Supporting Information

Size: px

Start display at page:

Download "Supporting Information"

Dwight Morgan
5 years ago
Views:

1 Supporting Information A Simple Descriptor to Rapidly Screen CO Oxidation Activity on Rare- Earth Metal Doped CeO 2 : from Experiment to First-Principles Kyeounghak Kim a,, Jeong Do Yoo b,, Siwon Lee b, Minseok Bae c, Joongmyeon Bae c, WooChul Jung b,* and Jeong Woo Han a,* a Department of Chemical Engineering, University of Seoul, Seoul 02504, Republic of Korea b Department of Materials Science and Engineering and c Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea These authors contributed equally to this work. Corresponding Authors * jwhan@uos.ac.kr (J. W. Han). * wcjung@kaist.ac.kr (W. Jung). S-1

CeO 2. The calculated vales of lattice parameters of CeO 2, NDC, PDC and SDC are 5.4135 Å, 5.4554 Å, 5.4192 Å and 5.

2 Experimental information Figure S1: The crystallite size of both SDC and CeO 2 NPs vs. solution ph (calcined at 750 C for 3 hours). The peak positions tend to shift toward lower angle (2 ) by adding RE dopants, indicative of the increase in a lattice parameter, compared to that of pure CeO 2. The calculated vales of lattice parameters of CeO 2, NDC, PDC and SDC are Å, Å, Å and Å, respectively, which as in good agreement with the reported values 1. Figure S2: (a) The XRD patterns of doped ceria NPs. (b) The peak positions shift toward lower angle (2 ) with dopants, compared to pure CeO 2. S-2

Figure S4: The minimum energy path between E4 and IS2 (Barrier 3) on RE-doped CeO 2 (111).

3 Figure S3: Light-off plots for CO oxidation over four different CeO 2 NPs, showing the degree of reproducibility in this work. (Insert) The average values of T 20, T 50 and T 80 with the corresponding standard deviations. Figure S4: The minimum energy path between E4 and IS2 (Barrier 3) on RE-doped CeO 2 (111). Note that pure ceria needs the highest endothermic reaction energy between E1 and IS1 (Barrier 1, red line) in the first oxidation process. S-3

4 To provide the detailed information of molecular configuration at the highest barrier step, we plotted minimum energy paths on each doped surfaces at Barrier 3. Since all dopants except for Ce have the highest reaction energy at Barrier 3 with the similar optimized structures at each elementary step, we exhibited the side views of Tm-doped surface as a representative case (Fig. S4). However, as shown in Table 3, pure ceria requires the highest endothermic reaction energy at Barrier 1 for the CO oxidation. Thus, we additionally considered both reaction processes (Barrier 1 and Barrier 3) for pure ceria to rigorously verify our screening scheme for the comparison of reactivity using E Rxn_max based on the BEP relationship (Fig. S5). Figure S5: BEP relationship between reaction barrier and reaction energy at a RDS step (Barrier 3) where most of doped surface revealed the highest reaction energy. S-4

5 E vf Barrier 1 Barrier 2 Barrier 3 Barrier 4 E Rxn_max La Ce Pr Gd Nd Pm Sm Eu Tb Dy Ho Er Tm Lu Yb Table S1: Reaction energies for the CO oxidation at endothermic elementary steps on REdoped CeO 2 (111). E vf is an oxygen vacancy formation energy (ev). Barrier 1~4 are the reaction energies (ev) for the endothermic reactions (E1 IS1, IS1 E2, E4 IS2 and IS2 E5 for Barrier 1, 2, 3 and 4, respectively). E Rxn_max is the maximum value obtained from each row. RE Ionic Radius RE Ionic Radius La Tb Ce Dy Pr Ho Gd Er Nd Tm Pm Lu Sm Yb Eu Ce O Table S2: Shannon ionic radius of RE cations (pm) and oxygen ion with coordination number (CN) of 8 and 4, respectively 2. We calculated all elementary steps of CO oxidation according to the MvK mechanism. Our results showed that the carbonate-like species formed at both E1 and E4 are more stable than the chemisorbed (IS1) and physisorbed (IS2) CO 2, respectively. The molecular adsorption and desorption are mostly observed near the RE-doped sites, implying that the oxidation activity may be controlled by surface doping. The details of optimized structures at each elementary step are shown as below; S-5

6 S-6

Figure S6: The optimized structures of reaction intermediates during the CO oxidation at each elementary step on M-doped CeO 2 (111) (M = Dy, Er, Eu, Gd, Ho, La, Lu Nd, Pm, Pr, Sm, Tb, Tm and Yb).

