Selective Activation of Methane on Single-Atom Catalyst of Rhodium Dispersed on Zirconia for Direct Conversion

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1 Supporting Information Selective Activation of Methane on Single-Atom Catalyst of Rhodium Dispersed on Zirconia for Direct Conversion Yongwoo Kwon a, Tae Yong Kim b, Gihun Kwon a, Jongheop Yi b *, and Hyunjoo Lee a * a Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, South Korea; b School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, South Korea Additional Data: Table S1-S4 Figure S1-S13 S1

2 Figure S1. Calibration curves for the quantification of (a) MeOH and (b) MeOOH using 1 H NMR data. The standard solutions were made with various MeOH and MeOOH concentrations, and the concentrations were plotted versus the peak (MeOH, δ=3.35ppm; MeOOH, δ=3.85ppm) area ratio to internal standard (DSS). The methyl hydroperoxide (MeOOH) was synthesized in the lab by using dimethyl sulfate. S1 Hydrogen peroxide solution (30%, Ducksan, 3.75g), H 2 O (6.25ml), and dimethyl sulfate (99.5%, Samcheon, 2.5g) were mixed in a round-flask with a condenser. KOH solution (40 wt%, Samcheon, 5.25g) was added to the solution and stirred for 1 h. The concentration of the synthesized MeOOH was estimated as a difference between the concentrations of total peroxides and H 2 O 2. The concentration of total peroxides was measured by titration with KI. The H 2 O 2 concentration was measured spectrophotometrically using titanium oxalate assay. S2 S2

3 Figure S2. (a) TEM image and (b) XRD pattern of the synthesized ZrO 2 support. S3

4 Figure S3. Rh K edge k 3 -weighted EXAFS oscillations of the 0.3 wt% Rh/ZrO 2, 2 wt% Rh/ZrO 2, 5 wt% Rh/SiO 2, and Rh foil in k-range. S4

5 Figure S4. (a) HAADF-STEM and (b) EDS mapping images of the 0.3 wt% Rh/ZrO 2 catalyst. (c) HAADF-STEM (b) EDS mapping images of the 0.3 wt% Rh/ZrO 2 catalyst after methane to methanol reaction. S5

6 Figure S5. HR-TEM images of (a) 0.3 wt% Rh/ZrO2, (b) 2 wt% Rh/ZrO2, and (c) 5 wt% Rh/SiO2 catalysts. S6

7 Figure S6. HAADF-STEM images of 2 wt% Rh/ZrO 2 with (a) high and (b) low magnification. The white arrows indicate Rh clusters on ZrO 2 support. S7

8 Table S1. IR peaks of the germinal dicarbonyl adsorption on the Rh sites of various Rh/ZrO 2 samples. The angle between the adsorbed CO molecules was estimated by using the area of the symmetric and asymmetric peak. Samples Rh(CO) 2 symmetric (cm -1 ) A sym a Rh(CO) 2 asymmetric (cm -1 ) A asym a 2ɑ( ) b RhCl 3 +ZrO wt% Rh/ZrO 2 2 wt% Rh/ZrO wt% Rh/ZrO 2 a The area of the symmetric or asymmetric peaks. b The angle between the adsorbed CO molecules (tan 2 ɑ = A asym /A sym ). S8

9 Table S2. Relative energies of various DFT models for a single atomic Rh on ZrO 2 surface. The density of hydroxyl groups on hydroxylated surface (Hyd) is 4.3 OH/nm 2 because this density showed the lowest surface free energy at room temperature under 1 atm of H 2 O partial pressure as shown in Figure S7a. Model structure Relative energy E (ev) Oxidation state e Adsorption A a Rh 1 /Clean Rh 1 /Hyd Adsorption B b Rh 1 O 2 /Clean Rh 1 O 2 /Hyd Adsorption C c Rh 1 /O v Rh 1 /O v -Hyd Zr Substitution d Rh Zr -Clean Rh Zr -Hyd a 1/2 Rh 2 O 3 (bulk) + Surface (clean or hydroxylated ZrO 2 (101) surface) Rh/surface + 3/4 O 2 (gas); E =E / + E E E b 1/2 Rh 2 O 3 (bulk) + Surface (clean or hydroxylated ZrO 2 (101) surface) + 1/4 O 2 RhO 2 /surface; E =E / E E E c 1/2 Rh 2 O 3 (bulk) + Surface with oxygen vacancy (O v ) Rh-O v + 3/4 O 2 (gas); E = E / + E E E d 1/2 Rh 2 O 3 (bulk) + Surface (clean or hydroxylated ZrO 2 (101) surface) + 1/4 O 2 Rh-Zr v + ZrO 2 (bulk); E =E +E E E E e Bader charge of Rh on each model was calculated, then the Rh oxidation state was estimated from the calibration line in Figure S6. S9

