Surface Composition Control of the Binary Au-Ag Catalyst for Enhanced Oxidant-Free Dehydrogenation

Effective control of surface composition of bimetallic catalysts is challenging yet important, as synergistic effects between two metals play vital roles in heterogeneous catalysis. This study involves methods of loading control, thermal activation, and selective surface etching to modify the bimetallic surface on the Au-Ag/SBA-15 catalyst and gains insight into the nature of the promotional effect of Ag modification over Au catalysts. The binary Au-Ag/SBA-15 catalyst prepared following a stepwise metal-loading procedure is found for the first time to be active in benzyl alcohol dehydrogenation without oxidant or hydrogen acceptor. A direct correlation is established between the surface compositions of the Au-Ag nanoparticles and their intrinsic catalytic activities. Au-Ag element analytical techniques, including X-ray fluorescence, X-ray photoelectron spectroscopy, and high-sensitivity low-energy ion scattering spectroscopy, are applied to acquire compositional information in bulk and on the surface. As a result, the pronounced improvement of Au catalyst by Ag is from an ensemble with a specific surface composition rather than the bulk composition. The synergy between Au and Ag for benzyl alcohol dehydrogenation is the most pronounced in the case of 4:1 Au-Ag surface compositions, corresponding to the prepared Au₁-Ag_0.111 catalyst. The electronic promoting effect is suggested as the natural origin of compositional enhancement by differentiating it from the structural effect. This work demonstrates the importance of surface composition control at an atomic level in developing highly efficient multicomponent catalysts.