Dilute Pd-in-Au alloy RCT-SiO₂ catalysts for enhanced oxidative methanol coupling

Dilute alloy catalysts have the potential to enhance selectivity and activity for large-scale reactions. Highly dilute Pd-in-Au nanoparticle alloys partially embedded in porous silica (“raspberry colloid templated” (RCT)-SiO₂) prove to be robust and selective catalysts for oxidative coupling of methanol. Palladium concentrations in the bimetallic nanoparticles as low as ~3.4 at.% catalyze the production of methyl formate with a selectivity of ~95% at conversions of ~55%, whereas conversions are low (<10%) for ~1.7 at.% Pd-in-Au nanoparticle and pure Au nanoparticle catalysts. Fractional reaction orders for both CH₃OH and O₂ measured for ~3.4 at.% Pd-in-Au nanoparticles supported on RCT-SiO₂ indicated a complex mechanism in which the sites for O₂ dissociation are not saturated. Optimal methyl formate production was found for an equimolar mixture. There is no conversion of methanol in the absence of O₂ between 360 and 450 K. All observations are consistent with a mechanism derived from model studies, requiring that clusters of Pd be available on the catalyst for O₂ dissociation.