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 TiO 2 systems. Although various procedures have been employed to synthesize efficient metal/TiO 2 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 TiO 2 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 TiO 2 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 TiO 2 for both deposition methods. For thermal catalytic glycerol oxidation, all catalysts show a similar selectivity to glycerate (>70%) regardless of the TiO 2 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.