Supporting Information Insights into Interfacial Synergistic Catalysis over Ni@TiO2-x Catalyst toward Water-Gas Shift Reaction Ming Xu, 1 Siyu Yao, 2 Deming Rao, 1 Yiming Niu, 3 Ning Liu, 1 Mi Peng, 2 Peng Zhai, 2 Yi Man, 4 Lirong Zheng, 5 Bin Wang,* 4 Bingsen Zhang,* 3 Ding Ma,* 2 Min Wei* 1 1 State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China 2 College of Chemistry and Molecular Engineering and College of Engineering, BIC-ESAT, Peking University, Beijing 100871, P. R. China 3 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P. R. China 4 Beijing Research Institute of Chemical Industry, Sinopec Group, Beijing 100013, P. R. China 5 Institute of High Energy Physics, the Chinese Academy of Sciences, Beijing 100049, P. R. China * Corresponding authors. Tel: +86-10-64412131; Fax: +86-10-64425385. E-mail addresses: wangbin.bjhy@sinopec.com (B. Wang); bszhang@imr.ac.cn (B. Zhang); dma@pku.edu.cn (D. Ma); weimin@mail.buct.edu.cn (M. Wei). S1
1. Experimental section 1.1. XAS characterization. In situ extend X-ray absorption fine structure spectroscopy (EXAFS) at the Ni and Ti K-edge was performed at the beamline 1W1B of the Beijing Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics (IHEP), Chinese Academy of Sciences (CAS). The typical energy of the storage ring was 2.5 GeV with a maximum current of 250 ma. The Si (111) double crystal monochromator was used. The IFFEFIT package was performed for the data analysis and fitting. The fitting was carried out in R-space after Fourier transformation of the k 2 -weighted data with the k-range from 2.7 to 12.5 Å. The powdered sample was firstly pressed into sheet and loaded into a reactor cell equipped with polyimide windows. Before the measurement on Ni K-edge and Ti K-edge EXAFS spectra of Ni@TiO2-x(450) catalyst, all the samples were pre-reduced in H2 atmosphere at 450 C for 1 h. The detailed experiment procedure is described as follows: (1) In situ Ni K-edge spectra of Ni@TiO2-x(450) catalyst in CO atmosphere were carried out in an in situ reaction cell. The powdered sample was pressed into sheet and loaded into a reactor cell equipped with polyimide windows. Then, the sample was reduced in 5% H2 (He, balance) at 450 C for 1 h at a heating rate of 5 C min 1 via a precise temperature controller, and subsequently flushed with He for 30 min. When the temperature was cooled to 150 C, CO atmosphere was switched into the in situ reaction cell, and the Ni K-edge EXAFS spectra were collected within 1 h. In addition, the Ni K-edge EXAFS spectra in CO atmosphere were performed with a similar procedure at 250 C, 350 C and 450 C, respectively. (2) Operando Ni K-edge spectra of Ni@TiO2-x(450) catalyst under reaction conditions were performed in an in situ reaction cell. The powdered sample was pressed into sheet and loaded S2
into a reactor cell equipped with polyimide windows. Then, the sample was reduced in 5% H2 (He, balance) at 450 C for 1 h at a heating rate of 5 C min 1 via a precise temperature controller, and subsequently flushed with He for 30 min. When the temperature was cooled to 150 C, a mixture of CO, H2O and He (molar composition: 6% CO, 24% H2O and 70% He) was introduced into the reactor with a total flux of 100 ml min 1, and the Ni K-edge EXAFS spectra were collected under stable operando conditions for 1 