Applied Catalysis A: General

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1 Applied Catalysis A: General 365 (2009) Contents lists available at ScienceDirect Applied Catalysis A: General journal homepage: Density functional theory study on water gas-shift reaction over molybdenum disulfide Xue-Rong Shi a,b, Sheng-Guang Wang c, Jia Hu a,d, Hui Wang a,b, Yan-Yan Chen a,b, Zhangfeng Qin a, Jianguo Wang a, * a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi , China b Graduate University of the Chinese Academy of Sciences, Beijing , China c Center for Atomic-scale Materials Physics, Technical University of Denmark, Lyngby DK-2800, Denmark d College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou , China ARTICLE INFO ABSTRACT Article history: Received 15 April 2009 Received in revised form 22 May 2009 Accepted 26 May 2009 Available online 6 June 2009 Keywords: Water gas-shift reaction Molybdenum disulfide DFT studies Density functional theory calculations have been carried out to investigate the adsorption of reaction intermediates appearing during water gas-shift reaction at the sulfur covered MoS 2 (1 0 0) surfaces, Motermination with 37.5% S coverage and S-termination with 50% S coverage using periodic slabs. The pathway for water gas-shift reaction on both terminations has been carefully studied where the most favorable reaction path precedes the redox mechanism, namely the reaction takes place as follows: CO + H 2 O! CO + OH + H! CO+O+2H! CO 2 +H 2. The most likely reaction candidates for the formate species HCOO formation is the surface CO 2 reaction with H as a side reaction of CO 2 desorption on S- termination with 50% S coverage. The formed HCOO species will react further with adsorbed hydrogen yielding H 2 COO followed by breaking its C O bond to form the surface CH 2 O and O species. ß 2009 Elsevier B.V. All rights reserved. 1. Introduction Sulfided Mo-based catalysts (MoS x ) have received much attention for CO hydrogenation (HYD) because of their high resistance to sulfur poisoning [1 5]. Hydrocarbons and alcohols synthesis from CO hydrogenation are complicated by various side reactions where the most important one is water gas-shift (WGS) reaction [6 12] H 2 from WGS can re-react with CO, and CO 2, another product from WGS, maybe also react with H 2 to produce methanol and H 2 O [13 15]. Over the years considerable efforts were devoted to the studies on the WGS reaction over MoS 2 -based catalysts, defining catalytically active species, a role of promoters, influence of preparation conditions, a role of supports and the reaction mechanism [6 16]. Two reaction pathways have been proposed regarding the mechanism for the WGS reaction, that is, the redox and associative mechanisms. In the redox mechanism, it includes the successive oxidation and reduction of the surface, namely, it consists of a surface oxidation, H 2 O+*$ H 2 + O* (* denotes an adsorption site), followed by a surface reduction, CO + O* $ CO 2 +* [17 21]. If one assumes a single type of * Corresponding author. address: iccjgw@sxicc.ac.cn (J. Wang). adsorption site, then the detailed mechanism can be written as follows: CO þ$co; (1) H 2 O þ$h 2 O; (2) H 2 O þ $OH þh; (3) OH þ $O þh; (4) CO þo $ CO 2 þ; (5) CO 2 $CO 2 þ; (6) 2H $ H 2 þ 2 : (7) In the associative mechanism, a formate species HCOO is formed from the surface CO and OH groups and then decomposes forming CO 2 and H 2 [22 25], namely, steps of CO* + OH* $ HCOO* + * and HCOO* + * $ CO 2 * + H* will replace steps (4) and (5). By developing a microkinetic model for the kinetic of the WGS reaction over sulfide Mo/Al 2 O 3 and suflide CoMo/Al 2 O 3 catalysts, Hou et al. [7] and Lund [26,27] supported the redox mechanism wherein the catalyst surface is alternately oxidized by water and then reduced by carbon monoxide on MoS 2 -based catalysts X/$ see front matter ß 2009 Elsevier B.V. All rights reserved. doi: /j.apcata

2 X.-R. Shi et al. / Applied Catalysis A: General 365 (2009) Although a considerable amount of experimental efforts are devoted to investigate on the WGS reaction, theoretical studies on this reaction over MoS 2 surface is still missing. For MoS 2 -based catalysts, theoretical work mainly focused on CO, H 2 adsorption and surface properties [28 37]. By means of IR spectroscopy and two-sheet periodic model Travert et al. [38] studied CO adsorption on the sulfided Mo catalysts and found under reductive condition one CO molecule adsorption on the sulfur edge with 50% S coverage is the most stable and exothermic by 0.7 ev. In the same literature, they also identified three thermodynamically stable surfaces under the different H 2 /H 2 S ratio, which also were studied by Schweiger et al. [32], Bollinger et al. [35], Raybaud et