Size-dependent activity and selectivity of carbon dioxide photocatalytic reduction over platinum nanoparticles

ARTICLE DOI: 10.1038/s41467-018-03666-2 OPEN Size-dependent activity and selectivity of carbon dioxide photocatalytic reduction over platinum nanoparticles 1234567890():,; Chunyang Dong1, Cheng Lian1, Songchang Hu1, Zesheng Deng1, Jianqiu Gong1, Mingde Li2, Honglai Liu1, Mingyang Xing 1 & Jinlong Zhang 1 Platinum nanoparticles (Pt NPs) are one of the most efficient cocatalysts in photocatalysis, and their size determines the activity and the selectivity of the catalytic reaction. Nevertheless, an in-depth understanding of the platinum’s size effect in the carbon dioxide photocatalytic reduction is still lacking. Through analyses of the geometric features and electronic properties with variable-sized Pt NPs, here we show a prominent size effect of Pt NPs in both the activity and selectivity of carbon dioxide photocatalytic reduction. Decreasing the size of Pt NPs promotes the charge transfer efficiency, and thus enhances both the carbon dioxide photocatalytic reduction and hydrogen evolution reaction (HER) activity, but leads to higher selectivity towards hydrogen over methane. Combining experimental results and theoretical calculations, in Pt NPs, the terrace sites are revealed as the active sites for methane generation; meanwhile, the low-coordinated sites are more favorable in the competing HER. 1 Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry & Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P. R. China. 2 Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University, Shantou 515063, P. R. China. These authors contributed equally: Chunyang Dong, Cheng Lian. Correspondence and requests for materials should be addressed to M.X. (email: ) NATURE COMMUNICATIONS | (2018)9:1252 | DOI: 10.1038/s41467-018-03666-2 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-03666-2 T he excessive emission of CO2, one of the most common green-house gases, has caused global warming, which in turn gives rise to severe environmental problems. Thus, it is clearly urgent to find possible solutions to address this problem1. On the basis of semiconductor photocatalysis technology, CO2 and H2O can be converted into some useful chemicals, such as CH4, under solar light irradiation, which appears to be an ideal approach to address the above-mentioned problematic CO2 emissions. However, since CO2 is a stable molecule, its photocatalytic reduction (CO2PR) with H2O yielding CH4 is an endothermic reaction and requires the participation of multiple electrons and protons. Hence, without modification, bare semiconductors often show poor CO2PR activity2, 3. Furthermore, the hydrogen evolution reaction (HER) often competes with the CO2PR for the photo-generated electrons and leads to lower CO2PR selectivity2, 4–6. According to previous studies, after loaded with proper cocatalysts, the performance of the photocatalyst composite can be significantly enhanced in CO2PR7, 8. As one of the mostly widely used cocatalyst, platinum nanoparticles (Pt NPs) are always considered as the electron trapping agents and active sites in photocatalysis9–11. Wang et al.12 adopted the tiltedtarget sputter method for the ultrafine Pt cluster preparation (0.5–2.0 nm) and found that 1.0-nm Pt loaded composite showed the highest CH4 yield due to optimal electron trapping ability; Xie et al.13 also studied the size effect of Pt NPs with different preparation methods and found that smaller Pt NPs showed better performance in CO2PR. Our previous preliminary work has found that the particle size of Pt affects both the activity and selectivity in CO2PR8. Although much effort has been devoted to explaining the roles and properties of Pt in photocatalysis, there is still a lack of objective and systematic studies on the size effect of Pt NPs with a wide size range and precise size distribution in CO2PR, especially for the deep understanding of the active sites on the Pt NPs surface. Generally, the electronic or geometric structures are altered when the particle size of Pt NPs is tuned. With the decrease in particle size, the proportion of the low-coordinated surface sites (corner or edge) increased14, 15. In addition, the metal oxidation states also exhibited size-dependent properties16, 17. Therefore, a good understanding of the size-dependent activity of Pt has been demonstrated in many catalytic reactions, such as ammonia borane dehydrogenation14, 18, regioselective hydrogenation of quinolone16, and the oxygen reduction reaction15, among others. Recently, size-dependent activity and selectivity in the CO2 electrocatalytic reduction reaction (CO2RR) have been reported for metal NPs, such as Pd, Au and Cu, due to the intrinsic free energy of key intermediate evolutions on different surface sites19– 21. Inspired by these works, we attempt to explore the size effect of Pt NPs in CO2PR by studying the diversity of electronic features and geometric properties of Pt NPs. Traditional size control methods of Pt NPs often involve variation of the amount of the Pt precursor or post deposition of presynthesized colloidal Pt NPs with controllable size on the support12, 14, 22, 23. However, neither of these approaches is ideal for CO2PR. For photocatalysis, loading with different amounts of Pt would cause the imbalance of light absorption of the supporting semiconductor24, 25; meanwhile, it also cannot tune the size of Pt NPs precisely. Smaller Pt NPs may still exist with excessive Pt loadings, and the size effect may not be reflected accurately. On the other hand, for colloidal Pt, polyvinyl pyrrolidone (PVP) was often introduced as the stabilizing agent in order to prevent the Pt NPs from aggregation; it is then difficult to remove PVP completely, and this may affect the performance of the Pt NPs26, 27. Therefore, two aspects of the objective investigation of the Pt size effect are challenging: precise control of the size of Pt NPs under the assumption of a constant loading 2 NATURE COMMUNICATIONS | (2018)9:1252 amount, and exclusion of the negative influence of the carbon impurities induced by the use of PVP during the preparation. In this work, various Pt NPs supported on hierarchically ordered TiO2–SiO2 porous materials (HTSO) are synthesized via a facile acid–base-mediated alcohol reduction (ABAR) method. With the increase in the Pt NPs size, different samples are denoted as xPHTSO (where x represents the Pt particle sizes of 1.8, 3.4, 4.3, and 7.0 nm). The variation of the geometric features and electronic properties with the size variation is carefully characterized. The size effect of Pt NPs is found to exist for both the activity and selectivity of CO2PR. Smaller Pt NPs favor charge transfer and show superior performance in both CO2PR and HER; a higher CH4 selectivity is achieved by the larger Pt NP (...truncated)