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 remain controversial. Here we investigate the activity of transition metals (TMs), metal–nitrogen-doped carbon catalysts (MNCs) and a supported phthalocyanine, and present a unified mechanistic picture of the CO2R to CO for these catalysts. Applying the Newns–Andersen model, we find that on MNCs, like TMs, electron transfer to CO2 is facile. We find CO2* adsorption to generally be limiting on TMs, whereas 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 the CO2R to CO activity. We present a unified activity volcano that includes the decisive CO2* and COOH* binding strengths. We show that the increased activity of MNC catalysts is due to the stabilization of larger adsorbate dipoles, which results from their discrete and narrow d states.