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Electrochemical reduction of carbon dioxide (CO2)
over transition metals follows a complex reaction network. Even for products
with a single carbon atom (C1 products), two bifurcated pathways
exist: initially between carboxyl (COOH*) and formate (HCOO*) intermediates and
the COOH* intermediate is further bifurcated by pathways involving either formyl
(CHO*) or COH*. In this study, we combine evidence from the experimental
literature with a theoretical analysis of energetics to rationalize that not
all steps in the reduction of CO2 are electrochemical. This insight
enables us to create a selectivity map for two-electron products (carbon
monoxide (CO) and formate) on elemental metal surfaces using only the CO and OH
binding energies as descriptors. In the further reduction of CO*, we
find that CHO* is formed through a chemical step only whereas COH* follows from
an electrochemical step. Notably on Cu(100), the COH pathway becomes dominant
at an applied potential lower than −0.5V vs. RHE. For the
elemental metals selective towards CO formation, the variation of the CO
binding energy is sufficient to further subdivide the map into domains that
predominantly form H2, CO, and ultimately more reduced products. We
find Cu to be the only elemental metal capable of reducing CO2 to
products beyond 2e− via the proposed COH pathway and we identify atomic
carbon as the key component leading to the production of methane. Our analysis also
rationalizes experimentally observed differences in products between thermal
and electrochemical reduction of CO2 on Cu.