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

Abstract

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.