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. [Figure not available: see fulltext.]