Enhancement of photocatalytic performance with the use of noble-metal-decorated TiO₂ nanocrystals as highly active catalysts for aerobic oxidation under visible-light irradiation

The use of noble metals loaded on semiconductor supports, through the absorption of visible light by metal nanoparticles, has opened new avenues for the improvement of catalytic performance under light irradiation. In this study, a series of different coinage metals such as gold, silver, platinum, and palladium loaded on TiO₂ were prepared by photo-deposition and characterized by transmission electron microscopy, X-ray diffraction, Brunauer–Emmett–Teller analysis, UV–vis diffuse reflectance spectroscopy, photoluminescence emission, and X-ray photoelectron spectroscopy. The photocatalytic activity of M–TiO₂ (M = Au, Ag, Pt, and Pd) samples was evaluated by the selective oxidation of benzyl alcohol under visible-light irradiation. In addition, the relationship between the light intensity, light wavelength, temperature of the reaction, and photocatalytic efficiency was investigated; the photocatalytic efficiency increased directly by increasing the light intensity or reaction temperature or by adjusting the irradiation wavelength in the most appropriate range. Particularly, Pt₂–TiO₂, with a Pt nanoparticle size ∼2 nm, created an Schottky barrier, so as to promote the electron transfer from platinum to titanium, which in turn promotes the aerobic oxidation of benzyl alcohol with an apparent quantum yield of 5.58% (at 400 nm). The mechanism of the oxidation process of benzyl alcohol over Pt₂–TiO₂ is also presented. In addition, platinum and palladium nanoparticles exhibited an even more profound improvement in catalytic performance at a high operating temperature (80 °C); these catalysts can be used as photo-thermocatalyst, which can more efficiently drive chemical conversion by coupling light and heat energy sources.