al. [39] and Cristol et al. [40]. Employing two-sheet periodic model, Cristol et al. [37] studied H 2 adsorption and dissociation on MoS 2 and found dissociative H 2 adsorption to be endothermic on Mo edge and S edge with 50% S coverage, while exothermic on Mo with 33% S coverage to yield one S H and one Mo H by 0.10 ev. Using onesheet periodic model, Sun et al. [36] found that homolytic H 2 dissociation to yield two S H groups is thermodynamically preferred over heterolytic dissociation to form one S H and one Mo H, and the latter is kinetically favored. However, Bollinger et al. [35] found the most stable hydrogen adsorption to be the formation of two S H groups on the S edge. In this paper, periodic density functional theory calculations are employed to identify the different intermediates and mechanism for WGS on MoS 2. The adsorption of reactants, products and intermediates has been investigated. Then the reaction pathway involves the dissociation of H 2 O, the reaction of CO with OH and the formation of CO 2 together with the transition states for the elementary steps have been studied. Finally, we will discuss the relevance of WGS reaction to CO hydrogenation on MoS 2 surface. 2. Method and models Periodic density functional theory (DFT) calculations were performed with DMol 3 program in Materials Studio 2.2 package [41,42]. The generalized gradient corrected functional by Perdew and Wang (PW91) [43,44] was employed. The real space cutoff of atomic orbital was set at 5.5 Å. The effective core potential (ECP) [45,46] was used for molybdenum, and the doubled numerical basis set with a set of polarization functions was used for other elements (DNP). For numerical integration the medium quality mesh size was used, i.e., the convergence criteria for structure optimization and energy calculation were set to (1) energy tolerance of au/atom, (2) maximum force tolerance of au/å, (3) maximum displacement tolerance of Å, (4) SCF tolerance of au/atom. Smearing value was set to be au. A k-point of (2 1 1) was used. All electronic states are determined by calculations including spin polarization for the open shell molecules in gas phase and for appropriate multiplet states of the adsorption systems. The transition states (TS) are located by using the complete LST/QST method [47]. Bulk MoS 2 forms a hexagonal crystal lattice with a layer-type structure described by weakly binding sulfide layers parallel to (0 0 1) netplanes. The reactive surface is assumed to be (1 0 0) [48] where the ideal bulk terminated geometry is described by alternative Mo termination and S termination, see Fig. 1a. Theoretical studies on the MoS 2 (1 0 0) surface at different H 2 S/ H 2 exposures [36 40,48] have shown that under hydrogen-rich conditions, i.e., for pressure ratios p(h 2 S/H 2 ) < 0.05, (1 0 0) surface has two possible edges, that is, both Mo termination and S termination with 50% sulfur coverage, respectively, however, with the surface sulfur positioned differently. Compared with the ideal surface this reconstructed model corresponds to sulfur addition at the initial Mo termination (denoted T 1 termination) and sulfur removal at the S termination (denoted T 2 termination), see Fig. 1b. Fig. 1. Crystal structure of hexagonal MoS 2 (1 0 0) surface with (a) ideal bulk termination, (b) sulfur reconstructed termination. Mo (S) centers are shown by small dark (large light) balls. The two different Mo and S terminations are labeled accordingly. It is also well accepted that the Mo coordinatively unsaturated sites (CUS) located at the edges play an important role in the adsorption and activation of the reactants [49,50]. In experiment, vacancy does form during the CO hydrogenation and WGS reactions [51]. However, at T 1 termination, it did not exhibit Mo coordinatively unsaturated sites. By DFT calculations, Paul and Payen [52] studied the sulfur vacancy formation on MoS 2 (1 0 0) and found that to create one sulfur vacancy on T 1 edge is possible with low activation energies for each elementary step. Our theoretical studies have shown that for low pressure ratios p(h 2 S/H 2 ), i.e., for pressure ratios p(h 2 S/H 2 ) < reflecting realistic reaction conditions, the T 1 surface with one sulfur vacancy can become more stable thermodynamically than the T 1 surface at 675 K corresponding to 37.5% relative coverage (100% S coverage corresponds two surface S atoms per Mo atom [39]) as sketched in Fig. 2a. This model, labeled T 0 1, is described by a sheet supercell containing four MoS x units along the surface edge and will be considered in the following. Here the T 0 1 (Fig. 2a) and T 2 (Fig. 2b) terminations containing four layers MoS x units along the (0 0 1)