h. The Ni K-edge EXAFS spectra of Ni@TiO2-x(450) catalyst in CO+H2O atmosphere were further collected with a similar procedure at 250 C, 350 C and 450 C, respectively. (3) In situ Ni K-edge spectra of Ni@TiO2-x(450) catalyst in H2O atmosphere were carried out in an in situ reaction cell. The powdered sample was pressed into sheet and loaded into a reactor cell equipped with polyimide windows. Then, the sample was reduced in 5% H2 (He, balance) at 450 C for 1 h at a heating rate of 5 C min 1 via a precise temperature controller, and subsequently flushed with He for 30 min. When the temperature was cooled to 150 C, H2O atmosphere was switched into the in situ reaction cell via a HPLC pump, and the Ni K-edge EXAFS spectra were collected within 1 h. The Ni K-edge EXAFS spectra in H2O atmosphere were further performed with a similar procedure at 250 C, 350 C and 450 C, respectively. (4) In situ Ni K-edge spectra of Ni particles in H2O atmosphere were carried out in an in situ reaction cell. The powdered Ni sample was pressed into sheet and loaded into a reactor cell equipped with polyimide windows. Then, the sample was reduced in 5% H2 (He, balance) at 450 C for 1 h at a heating rate of 5 C min 1 via a precise temperature controller, and subsequently flushed with He for 30 min. When the temperature was cooled to 150 C, H2O S3
atmosphere was switched into the in situ reaction cell via a HPLC pump, and the Ni K-edge EXAFS spectra were collected within 1 h. The Ni K-edge EXAFS spectra in H2O atmosphere were further performed with a similar procedure at 250 C, 350 C and 450 C, respectively. 1.2. DRIFTS experiments. In situ diffuse reflectance Fourier transform infrared spectroscopy (in situ DRIFTS) was performed in an in situ reaction cell on a VERTEX 70 spectrometer equipped with a MCT narrow-band detector, with a resolution of 4 cm 1. The detailed pre-treatment and test conditions are given as the follows: (1) In situ DRIFTS spectra were performed to further explore the electronic structure of Ni δ+ after H2O dissociation on the surface of Ni@TiO2-x(450), by using CO as a probe molecule. The powdered sample was filled into the reactor, pre-reduced in 5% H2 (He balance) at 450 C for 1 h and flushed with high purity He for 30 min. When the temperature was cooled to 25 C, 5% CO (He balance) was induced to the reactor for adsorption on the surface of fresh Ni@TiO2-x(450) catalyst for 30 min, and followed by flushing with He for another 30 min (as shown in Figure 3a). The fresh Ni@TiO2-x(450) sample (after reduction at 450 C mentioned above) was slowly cooled to 150 C, 200 C, 250 C, 300 C, 350 C, and 450 C, respectively, and H2O vapor was introduced for 30 min and subsequently flushed with He for another 30 min. Afterwards, the result sample was slowly cooled to 25 C, and then 5% CO (He balance) atmosphere was introduced into the reactor. The DRIFTS spectra were collected after CO adsorption on the surface of Ni@TiO2-x(450) for 30 min and subsequently flushed with He for another 30 min at 25 C (as shown in Figure 3b h). S4
