Formation of PdO on Au-Pd bimetallic catalysts and the effect on benzyl alcohol oxidation

A series of catalysts consisting of gold-palladium bimetallic nanoparticles (Au-Pd NPs in the range of 1-6 nm) anchored on foamlike mesoporous silica were used for the aerobic oxidation of benzyl alcohol. A remarkable synergistic effect was observed on these Au-Pd NP catalysts prepared by a one-pot method. Both experimental and theoretical study revealed a close relationship between the surface PdO species on the catalysts and their catalytic performance; that is, higher surface PdO content leads to lower catalytic activity. The surface content of PdO species on the catalysts could be tuned by controlling the Au/Pd ratios, because the formation of Au-Pd alloy NPs and electron transfer between surface Au and Pd atoms prevented the oxidation of surface Pd and retarded the formation of PdO species. An optimal Au/Pd ratio of 1/4.5 on the foamlike mesoporous silica support was obtained, with nearly no surface PdO species formed, and resulted in the highest benzyl alcohol conversion of 96%. The bimetallic Au-Pd catalysts exhibited much higher catalytic activity for benzyl alcohol oxidation (TOF = 50,000-60,000 h(-1)) than the monometallic Pd catalyst (TOF= 12,500 h(-1)), on which surface Pd is easily oxidized to PdO. These results provide direct evidence for the synergistic effect of Au-Pd bimetallic catalysts in benzyl alcohol oxidation. (C) 2019 Elsevier Inc. All rights reserved.