3 64 X.-R. Shi et al. / Applied Catalysis A: General 365 (2009) where E tot (A/slab), E tot (B/slab) and E tot (AB/slab) are the total energies for the separately adsorbed A/slab, B/slab and AB/slab, respectively; and ða þ BÞ AB DE c ¼ E tot E tot ; (10) slab slab where E tot ((A + B)/slab) is the total energy for the co-adsorbed (A + B)/slab. DE s represents the reaction of energy of a formal reaction at the most stable adsorption states without considering coverage effect and reaction process (idealized reactant and product states without surface effect or at very low coverage); and DE c represents the reaction energy of a reaction at defined coverage considering the lateral interaction during co-adsorption for bond breaking or bond formation. The difference between DE s and DE c is the interaction between A and B in the co-adsorbed states. It reflects the thermodynamics at a defined coverage and includes the effect of lateral interactions, which could be repulsive or attractive. Therefore, we used DE c for discussion and keep DE s for comparison. 3. Results and discussion Our calculation results will be divided into three sections. In Section 3.1, the adsorption of reactants, products and intermediates will be given. In Section 3.2 we will examine the reaction pathway involves the dissociation of H 2 O, the reaction of CO with OH and the formation of CO 2 where the transition states for the elementary steps have been studied. In Section 3.3, we will focus on the relevance of the WGS reaction to the CO hydrogenation reaction Reactants, intermediates and products adsorption Fig. 2. Slabs of (a) T 0 1 termination, (b) T 2 termination used for the supercell model systems. Mo (S) centers are shown by small dark (large light) balls. Vector R s denotes the supercell periodicity along the sheet edge including four MoS x units. direction, and only one sheet along the (1 0 0) direction. Vacuum thickness of 15 Å is applied to separate the slab in these two directions, respectively, which were found to be sufficient to avoid electronic coupling between adjacent slabs in test calculations. Adsorption of reaction intermediates at the T 0 1 edge and T 2 edge is considered in corresponding geometry optimizations where the two bottom layers of the slabs are fixed at the bulk position while the two top layers along with the adsorbates are allowed to relax freely. The adsorption energies E ads were calculated using the Eq. (8) where E tot (ads/slab) is the total energy for the slab with absorbates in its equilibrium geometry, E tot (slab) is the total energy of the corresponding slab, and E tot (ads) is the total energy of the free adsorbates in the gas phase, respectively. A negative E ads value corresponds to an exothermic adsorption. ads E ads ¼ E tot E tot ðslabþ E tot ðadsþ (8) slab For reactions like AB = A + B, the reaction energy is given under two definitions: A DE s ¼ E tot slab þ E tot B slab E tot AB slab þ E tot ðslabþ ; (9) Lots of efforts have been denoted to study CO adsorption; the most stable structure is given in Fig. 3a. Here the most stable adsorbate geometry for T 0 1 and T 2 yields the carbon end of the radical binding to one Mo center resulting in adsorption energies of 1.11 ev ðt 0 1 Þ and 0.95 ev (T 2)(Table 1). As shown in Fig. 3b, for T 0 1 termination, water species H 2Ois found to bridge two adjacent Mo atoms with its oxygen center which accounts for adsorption energy of 0.81 ev. The calculated O Mo distances are 2.50 and 2.53 Å, and the obtained H O distances in the adsorbed H 2 O molecule (0.98 Å) hardly change with respect to those in the gas phase (0.97 Å). The H O H angle increases from (gas-phase H 2 O) to (adsorbed H 2 O). On the basis of these data, it can be concluded that water chemisorbs on T 0 1 edge without significantly altering its structure with respect to the gas-phase molecule, i.e., the adsorption is a nonactivated process. The net Mulliken charge of the adsorbed H 2 O molecule is 0.24e indicating there is electron transfer from the surface to the adsorbed H 2 O molecule. At the T 2 termination the oxygen end of water molecule binds only with one Mo center. As a result, the corresponding adsorption energy is 0.34 ev which is 0.47 higher than that on T 0 1 edge yielding an energetic preference for T 0 1 termination of the MoS 2 surface. The calculated O Mo distance is 2.44 Å, and the obtained H O distances in the adsorbed H 2 O molecule (0.98 Å) hardly change with respect to those in the gas phase (0.97 Å). The H O H angle increases from (gasphase H 2 O) to (adsorbed H 2 O). The net Mulliken charge of the adsorbed H 2 O molecule is 0.21e indicating there is charge redistribution between the surface and the adsorbed H 2 O molecule. Geometry optimizations of hydroxy species OH at the MoS 2 surface yield for T 0 1 and T 2 terminations two different local energy minima each, see Fig. 3c. In both cases the radical is found to prefer

