Mechanistic Pathway in the Electrochemical Reduction of CO 2 on RuO 2

RuO 2 has been reported to reduce CO 2 electrochemically to methanol at low overpotential. Herein, we have used density functional theory (DFT) to gain insight into the mechanism for CO 2 reduction on RuO 2 (110). We have investigated the thermodynamic stability of various surface terminations in the electrochemical environment and found CO covered surfaces to be particularly stable, although their formation might be kinetically limited under mildly reducing conditions. We have identified the lowest free energy pathways for CO 2 reduction to formic acid (HCOOH), methanol (CH 3 OH), and methane (CH 4 ) on partially reduced RuO 2 (110) covered with 0.25 and 0.5 ML of CO*. We have found that CO 2 is reduced to formic acid, which is further reduced to methanol and methane. At 0.25 ML of CO*, the reduction of formate (OCHO*) to formic acid is the thermodynamically most difficult step and becomes exergonic at potentials below −0.43 V vs the reversible hydrogen electrode (RHE). On the other hand, at 0.5 ML of CO*, the reduction of formic acid to H 2 COOH* is the thermodynamically most difficult step and becomes exergonic at potentials below −0.25 V vs RHE. We have found that CO 2 reduction activity on RuO 2 changes with CO coverage, which suggests that CO coverage can be used as a tool to tune the CO 2 reduction activity. We have shown the mechanism for CO 2 reduction on RuO 2 to be different from that on Cu. On Cu, hydrocarbons are formed at high Faradaic efficiency through reduction of CO* at ∼1 V overpotential, while on RuO 2, methanol and formate are formed through reduction of formic acid at lower overpotentials. Using our understanding of the CO 2 reduction mechanism on RuO 2, we suggest reduction of formic acid on RuO 2, which should lead to methanol and methane production at relatively low overpotentials.