(2) In situ time-resolved DRIFTS spectra were carried out to directly observe the catalytic performance of active sites (interfacial sites) under operando condition (as shown in Figure 5). The powdered sample was filled into the reactor, pre-reduced in 5% H2 (He balance) at 450 C for 1 h and flushed with high purity He for 30 min. Then, the sample was slowly cooled to 150 C, 200 C, 350 C, respectively. Afterwards, 5% CO (He balance) atmosphere was induced into the reactor for adsorption on the surface of catalyst for 30 min, followed by subsequent flushing with He for 30 min. Finally, H2O vapor was induced into the reactor, and time-resolved DRIFTS spectra were collected using a 64-scan quick sweep mode with a resolution of 4 cm 1. The time interval is 10 s between two collected spectra as shown in Figure 5 and Figure S14 S19. (3) In situ DRIFTS spectra were performed to reveal the CO adsorbate-induced surface reconstruction of Ni@TiO2-x(450) catalyst (Figure S3). The powdered sample was filled into the reactor, pre-reduced in 5% H2 (He balance) at 450 C for 1 h and flushed with high purity He for 30 min. Then, 5% CO (He balance) was introduced at 25 C, 150 C, and 350 C, respectively, for 30 min. Afterwards, DRIFTS spectra were collected after flushing with He for 30 min. 1.3. H2O TPD-MASS and H2O Pulse-MASS experiments. H2O TPD-MASS and H2O Pulse-MASS measurements were performed on a Micromeritics ChemiSorb 2070 with a thermal conductivity detector (TCD). The detailed experimental process is given as follows: In a typical process, 100 mg of sample was sealed in a quartz tube reactor and pretreated in a Ar atmosphere at 200 C for 2 h, followed by reduction in a stream of H2/Ar (1:9, v/v; a total flow rate of 40 ml min 1 ) at 450 C for 1 h at a heating rate of 5 C min 1 and flushed with S5
high purity He for 30 min. Then, the sample was slowly cooled to 120 C, and H2O vapor was induced into the reactor. After upon exposure in H2O atmosphere for 15 min and flushed with He for another 15 min, the temperature was increased from 120 C to 500 C at a heating rate of 5 C min 1 (as shown in Figure 4a). Subsequently, H2O Pulse-Mass experiments were carried out to further investigate the process of H2O dissociation on the surface of Ni@TiO2-x(450) catalyst. The sample was reduced in H2 atmosphere and flushed with He for 30 min, followed by pulsing water with a time interval of 3 min (cycle times: 30) at 150 ºC, 200 ºC, 250 ºC, 350 ºC, and 450 ºC, respectively. The Mass signals for H2 and H2O were collected during the water pulsing (as shown in Figure 4b f). 1.4. TPSR-MASS and cycling tests of H2O and CO Pulse-MASS experiments. The temperature-programed surface reaction (TPSR) and cycling test of CO and H2O Pulse-MASS were carried out to further explore the reaction mechanism of WGS over the Ni@TiO2-x(450) catalyst. All the measurements were carried out on a Micromeritics ChemiSorb 2070 with a thermal conductivity detector (TCD). The detailed pre-treatment conditions, test conditions and experimental process are given as follows: In a typical process, 100 mg of sample was sealed in a quartz tube reactor and pretreated in a Ar atmosphere at 200 C for 2 h, followed by reduction in a stream of H2/Ar (1:9, v/v; a total flow rate of 40 ml min 1 ) at 450 C for 1 h at a heating rate of 5 C min 1 and flushed with High purity He for 30 min. The sample was slowly cooled to 120 C in the He atmosphere. Then, a mixture of CO, H2O and Ar (molar composition: 6% CO, 24% H2O and 70% Ar; Ar as an internal standard) was introduced into the reactor with a total flux of 100 ml min 1. The Mass signals of CO, H2O, CO2, and H2 were collected within the temperature range from 120 S6
