Highly active and selective CO2 conversion into useful chemicals is desirable to generate valuable products out of greenhouse gases. To date, various metal-based heterogeneous catalysts have shown promising electrochemical catalytic activities for CO2 reduction, yet there have been no systematic studies of the active sites of these metal catalysts that can guide further experiments. In this study, we use first-principles calculations to identify active sites for the CO2 reduction reaction for Ag and Au metals, the two metals that have been shown to be the most active in producing CO. We compare the catalytic activity and selectivity of three reaction sites of nanoparticles, namely, low-index surfaces, edge sites, and corner sites of these metals. For nanoparticle corner sites, in particular, we find that the size effect is critical, and 309-atom (or larger) nanoparticles should be used to appropriately describe realistic metal nanocatalysts. However, a 55-atom cluster model is often used in the literature to model nanoparticles. From a comparative study, we reveal that corner sites are the most active for the CO2 reduction reaction in the case of Au, whereas edge sites are the most active in the case of Ag. Although Au is generally the more active CO2 reduction catalyst than Ag due to the intrinsically stronger binding of *C-species, our results indicate that reducing the size of Au nanoparticles up to 2 nm also increases the unwanted H2 evolution reaction, as observed in a recent experiment. However, reducing the size of Ag nanoparticles up to 2 nm enhances the CO2 reduction reaction without suffering from the H2 evolution reaction, and on this basis, Ag nanoparticles are a comparable or even better-performing, inexpensive catalyst than Au for electrochemical CO production. Our findings suggest that the catalyst design principle (elemental composition, morphology, and size) is metal-dependent and should be carefully tailored for each system.
density functional calculations; CO2 electroreduction; active sites; electrocatalysts; nanoparticles; metal catalysts