positon and all O in a shifted to position was most favorable with a Gibbs free formation energy of -18 kj/mol/mno. Examination of monolayers showed that these are not formed as the most favorable configuration examined has a Gibbs free formation energy of 13 kj/mol/mno. Lastly, non-stoichiometric patches were examined, triangular shaped patches were found to be unfavorable as the most favorable configuration has a Gibbs free formation energy of 5 kj/mol/mno. Hexagonal patches appeared are stable with a Gibbs free formation energy of -13 kj/mol/mno for the most favorable configuration. Hydrogen termination was also examined, however, this proved unfavorable due to the high entropic cost of hydrogen addition. Overall, the six MnO ring was shown to be the most favorable configuration on a terrace surface. However as enthalpy and entropy corrections were not considered for adsorbed structures it is thought that the cost for adding extra oxygen and hydrogen to these structures is overestimated and that they may be more favorable than reported. For step surfaces the substitution of Mn into the edge proved to be unfavorable with a Gibbs free energy of formation of 236 kj/mol/mno. Interestingly, the substation of MnO into the edge was shown to be favorable, where the most favorable configuration has O in an HCP position on the terrace above the edge with a Gibbs free energy of formation of -31 kj/mol/mno. Coverage of this structure was examined, however, the initial addition was found to be the most favorable indicating that one Mn substitution every four atoms of the edge is preferred. Bader charge analysis showed that the six MnO ring is most favorable on a terrace because due to Mn interference with the Co-O bond limiting the extent to which this bond can deform the Co slab. The Mn interference occurs equally on both sides allowing a strong Co-O bond but still limiting the extent of slab deformation. Lastly, Mn donates charge to the Co atoms surrounding the Co atom bonded to oxygen, decreasing the Co-Co bond strength which limits Conclusion 125

154 the destabilization due to slab deformation. Single MnO substitution into the edge is most favorable on a step surface as this configuration allows oxygen to bind strongly to all its bonded atoms, allowing it to keep the Mn atom in the edge without stretching the cobalt manganese bonds limiting the impact on the deformation of the cobalt slab. 126 Conclusion

155 7.3 Kinetics Mn promotion on a terrace surface has been shown to affect H2O dissociations as H and OH adsorption is respectively 21 kj/mol and 17 kj/mol less favorable and H2O adsorption is 26 kj/mol more favorable. H and OH adsorption have been shown to destabilize the six MnO ring while H2O adsorbs onto the six MnO ring instead of onto the surface causing changes in the adsorption energies. Mn promotion in a step surface is found to promote H2O dissociation by stabilizing the adsorption of H and OH. The effect on CO dissociation is expected to be positive, however, further analysis is required. Conclusion 127

156 7.4 Future work Additional functionals could be examined for construction of the phase diagram, where a functional outperforming vdw-df with similar computational requirements might be found. Future work for the location of the promoter is the examination of additional patches. Furthermore, an examination of the impact of including entropy and enthalpy corrections on current and new structures could show that addition of extra oxygen and hydrogen is more favorable or unfavorable than reported in this work. Further examination of adsorption energies for CO, H, OH and H2O on Co(111) and Co(211) surfaces could be performed, finding a more optimal configuration than the one reported in this work. Additionally, the effect of Mn-promotion on CO and H2O dissociation using transition state theory could be examined. 128 Conclusion

157 Appendix A: Ekvicalc memo MEMO To: [To] From: Maarten Van Doorslaer Cc: [CC] Our telephone n : [Telephone] Date: [Date] Our Maarten.vandoorslaer@ugent.be Subject: Constructing a phase diagram using Ekvicalc 1) Introduction Using Ekvicalc software and excel data processing it is possible to construct a phase diagram. This memo contains a short explanation on how to use the Ekvicalc software and how the results can be used to create a phase diagram. 2) Setting up the software The Ekvicalc software can be found on the LCT share folder, there are different version of the software available, it is recommended to use version 4.3. You can copy the install package to your PC and install, no license is required. After installation the installation folder contains the file EkviCalc Reference Manual, here all possible commands are listed together with their usage. The file EkviBase Index contains a list of the available species together with the temperature range in which Ekvicalc possess data. 3) Introduction Ekvicalc uses a command based input. As shown in the Figure 1, commands can be entered in the commands and status window. While entering a command, Ekvicalc proposes possible suggestions for a command in the green field. Once the desired command is displayed in the green field, it can be used, an example is using n H2O instead of name H2O. However, it is not possible to type in commands which are not inbuilt in the software. For example none of the commands uses z in the beginning and as such it is not allowed to type z.. Implemented settings can be saved by saving a job file. This function is found in the file menu. To load a job file use the read job file option in the same menu. The only exception for the command based input is the pressure and temperature settings This can be adapted via the settings menu. Do note that adapting the unit there will not change the unit in the user interface and neither in the output to this end use the temperature-unit and pressure-unit commands, followed by the desired unit (available units, Temperature: C, K ; Pressure: atm, bar, torr, kpa ). To save results obtained from the calculation, it is advised to use the save partial results option under the file menu to reduce the amount of data written out (see further for the data that is required to construct a phase diagram). Appendix A 129

