Experimental and mechanistic understanding of photo-oxidation of methanol catalyzed by CuO/TiO₂-spindle nanocomposite: Oxygen vacancy engineering

We report experimental and mechanistic understanding of methanol oxidation to produce methyl formate using CuO/TiO₂-spindle composite as a promising photocatalyst under mild conditions with over 97% conversion and 83% selectivity. The catalysts are obtained via precise depositing of CuO nanoclusters (size: ~ 3.5 nm) at the {101} facet of the TiO₂ to optimally tune exciton recombination through oxygen vacancies generation, evidenced by photoluminescence and Raman spectroscopy measurements. The turnover frequency (TOF) and the apparent quantum efficiency (AQE) of the 7%CuO/TiO₂-spindle composites reach up to 23.8 mol_methanol·g_cat^-1·h^-1 and 55.2% at 25 °C, respectively, which are substantially higher than these previously reported photocatalysts. Further, the in-situ attenuated total reflection infrared spectroscopy analysis reveals that the methanol oxidation most likely takes place through the conversion of adsorbed methoxy (CH₃O*) to formaldehyde (CHO*) intermediate, a subject of major debate for a long time. The adsorbed formaldehyde (CHO*) thus produced reacts with another CH₃O* species in its close proximity to form the final product of methyl formate. Results of this study provide insights into the reaction mechanism, and offer guidelines to systematically develop and apply photocatalysts for methanol conversion and related reactions via surface engineering.