Unified Mechanistic Understanding of CO2 Reduction to CO on Transition Metal and Single Atom Catalysts

CO is the simplest product from CO2 electroreduction (CO2R), but the identity and nature of its rate limiting step remains controversial. Here we investigate the activity of both transition metals (TMs) and metal-nitrogen doped carbon catalysts (MNCs), and a present unified mechanistic picture of CO2R to for both these classes of catalysts. By consideration of the electronic structure through a Newns-Andersen model, we find that on MNCs, like TMs, electron transfer to CO2 is facile, such that CO2 (g) adsorption is driven by adsorbate dipole-field interactions. Using density functional theory with explicit consideration of the interfacial field, we find CO2 * adsorption to generally be limiting on TMs, while MNCs can be limited by either CO2* adsorption or by the proton-electron transfer reaction to form COOH*. We evaluate these computed mechanisms against pH-dependent experimental activity measurements on CO2R to CO activity for Au, FeNC, and NiNC. We present a unified activity volcano that, in contrast to previous analyses, includes the decisive CO2* and COOH* binding strengths as well as the critical adsorbate dipole-field interactions. We furthermore show that MNC catalysts are tunable towards higher activity away from transition metal scaling, due to the stabilization of larger dipoles resulting from their discrete and narrow d-states. The analysis suggests two design principles for ideal catalysts: moderate CO2* and COOH* binding strengths as well as large dipoles on the CO2* intermediate. We suggest that these principles can be exploited in materials with similar electronic structure to MNCs, such as supported single-atom catalysts, molecules, and nanoclusters, 2D materials, and ionic compounds towards higher CO2R activity. This work captures the decisive impact of adsorbate dipole-field interactions in CO2R to CO and paves the way for computational-guided design of new catalysts for this reaction.