Investigation of Reaction Mechanism of

Transcription

1 Supporting Information: Investigation of Reaction Mechanism of NO C 3 H 6 CO O 2 Reaction over NiFe 2 O 4 Catalyst Kakuya Ueda, a Junya Ohyama, a,b Atsushi Satsuma *,a,b a Graduate School of Engineering, Nagoya University, Nagoya , Japan. b Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto , Japan. S1

2 Figure S1. Synchrotron XRD pattern (wavelength = 0.69 Å) of NiFe 2 O 4 catalyst and Rietveld refinement using RIETAN-FP 1 (simulated NiFe 2 O 4 to form inverse spinel structure with Fd3m space group, R wp = 5.152, S = , R B = 1.805, R F = 1.044). From Rietveld refinement, it was confirmed that the synthesized NiFe 2 O 4 formed a single spinel phase. S2

3 (A) (B) Figure S2. Temperature dependence of (A) C 3 H 6 and (B) CO oxidation conversion in a flow of each stoichiometric gas mix over NiFe 2 O 4. C 3 H 6 conversions under each condition were almost the same. CO conversion decreased in the presence of C 3 H 6. S3

4 Figure S3. Dependence of NO conversion on O 2 concentration over NiFe 2 O 4 at 325 C. (NO = 1000 ppm, and the C 3 H 6 concentration is a quarter of the CO concentration for balanced stoichiometry.) The broken line shows the NO conversion under standard TWC conditions. Decreasing the O 2 concentration resulted in slightly decreased NO conversion. S4

5 Figure S4. IR spectra in a flow of NO C 3 H 6 O 2, with or without 10 vol.% H 2 O over NiFe 2 O 4 at 300 C. S5

6 (A) (B) Figure S5. Dynamic changes of IR spectra in a flow of C 3 H 6 O 2 at 300 C over NiFe 2 O 4 at (A) cm 1 and (B) cm 1. S6

7 (A) (B) Figure S6. Dynamic changes of IR spectra in a flow of O 2 at 300 C over C 3 H 6 O 2 treated NiFe 2 O 4 at (A) cm 1 and (B) cm 1. S7

8 (A) (B) Figure S7. Dynamic changes of IR spectra in a flow of NO O 2 at 300 C over NiFe 2 O 4 at (A) cm 1 and (B) cm 1. S8

9 Figure S8. Concentration of adsorbed CH 3 COO as a function of time under each conditions. The slope indicated the reaction rate of CH 3 COO under each conditions. S9

10 Figure S9. Dynamic changes of IR spectra in a flow of NO O 2 at 300 C over CH 3 COO pre-adsorbed NiFe 2 O 4. S10

11 Figure S10. Dynamic changes of IR spectra in a flow of NO O 2 at 300 C over HCOO preadsorbed NiFe 2 O 4. S11

12 (A) (B) (C) Figure S11. Dynamic changes of IR spectra at (A) cm 1 and (B) cm 1 and (C) effluent gas composition of the in-situ IR cell in a flow of NO at 300 C over C 3 H 6 O 2 treated NiFe 2 O 4. S12

13 (A) (B) (C) Figure S12. Dynamic changes of IR spectra at (A) cm 1 and (B) cm 1 and (C) effluent gas composition of the in-situ IR cell in a flow of NO 2 at 300 C over C 3 H 6 O 2 treated NiFe 2 O 4. S13

14 Figure S13. CH 3 COO consumption rate dependent on O 2 concentration in the mixture of NO O 2 at 300 C over C 3 H 6 O 2 treated NiFe 2 O 4. A vertical line shows relative CH 3 COO consumption rate with respect to that under NO O 2 (O 2 = 4000ppm). S14

15 Figure S14. Dynamic changes of IR spectra in a flow of NO O 2 with 10 vol.% H 2 O at 300 C over C 3 H 6 O 2 treated NiFe 2 O 4. S15

16 Table S1. CH 3 COO consumption rate under various atmospheres at 300 C over NiFe 2 O 4. Atmosphere CH 3 COO consumption rate / µmols 1 NO O NO 0.38 O NO NO (with 10 vol.% H 2 O) 1.3 The addition of H 2 O decreased the CH 3 COO consumption rate, which leads to the lower NO reduction activity of NiFe 2 O 4 than that without H 2 O. 2 S16

17 0.76 Figure S15. Temperature dependence of the selectivity for the reaction of C 3 H 6 with O 2 in NO C 3 H 6 O 2. C 3 H 6 reacts with NOx or O 2 in NO C 3 H 6 O 2 as following equation: C 3 H 6 + NO + 4O 2 1/2N 2 + 3CO 2 + 3H 2 O (1) C 3 H 6 + 9/2O 2 3CO 2 + 3H 2 O (2) Considering NO only reacts with C 3 H 6 under stoichiometric conditions (NO C 3 H 6 4O 2 ), the NO reduction rate equals the C 3 H 6 reaction rate with NO(x). Therefore, the C 3 H 6 reaction rate of eq. (1) can be roughly estimated as follows: C 3 H 6 reaction rate of eq. (2) = C 3 H 6 reaction rate NO reduction rate We determined the selectivity for the C 3 H 6 reaction with O 2 in NO C 3 H 6 O 2 from the ratio of C 3 H 6 reaction rate of eq. (2) to that of eq. (1). The ratio of CH 3 COO consumption rate in O 2 to that in NO O 2 (0.76) was almost the same as the selectivity for the C 3 H 6 reaction with O 2 (0.76) at 300 C. These results suggested the selectivity for the reaction of CH 3 COO with NOx or O 2 determined the selectivity for the reaction of C 3 H 6 with NOx or O 2 under TWC conditions. S17

18 Figure S16. IR spectra in a flowing of NO C 3 H 6 CO O 2 on Rh(1wt.%)/Al 2 O 3. S18

19 Scheme S1. Assumed detailed reaction mechanism The detailed overall reaction mechanism might proceed as scheme S1 based on the preceding works. 3 7 Partial oxidization of C 3 H 6 to CH 3 COO over metal oxide surface proceeded via two type pathways, C 3 H 6 activation at C(1) and at C(2). 3,4 The former accompanies with the scission of the allylic C H bond and a subsequent oxygen insertion to give allyl alcoholate species, finally these species are oxidized to acetate species via acrylate. In the latter case, the alkene double bond of C 3 H 6 is electrophilically attacked by weakly Brønsted acidic OH groups according to Markovnikov rule and secondary propoxy species are generated. These species are oxidized to acetone and undergo oxidative breaking of the C C bond causes the formations of CH 3 COO and HCOO. In the subsequent CH 3 COO reaction step, the reaction between CH 3 COO and NO 2 leads to aci-anion of nitromethane (and CO 2 ), 5,6 which easily decomposes to NCO and H 2 O. 6,7 Finally, the NCO species reacted with HNO 2 to form N 2 via NH 3. 6,7 S19

20 Figure S17. Top view of the M Oh-Td terminated NiFe 2 O 4 (111) surface. M (Fe or Ni) ions are in red, O ions are in black, and O cus ions are in purple. NiFe 2 O 4 preferentially exposed (111) surface. 2 The density of cus oxygen for the (111) surface was 3.31 nm -2. Cus oxygen and neighboring M cation are required for the adsorption of bidentate chelating CH 3 COO. 3 The density of the adsorbed CH 3 COO on NiFe 2 O 4 in C 3 H 6 O 2 was 3.02 nm -2, which is very close to that of cus oxygen. Therefore, CH 3 COO species almost completely covered the M O site with an anion vacancy of NiFe 2 O 4 surface. S20

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