Ag/Au Mixed Sites Promote Oxidative Coupling of Methanol on the Alloy Surface

Abstract

Nanoporous gold, a dilute alloy of Ag in Au, activates molecular oxygen and promotes the oxygen-assisted catalytic coupling of methanol. Because this trace amount of Ag inherent to nanoporous gold has been proposed as the source of oxygen activation, a thin film Ag/Au alloy surface was studied as a model system for probing the origin of this reactivity. Thin alloy layers of Ag(x)Au(1-x), with 0.15≤x≤0.40, were examined for dioxygen activation and methanol self-coupling. These alloy surfaces recombine atomic oxygen at different temperatures depending on the alloy composition. Total conversion of methanol to selective oxidation products, that is, formaldehyde and methyl formate, was achieved at low initial oxygen coverage and at low temperature. Reaction channels for methyl formate formation occurred on both Au and Au/Ag mixed sites with a ratio, as was predicted from the local 2-dimensional composition.

Dynamic restructuring drives catalytic activity on nanoporous gold–silver alloy catalysts

“…The O1s binding energy is in agreement with previously reported oxygen states for bulk Au and Ag oxides ( Fig. 5a and Supplementary Table 4, O x species) 22,[25][26][27] . Following reduction by CO (0.1 torr, 150 • C), ∼90% of this oxidic oxygen is removed (Fig.…”

Catalyst design for enhanced sustainability through fundamental surface chemistry

“…Coupling on mixed sites increases with Ag content at the expense of reaction on Au-like sites, but only on alloys with very high Ag content (Ag 0.6 Au 0.4 ) do Ag-like sites begin to contribute even a small amount of methyl formate production (less than 1%). The production of hydrogen confirms that surface-assisted β-H elimination does occur on mixed sites, as H recombination to form H 2 occurs on Ag single crystals but not on Au [29]. The yield of methyl formate also increases with increasing Ag content, and methyl formate production on O/Ag 0.6 Au 0.4 is 50% higher than on O/Au(111) [29].…”

“…The production of hydrogen confirms that surface-assisted β-H elimination does occur on mixed sites, as H recombination to form H 2 occurs on Ag single crystals but not on Au [29]. The yield of methyl formate also increases with increasing Ag content, and methyl formate production on O/Ag 0.6 Au 0.4 is 50% higher than on O/Au(111) [29]. However, owing to a concomitant increase in combustion with higher Ag content, AgAu materials with a large percentage of Ag are not favourable for selective catalysis, as described for npAu, below.…”

“…At low O coverage, the selectivity to methyl formate is high [29]. Formaldehyde is also formed as a product, but there is no detectable CO 2 produced.…”

“…This understanding of O binding to Au(111) surfaces is key in deconvoluting the multiple O recombination features observed during desorption experiments involving AgAu single-crystal alloys [29]. These single-crystal alloy studies in turn provide a bridge to understanding O 2 dissociation on AgAu alloy materials with a greater degree of structural complexity, such as npAu, under well-controlled low-pressure conditions [30].…”