Reversed selectivity of photocatalytic CO2 reduction over metallic Pt and Pt(ii) oxide cocatalysts

Abstract

The chemical state of Pt in cocatalysts has a major influence on the activity and selectivity of the photocatalytic reduction of CO₂; however, the underlying mechanism is unclear owing to the co-existence of different Pt chemical states and mutual transformation between them. In this study, PtO/TiO₂ catalysts were prepared through photodeposition and Pt/TiO₂ was prepared by the photoreduction of PtO/TiO₂ to avoid interference arising from co-existing Pt forms and different loading amounts. These catalysts exhibited completely reversed selectivity for CO and CH₄ production during CO₂ photoreduction: PtO/TiO₂ tended to produce CO (100%), whereas Pt/TiO₂ favored the production of CH₄ (66.6%). By combining experimental analysis and theoretical calculations, the difference in selectivity was ascribed to the different charge transfer/separation and CO/H adsorption properties of PtO/TiO₂ and Pt/TiO₂. Photoelectric and photoluminescence (PL) analysis showed that Pt was more advantageous to the photogenerated carrier separation compared with PtO, which was conducive to the multi-electron CH₄ reduction reaction. Fourier transform-infrared spectroscopy, temperature-programmed desorption/temperature-programmed reduction, and density functional theory calculations indicated that the adsorption of CO and hydrogen on Pt was stronger than that on PtO, which favored the further reduction of CO to CH₄. Based on the above results, a mechanism was proposed to explain the reversed selectivity of the photocatalytic reduction of CO₂ over Pt/TiO₂ and PtO/TiO₂.