Electrochemical CO2 reduction towards formic acid and methanol on transition metal oxide surfaces as a function of CO coverage

Density functional theory is used to study the effect of varying CO coverage on the selectivity and activity of the CO2 reduction reaction (CO2RR) towards methanol and formic acid formation on transition metal oxide (TMO) surfaces. The model system uses twelve TMO surfaces in the rutile crystal structure and (110) facet. Scaling relations and volcano plot analysis are used to determine the TMO catalytic activity trend (overpotential) as CO coverage varies. In agreement with previous results on RuO2, we find that varying CO coverage on TMO surfaces alters the CO2RR selectivity towards methanol, formic acid and H2 suggesting that the product selectivity can be tuned by CO coverage. For all TMOs in this study, 50% CO coverage is optimal requiring lower overpotentials for formic acid and methanol production. However, the selectivity for TiO2 changes towards formic acid at 50% CO coverage. MoO2 and HfO2 are predicted to reduce CO2 to methanol at lower overpotentials having 50% or 75% CO coverage compared to 25% CO coverage. Furthermore, higher CO coverage on TMOs can suppress the hydrogen evolution reaction (HER) slightly. The present study reveals that a moderate amount of CO coverage on TMO surfaces is compulsory to reduce the overpotentials for methanol formation. On the other hand, higher CO coverage on most of the TMOs results in lower overpotentials for formic acid formation since the binding free energy of adsorbates changes significantly due to CO repulsion interactions. The present study highlights the importance of the reaction conditions for CO2 conversion to liquid fuels using TMO-based electrocatalysts.