Can a single valence electron alter the electrocatalytic activity and selectivity for CO₂ reduction on the subnanometer scale?

Electrocatalytic reduction of CO₂ (CO₂RR) is an excellent strategy for addressing the issue of both ever-increasing anthropogenic CO₂ emissions as well as the rapid diminishing of nonrenewable fossil reserves. Recently, significant attention has focused on the development of size-selected subnanometer nanocatalysts because of the unique electronic, geometric, and catalytic properties of these clusters, which often exhibit enhanced catalytic activities and selectivities compared to bulk metal catalysts and larger nanoparticles. In this paper, we investigate in detail the electrocatalytic activity of size-selected Cu_n clusters (n = 3–6) employing the computational hydrogen electrode model. We have found a striking similarity between the CO₂RR activity of Cu₃ and Cu₅ and between Cu₄ and Cu6nanoclusters. The reaction proceeds through * + CO₂ → COOH* → CO* + H₂O → CHO* → CH₂O* → CH₃O* → O* + CH₄ → OH* → * + H₂O as in the case of copper surface on all Cu clusters. The rate-limiting potential on Cu₄ and Cu6clusters is the proton–electron (H⁺ + e^–) transfer to CO* to form the CHO* adsorbed species, which is also the rate-limiting step on Cu surfaces, whereas on Cu₃ and Cu₅ clusters, it is the removal of the adsorbed OH* from the cluster surface (OH* → * + H₂O). Most importantly, we have identified a general trend in the exergonicity and endergonicity of each step with the spin-state of the nanocluster. In general, electrochemical steps corresponding to an odd total number of (H⁺ + e^–) pair transfers, leading to the formation of the doublet adsorbed species on Cu₄ and Cu₆ clusters, are highly endergonic uphill processes relative to the same steps on Cu₃ and Cu₅ clusters. However, the steps corresponding to an even total number of proton–electron pair transfers, leading to the formation of singlet adsorbed species on Cu₄ and Cu₆ clusters, are highly exergonic downhill processes relative to the same steps on Cu₃ and Cu₅. We have also found that the competing hydrogen evolution reaction is more hindered on Cu₃ and Cu₅ compared to Cu₄ and Cu₆ clusters. There is also a general qualitative relationship between the exer/endergonicity of an electrochemical step and the HOMO–LUMO gap of the various cluster–adsorbate complexes. We have found that an increase or decrease of a single valence electron can significantly alter the electrocatalytic activity and reactivity on the subnanometer level, and this has great implications in the design and development of size-selected nanoclusters for CO₂RR and similar reactions.