Probing the Active Site Chemistry of Supported Gold Catalysts with Charged Au25q 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 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 Au^n+/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 Au_25‾ 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.