4 X.-R. Shi et al. / Applied Catalysis A: General 365 (2009) binding scheme is similar in both cases: oxygen is more likely to bridge at two molybdenum centers compared with the adsorption at one top sulfur center. This is in analogy with the result discussed earlier for OH species adsorption. At T 0 1 termination, the adsorption energy of the structure in which O bridge two adjacent Mo atoms is 5.45 ev with calculated O Mo distances of 1.98 and 1.99 Å, respectively. For the structure with O top adsorption on the surface S atom, the adsorption energy is 3.43 ev with the calculated O S distance of 1.51 Å. On the T 2 edge, the adsorption energy of the structure in which O adsorbed on the adjacent Mo atoms is 5.17 ev. The calculated two O Mo distances are both 1.96 Å. For the structure with O top adsorption on the surface S atom, the adsorption energy is 3.98 ev with the calculated O S distance of 1.52 Å. Lots of efforts have been denoted to study H 2 adsorption on the MoS 2 (1 0 0) surface while the investigation on atomic H adsorption is still missing. Our calculations show that atomic H prefers to bridging adsorption on the unsaturated coordinately Mo sites to form MoH species with bond lengths of 2.00 and 1.99 Å over top adsorption on S sites to form SH species on T 0 1 edge (E ads = 2.45 vs ev). The same result was obtained on T 2 edge (E ads = 2.55 vs ev), see Fig. 3e. For carbon dioxide CO 2, it is found to bridge two adjacent molybdenum centers by CO part with the C Mo and O Mo distances of 2.09 and 2.14 Å resulting in the adsorption energy of 0.42 ev on T 0 1 edge. The similar structure was obtained on T 2 edge with the C Mo and O Mo distances of 2.20 and 2.16 Å yielding the adsorption energy of 0.53 ev. This indicates that these two structures are meta-stable, see Fig. 3f. Further, the C O bond of the adsorbate is activated with respect to the free CO 2 molecule in the gas phase. The C O bond distances increase from 1.18 Å (gasphase CO 2 ) to 1.24 and 1.27 Å on T 0 1 edge. On T 2 edge, they increase to 1.22 and 1.28 Å. The O C O angle increases from (gasphase CO 2 ) to (adsorbed CO 2 ) on T 0 1 edge and (adsorbed CO 2 )ont 2 edge. On the basis of these data, it can be concluded that the adsorption of CO 2 molecule on the two edges is an activated process. The adsorption of the formate species HCOO has been studied in our previous study [53]. The most stable structure with oxygen of the corresponding OCO part bridging two adjacent molybdenum centers of the substrate is shown in Fig. 3g. The adsorption energies are 2.84 ev ðt 0 1 Þ and 2.75 ev (T 2) The WGS mechanism Fig. 3. Computed equilibrium geometries of reaction intermediates at T 0 1 and T 2 terminations of the MoS 2 (1 0 0) surface, (a) CO, (b) H 2 O, (c) OH, (d) O, (e) H, (f) CO 2 and (g) HCOO. The atom centers are shown by shaded balls of different size and labeled in Fig. 3a. bridging two adjacent molybdenum centers with its oxygen center over binding to one top sulfur site which accounts for adsorption energies of 3.96 ev ðt 0 1 Þ and 3.49 ev (T 2) against 1.67 ev ðt 0 1 Þ and 2.09 ev (Table 1). The energy difference between the stable structures on two edges suggests that OH radical prefers adsorption at T 0 1 termination over T 2. The adsorption geometries of atomic O at the two different terminations are sketched in Fig. 3d. Obviously, the resulting Dissociation of H 2 O Experiments supported the redox mechanism for the WGS reaction on MoS 2 surface where the catalyst surface is alternately oxidized by water and then reduced by carbon monoxide. Therefore we will focus on the dissociation of H 2 O on the MoS 2 (1 0 0) slab to decompose two H atoms and one O atom, firstly. The results of the adsorption of H 2 O and the subsequent dissociation for the elementary step are shown in Figs. 4 and 5 and Table 2. The results of the adsorption of H 2 O near CO are shown for comparison, see Figs. 6 and 7. The reaction energies are calculated according to Eqs. (9) and (10). The results are given in Table 2. The calculations show that the dissociation of H 2 O to yield OH and H on T 0 1 edge is connected with a barrier of 1.01 ev and is slightly endothermic by 0.03 ev, see Table 2 and Fig. 4(1). The same qualitative results, with, however, larger barriers (1.70 ev) are found for the T 2 termination, see Table 2 and Fig. 5(1). Subsequent dissociation of adsorbed OH to yield O and H groups is connected with a barrier of 1.93 ev and is endothermic by 0.68 ev, see Table 2 and Fig. 4(2a). Qualitatively similar results are found on T 2 edge, see Table 2 and Fig. 5(2a). However, it is with a smaller barrier of 1.25 ev.

