Preparation of Au/CeO2 exhibiting strong surface plasmon resonance effective for selective or chemoselective oxidation of alcohols to aldehydes or ketones in aqueous suspensions under irradiation by green light

Article abstract of DOI:10.1021/ja305225s

Au/CeO₂ samples with various Au contents were prepared by the multistep (MS) photodeposition method. Their properties including Au particle size, particle dispersion, and photoabsorption were investigated and compared with properties of samples prepared by using the single-step (SS) photodeposition method. The MS- and SS-Au/CeO₂ samples were used for selective oxidation of benzyl alcohols to corresponding benzaldehydes in aqueous suspensions under irradiation by visible light from a green LED, and the correlations between reaction rates and physical properties of the MS- and SS-Au/CeO₂ samples were investigated. Difference in the two photodeposition methods was reflected in the average size and number of Au nanoparticles, for example, 92 nm and 1.3 × 10¹² (g-Au/CeO ₂)^-1 for MS photodeposition and 59 nm and 4.8 × 10¹² (g-Au/CeO₂)^-1 for SS photodeposition in the case of 1.0 wt % Au samples. Fixation of larger Au particles resulted in strong photoabsorption of the MS-Au/CeO₂ samples at around 550 nm due to the surface plasmon resonance, and the Kubelka-Munk function of the photoabsorption linearly increased with increase in Au content up to 2.0 wt %, in contrast to the photoabsorption of SS-Au/CeO₂ samples, which was weak and was saturated even at around 0.5 wt %. Due to the strong photoabsorption, the MS-Au/CeO₂ samples exhibited reaction rates approximately twice larger than those of SS-Au/CeO₂ samples with the same Au contents, and apparent quantum efficiency of MS-Au/CeO₂ reached 4.9% at 0.4 mW cm^-2. Linear correlations were observed between reaction rates (r) and surface area of Au nanoparticles (S) in both MS- and SS-Au/CeO₂ samples, though the two slopes of r versus S plots were different, suggesting that oxidation of benzyl alcohol occurred on the Au surface and that S was one of the important factors controlling the reaction rate. Photocatalytic oxidation of benzyl alcohol having an amino group revealed that the Au/CeO₂ photocatalyst exhibited high chemoselectivity toward the hydroxyl group of alcohol, i.e, the Au/CeO₂ photocatalyst almost quantitatively converted aminobenzyl alcohol to aminobenzaldehyde with 99% yield.