Supporting Information

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1 Supporting Information Wiley-VCH Weinheim, Germany Photocatalytic Conversion of Carbon Dioxide with Water to Methane: Platinum and Copper(I) Oxide Co-catalysts with a Core Shell Structure** Qingge Zhai, Shunji Xie, Wenqing Fan, Qinghong Zhang,* Yu Wang, Weiping Deng, and Ye Wang* anie_ _sm_miscellaneous_information.pdf

2 [Supporting Information] 1. Experimental Details (1) Preparation of Photocatalysts TiO 2 (P25), which contained 20% rutile and 80% anatase, was purchased from Degussa. The Cu/Pt/TiO 2 series of catalysts were prepared by a stepwise photodeposition technique. A Pt/TiO 2 was first prepared by the following procedures. A certain amount of TiO 2 was suspended in an aqueous solution of hexachloroplatinic acid (H 2 PtCl 6 ) with a concentration of mol dm -3. The suspension was irradiated with a 300 W Hg lamp for 1 h. The obtained sample was recovered by filtration, followed by washing repeatedly with deionized water and drying overnight in vacuum. Then, the obtained Pt/TiO 2 was dispersed in an aqueous solution of CuSO 4 (0.63 mmol dm -3 ). After evacuation, the suspension was exposed to irradiation with the 300 W Hg lamp to deposit Cu species onto the Pt/TiO 2. After a certain irradiation time, the obtained sample was washed thoroughly with deionized water and dried overnight in vacuum. The irradiation time was changed from 1 to 10 h to regulate the content of Cu in the final sample, which was denoted as Cu/Pt/TiO 2 xh. A Cu/TiO 2 was also prepared by the same procedure by using TiO 2 instead of the Pt/TiO 2. The irradiation time for the preparation of the Cu/TiO 2 was 5 h. A hydrazine-reduction method was employed for the preparation of TiO 2 -loaded Pt and Cu binary cocatalysts. [1] In brief, an aqueous solution of hydrazine was added into the mixed aqueous solution containing H 2 PtCl 6 and CuSO 4 with a small amount of cetyltrimethylammonium bromide. The mixture was stirred for 2 h. Then, TiO 2 was added into the mixture, and the suspension was further stirred for 2 h. The sample was recovered by filtration, washing thoroughly with deionized water and drying overnight in vacuum. (2) Characterization The photocatalysts were characterized by Powder X-ray diffraction (XRD), N 2 physisorption, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and high-sensitivity low-energy ion scattering (HS-LEIS) techniques. XRD patterns were collected on a Panalytical X pert Pro diffractometer using Cu K α radiation (40 kv, 30 ma). N 2 physisorption was carried out with a Micromeritics Tristar 3020 surface area and porosimetry analyzer. TEM and high-resolution TEM (HRTEM) measurements were performed on a Tecnai F30 electron microscope (Phillips Analytical) operated at an acceleration voltage of 300 kv. XPS measurements were performed on a Quantum 2000 Scanning ESCA Microprobe (Physical Electronics) using Al K α radiation ( ev) as the X-ray source. The HS-LEIS measurements were carried out on an IonTOF Qtac100 low-energy ion scattering analyzer. 4 He + ions with a kinetic energy of 3 kev were applied at a low ion flux equal to 1325 pa cm -2, which was necessary to avoid the sputtering of surfaces. 20 Ne + ions with a kinetic energy of 5 kev were applied at a low ion flux equal to 445 pa cm -2. The scattering angle was 145. In situ CO-adsorbed FT-IR spectroscopic studies were carried out with a Nicolet 6700 instrument equipped with an MCT detector to determine the oxidation state of Cu species in the catalysts after the irradiation (without exposure to air). Transmission FT-IR spectra were recorded with a resolution of 4 cm -1. The sample was pressed into a self-supporting wafer and was placed in an in situ IR cell, where it could be treated directly under different conditions including irradiation with a Xe lamp. 1

