Mechanistic insights into CO₂ reduction to CO by group 5 transition metal monoxide cations

The reduction of carbon dioxide (CO₂) by transition-metal oxides in the gas phase serves as a unique model system for understanding transition metal-based catalytic systems in CO₂ utilization. In this work, thermochemistry and reaction mechanisms attributed to the two-state reactivity scenario of CO₂ reduction by group 5 transition metal monoxide cations are extensively investigated using quantum chemical calculations. The interaction between the VO⁺ cation with CO₂ exhibits an endothermic feature, whereas the reaction involving the TaO⁺ cation showcases a more pronounced exothermic behavior than the NbO⁺ cation, in accordance with previously reported reaction rates. Based on in-depth examinations of potential energy surfaces and spin-orbit couplings, it has been revealed that the reaction kinetics of CO₂ reduction to CO by the VO⁺ cation is restricted not only by a significant energy barrier related to the singlet transition state, but also by the limited probability of intersystem crossing. For NbO⁺ and TaO⁺ cations, the spin inversion from triplet to singlet pathways becomes the rate-limiting step. The reaction with the TaO⁺ cation represents a different case from typical two-state reactivity patterns, where the minimum energy crossing point submerged relative to the reactants level stands for the exclusive barrier. A considerably higher probability of intersystem crossing was identified for the reaction of the TaO⁺ cation with CO₂, elucidating the basis for the substantial increase in the rate constant compared to that of the NbO⁺ cation.