Selective reductions using visible light photocatalysts of supported gold nanoparticles

Abstract

Photocatalytic synthesis using visible light is a desirable chemical process because of its potential to utilize sunlight. Supported gold nanoparticles (Au-NPs) were found to be efficient photocatalysts and the effects of the supports were identified including CeO₂, TiO₂, ZrO₂, Al₂O₃, and zeolite Y. In particular Au/CeO₂ exhibited the high catalytic activity to reduce nitro-aromatics to azo compounds, hydrogenate azobenzene to hydroazobenzene, reduce ketones to alcohols, and deoxygenate epoxides to alkenes at ambient temperatures, under irradiation of visible light (or simulated sunlight). The reactive efficiency depends on two primary factors: one is the light adsorption of catalysts and the other is the driving ability of catalysts corresponding to the reactants. The light absorption by Au-NPs is due to the surface plasmon resonance effect or interband electron transition; this is related to the reduction ability of the photocatalysts. Irradiation with shorter wavelengths can excite the conduction electrons in Au-NPs to higher energy levels and, as a result, induce reduction with more negative reduction potentials. It is known that when irradiated with light the Au-NPs can abstract hydrogen from isopropanol forming Au–H species on the Au-NP surface. Hence, we proposed that the active Au–H species will react with the NO, NN, CO double bonds or epoxide bonds, which are weakened by the interaction with the excited electrons in the Au-NPs, and yield the final reductive products. The reacting power of the Au–H species depends on the energy of the excited electrons in Au-NPs: the higher the electronic energy, the stronger the reduction ability of the Au–H species. This finding demonstrates that we can tune the reduction ability of the photocatalysts by manipulating the irradiation wavelength.