Preparation of ultra low loaded Au catalysts for oxidation reactions

Cyanide leaching was used to obtain Au/SiO2 catalysts with very low gold loadings. A number of catalysts with a nominal 5 wt% target loading were prepared using impregnation and deposition precipitation techniques and these were found to be active catalysts for the solvent-free aerobic oxidation of benzyl alcohol. Exposure of these catalysts to a basic solution of NaCN for 10 min leached gold from the materials to give very pale pink catalysts which were found to contain just 0.06 wt% Au. The concentration of Au removed from the catalyst was constant regardless of the length of NaCN exposure. When the cyanide leached materials were employed for benzyl alcohol oxidation under the same conditions as the unleached parent catalysts, the conversions were identical. However, when the catalytic activity was normalised to the Au content (determined by ICP analysis) the TOFs were much higher for the NaCN treated catalysts (> 400,000 h(-1)). These results clearly demonstrate that NaCN leaching is an effective route to the development of catalysts containing very low gold content, whilst maintaining high activity. The leached materials were found to comprise metallic Au nanoparticles. The successful utilisation of ultra low loaded Au catalysts for selective oxidation, where the majority of the Au present is active and 98% of the spectator Au is removed, makes the industrial application of such materials more economically viable. Information obtained through the identification of these active structures using state of the art techniques may provide useful insights into how the reaction proceeds on the Au surface.