5 66 X.-R. Shi et al. / Applied Catalysis A: General 365 (2009) Table 1 Adsorption energies E ads (in ev) of all reaction intermediates at the MoS 2 (1 0 0) surface considered in this work and selected bond lengths (in Å) of the Mo Atom with C and O of MoS 2, O H bond of H 2 O molecule, and C O Bond of CO. The data are listed for both the T 0 1 and T 2 termination. Corresponding adsorption geometries are shown in Figs. 3, 6 and 7. Species T 0 1 T 2 E ads d C O/O H/Mo C/Mo O E ads d C O/O H/Mo C/Mo O CO / /2.05/ / /2.02/ H 2 O 0.81 /0.98,0.98/ /2.50, /0.98,0.98/ /2.44 OH Mo 3.96 /0.92/ /2.17, /0.97/ /2.20,2.20 OH S 1.67 /0.98/ / 2.09 /0.98/ / O Mo 5.45 / / /1.98, / / /1.96,1.96 O S 3.43 / / / 3.98 / / / H Mo H S CO ,1.27/ /2.09/ ,1.28/ /2.20/2.16 HCOO ,1.27/ / /2.15, ,1.26/ / /2.17,2.21 CO + H 2 O /0.98,0.98/2.09/3.57, /0.98,0.98/2.00/2.59 CO + OH + H /0.98/2.15/2.31, /0.98/2.04/2.19,2.17 CO+O+2H / /2.11/2.22, / /2.06/2.05,1.93 Table 2 Reaction energies, DE s and DE c, and reaction barriers, DE a, of different elementary steps for the WGS reaction at the MoS 2 (1 0 0) surface. For definitions of DE s, DE c, DE a, see text. All energies are given in ev. Step Elementary reaction step T 0 1 T 2 DE s DE c DE a DE s DE c DE a 1 H 2 O! OH + H (0.31 a ) 1.01 (0.98 a ) (0.51 a ) 1.70 (1.77 a ) 2a OH! O + H (0.60 b ) 1.93 (2.10 b ) (0.49 b ) 1.25 (1.24 b ) 2b CO + OH! HCOO CO+O! CO (0.71 c ) (0.71 c ) 4a CO 2! (CO 2 ) gas (0.70 c ) (0.70 c ) 4b CO 2 +H! HCOO a Co-adsorbed CO. b Co-adsorbed CO and H. c Experimental data in Ref. [26] The dissociation of H 2 O near the adsorbed CO has been investigated for comparison. For the first step, namely, the dissociation of H 2 O species to form OH and H species, in the initial state, CO was set at one 5-fold coordinated Mo1 site through the C end, and H 2 O was set to bridge over the two 5-fold Mo sites with two Mo O bonds of 2.28 Å employing the same procedure with respect to those separated molecule adsorption alone. After optimization, H 2 O adsorbed at 2.37 Å over Mo2 position, and the O Mo1 distance is 3.57 Å while for the H 2 O adsorption alone, two corresponding Mo O bonds are 2.50 and 2.53 Å, respectively, see Table 1 and Fig. 6. This means H 2 O was pushed away by CO molecule during optimization showing the repulsion of H 2 O from CO molecule. The co-adsorption energy is 1.36, 0.56 ev higher than the sum of their separate adsorption energies. On the basis of these data, it can be concluded that there is repulsion between the co-adsorbed CO and H 2 O species. For the co-adsorbed CO + OH + H system, CO and OH co-adsorbed on the same Mo site through the C center of CO molecule and the O end of OH while H co-asorbed on the top S site, see Fig. 6. The co-adsorption energy is 6.56, 0.84 ev higher than the sum of the separate CO and OH + H structures indicating there is repulsion between the co-adsorbed species. The C Mo1 distances increased to 2.15 Å with respect to 2.05 Å in the CO adsorption only, and the O Mo1 of the OH specie increased to 2.31 Å with respect to 2.17 Å in the OH adsorption only meanwhile the O Mo2 distance decreased to 2.11 Å with respect to 2.17 Å in the OH adsorption only. On the basis of these data, it can be concluded that there is repulsion between adsorbates for the CO + OH + H system. This kind of interaction between the adsorbates for the CO + H 2 O and CO + OH + H system yields the different reaction energies from the case wherein there is H 2 O species alone, 0.31 ev on T 0 1 edge and 0.51 ev on T 2 edge, while the corresponding barriers are quite similar with the difference of 0.03 ev on T 0 1 edge and 0.07 ev on T 2 edge. The bond lengths of the corresponding TS are listed in Table 3 and the structures are shown in Figs. 4 and 6. As shown in Table 3, Figs. 4 and 6, although the barrier energies are similar, the corresponding TS structures are different. The Mo1 O bond length shifts from 2.41 Å for H 2 O alone to 3.42 Å for the co-adsorbed CO + H 2 O system in the TS structures. And the Mo1 O and S