ºC to 450 C at a heating rate of 5 C min 1 (as shown in Figure S20). The H2O and CO cycling Pulse-Mass experiments were carried out to further reveal the reaction mechanism. After the sample was reduced in H2 atmosphere and flushed with He for 30 min, the temperature was slowly cooled to 200 C, 250 C, 350 C, and 450 C, respectively. Subsequently, water pulses were induced into the reactor for 15 20 cycles with time interval of 2 min, followed by the introduction of CO pulses for 15 20 cycles at different temperatures. The cycle pulse atmosphere was H2O, CO, H2O, CO in sequence, respectively. S7
Figure S1. (a) HAADF-STEM image of the Ni@TiO2-x(450) catalyst, (b) EDS line scan profile of Ni-K (black), Ti-K (red) and O-K (blue) along the red line in (a). Figure S2. (a) In situ XPS Ni2p and (b) In situ XPS Ti2p of the Ni@TiO2-x(450) catalyst. (c) XAS spectra of Ni L-edge for Ni@TiO2-x(450) and the reference Ni foil. (d) XAS spectra of Ti L-edge for Ni@TiO2-x(450) and the reference Anatase. S8
Figure S3. In situ DRIFTS spectra of CO adsorption on the Ni@TiO2-x(450) catalyst recorded in 2250 1725 cm 1 at 25 C, 150 C, and 350 C, respectively, after flushing with He for 30 min. Figure S4. (a) Top side view of Ni(111); CO adsorption on (b) the top sites of Ni(111) and (c) the hollow sites of Ni(111). (d) Top side view of Ni(111)@TiO2 x(101); CO adsorption on (e) the top sites of Ni(111)@TiO2 x(101) and (f) the hollow sites of Ni(111)@TiO2 x(101). S9
Figure S5. (a) TEM image of the 20% Ni/Al2O3 sample. In situ DRIFTS spectra of CO adsorption on the 20% Ni/Al2O3 sample recorded in 2250 1725 cm 1 at (b) 25 C, (c) 50 C, and (d) 150 C, respectively, during the flushing process with He atmosphere within 30 min. Figure S6. Fourier-transform EXAFS spectra and fitting at Ni K-edge for the reference sample of: (a) Ni-foil, (b) NiO sample at 25 C (the dashed line: experimental data; the solid line: fitting curve). S10
Figure S7. Fourier-transform EXAFS spectra and fitting at Ni K-edge for the Ni@TiO2-x(450) catalyst after reduction in 5% H2 (He balance) at 450 C for 1 h (the dashed line: experimental data; the solid line: fitting curve). S11
Figure S8. Fourier-transform EXAFS spectra and fitting at Ni K-edge for the Ni@TiO2-x(450) catalyst in CO atmosphere at: (a) 150 C, (b) 250 C, (c) 350 C and (d) 450 C, respectively. (The dashed line: experimental data; the solid line: fitting curve). S12
Figure S9. Normalized XANES spectra at Ni K-edge for the Ni@TiO2-x(450) catalyst in: (a1) H2 atmosphere, (b1) He atmosphere at 150 C, 250 C, 350 C, 450 C, respectively. Fourier-transform EXAFS spectra at Ni K-edge for the Ni@TiO2-x(450) catalyst in: (a2) H2 atmosphere, (b2) He atmosphere at 150 C, 250 C, 350 C, 450 C, respectively. S13
Figure S10. Fourier-transform EXAFS spectra and fitting at Ni K-edge for Ni@TiO2-x(450) catalyst in CO+H2O atmosphere (operando conditions) at: (a) 150 C, (b) 250 C, (c) 350 C and (d) 450 C, respectively (the dashed line: experimental data; the solid line: fitting curve). S14
Figure S11. Fourier-transform EXAFS spectra and fitting at Ni K-edge for the Ni@TiO2-x(450) catalyst in H2O atmosphere at: (a) 150 C, (b) 250 C, (c) 350 C and (d) 450 C, respectively (the dashed line: experimental data; the solid line: fitting curve). S15
