Promotion Mechanisms of Au Supported on TiO2 in Thermal- and Photocatalytic Glycerol Conversion

Catalytic glycerol conversion by means of either photon or thermal energy is of great importance and can be realized by metal supported on TiO2 systems. Although various procedures have been employed to synthesize efficient metal/TiO2 catalysts, the promotional mechanisms for both reactions are still unclear due to the lack of well-defined systems. Here, we have deposited gold nanoparticles on a series of highly crystalline anatase TiO2 substrates with different crystallite sizes (7, 12, 16, 28 nm) by both direct precipitation and sol-immobilization methods to examine the effect of metal deposition methods and TiO2 sizes on both photo- and thermal catalytic glycerol reforming. For photocatalytic H-2 evolution from glycerol, optimum performance was observed for the Au supported on 12 nm TiO2 for both deposition methods. For thermal catalytic glycerol oxidation, all catalysts show a similar selectivity to glycerate (>70%) regardless of the TiO2 size and metal deposition method; however, the metal deposition method significantly influences the catalytic activity. In situ UV-vis spectrometry reveals that the optimized photocatalytic performance originates from enhanced charge transfer kinetics and a more negative Fermi level for proton reduction, whereas electrochemical analysis reveals that the promoted glycerol oxidation is caused by the enhanced oxygen reduction half-reaction.