H bond distances also changed. The same qualitative results are found for the T 2 termination, see Table 3 and Figs. 5 and 7. We can see clearly that the surface S2 coordinated with Mo1 was pushed to the other side. It should be noted that at T 2 edge, the dissociation of water species to form OH and one SH Table 3 Selected bond lengths (in Å) of the Mo atoms with O of H 2 O and S atoms with H for the transition states during the dissociation of water species with and without coadsorbed CO. The data are listed for both the T 0 1 and T 2 termination. TS1 denotes the transition state for the dissociation of H 2 O to form OH and H and TS2 denotes the transition state for the dissociation of OH to form O and H. Corresponding adsorption geometries are shown in Figs Edge d Mo1 O d Mo2 O d S1 H d S2 H d S3 H T 0 1 TS TS1 a TS TS2 b T 2 TS TS1 a TS TS2 b a Co-adsorbed CO. b Co-adsorbed CO and H.

6 X.-R. Shi et al. / Applied Catalysis A: General 365 (2009) Fig. 4. Elementary reaction steps for the WGS reaction at the T 0 1 edge: The structures on the left refer to initial reactants, those on the right denote products, and the center structures are corresponding transition states. The atom centers are shown by shaded balls of different size and labeled in Fig. 3a. species firstly, and then the H atom will diffuse from the top S site to the more favorable Mo site. The atomic H has been confirmed to be quite mobile by other groups [52], hence its diffusion was ignored in our present studies. In a subsequent reaction, the dissociation of OH species yield similar reaction energies for the system with and without co-adsorbed CO (DE c = 0.60 vs ev) and the corresponding barriers are different (DE a = 2.10 vs ev) on T 0 1 edge. The bond lengths of the corresponding TS are listed in Table 3 and the structures are shown in Figs. 4 and 6.As shown in Table 3, Figs. 4 and 6, the corresponding TS structures are different. The Mo1 O bond length shifts from 2.10 Å for H 2 O alone to 2.26 Å for the co-adsorbed CO + H 2 O system in the TS structures. And the Mo1 O and S H bond distances also changed. On T 2 edge, as shown in Figs. 5 and 7, the surface S2 coordinated with Mo1 was pushed to the other side near the co-adsorbed CO yielding the reaction energy of 0.49, 0.08 ev higher than that in the system without co-adsorbed CO, while the corresponding Mo1 O, Mo2 O and S1 H distances are quite similar with the difference below 0.03 Å in the TS structures yielding the barriers nearly the same (DE a = 1.25 vs ev). On the basis of the above discussions, it can be concluded that the co-adsorbed CO will affect the surface structures, however, it does not facilitate or hinder the dissociation of the water species on the (1 0 0) surface of MoS 2 catalyst Reaction of CO with OH As a competing step for the dissociation of OH to produce the surface O and H species in the redox mechanism, the surface OH species will react with co-adsorbed CO to form the formate species HCOO in the associative mechanism. As shown in Table 2, compared with the dissociation of OH, HCOO formation is more favorable thermodynamically (DE c = 0.33 vs ev) while less favorable kinetically (DE a = 3.04 vs ev) on T 0 1 edge, see Fig. 4(2b). Qualitatively similar results are found for T 2 edge, see Table 2 and Fig. 5(2b). Since the barrier for the formation of HCOO is quite high (3.04 ev), the second step for WGS reaction will be the dissociation of OH. Namely, on the pure MoS 2 (1 0 0) surface, the

7 68 X.-R. Shi et al. / Applied Catalysis A: General 365 (2009) Fig. 5. Elementary reaction steps for the WGS reaction at the T 2 edge: the structures on the left refer to initial reactants, those on the right denote products, and the center structures are corresponding transition states. The atom centers are shown by shaded balls of different size and labeled in Fig. 3a. Fig. 6. Schematic potential energy surface for the dissociation of H 2 O with coadsorbed CO at the T 0 1 edge. The atom centers are shown by shaded balls of different size and labeled in Fig. 3a. Fig. 7. Schematic potential energy surface for the dissociation of H 2 O in the presence of co-adsorbed CO at the T 2 edge. The atom centers are shown by shaded balls of different size and labeled in Fig. 3a.

