Selective lactose oxidation in aqueous-phase over Ag-Au bimetallic nanoparticles supported on Al 2 O 3 under mild reaction conditions

Abstract

In this work, Au-Ag based catalysts supported on Al 2 O 3 were prepared by a precipitation-deposition method at controlled pH and tested in the selective lactose (LA) oxidation to lactobionic acid (LB) in aqueous phase. Combining XPS and STEM-HAADF, it was found that there is an important surface Ag enrichment of bimetallic nanoparticles for bulk atomic ratios Au/(Au + Ag)>0.5. These bimetallic nanoparticles were active for the selective LA oxidation into LB and the maximum activity was reached with Au/(Au + Ag) = 0.9. In this case, the surface Au/(Au + Ag) atomic ratio was about 0.5, which indicates that there is the same amount of both elements at the active surface of the catalyst. Assuming that LA is chemisorbed on Au sites, and O 2 on Ag ones, this particular surface atomic ratio would favor the interaction and reaction between both reactants. Thus, the synergistic effect between Au and Ag could explain the results of this study. A compensation effect between frequency factor and activation energy supports the existence of such synergy. If the atomic ratio is Au/(Au + Ag)≤0.5, a layer of Ag deposits over the Au nanoparticles (core-shell structure) and the conversion of LA into LB drops to zero.