Selective Photocatalytic Activation of Ethanol C-H and O-H Bonds over Multi-Au@SiO 2 /TiO 2 : Role of Catalyst Surface Structure and Reaction Kinetics.

The chemical bond diversity and flexible reactivity of biomass-derived ethanol make it a vital feedstock for the production of value-added chemicals but result in low conversion selectivity. Herein, composite catalysts comprising SiO 2 -coated single- or multiparticle Au cores hybridized with TiO 2 nanoparticles (mono- or multi-Au@SiO 2 /TiO 2 , respectively) were fabricated via electrostatic self-assembly. The C-H and O-H bonds of ethanol were selectively activated (by SiO 2 and TiO 2 , respectively) under irradiation to form CH 3 CH • (OH) or CH 3 CH 2 O • radicals, respectively. The formation and depletion kinetics of these radicals was analyzed by electron spin resonance to reveal marked differences between mono- and multi-Au@SiO 2 /TiO 2 . Consequently, the selectivity of these catalysts for 1,1-diethoxyethane after 6 h irradiation was determined as 81 and 99%, respectively, which was attributed to the more pronounced effect of localized surface plasmon resonance for multi-Au@SiO 2 /TiO 2 . Notably, only acetaldehyde was formed on a Au/TiO 2 catalyst without a SiO 2 shell. Fourier transform infrared (FTIR) spectroscopy indicated that the C-H adsorption of ethanol was enhanced in the case of multi-Au@SiO 2 /TiO 2 , while NH 3 temperature-programmed desorption and pyridine adsorption FTIR spectroscopy revealed that multi-Au@SiO 2 /TiO 2 exhibited enhanced surface acidity. Collectively, the results of experimental and theoretical analyses indicated that the adsorption of acetaldehyde on multi-Au@SiO 2 /TiO 2 was stronger than that on Au/TiO 2 , which resulted in the oxidative coupling of ethanol to afford 1,1-diethoxyethane on the former and the dehydrogenation of ethanol to acetaldehyde on the latter.