3 Photoelectrochemical measurements were carried out with an Ivium CompactStat (Holland) using a standard threeelectrode cell with a working electrode, a Pt wire as the counter electrode, and an SCE electrode as the reference electrode. [2] A 0.5 M solution of Na 2 SO 4 was used as the electrolyte. The working electrode was prepared by cleaning an F-doped SnO 2 -coated glass (FTO glass, 1 cm 2 cm). The photocatalyst was dispersed in ethanol, and the suspension was added dropwise directly onto the FTO by microsyringe with a gentle stream of air to speed drying. The film was dried at 393 K for 1 h, and the typical surface density of the photocatalyst was 1 mg cm -2. (3) Photocatalytic Reaction Photocatalytic reduction of CO 2 in the presence of H 2 O was carried out in a stainless-steel reactor (volume, ~100 ml) with a quartz window on the top of the reactor (Figure S0). The light source was 200 W Xe lamp (Beijing Trusttech Co., Ltd.) at UV-vis (λ = nm). The photocatalytic reaction was performed in a gas (vapor)-solid heterogeneous reaction mode. As displayed in Figure S0, typically, 20 mg of solid catalyst was placed on a Teflon catalyst holder in the upper region of the reactor. Liquid water with a volume of 4 ml was pre-charged in the bottom of the reactor. It should be noted that the catalyst was not immersed into the liquid water. Instead, the catalyst was surrounded by H 2 O vapor and CO 2. The pressure of CO 2 was typically regulated to 0.2 MPa. The temperature of the reactor was kept at 323 K, and the vapor pressure of H 2 O was 12.3 kpa under such a circumstance. With this kind of reaction mode, the leaching of CuO x into the liquid phase was negligible. The photocatalytic reaction was typically performed for 4 h. The amounts of CO, CH 4 and H 2 formed were analyzed by gas chromatography. We adopted a flame ionization detector (FID) for quantifying the amounts of CO and CH 4 formed from CO 2 to ensure high sensitivities. After the effluents containing CO 2, CO and CH 4 were separated by a carbon molecular sieve (TDX-01) column, CO and CO 2 were further converted to CH 4 by a methanation reactor, and were then analyzed by the FID detector. The detection limits of our analytic method for CH 4 and CO were both 2 nmol (0.002 µmol). H 2 was analyzed by an Agilent Micro 3000-GC equipped with a Molecular Sieve 5A column and a high-sensitivity thermal conductivity detector (TCD). Ar was used as the carrier gas. We performed the same experiment (including both reaction and detection) for at least 3 times for each catalyst, and the relative error was <5%. Liquid products such as CH 3 OH, HCHO and HCOOH possibly formed and dissolved in water were also analyzed carefully by gas chromatography or high-performance liquid chromatography, but no such products were detected under our reaction conditions. We observed the formation of O 2 during the photocatalytic reduction of CO 2 with H 2 O. The amount of O 2 formed was also quantified for several catalysts using an Agilent Micro 3000-GC equipped with 5A column and TCD detector. For example, over the Cu/Pt/TiO 2-5h catalyst, the rate of O 2 formation was 97 µmol g -1 h -1 under the following reaction conditions: photocatalyst, g; CO 2 pressure, 0.2 MPa; H 2 O, 4.0 ml; light source, nm; irradiation time, 4 h. On the other hand, from the rates of the formations of reductive products including H 2, CO and CH 4, we can calculate the stoichiometric formation rates of O 2 by assuming that the oxidation of water to O 2 is the sole reaction to consume the photogenerated holes. The result is as follows: stoichiometric rate of O 2 formation = [(H 2 formation rate)/2 + (CO formation rate)/2 + 2 (CH 4 formation rate)] 83 µmol g -1 h -1. Considering that the actual rate of O 2 formation is similar to the stoichiometric rate, we propose that the following reactions mainly occur over our photocatalysts: CO 2 + 2H + + 2e CO + H 2 O (1) CO 2 + 8H + + 8e CH 4 + 2H 2 O (2) 2H + + 2e H 2 (3) 2