Figure S12. (a) HRTEM image of the powdered Ni sample (30 50 nm), (b) HRTEM image of one single Ni particle, (c) enlarged HRTEM image of the box dashed line area in (b), (d) selected electron diffraction image of (c). S16
Figure S13. (A) Normalized XANES spectra at Ni K-edge for (a) Ni foil at 25 C, the powdered Ni sample (after reduced in 5% H2 atmosphere at 450 C for 1 h in H2O atmosphere at (b) 150 C, (c) 250 C, (d) 350 C, (e) 450 C, respectively, and (f) the fresh powdered Ni sample. (B) Fourier-transform EXAFS spectra at Ni K-edge for (a) Ni foil at 25 C, the powdered Ni sample in H2O atmosphere at (b) 150 C, (c) 250 C, (d) 350 C, (e) 450 C, respectively, and (f) the fresh powdered Ni sample. Figure S14. In situ DRIFTS spectra of: (a) CO adsorbed on the Ni@TiO2-x(450) catalyst recorded in 2200 1700 cm 1 within the temperature range 30 200 C, (b) flushing with He for 30 min. S17
Figure S15. In situ time-resolved DRIFTS spectra of CO adsorption on Ni@TiO2-x(450) catalyst recorded in 3000 2800 cm 1 upon exposure to H2O atmosphere at 200 C for various time. Figure S16. In situ DRIFTS spectra of CO adsorbed on the Ni@TiO2-x(450) catalyst recorded in 2200 1700 cm 1 at 150 C during the He flushing process for 30 min, after the adsorption of CO. S18
Figure S17. In situ DRIFTS spectra of CO adsorbed on the Ni@TiO2-x(450) catalyst recorded in (a) 3800 3500 cm 1, (b) 2400 2300 cm 1, (c) 2200 1700 cm 1, (d) 1700 1200 cm 1 upon exposure in H2O atmosphere at 150 C for 3 h. Figure S18. In situ DRIFTS spectra of CO adsorbed on the Ni@TiO2-x(450) catalyst recorded in 2200 1700 cm 1 at 350 C during the He flushing process for 30 min, after the adsorption of CO. S19
Figure S19. In situ DRIFTS spectra of CO adsorbed on the Ni@TiO2-x(450) catalyst recorded in (a) 3950 3500 cm 1, (b) 2400 2300 cm 1, (c) 2200 1700 cm 1, (d) 1700 1200 cm 1 upon exposure in H2O atmosphere at 350 C for 120 s. S20
Figure S20. Spectra of CO+H2O TPSR within the temperature range from 125 C to 400 C. Figure S21. Mass signals of CO2 and H2 during alternate pulsing H2O and CO at (a) 200 C, (b) 250 C, (c) 350 C, (d) 450 C, respectively. S21
Table S1. Bader charge analysis of Ni(111) surface and Ni@TiO2 x interface before and after CO adsorption Sample Atom Charge (free) Charge (after CO adsorption) Charge Ni(111) Ni a 2 10.0347 9.9372 0.0975 Ni(111)@TiO 2 x Ni a 2 10.0413 9.9246 0.1167 Ni 1 b 10.0157 9.9083 0.1074 Ni(111) Ni 2 b 10.0347 9.9199 0.1148 Ni 3 b 10.0569 9.9118 0.1451 Ni b 1 10.2996 10.0112 0.2844 Ni(111)@TiO 2 x b Ni 2 10.0413 9.9221 0.1192 b Ni 3 10.0710 9.8991 0.1719 a The Ni atom located at the Top site of Ni(111) surface or Ni@TiO 2 x interface, as shown in Figure S4. b The three Ni atoms located at the Hollow sites of Ni(111) surface or Ni@TiO 2 x interface, as shown in Figure S4. Table S2. EXAFS fitting parameters at the Ni K-edge for Ni@TiO2-x(450) in CO atmosphere at different temperatures Sample Shell N a R (Å) b σ 2 (Å 2 10 3 ) c ΔE 0 (ev) d R factor (%) Ni@TiO2-x (CO 150 C) Ni Ni (in metallic state Ni) 10.2(±0.6) 2.48(±0.01) 8.2(±0.5) 6.5 0.43 Ni@TiO2-x (CO 250 C) Ni Ni (in metallic state Ni) 10.1(±0.8) 2.48(±0.01) 9.9(±0.7) 6.4 0.59 Ni O 0.5(±0.5) 1.83(±0.02) 6.3(±0.4) 8.8 Ni@TiO2-x (CO 350 C) Ni Ni (in