A series of catalysts containing of gold-palladium bimetallic nanoparticles (Au-Pd NPs in the range of 1-6 nm) anchored on foam-like mesoporous silica were used for the aerobic oxidation of benzyl alcohol. A remarkable synergistic effect was observed on these Au-Pd NPs catalysts prepared by one-pot method. Both the experimental and theoretical study revealed a close relationship between the surface PdO species on the catalysts and their catalytic performance, that is, a higher surface PdO content leads to a 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 foam-like mesoporous silica support was obtained, with nearly no surface PdO species formed and resulted the highest benzyl alcohol conversion of 96%.The bimetallic Au-Pd catalysts exhibited much higher catalytic activity for benzyl alcohol oxidation (TOF = 50000 -60000 h -1 ) than the monometallic Pd catalyst (TOF = 12500 h -1 ) on which surface Pd is easily oxidized to PdO. These results provide direct evidence for the synergistic effect of the Au-Pd bimetallic catalyst in benzyl alcohol oxidation.
A balance between the catalytic activity and product selectivity remains a dilemma for the partial oxidation processes because the products are prone to be over-oxidized. In this work we report on the partial oxidation of benzyl alcohol using a modified catalyst consisting of nanosized Au-Pd particles (NP) with tin oxide (SnO x ) deposited on mesoporous silica support. We found that the SnO x promotes the autogenous reduction of PdO to active Pd 0 species on the Au-Pd NP catalyst (SnO x @AP-ox) before H 2 reduction, which is due to the high oxophilicity of Sn. The presence of active Pd 0 species and the enhancement of oxygen transfer by SnO x led to a high catalytic activity. The benzaldehyde selectivity was enhanced with the increase of SnO x content on catalyst SnO x @AP-ox, which is ascribed to the modulated affinity of reactants and products on the catalyst surface through the redox switching of Sn species. After H 2 reduction, the SnO x was partially reduced and Au-Pd-Sn alloy was formed. The formation of Au-Pd-Sn alloy weakened both the catalytic synergy of Au-Pd alloy NPs and the adsorption of benzyl alcohol on the reduced catalyst, thus leading to a low catalytic activity.
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