4 H 2 O + 2h + 1/2O 2 + 2H + (4) Considering that the reduction of H 2 O to H 2 is competitive with the reduction of CO 2 to CO and CH 4, we have evaluated the selectivity for CO 2 reduction on an electron basis using the follow equation: Selectivity (%) = [2n(CO) + 8n(CH 4 )]/[2n(CO) +8n(CH 4 ) + 2n(H 2 )] 100% (5) where n(co), n(ch 4 ) and n(h 2 ) are the amounts (moles) of CO, CH 4, and H 2 formed. Figure S0. Reactor used for photocatalytic reduction of CO 2 in the presence of H 2 O. 1: CO 2 gas cylinder, 2: pressure digital meter, 3: stainless steel reactor, 4: O-ring quartz window, 5: Xe lamp, 6: magnetic stirrer, V 1 and V 2 : valves. 2. TEM Micrograph and Pt Particle Size Distribution for Pt/TiO 2 Figure S1 shows the typical TEM image and the corresponding Pt particle size distribution for the 0.89 wt% Pt/TiO 2 catalyst prepared by the photodeposition method. The mean size of Pt nanoparticles was evaluated to be 3.1 nm by counting ~100 Pt particles. Figure S1. TEM micrograph and Pt particle size distribution for the 0.89 wt% Pt/TiO Cu Contents in the Cu/Pt/TiO 2 xh Series of Catalysts Table S1 shows the contents of Pt and Cu in the Cu/Pt/TiO 2 xh series of catalysts prepared by changing the irradiation time for photodeposition of Cu species. The contents of Pt in these catalysts were ~0.9 wt%. The increase in the irradiation time for the deposition of Cu species from 1 to 10 h increased the content of Cu from 0.59 to 3.0 wt%. 3

5 Table S1: Contents of Pt and Cu in the Cu/Pt/TiO 2 xh series of catalysts. [a] Photocatalyst Pt content [wt%] Cu content [wt%] Pt/TiO Cu/TiO Cu/Pt/TiO 2 1h Cu/Pt/TiO 2 2h Cu/Pt/TiO 2 5h Cu/Pt/TiO 2 10h Pt Cu/TiO [a] Measured by ICP-MS. 4. HRTEM Micrograph for the Pt Cu/TiO 2 Catalyst Figure S2 shows the typical HRTEM image for the Pt Cu/TiO 2 catalyst prepared by the hydrazine-reduction method. Figure S2. HRTEM micrograph for the Pt Cu/TiO 2 catalyst prepared by the hydrazine-reduction method. 5. XRD Patterns Figure S3A shows the XRD patterns for TiO 2, Pt/TiO 2, Cu/TiO 2, Cu/Pt/TiO 2 5h, and Pt Cu/TiO 2 catalysts. Figure S3B shows the XRD patterns for the Cu/Pt/TiO 2 xh series of catalysts together with TiO 2. Only diffraction lines ascribed to TiO 2 (including both anatase and rutile phases) can be observed for each sample. 4

6 Figure S3. XRD patterns. (A) TiO 2, Pt/TiO 2, Cu/TiO 2, Cu/Pt/TiO 2 5h, and Pt Cu/TiO 2. (B) Cu/Pt/TiO 2 xh and TiO Pt 4f XPS Spectra Figure S4 shows the Pt 4f XPS spectra for Pt/TiO 2, Cu/Pt/TiO 2 5h, and Pt Cu/TiO 2 catalysts. The binding energies of Pt 4f 7/2 for these samples were ~70.9 ev. Figure S4. Pt 4f XPS spectra for Pt/TiO 2, Cu/Pt/TiO 2 5h, and Pt Cu/TiO 2 catalysts. 7. Cu 2p XPS Spectra and Cu L 3 VV Auger Spectra Figure S5 shows the Cu 2p XPS and Cu L 3 VV Auger spectra for the Cu/TiO 2, Cu/Pt/TiO 2 5h, and Pt Cu/TiO 2 catalysts. The binding energy values of Cu 2p 3/2 were ~932.5 ev. The positions of the Cu L 3 VV Auger lines for these samples were centered at ev. These observations suggest that the Cu species in these catalysts are in the state of Cu 2 O. 5

