Au@Pd/TiO2 with atomically dispersed Pd as highly active catalyst for solvent-free aerobic oxidation of benzyl alcohol

Abstract

In this study, bimetallic Au@Pd/TiO2 catalysts with an Au loading amount of 0.94wt.% and Pd loading amounts varying from 0.017 to 0.13wt.% were prepared using a two-step photocatalytic deposition method, and the solvent-free aerobic oxidation of benzyl alcohol was investigated on the catalysts. For comparison, monometallic Pd/TiO2 with a Pd loading amount of 0.048wt.% (denoted as Pd(0.048)/TiO2) and Au/TiO2 with a Au loading amount of 0.94wt.% were also prepared. The catalysts were characterized using X-ray diffraction, transmission electron microscope, UV–Vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, infrared spectroscopy of CO adsorption, and energy dispersive spectroscopy. Characterization results suggested that in the bimetallic catalysts Pd atoms were preferentially deposited on exposed Au surface and core-shell structured Au@Pd particles with atomically dispersed Pd as the shell were formed. Additionally, monomeric and paired Pd sites were dominated in bimetallic catalysts with Pd contents below 0.066wt.%, and increasing Pd deposition amount led to gradual development of Pd ensembles. For the solvent-free aerobic oxidation of benzyl alcohol, Au/TiO2 had negligible catalytic activity, while Pd(0.048)/TiO2 exhibited marked conversion of benzyl alcohol. As for bimetallic catalysts, the conversion of benzyl alcohol increased with the increase of Pd deposition amount from 0.017 to 0.049wt.%, whereas kept constant when Pd deposition amount was further increased to 0.13wt.%. Accordingly, Au@Pd(0.049)/TiO2 had a highest turnover frequency value of 21961h−1 for the oxidation of benzyl alcohol among the test bimetallic catalysts.