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

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 Au-0-Cu+ synergy. Support calcination at 700 degrees 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 H-2- or ethanol-induced catalyst restructuring and the stabilization of surface Cu+ species in well-crystallized spinel without reduction to Cu, which leads to a higher surface Cu+ fraction and enhanced Au-0-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 O-2 is activated on defective Cu+ sites at the AuNP/support interface to form peroxide-type O-2(-) species, which serve as active sites for removing hydride from the gold surface and breaking the 0-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 1245 g g(aldehyde) g(AU)(-1) h(-1) at 250 degrees C with air as oxidant. (C) 2016 Elsevier Inc. All rights reserved.