Modified preparation method for highly active Au/SiO2 catalysts used in CO oxidation

In this study we report a novel synthesis method for the preparation of silica supported nanosized gold catalysts using HAuCl4 gold precursor and ammonia solution. Indirect evidences were obtained for the formation of gold ammine cation complexes in the presence of ammonia solution. Urea was also effective for the synthesis of Au/SiO2 catalysts, but the particle size of supported gold was much larger compared to the samples prepared by using ammonia solution. The results show that the key step of the preparation using ammonia solution is the electrostatic interaction between the positively charged gold precursor and the negatively charged silica surface. The partial dissolution of the silica surface at high pH may provide unique binding sites for the gold complexes, which prevents the particle growth during the treatment in hydrogen at 350 deg C and results in highly dispersed gold particles on the silica surface. The new method of synthesis resulted in highly dispersed gold particles on silica with high catalytic activity in carbon monoxide oxidation. XPS and FTIR spectroscopic results show that the gold is in metallic state after hydrogen treatment at 350 deg C, while operando FTIR measurements in the presence of oxygen revealed that ionic forms of gold are formed. The formation of these surface entities was strongly hindered over inactive Au/SiO2 catalyst containing large gold particles.