XAS study of au supported on TiO ₂: Influence of oxidation state and particle size on catalytic activity

The effect of synthesis and reaction conditions on the structure and activity of Au clusters supported on nanocrystalline and mesoporous TiO ₂ was investigated. X-ray absorption spectroscopy was applied to correlate the particle size and the oxidation state with several parameters, such as pH of the precursor solution, Au loading, pretreatment, and support structure. The study using mesoporous TiO ₂ as the support shows that lower Au loadings resulted in bigger Au aggregates with lower reducibility. The high activity state for Au supported in different allotropic forms of TiO ₂ corresponds to Au in a fully reduced state. Furthermore, once reduced, no reoxidation occurs under reaction conditions, even after flowing air at higher temperatures (150 and 300°C), Therefore, our results indicate that oxidized Au is not necessary for high activity. The activity decreased with particle growth, and, compared to the other allotropic TiO ₂, Au on brookite exhibited no significant particle agglomeration and was the most active catalyst after treatment at 300°C.