The catalytic conversion of CO2 into industrially relevant chemicals is one strategy for mitigating greenhouse gas emissions. Along these lines, electrochemical CO2 conversion technologies are attractive because they can operate with high reaction rates at ambient conditions. However, electrochemical systems require electricity, and CO2 conversion processes must integrate with carbon-free, renewable-energy sources to be viable on larger scales. We utilize Au25 nanoclusters as renewably powered CO2 conversion electrocatalysts with CO2 → CO reaction rates between 400 and 800 L of CO2 per gram of catalytic metal per hour and product selectivities between 80 and 95%. These performance metrics correspond to conversion rates approaching 0.8-1.6 kg of CO2 per gram of catalytic metal per hour. We also present data showing CO2 conversion rates and product selectivity strongly depend on catalyst loading. Optimized systems demonstrate stable operation and reaction turnover numbers (TONs) approaching 6 × 10(6) molCO2 molcatalyst(-1) during a multiday (36 h total hours) CO2 electrolysis experiment containing multiple start/stop cycles. TONs between 1 × 10(6) and 4 × 10(6) molCO2 molcatalyst(-1) were obtained when our system was powered by consumer-grade renewable-energy sources. Daytime photovoltaic-powered CO2 conversion was demonstrated for 12 h and we mimicked low-light or nighttime operation for 24 h with a solar-rechargeable battery. This proof-of-principle study provides some of the initial performance data necessary for assessing the scalability and technical viability of electrochemical CO2 conversion technologies. Specifically, we show the following: (1) all electrochemical CO2 conversion systems will produce a net increase in CO2 emissions if they do not integrate with renewable-energy sources, (2) catalyst loading vs activity trends can be used to tune process rates and product distributions, and (3) state-of-the-art renewable-energy technologies are sufficient to power larger-scale, tonne per day CO2 conversion systems.

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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.