Impact of Particle Size on the Vapor-Phase Oxidative Coupling of Methanol and Dimethylamine over Palladium–Gold Nanoparticles

Abstract

Oxidative coupling of methanol and dimethylamine in the presence of O₂ in the vapor phase over dilute Pd in Au bimetallic catalysts occurs via the dissociation of O₂ on Pd and selective oxidation of methanol on Au. Here, we synthesize a series of silica-supported PdAu alloy nanoparticle catalysts of varied Pd:Au ratios with ∼5 nm particle diameter and show that these catalysts have increased selectivity to dimethylformamide across all Pd:Au ratios (∼95%), distinct from observations over larger PdAu nanoparticles (∼15–25 nm diameter) of similar Pd:Au ratios. Small monometallic Pd particles are more selective than large monometallic Pd particles, and small Au nanoparticles are reactive and selective for oxidative coupling (while large Au nanoparticles are inactive). Rates per surface metal atom were similar over PdAu nanoparticles of all sizes and increased monotonically with increasing Pd content for the small nanoparticles. Apparent reaction kinetics demonstrate distinct apparent methanol reaction order and apparent activation energy relative to those reported over larger nanoparticles of similar Pd:Au ratios. Unlike larger PdAu nanoparticles, the rate of dimethylformamide formation is not promoted by cofed water over small PdAu nanoparticles. The results of the kinetic studies are used to propose a series of elementary steps, derive a plausible rate expression, and regress rate and equilibrium constants. These results suggest high coverages of surface methoxy species and low coverages of adsorbates derived from dimethylamine. Taken together, these results demonstrate the sensitivity of the rates, selectivities, and kinetics of oxidative coupling reactions to the size of bimetallic nanoparticles.