metallic state Ni) 4.9(±0.6) 2.49(±0.03) 11.6(±0.4) 2.3 Ni Ti (O-bridged ) 2.1(±0.4) 2.98(±0.01) 10.0(±0.6) 5.5 0.26 Ni@TiO2-x (CO 450 C) Ni Ni (in metallic state Ni) 9.9(±0.6) 2.48(±0.01) 13.2(±0.7) 5.0 0.31 Ni@TiO2-x (H2 450 C) Ni Ni (in metallic state Ni) 10.2(±0.8) 2.48(±0.01) 13.7(±0.9) 5.5 0.64 NiO reference (25 C ) Ni O 6.0(±0.3) 2.09(±0.01) 6.1(±0.6) Ni Ni (O-bridged) 11.6(±0.4) 2.95(±0.01) 6.4(±0.4) 4.0 0.06 Ni-foil (25 C ) Ni Ni (in metallic state Ni) 11.8(±0.4) 2.48(±0.01) 6.1(±0.3) 6.8 0.01 a N: coordination number; b R: bond distance; c σ 2 : Debye-Waller factor; d ΔE 0: the inner potential correction. R factor: goodness of fit. Ѕ 02, 0.912, was obtained from the experimental EXAFS fitting over NiO reference with known crystallographic value, which was then used to all these samples. S22
Table S3. EXAFS fitting parameters at the Ni K-edge for Ni@TiO2-x(450) in CO+H2O (operando conditions) at different temperatures Sample Shell N a R (Å) b σ 2 (Å 2 10 3 ) c ΔE 0 (ev) d R factor (%) Ni@TiO2-x (Operando 150 C) Ni Ni (in metallic state Ni) 9.7(±0.6) 2.48(±0.01) 10.8(±0.8) 5.7 0.12 Ni@TiO2-x (Operando 250 C) Ni Ni (in metallic state Ni) 9.7(±0.8) 2.48(±0.01) 10.9(±0.8) 5.7 0.12 Ni@TiO2-x (Operando 350 C) Ni Ni (in metallic state Ni) 10.3(±0.5) 2.49(±0.01) 12.9(±0.2) 7.1 0.25 Ni@TiO2-x (Operando 450 C) Ni Ni (in metallic state Ni) 9.2(±0.5) 2.48(±0.01) 14.0(±0.7) 4.8 0.68 Ni@TiO2-x (H2 450 C) Ni Ni (in metallic state Ni) 10.2(±0.8) 2.48(±0.01) 13.7(±0.9) 5.5 0.64 NiO reference (25 C ) Ni O 6.0(±0.3) 2.09(±0.01) 6.1(±0.6) Ni Ni (O-bridged) 11.6(±0.4) 2.95(±0.01) 6.4(±0.4) 4.0 0.06 Ni-foil (25 C ) Ni Ni (in metallic state Ni) 11.8(±0.4) 2.48(±0.01) 6.1(±0.3) 6.8 0.01 a N: coordination number; b R: bond distance; c σ 2 : Debye-Waller factor; d ΔE 0: the inner potential correction. R factor: goodness of fit. Ѕ 02, 0.912, was obtained from the experimental EXAFS fitting over NiO reference with known crystallographic value, which was then used to all the samples. Table S4. EXAFS fitting parameters at the Ni K-edge for Ni@TiO2-x(450) in H2O atmosphere at different temperatures Sample Shell N a R (Å) b σ 2 (Å 2 10 3 ) c ΔE 0 (ev) d R factor (%) Ni@TiO2-x (H2O 150 C ) Ni Ni (in metallic state Ni) 10.0(±1.2) 2.48(±0.01) 8.3(±1.2) 6.4 0.1 Ni@TiO2-x (H2O 250 C) Ni@TiO2-x (H2O 350 C) Ni@TiO2-x (H2O 450 C) Ni O 0.62(±0.6) 2.03(±0.05) 9.5(±0.5) 8.0 Ni Ni (in metallic state Ni) 10.1(±0.9) 2.48(±0.01) 10.7(±0.9) 6.8 Ni O 0.65(±0.5) 1.99(±0.07) 2.3(±0.7) 4.6 Ni Ni (in metallic state Ni) 8.65(±0.7) 2.49(±0.01) 12.2(±1.0) 7.7 Ni O 1.46(±0.9) 1.94(±0.05) 2.8(±0.2) 6.6 Ni Ni (in metallic state Ni) 4.0(±0.6) 2.55(±0.08) 10.5(±0.6) 4.7 Ni Ti(O-bridged ) 2.7(±0.9) 3.08(±0.1) 9.7(±1.2) 5.6 0.15 1.3 0.1 Ni@TiO2-x (H2 450 C) Ni Ni (in metallic state Ni) 10.2(±0.8) 2.48(±0.01) 13.7(±0.9) 5.5 0.64 NiO reference (25 C ) Ni O 6.0(±0.3) 2.09(±0.01) 6.1(±0.6) Ni Ni (O-bridged) 11.6(±0.4) 2.95(±0.01) 6.4(±0.4) 4.0 0.06 Ni-foil (25 C ) Ni Ni (in metallic state Ni) 11.8(±0.4) 2.48(±0.01) 6.1(±0.3) 6.8 0.01 a N: coordination number; b R: bond distance; c σ 2 : Debye-Waller factor; d ΔE 0: the inner potential correction. R factor: goodness of fit. Ѕ 02, 0.912, was obtained from the experimental EXAFS fitting over NiO reference with known crystallographic value, which was then used to all the samples. S23