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

Charged Au n+/− sites are hypothesized as key reaction centers in gold catalysis, but their charge state and mechanistic roles remain controversial. Two examples include CO 2 reduction and CO oxidation. Converting CO 2 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 + H 2 O → CO 2 + H 2 ). Debate surrounds the charge state of Au active sites, and variously charged Au n+/0/− species and/or the catalyst-support have all been proposed as reaction centers for CO oxidation and CO 2 reduction. We used differently charged Au 25 q clusters ( q = −1, 0, +1) to precisely identify the role of active site charges in heterogeneous gold catalysis. Au 25 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, Au 25 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 CO 2 reduction, CO oxidation and O 2 reduction reactions in aqueous media. Anionic Au 25‾ promoted CO 2 reduction by stabilizing CO 2 + H + coadsorption. Cationic Au 25 + promoted CO oxidation by stabilizing CO + OH − coadsorption. Finally, stronger product adsorption at Au 25 + inhibited O 2 reduction. These results provide insight into the role of charged active sites and should help guide future catalyst design.