Optimization of Au0–Cu+ synergy in Au/MgCuCr2O4 catalysts for aerobic oxidation of ethanol to acetaldehyde

Abstract

A ternary MgCuCr2O4 spinel-supported gold nanoparticle catalyst is optimized toward high acetaldehyde productivity in gas-phase aerobic oxidation of ethanol. We investigate the structure–performance relationships of Au/MgCuCr2O4 catalysts by changing support and catalyst pretreatment to gain further insight into the Au0–Cu+ synergy. Support calcination at 700°C and catalyst prereduction result in the most active and stable ethanol oxidation catalyst. Extensive characterization shows this to be mainly due to the enrichment of Cu in the surface by H2- or ethanol-induced catalyst restructuring and the stabilization of surface Cu+ species in well-crystallized spinel without reduction to Cu0, which leads to a higher surface Cu+ fraction and enhanced Au0–Cu+ interaction. Kinetic studies show that the apparent activation energies of prereduced catalysts are higher than those of preoxidized catalysts, suggesting that oxygen vacancy formation via water removal from Au–H and active oxygen species is a dominant rate-limiting step. Molecular O2 is activated on defective Cu+ sites at the AuNP/support interface to form peroxide-type O2− species, which serve as active sites for removing hydride from the gold surface and breaking the O-H bond of ethanol. The reaction rate increases with space velocity and reactant concentration, achieving a lower boundary estimate of space–time yield of up to 1245galdehyde gAu−1h−1 at 250°C with air as oxidant.