Effects of Au Loading and CO2 Addition on Photocatalytic Selective Phenol Oxidation over TiO2-Supported Au Nanoparticles

The photocatalytic oxidation of aqueous phenol to hydroquinone on TiO2-supported Au nanoparticles (Au/P25) and P25 under solar simulator (>320nm) and monochromated light irradiation was conducted in air and under a CO2 atmosphere to investigate the effects of Au loading and CO2 atmosphere on the photocatalysis. The action spectrum in hydroquinone formation on Au/P25 under a CO2 atmosphere was in good agreement with the absorption spectrum of P25. A larger amount of hydroquinone formed and a smaller amount of CO2, which was the completely oxidized product of phenol, evolved if Au was loaded on P25. These observations indicated that this photocatalysis did not involve a visible light-induced step by the localized surface plasmon resonance of Au and Au loading did not improve the charge separation efficiency on P25 for the reaction. On the other hand, the amount of the surface titanol group on P25 decreased dramatically after Au loading, which was confirmed by analysis of the temperature-programmed desorption of ammonia curves for P25 and Au/P25. Comparison of the adsorption isotherms of hydroquinone from water on P25 and Au/P25 revealed that the surface modification of P25 with Au reduced the amount of hydroquinone adsorbed from a relatively low concentration solution dramatically. Moreover, the adsorption of hydroquinone from water on Au/P25 under various pH values was investigated to find that there was a partially opposite relationship between the pH-dependent hydroquinone adsorption and the CO2 pressure- (and possibly pH-)dependent photocatalytic hydroquinone formation on Au/P25. Similar results were obtained for the photocatalytic oxidation of aqueous benzene to phenol on Au/P25 under solar simulator irradiation. All results obtained suggested that Au nanoparticle loading on P25 promoted the desorption of the formed hydroquinone from the surface to prevent the successive oxidation of the product, and the presence of CO2 further promoted the hydroquinone desorption probably by lowering the pH of the reaction solution below the pKa of Au/P25.