8 X.-R. Shi et al. / Applied Catalysis A: General 365 (2009) WGS reaction is through the redox mechanism wherein it is consist with the experimental conclusion Formation of CO 2 The formed O species will react with surface CO to form surface CO 2 which is connect with a small barrier of 0.53 ev and is exothermic by 0.28 ev on T 0 1 edge, see Fig. 4(3). The same qualitative results, with, however, larger barriers (1.23 ev) are found for T 2 edge, see Fig. 5(3). In a subsequent reaction the adsorbed CO 2 can desorb from the surface where the reaction is exothermic (DE c = 0.42 ev) and connected with a barrier of only DE a = 0.22 ev, see Fig. 4(4a). For comparison, adsorbed CO 2 reacting with surface H to form HCOO is found to yield a rather high barrier, DE a = 3.97 ev, and is much more exothermic, DE c = 0.79 ev, than CO 2 desorption, see Fig. 4(4b). The same qualitative results, with, however, smaller barriers (1.64 ev) are found for T 2 edge, see Fig. 5(4a) and (4b). Altogether, the calculations show that CO 2 desorption is a very likely process. Fig. 8 shows energetic reaction schemes for sequential WGS reaction at the MoS 2 (1 0 0) surface for both T 0 1 (Fig. 8a) and T 2 edges (Fig. 8b) which consider all reaction steps discussed above. This result in optimized reaction paths combining energetically lowest reaction intermediates and lowest reaction barriers are highlighted by thick lines. Overall, the results in Table 2 show that reaction energies calculated according to DE s of definition (9), and DE c of definition (10), yield the same trends. (The same conclusions have been obtained in studies on the dehydrogenation of methanol over Pt (1 1 1) surface [54].) The energy difference for the same adsorbated species can be explained by the interaction between the adsorbates. Obviously, the most favorable reaction path is qualitatively the same for both terminations and the WGS reaction is through the redox mechanism in the sequence CO þ H 2 O! CO þ OH þ H! CO þ O þ 2H! CO 2 þ H 2 (11) The available experimental data [26] has been listed in Table 2 for comparison. In Ref. [26], Lund considered the H 2 S in the gas phase therefore the detailed mechanism is different from our model resulting only two experimental data available. However the key step for the redox mechanism, namely step (5), is the same. As shown in Table 2, compared with the experimental data, the calculated barrier for CO reaction with surface O is higher on T 0 1 edge while it is lower on T 2 edge. For CO 2 desorption from the surface, the calculated barriers are both higher than the experimental data. The difference may be attributed to two possible reasons: our calculations are performed at T = 0 K while experiments are carried out at about T = K; the real catalyst surface is quite complex while we just consider the reaction at (1 0 0) surface Relevance to CO hydrogenation reaction Herein we have investigated the WGS reaction on the pure MoS 2 (1 0 0) surface. In our previous study [53], we confirmed the formation of the formate species HCOO during CO hydrogenation on the pure MoS 2 (1 0 0) surface that has been proposed in experiment [55] by frequency calculations. Then it raises the question where the formate species is from and where it will go. Two possible reaction paths may produce the formate species, one is the surface CO reacting with the OH species and the other is the adsorbed CO 2 molecule reacting with surface H. As shown in Fig. 8c, our present studies show that the formation of the formate species on T 0 1 edge is quite difficult where it is hindered by the high barriers of 3.04 ev from the surface CO and OH species and 3.97 ev from the surface CO 2 and atomic H. While on T 2 edge, the barriers Fig. 8. Schematic potential energy surfaces for (a) the WGS reaction on the T 0 1 edge, (b) the WGS reaction on the T 2 edge, and (c) the reaction relevant to the formate HCOO species where dash lines denote the T 0 1 edge and solid lines denote the T 2 edge. The most favorable reaction paths are highlighted by thick lines. The barriers between the reaction intermediates refer to the corresponding reaction barriers. for the formation of the formate species HCOO are relatively lower with 2.26 ev from the surface CO and OH species and 1.64 ev from the surface CO 2 molecule and atomic H. That indicates the formate species is most likely from the surface CO 2 molecule reaction with atomic H on T 2 edge. As discussed formaldehyde and methanol synthesis from CO and H on Ni (1 1 1) by Remediakis et al. [56], although the energy barrier of 1.64 ev may look high, it is possible to overcome this barrier under reasonable reaction conditions. To estimate the reaction rate, we use the Arrhenius formula, r ¼ vexpð E a =k B TÞ. By assuming a standard value of preexponential factor, n 10 13, and setting E a = 1.64 ev, we obtain a reaction rate that become equal to 1 s 1 at T = 636 K which is typically lower than the temperature of 714 K used for CO hydrogenation reaction [57]. Indeed, CO 2 desorption is more competitive, and the

9 70 X.-R. Shi et al. / Applied Catalysis A: General 365 (2009) formation of HCOO by CO 2 hydrogenation is not favorable. This also agrees with the fact that WGS is a quick and reversible reaction, while formation of formate is just a side reaction. However, WGS will finally reach its equilibrium, and then CO 2 desorption is thermodynamically resisted. At this situation, formation of HCOO becomes favorable, although it is a slow reaction. The formed HCOO species can react further with adsorbed hydrogen yielding H 2 COO, alternatively, it can break its C O bond yielding surface CHO and O, see Fig. 8c. Our calculations show that the former is more favorable than the latter both kinetically (DE a = 1.65 vs ev) and thermodynamically (DE c = 0.86 vs ev). Thus the formed HCOO species will react further with adsorbed hydrogen yielding H 2 COO. In a subsequent reaction the adsorbed H 2 COO can break its C O bond to produce the surface CH 2 O and O groups where the reaction is exothermic (DE c = 0.23 ev) and connected with a barrier of DE a = 1.28 ev, see Fig. 8c. The formed CH 2 O is the typical intermediates in the CO hydrogenation on the pure MoS 2 surface, which has been confirmed by our previous study [53]. 4. Conclusions By using the generalized gradient approximation (GGA) and Perdew Wang functional (PW91), the adsorption structures of the WGS reaction intermediates have been investigated at the level of density functional theory. H 2 O, OH and O species are more stable on T 0 1 edge than on T 2 edge while the other species exhibit similar stabilities on two edges. Among them, CO 2 is found to be the least stable with positive adsorption energies on two edges suggesting that it may easily desorb from the surface. We have also performed a systematic density functional theory study on the WGS reaction over pure MoS 2 (1 0 0) surface. Our calculated results show that the WGS reaction on two edges will proceed as follows: (1) in the first step, water species H 2 O dissociates into OH and H species. (2) In the second step, OH species will dissociate into O and H group instead to react with CO to form the formate species HCOO. (3) The formed O species will convert to the meta-stable CO 2 by reacting with the surface CO species. (4) The formed CO 2 will desorb from the surface in preference over reaction with surface H to produce the formate species HCOO. Namely, on the pure MoS 2 (1 0 0) surface, the WGS reaction is through the redox mechanism wherein it is consist with the experimental conclusion. The rate-determining step on T 0 1 edge is the dissociation of OH to form O and H groups while on T 2 edge it is the dissociation of H 2 O to form OH and H groups. 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