Bimetallic Ag-Au/SiO2 catalysts: Formation, structure and synergistic activity in glucose oxidation

SiO2 supported Ag-Au bimetallic catalysts were prepared by sol adsorption method with 10/90, 20/80, 33/67, and 50/50 Ag/Au molar ratios. Reduction of HAuCl4 in Ag sol resulted in alloyed Ag-Au colloid particles and that structure remained after calcination and reduction treatment. The alloy structure of the catalysts was confirmed by UV-visible spectroscopy and high resolution transmission electron microscopy. The Au-Ag bimetallic effect and its dependence on the Ag/Au molar ratio was studied in glucose oxidation where synergistic activity increase was observed compared to the Au/SiO2 reference sample in the case of the bimetallic samples with less than Ag/Au = 50/50 molar ratio. The Ag/SiO2 was inactive at the same conditions. The Ag/Au surface atomic ratios - calculated by X-ray photoelectron spectroscopy (XPS) - were slightly higher than in the bulk-determined by prompt gamma activation analysis (PGAA). The higher activity of the bimetallic samples is suggested to be caused by the improved O-2 activating ability provided by Ag sites. The further increase of Ag loading above the optimal concentration may dilute or cover the Au to such an extent that the number of gold ensembles necessary for glucose activation decreases deteriorating the activity. (C) 2014 Elsevier B.V. All rights reserved.