Selective photocatalytic reduction of CO₂ by H₂O/H₂ to CH₄ and CH₃OH over Cu-promoted In₂O₃/TiO₂ nanocatalyst

Highlights: • Cu-promoted In₂O₃/TiO₂ nanocatalysts tested for CO₂ photoreduction with H₂O/H₂. • Production of CH₄ and CH₃OH depends on reductants type and metal-loading to TiO₂. • CH₄ production over Cu-In/TiO₂ was 1.5 fold more than In/TiO₂ and 5 times the TiO₂. • The Cu-promoted CH₃OH production while In gave more CH₄ with water vapors. • The H₂ reductant gave negative effect for CH₄ but enhanced CH₃OH production. - Abstract: Photocatalytic CO₂ reduction by H₂O and/or H₂ reductant to selective fuels over Cu-promoted In₂O₃/TiO₂ photocatalyst has been investigated. The samples, prepared via a simple and direct sol-gel method, were characterized by XRD, SEM, TEM, XPS, N₂ adsorption-desorption, UV–vis diffuse reflectance, Raman and PL spectroscopy. Cu and In loaded into TiO₂, oxidized as Cu²⁺ and In³⁺, promoted efficient separation of photo-generated electron/hole pairs (e⁻/h⁺). The results indicate that the reduction rate of CO₂ by H₂O to CH₄ approached to 181 μmol g⁻¹ h⁻¹ using 0.5% Cu-3% In₂O₃/TiO₂ catalyst, a 1.53 fold higher than the production rate over the 3% In₂O₃/TiO₂ and 5 times the amount produced over the pure TiO₂. In addition, Cu was found to promote efficient production of CH₃OH and yield rate reached to 68 μmol g⁻¹ h⁻¹ over 1% Cu-3% In₂O₃/TiO₂ catalyst. This improvement was attributed to charge transfer property and suppressed recombination rate by Cu-metal. More importantly, H₂ reductant was less favorable for CH₄ production, yet a significant amount of CH₄ and CH₃OH were obtained using a mixture of H₂O/H₂ reductant. Therefore, Cu-loaded In₂O₃/TiO₂ catalyst has shown to be capable for methanol production, whereas product selectivity was greatly depending on the amount of Cu-loading and the type of reductant. A photocatalytic reaction mechanism was proposed to understand the experimental results over the Cu-loaded In₂O₃/TiO₂ catalyst.