Switching Selectivity in Oxidation Reactions on Gold: The Mechanism of C-C vs C-H Bond Activation in the Acetate Intermediate on Au(111)

摘要

Carboxylates are important intermediates in oxidative reactions on gold, as they are precursors to carboxylic acids and CO2; they may also act as site-blockers in oxidative coupling of alcohols, thereby decreasing both catalyst activity and selectivity. We demonstrate that the reaction selectivity and pathways for a prototype carboxylate, acetate, adsorbed on Au(111), are dramatically altered by the presence of coadsorbed atomic O. Finely tuning the initial oxygen coverage affords control of the product selectivity and the reaction pathway. Oxygen-assisted γ-C-H activation occurs with coadsorbed oxygen near 425 K, yielding mainly CO2 and formaldehyde, and a kinetic isotope effect is observed for these products. In the absence of coadsorbed oxygen, acetate reacts at 530 K by C-C bond cleavage to form CO2, methyl, and methyl acetate as well as minor products. These studies have led to the identification of a new synthetic pathway for ester formation, in which methyl (either produced in the reaction or introduced externally using methyl iodide) reacts with surface acetate to form methyl acetate. Detailed isotopic labeling studies using d3- acetate, ~(13)C-acetate, and ~(18)O show that the methyl carbon forms mainly formaldehyde in the oxygen assisted reaction and methyl in the clean-surface reaction and that surface oxygen is incorporated into products in the low temperature, oxygen-assisted pathway. A complete mechanism is proposed and compared to the reaction of acetate on silver. These studies provide a detailed fundamental understanding of acetate chemistry on gold and demonstrate how the oxygen concentration can be used to tune selectivity.