Effect of Coverage on Catalytic Selectivity and Activity on Metallic and Alloy Catalysts; Vinyl Acetate Monomer Synthesis

The repulsive lateral interactions that occur at the high coverages present on catalysts under reaction conditions are likely to exert a significant influence on the reaction energetics. The effect of such lateral interactions are explored for the synthesis of vinyl acetate monomer (VAM) on Pd(111) and Au/Pd(111) alloy model single-crystal catalysts, where the reaction kinetics are monitored by reflection-absorption infrared spectroscopy (RAIRS). It is shown by comparing the reactivity of ethylene and its deuterated isotopomers, and by trapping reaction intermediates, that VAM is formed on Pd(111) by a mechanism first proposed by Samanos, where the reaction is initiated by coupling between ethylene and adsorbed acetate species to form an acetoxyethyl intermediate, followed by a rate-limiting beta-hydride elimination step to produce VAM. The reaction comprises two sequential steps; a bond-forming reaction, followed by a bond-breaking step. Repulsive lateral interactions are expected to facilitate bond-forming reactions that reduce the total coverage, but inhibit bond-breaking steps, so that VAM formation provides an ideal reaction to explore these effects. Density functional theory (DFT) calculations confirm the proposed coverage effects and yield energy barriers for the reaction of ethylene with acetate-saturated Pd(111) surfaces that are in excellent agreement with experiment. It is also found that bond-breaking in VAM decomposition is inhibited on a crowded Pd(111) surface, indicating that high adsorbate coverages influence both reactivity and selectivity. VAM formation is also explored on Au/Pd(111) alloys. The catalytic activity of alloys is conventionally discussed by invoking ensemble (geometric) and electronic (ligand) effects. However, the acetate coverage also decreases with increasing gold content of the Au/Pd(111) alloy, thereby decreasing the repulsive lateral interactions. The coverage effects on alloys are found to be as large as the ensemble + electronic effects and induce a change in the rate-limiting step for VAM formation from beta-hydride elimination to the ethylene-acetate coupling step.