Here, we elucidate the relationship between E vf and E Rxn_max by examining the bond strength between lattice oxygen (O latt ) to the C atom in carbonate species (Fig S7(a)) or the nearest neighbor

7 Figure S6: The optimized structures of reaction intermediates during the CO oxidation at each elementary step on M-doped CeO 2 (111) (M = Dy, Er, Eu, Gd, Ho, La, Lu Nd, Pm, Pr, Sm, Tb, Tm and Yb). At a rating determining step (RDS, E4 IS2), CO 2 experiences the phase transition from carbonate species to the CO 2 species that will be desorbed from the surface. Here, we elucidate the relationship between E vf and E Rxn_max by examining the bond strength between lattice oxygen (O latt ) to the C atom in carbonate species (Fig S7(a)) or the nearest neighbor (NN) cations (Fig. S7(b)), respectively. In general, the strong bond between lattice cation and oxygen (d cation-o ) increases E vf. Since one surface oxygen atom is surrounded by three cations (Fig. S7(b)), we can estimate the bond strength between O latt and three NN cations by comparing how much the average bond length between them is changed upon the existence S-7

of carbonate species at E4, compared to the one on clean surface. Our results show that the lower change of Cation-O latt in E4 results in higher E vf (Fig. S7(b)).

8 of carbonate species at E4, compared to the one on clean surface. Our results show that the lower change of Cation-O latt in E4 results in higher E vf (Fig. S7(b)). Once lattice oxygen has strong binding to other surface cations, it may weakly interact with the CO 2 part in the carbonate species (Fig. S7(a)). The more elongated bond length between C atom in carbonate species and O latt (d C-O ) implies the lower reaction energy required for the phase transition from carbonate species to the CO 2 species. For this reason, the carbonate species more easily transforms to the physisorbed CO 2 with higher E vf, inducing to the easier desorption of CO 2 species from ceria surface to the gas phase. Figure S7: The relationship of bond length between (a) C atom in carbonate species or (b) lattice cation and O latt with E vf at E4 in the second oxidation process. The right side in (a) represents the position of each atom. O CO is an oxygen atom from the gas phase of CO, O C is an excess oxygen bonded to the C atom, and O latt is a lattice oxygen in CeO 2 (111). The black arrows in the right side in (b) show the bond length we used for the calculation of average bond length. S-8

Cerium nitrate hexahydrate (Ce(NO 3 ) 3 6H 2 O, Kanto Chemical, 99.99 %), Neodymium nitrate hexahydrate (Nd(NO 3 ) 3 6H 2 O, Alfa Aesar, 99.

Ethylenediaminetetraacetic acid (EDTA, C 10 H 16 N 2 O 8, 99.5 %) and citric acid (C 6 H 8 O 7, 99.5 %) and an ammonia solution (NH 3 H 2 O, 28-30 wt%) were from Junsei Chemical.

9 Cerium nitrate hexahydrate (Ce(NO 3 ) 3 6H 2 O, Kanto Chemical, %), Neodymium nitrate hexahydrate (Nd(NO 3 ) 3 6H 2 O, Alfa Aesar, 99.9 %), Samarium nitrate hexahydrate (Sm(NO 3 ) 3 6H 2 O, Alfa Aesar, 99.9 %), Praseodymium nitrate hexahydrate (Pr(NO 3 ) 3 6H 2 O, Alfa Aesar, 99.9 %) were used as metal sources. Ethylenediaminetetraacetic acid (EDTA, C 10 H 16 N 2 O 8, 99.5 %) and citric acid (C 6 H 8 O 7, 99.5 %) and an ammonia solution (NH 3 H 2 O, wt%) were from Junsei Chemical. Silica beads (50-70 mesh particle size) were from Sigma-Aldrich. Quartz wool (8-15 micro-coarse porosity) was purchased from Chemglass. All reagents employed for the experiments were of analytic grade and were used without further purification. Figure S8: The overall processes to synthesize Ce 0.8 RE 0.2 O 2- NPs in this work. S-9

The DTA curve (solid line) indicate an endothermic peak around 200 C which is associated with the formation of EDTA-citrate complex and two exothermic peaks 3.

