Mechanistic Pathway in the Electrochemical Reduction of CO2 on RuO2

RuO2 has been reported to reduce CO2 electrochemically to methanol at low overpotential. Herein, we have used density functional theory (DFT) to gain insight into the mechanism for CO2 reduction on RuO2(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 CO2 reduction to formic acid (HCOOH), methanol (CH3OH), and methane (CH4) on partially reduced RuO2(110) covered with 0.25 and 0.5 ML of CO*. We have found that CO2 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 H2COOH* is the thermodynamically most difficult step and becomes exergonic at potentials below -0.25 V vs RHE. We have found that CO2 reduction activity on RuO2 changes with CO coverage, which suggests that CO coverage can be used as a tool to tune the CO2 reduction activity. We have shown the mechanism for CO2 reduction on RuO2 to be different from that on Cu. On Cu, hydrocarbons are formed at high Faradaic efficiency through reduction of CO* at similar to 1 V overpotential, while on RuO2, methanol and formate are formed through reduction of formic acid at lower overpotentials. Using our understanding of the CO2 reduction mechanism on RuO2, we suggest reduction of formic acid on RuO2, which should lead to methanol and methane production at relatively low overpotentials.