(515f) Structural and Environmental Effects on the Activity of Nanoporous Gold Catalysts for Sustainable Oxidation Reactions

Unsupported nanoporous gold (npAu) is currently being investigated as a promising material for more sustainable selective oxidation processes. This material provides a unique catalytic surface that takes advantage of the surface reactivity of pure gold to guide product selectivity – for example, for the selective coupling of alcohols to form esters. The focus of this work is to improve our understanding of the factors that control npAu activity and stability for such reactions. Nanoporous Au, typically prepared by the dealloying of sacrificial Ag from a bulk alloy, is a dilute gold-silver alloy. The residual silver is thought to produce adsorbed atomic oxygen (via O₂ dissociation) which spills over onto the gold and is then consumed in the activation and further reaction of alcohols (e.g. methanol). When used for alcohol oxidation reactions, npAu requires a lengthy activation period prior to use by exposure to reaction gases (e.g. exposure to methanol and oxygen at 70 °C for up to 15 hours). The effect of this treatment on the catalyst has remained unclear. Furthermore, the activated catalysts are prone to deactivation upon exposure to higher alcohols (e.g. ethanol, 1-butanol) or alternative coupling reactants (e.g. acetaldehyde). Here we present recent developments on the activation of nanoporous gold using ozone and its effect on the ligament size, surface composition, and catalyst activity and selectivity. We show that ozone treatment at 150 °C greatly increases the stability of the catalyst, while maintaining high selectivity to methyl formate during methanol oxidation. We provide evidence for the formation of silver oxide features at the catalyst surface, which appear to affect the conditions under which oxygen activation and catalytic methanol coupling occur. By using a combination of electron microscopy, including environmental TEM, and X-ray photoelectron spectroscopy we relate dynamic changes in the material to activity and selectivity. These studies provide detailed insight into the origin of catalytic activity and illustrate a methodology for investigating this class of nanoporous metallic catalysts.