Alternative Methods for the Preparation of Gold Nanoparticles Supported on TiO₂

The best current way to prepare Au/TiO2 catalysts is the method of deposition−precipitation with NaOH (DP NaOH) developed by Haruta and co-workers. With this method, it is possible to obtain small gold metal particles (2−3 nm), but the corresponding gold loading remains rather low (∼3 wt %). The main goal of this work is to investigate other methods of preparation of Au/TiO2 catalysts to obtain small gold metal particles (2−3 nm) and a higher Au loading. It is shown that anion adsorption with AuCl4- (AA) does not produce Au loading higher than 1.5 wt % and the average particle size is not very small (∼4 nm). Cation adsorption with Au(en)23+ (CA) leads to small particles (2 nm) when the solution/support contact time is moderate (1 h), but the Au loading does not exceed 2 wt %. The most promising method of preparation appears to be deposition−precipitation with urea (DP urea). Indeed, samples with gold particles as small as those obtained with DP NaOH (∼2 nm) can be prepared, and all gold in solution is deposited on TiO2 in contrast to DP NaOH. The DP urea samples reported in this paper can reach a Au loading as high as 8 wt % using a TiO2 support with a surface area of 45 m2 g-1. The possible mechanisms of deposition of gold on the TiO2 support by the different methods of preparation are discussed.