158 Lastly, it is important to note that Ekvicalc determines the equilibrium composition by minimizing the global Gibbs free energy of the system. Note that when temperatures outside of the database temperature interval for a specific species is used, Ekvicalc will approximate the Gibbs free energy of these species. This will be indicate in the software by an star symbol (*) next to the considered species. Figure 1: The user interface of Ekvicalc 4) Running a calculation to create a phase diagram It is important to note that Ekvicalc does not create a phase diagram for you. Further data processing is required to create a phase diagram(see section 4). To start a calculation several inputs are required and the order in which they are entered is not-restricted. 1) Specifying the species To specify the species the name command is used, e.g. name H2O. The species to enter are the ones under consideration and a reducer/oxidizer system, in Figure 1 these are respectively the Manganese oxides (incl Mangenese metal) and the H2O/H2 system. The H2O/H2 system is used as a convention. The formation of liquid and gaseous phases of the examined species is not desired for the construction of a phase diagram as this will impact the equilibrium. For the same reason only gaseous H2O and H2 are considered. To this end the phases have to be specified in the name <species> command, solid is used for the considered species and gas for H2O and H2, it is possible to enter the desired phase by typing name <species>(<phase>), eg: name H2O(g). Note that in the example (Figure 1), name Mn(s) was used, giving rise to four solid metallic Mn phases, to enter these one by one use name <species>(<desired phase>). Specifying no phase will include all possible phases. 2) Specifying the amount of the species To be able to enter the amount of the species present the amount <amount of mol> command is used, however it is necessary to select a species using the name command (same as with specifying species) before using amount. First one of the species under consideration is taken as starting species, in the example (Figure 1) Mn(alpha) is used. The amount of moles of these species has to be very low, so that any conversion will only have a small impact on the amount of H2O and H2, likewise a high amount of the H2O 130 Appendix A

159 and H2 has to be used. In the example 10 mol of Mn(alpha) and mol of H2O is used. The amount of H2 than needs to be varied over a certain range, this range is set by firstly selecting the species ( name H2(g) ) than using minimum amount <# mol> and maximum amount <# mol> to set a minimum and a maximum. It is advised to use the factor option to increase the amount of H2 in the calculation using the amount factor <factor> command. This uses the following formula to adjust the amount: Note that it is also possible to vary the amount of H2O and keep H2 constant, however this can be problematic because at very low H2O levels it is possible that there is not enough oxygen in the system to equilibrate the system, resulting in errors. 3) Setting the conditions The phase diagram is constructed at a constant pressure as such the command pressure is used to set the pressure. Using the command pressure-unit allows changing the pressure unit to bar, atm, torr or kpa. The temperature has to be varied as well, to this end commands similar as those used for varying the amount are used: minimum-temperature, maximum-temperature and temperature-step. Here a step increase can be used as the interval is not as large as with the amounts. The temperature-unit command allows changing between Kelvin (K) and Celcius (C). 4) Running the calculation and saving the results Using the calculate command the calculation will commence, note that depending on the chosen temperature step and amount factor this calculation can take some time. To save the results from the calculation it is advised to us the save partial results option in the file menu, this will decrease the size of the resulting document. Figure 2 shows the required data to construct a phase diagram. Figure 2: the minimal save options to construct a phase diagram Appendix A 131

160 5) Processing the data The most effective way to construct a phase diagram from the acquired data is to keep only the data points which denote an equilibrium between two phases. Equilibrium occurs when two solid phases have an activity of 1, note that due to the sampling some equilibrium points are missed and only the transition from one phase to the next is seen, naturally these points need to be included as well. The case in which the equilibrium is detected is shown in Figure 3 right, the case where the equilibrium is missed is shown in Figure 3 right. After linking these equilibrium points to a certain transition, it is possible to use the partial pressures of H2 and H2O to calculate ln, combining this with the temperature gives one equilibrium point in the phase diagram. After organizing all points of each equilibrium, equilibrium lines can be plotted yielding a figure similar to Figure 4. To change this into a real phase diagram some coloring can be added, yielding Figure 5. Figure 3: Left: the equilibrium between the two phases is missed, right equilibrium between two phases Figure 4: The data from Ekvicalc after plotting 132 Appendix A

161 Figure 5: The finished phase diagram Appendix A 133