10 Figure S7. Surface free energy of tetragonal ZrO 2 (101) surface as a function of temperature under (a) 1 atm and (b) 10-5 atm of H 2 O partial pressure. Dotted lines in (a) and (b) indicate reaction temperature for liquid- and gas-phase methane oxidation, respectively. S10

11 Figure S8. Top views of the simulated structures of (a) Rh 1 /Clean, (b) Rh 1 /Hyd, (c) Rh 1 O 2 /Clean, (d) Rh 1 O 2 /Hyd, (e) Rh 1 /O v, (f) Rh 1 /O v -Hyd. The number in the inset indicates Rh oxidation state estimated by Bader charge analysis. S11

12 Figure S9. A relation between Bader charge of Rh calculated by DFT and its oxidation state for metallic Rh, Rh 2 O 3, and RhO 2. S12

13 Table S3. The direct methane oxidation results for various kinds of metal supported on ZrO 2. Reaction condition: 30 bar of 95% CH 4 /He, 70 C, 30 min, 0.5 M H 2 O 2, 30 min, and catalyst 30 mg. Catalyst Product (µmol/ µmol metal ) MeOH MeOOH CO wt% Rh/ZrO wt% Pd/ZrO wt% Pt/ZrO wt% Ir/ZrO S13

14 Table S4. The effect of the support for the direct methane oxidation. Reaction condition: 30 bar of 95% CH 4 /He, 70 C, 30 min, 0.5 M H 2 O 2, 30 min, and catalyst 30 mg. Catalyst Product (µmol/µmol Rh ) MeOH MeOOH CO wt% Rh/ZrO wt% Rh/CeO wt% Rh/TiO wt% Rh/SiO S14

15 Figure S10. The change in the product distribution for the direct methane oxidation over reaction time on the 0.3 wt% Rh/ZrO 2 catalyst. Reaction condition: 30 bar of 95% CH 4 /He, 70 C, 0.5 M H 2 O 2, and catalyst 30 mg. S15

16 Figure S11. (a) MeOOH and (b) MeOH decomposition reaction results on the 0.3 wt% Rh/ZrO 2, 2 wt% Rh/ZrO 2, and 5 wt% Rh/SiO 2 catalysts. Reaction condition: 0.8 mm of MeOOH or 0.5 mm of MeOH, 3 bar of He, 70 C, 30 min, 0.5 M H 2 O 2, and catalyst 30 mg S16

17 Figure S12. (a) Optimized model structure and (b) energy diagram of methane oxidation to oxygenates on Rh 1 /ZrO 2 (Rh Zr -Hyd). The density of hydroxyl groups on hydroxylated surface (Hyd) is 4.3 OH/nm 2 because this density showed the lowest surface free energy under reaction condition as shown in Figure S7a. S17

18 Figure S13. (a) Optimized model structure and (b) energy diagram of methane conversion to ethane on Rh 1 /ZrO 2 (Rh Zr -Hyd). The density of hydroxyl groups on hydroxylated surface (Hyd) is 2.9 OH/nm 2 because this density showed the lowest surface free energy under reaction condition as shown in Figure S7b. S18

19 References S1. Davies, D. M.; Deary, M. E., J. Chem. Soc. Perkin Transaction , 4, S2. Sellers, R. M., Analyst 1980, 105, S19