7 Figure S5. (A) Cu 2p XPS spectra and (B) Cu L 3 VV Auger spectra for the Cu/TiO 2, Cu/Pt/TiO 2 5h, and Pt Cu/TiO 2 catalysts. 8. CO-adsorbed FT-IR Spectra for the Cu/TiO 2, Cu/Pt/TiO 2, and Pt Cu/TiO 2 Catalysts after the Irradiation In situ CO-adsorbed FT-IR spectroscopic studies were carried out to gain information about the oxidation state of Cu species in the Cu/TiO 2, Cu/Pt/TiO 2 5h, and Pt Cu/TiO 2 catalysts after the irradiation with Xe lamp (λ = nm) in the in situ IR cell. It is known that CO can be adsorbed strongly onto Cu I sites, whereas the adsorption of CO onto Cu 0 sites is weaker at room temperature. [3,4] We first studied the IR bands of CO adsorbed on Cu I, Cu 0, and Pt 0 sites supported on TiO 2 using reference samples including Cu 2 O/TiO 2, Cu/TiO 2, and Pt/TiO 2. The Pt/TiO 2 was prepared by the photodeposition, while the reference Cu/TiO 2 containing Cu 0 was prepared by reducing the Cu/TiO 2 obtained by photodeposition with H 2 at 773 K. The Cu 2 O/TiO 2 was prepared by a procedure established in our laboratory. [5] We confirmed that the positions of IR bands for CO adsorbed on Cu I, Cu 0, and Pt 0 sites were different, and were located at 2105 cm -1 (strong), 2093 cm -1 (weak), 2096 cm -1 (medium), respectively (Figure S6). We then performed in situ CO-adsorbed FT-IR studies for our Cu/TiO 2, Cu/Pt/TiO 2 5h, and Cu Pt/TiO 2 catalysts after irradiation with Xe lamp (λ = nm) for 3 h in the in situ IR cell. The IR spectra of CO adsorbed on all the three catalysts showed a strong absorption band at 2105 cm -1 (Figure S6). This demonstrates that the Cu species in these catalysts are in Cu I state. The Cu Pt/TiO 2 catalyst exhibited a weak shoulder band at 2096 cm -1, which could likely be ascribed to the CO adsorbed on the Pt sites on its surface. Therefore, we conclude that the Cu species in our catalysts are in the state of Cu I or Cu 2 O. 6

8 Figure S6. CO-adsorbed FT-IR spectra. (a), (b), and (c): reference samples of Pt/TiO 2, Cu 0 /TiO 2, and Cu I /TiO 2. (d), (e), and (f): Cu/TiO 2, Cu/Pt/TiO 2 5h, and Pt Cu/TiO 2 catalysts after the irradiation with Xe lamp for 3 h in the in situ IR cell. 9. HS-LEIS Spectra for the Cu/Pt/TiO 2 xh Series of Catalysts Figure S7 shows the HS-LEIS spectra for the Cu/Pt/TiO 2 xh series of catalysts and the Pt/TiO 2 catalyst. The signals in the HS-LEIS spectra reflect the elements on the outmost layer of the sample surface. Figure S7 demonstrates that the increase in the irradiation time for photodeposition of Cu species onto the Pt/TiO 2 increases the signal for Cu and decreases that for Pt. The signal for Pt disappears as the irradiation time reaches 5 h. This provides further evidence for the formation of the core-shell structure of Pt@Cu 2 O in the Cu/Pt/TiO 2 5h and Cu/Pt/TiO 2 10h catalysts. Figure S7. HS-LEIS spectra. (A) 3 kev 4 He +, (B) 5 kev 20 Ne +. Catalysts: (a) Pt/TiO 2, (b) Cu/Pt/TiO 2 1h, (c) Cu/Pt/TiO 2 2h, (d) Cu/Pt/TiO 2 5h, (e) Cu/Pt/TiO 2 10h. References [1] W. Wang, X. Tian, K. Chen, G. Cao, Colloids Surf. A: Physicochem. Eng. Aspects 2006, 273, [2] A. Ye, W. Fan, Q. Zhang, W. Deng, Y. Wang, Catal. Sci. Technol. 2012, 2,

9 [3] K. Hadjiivanov, H. Knöinger, Phys. Chem. Chem. Phys. 2001, 3, [4] K. I. Hadjiivanov, G. N. Vayssilov, Adv. Catal. 2002, 47, [5] J. He, Q. Zhai, Q. Zhang, W. Deng, Y. Wang, J. Catal. 2013, 299,