Probing the Active Site Chemistry of Supported Gold Catalysts with Charged Au25 q Nanoclusters (q = -1, 0, +1)

Abstract

Charged Aun+/− sites are hypothesized as key reaction centers in gold catalysis, but their charge state and mechanistic roles remain controversial. Two examples include CO2 reduction and CO oxidation. Converting CO2 into value-added products is critical for green-house gas mitigation and renewable fuels discovery, and oxidizing CO in the presence of water is central to the industrially important water gas shift reaction (WGS: CO + H2O → CO2 + H2). Debate surrounds the charge state of Au active sites, and variously charged Aun+/0/− species and/or the catalyst-support have all been proposed as reaction centers for CO oxidation and CO2 reduction. We used differently charged Au25 q clusters (q = −1, 0, +1) to precisely identify the role of active site charges in heterogeneous gold catalysis. Au25 q are unique because they have three stable charge states, their crystal structure has been solved, and their small size (~1nm) allows computational modeling of realistic cluster-adsorbate systems. In this regard, Au25 q can serve as well-defined active sites for probing the chemistry of charged catalyst species. Experimental studies and density functional theory identified a relationship between the active site charge, the stability of adsorbed reactants or products and the reaction rate. We found charge-dependent electrocatalytic activity for CO2 reduction, CO oxidation and O2 reduction reactions in aqueous media. Anionic Au25‾ promoted CO2 reduction by stabilizing CO2 + H+ coadsorption. Cationic Au25 + promoted CO oxidation by stabilizing CO + OH− coadsorption. Finally, stronger product adsorption at Au25 + inhibited O2 reduction. These results provide insight into the role of charged active sites and should help guide future catalyst design.