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

Abstract

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 d 3-acetate, 13C-acetate, and 18O 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.

Adsorption of Acetate on Au(111): An in‐situ Scanning Tunnelling Microscopy Study and Implications on Formic Acid Electrooxidation

“…The electrosorption of anions, in particular oxoanions and halides on noble metal electrode surfaces, is a major topic in the field of interfacial electrochemistry and electrocatalysis. The presence of adsorbed species and their adsorption strength can strongly affect reaction kinetics, activity and selectivity . Electrocatalytic reactions are often influenced by nonreactive specifically adsorbed anions (spectator species), that lead to changes in structure and composition of the electrical double‐layer and alter the electronic properties of a catalyst's surface or even block active surface sites .…”

“…Unlike formate, acetate is unreactive on gold . This is because the C−H bond of formate is much easier to split than the C‐C bond of acetate . However, as is the case for formate, also acetate can exert an influence on electrocatalytic reactions.…”

XPS and STM studies of the oxidation of hydrogen chloride at Cu(100) surfaces

“…surfaces in particular [1][2][3][4][5][6]. The ability of oxygen to act as a base to ab-41 stract hydrogen was recognized early on [7][8][9][10], and a huge number of 42 reactions have been studied subsequently, ranging from amines and 43 sulfides to alcohols, alkenes, and acids [9,10,5,11].…”

Acetaldehyde partial oxidation on the Au(111) model catalyst surface: C–C bond activation and formation of methyl acetate as an oxidative coupling product

“…In order to optimize the performance of these uniquely active and selective catalytic materials, mechanisms of the corresponding reactions occurring on gold surfaces should be understood at the molecular level. Fundamental surface science studies on single crystal gold model catalysts provide valuable insights regarding the reaction mechanisms operating during the catalytic reactions [18][19][20][21][22][23][24]. Selective oxidation and oxidative coupling reactions of alcohols and aldehydes can produce commercially valuable partial oxidation products such as esters which are of high demand by the chemical industry.…”