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

Abstract

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.