Effect of redox treatments on activation and deactivation of gold nanospecies supported on mesoporous silica in CO oxidation

The influence of pretreatments with He, O2 and H2 on the physicochemical and catalytic properties of gold supported on hexagonal mesoporous silica (HMS) has been studied by a variety of methods including: XPS, TPR, N2 sorption, UV–visible spectroscopy, FTIR and catalytic testing. The investigations show supported gold to form different states under redox treatments: Au3+ and Au+ ions, neutral and partly charged gold clusters and metal nanoparticles of different sizes. Reducing pretreatments increase the catalytic activity of the samples; while oxidative treatments deactivate the catalyst. Catalytic tests show several regions of the reduced samples with various catalytic behaviours: 20–200 °C, 200–400 °C and >400 °C; this situation is explained by the co-existence of gold active sites of different types. A comparative analysis among spectroscopic and catalytic data allows suggesting Au n δ + clusters to be responsible for the activity in the low-temperature region (<200 °C); neutral Aun clusters are active in the range 200–400 °C; and gold nanoparticles may catalyze high-temperature CO oxidation. Au3+ ions are inactive in this process.