Selective oxidation of methanol to methyl formate over bimetallic Au-Pd nanoparticles supported on SiO₂

Selective oxidation of methanol to methyl formate (MF) is one of the most attractive processes to get valuable methanol-downstream products, where the supported Au and Pd catalysts were proved rather effective at low temperature. To search for highly active, regenerable and practical catalysts as well as to reveal the synergy of Au-Pd and reaction mechanism for the methanol oxidation, a series of silica supported Au-Pd nanoparticles (Au-Pd/SiO₂) were prepared and their catalytic performance in the oxidation of methanol to MF with molecular oxygen was investigated in this work. The results indicate that the Au₂-Pd₁/SiO₂ catalyst with an Au+Pd loading of only 0.6% and a Au/Pd mass ratio of 2 exhibits excellent performance in the methanol oxidation with oxygen; the conversion of methanol over Au₂-Pd₁/SiO₂ reaches 57.0% at 130℃, with a selectivity of 72.7% to MF. Various characterization results illustrate that the Au-Pd bimetallic nanoparticles (2-4 nm) are highly dispersed on the silica surface, inclined to take a twinned structure and present the (111) planes, which may contribute to the high activity of Au-Pd/SiO₂ in the oxidation of methanol to MF. A possible reaction mechanism was proposed on the basis of DRIFTS results:methanol was first activated by surface oxygen on the interface of Au-Pd nanoparticles, forming the chemisorbed methoxy species; the methoxy species was then deprotonated to adsorbed formaldehyde species, which reacted with another methoxy species, producing MF by nucleophilic attack and subsequent β-H elimination.