Role of pH in the formation of structurally stable and catalytically active TiO ₂-supported gold catalysts

We report the investigation of titania (Degussa P25) supported gold catalysts prepared by magnetron sputtering. Catalysts grown on natural fumed titania were structurally unstable, resulting in the rapid coarsening of 2.4 nm gold clusters into large ̃20 nm gold clusters in a few days at room temperature under normal atmospheric conditions. However, treating the titania support powder to a mock deposition-precipitation process, at pH 4, followed by the subsequent deposition of gold onto this treated powder produced a remarkable enhancement in gold particle stability and a 20-fold enhancement of catalytic activity. Furthermore, it was found that treating the titania under basic conditions (pH 10) resulted in a further enhancement of structural stability and a further doubling of the reaction rate to 0.28 mol of CO/mol of Au×s. This enhancement cannot be attributed to removing surface Cl ^- species from the titania, the formation of oxygen vacancies on the TiO ₂ surface, or an electronic effect. Instead, it appears to be associated with the formation of strongly bound hydroxyl species on the TiO ₂ surface. The formation of surface hydroxyls during the deposition-precipitation method is coincidental and contributes significantly to the properties of Au/TiO ₂ catalysts.