The peak at around 300 C can be attributed to the decomposition/oxidation of the metal-chelates and the other peak at around 380 C, which is very sharp and distinctive, is associated with the

10 The DTA curve (solid line) indicate an endothermic peak around 200 C which is associated with the formation of EDTA-citrate complex and two exothermic peaks 3. Two exothermic peaks are also shown in the DTA curve. The peak at around 300 C can be attributed to the decomposition/oxidation of the metal-chelates and the other peak at around 380 C, which is very sharp and distinctive, is associated with the pyrolysis of metal/nitrate/citrate/edta complexes and formation of cubic fluorite phase 4. The TGA curve can be divided into three well separated steps with large amount of mass loss (83 %). The first mass loss of about 10% is observed during the heating step up to 180 C, presumably due to the dehydration and decomposition of nitrates. The next two mass loss of about 73% was occurred in the temperature range of C and C, the former correspond to the evaporation of trapped water in sticky gel and decomposition of ammonium nitrate, and the latter is attributed to the combustion of the metal/nitrate/citrate/edta complexes accompanied by the exothermic reaction 4 No further mass loss was observed above 410 C, which indicate that no impurities are remained in the samples after calcined at 750 C. Figure S9: DTA-TGA data of Ce 0.8 Sm 0.2 O 2- (SDC) gel precursor (solution of ph 10, overnight heat-treatment at 80 C). S-10

The oxygen vacancy formation energy (E vf ) was calculated from the total energies of the supercells with various defect positions; 1 E ( E E ) E, (1) vf vac O 2 g( ) surf 2 where E vac is the total

The adsorption energy of adsorbates with/without the oxygen vacancy is defined as the total energies difference between before and after the molecular adsorption at each surface; E E E E, (2) ads ads

11 The oxygen vacancy formation energy (E vf ) was calculated from the total energies of the supercells with various defect positions; 1 E ( E E ) E, (1) vf vac O 2 g( ) surf 2 where E vac is the total energy of the system containing an oxygen vacancy, E O2(g) the total energy of an isolated oxygen molecule in the gas phase, and E surf the total energy of optimized perfect slab structures. The adsorption energy of adsorbates with/without the oxygen vacancy is defined as the total energies difference between before and after the molecular adsorption at each surface; E E E E, (2) ads ads surf surf adsorbate where E ads-surf is the total energy of an adsorbate adsorbed on the surface and E adsorbate the total energy of an isolated adsorbate in the gas phase. E des is the desorption energy, which is assumed to be reversible to the adsorption, E ads. With our definition, a negative value of E vf or E ads indicates that the process energetically prefers to occur spontaneously. We used a (2 2) surface unit cell of CeO 2 (111) with the lattice constant of 5.46 Å. We substituted one Ce atom at the top surface layer (dotted line in Fig. S11(a)) with a RE-metal atom. Figure S10: Top (a) and side (b) views of CeO 2 (111) slab model. A surface unit cell we considered is shown as the line in (a). The black dotted circle represents the Ce atom substituted by dopant metal. S-11

12 REFERENCES (1) Zhao, S.; Gorte, R. J., The effect of oxide dopants in ceria on n-butane oxidation. Appl. Catal. A: Gen. 2003, 248 (1 2), (2) Shannon, R., Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Cryst. A 1976, 32 (5), (3) Harshini, D.; Lee, D. H.; Jeong, J.; Kim, Y.; Nam, S. W.; Ham, H. C.; Han, J. H.; Lim, T.-H.; Yoon, C. W., Enhanced oxygen storage capacity of Ce 0.65 Hf 0.25 M 0.1 O 2-δ (M = rare earth elements): Applications to methane steam reforming with high coking resistance. Appl. Catal. B: Environ. 2014, , (4) Prasad, D. H.; Park, S. Y.; Ji, H. I.; Kim, H. R.; Son, J. W.; Kim, B. K.; Lee, H. W.; Lee, J. H., Structural Characterization and Catalytic Activity of Ce 0.65 Zr 0.25 RE 0.1 O 2 δ Nanocrystalline Powders Synthesized by the Glycine-Nitrate Process. J. Phys. Chem. C 2012, 116 (5), S-12

Electron-hopping brings lattice strain and high catalytic activity in low temperature oxidative coupling of methane in an electric field

Supporting information Electron-hopping brings lattice strain and high catalytic activity in low temperature oxidative coupling of methane in an electric field Shuhei Ogo*, Hideaki Nakatsubo, Kousei Iwasaki,