Impact of nonadiabatic charge transfer on the rate of redox chemistry of carbon oxides on rutile TiO₂(110) surface

We present the results of a density functional theory (DFT) within the LDA+U approximation on large models of the partially reduced TiO₂(110) rutile surface to investigate the nature of charge transfer and the role of nonadiabatic effects on three prototypical redox reactions: (i) O₂ adsorption, (ii) CO oxidation, and (iii) CO₂ reduction. Charge-constrained DFT (cDFT) is used to estimate kinetic parameters for a Marcus theory rate law that accounts for adiabatic coupling effects on reaction rates. We find that for O₂ adsorption, the coupling between adiabatic states is strong, leading to fast charge transfer rates. The lowest energy structures at high coverage consist of two chemisorbed O₂^-, one adsorbed at a V_O site and the other adsorbed at an adjacent Ti_5C site. For CO oxidation, however, all reactions are kinetically hindered on the ground state because of the weak adiabatic coupling at the state crossing, such that one has to overcome two kinetically unfavorable charge transfer events to drive the process (nonadiabatically) on the thermal ground state. The process can be driven by photochemical means but would result in an adsorbed radical [OCOO^-] intermediate species. Similarly, CO₂ reduction also proceeds via a nonadiabatic charge transfer to form an adsorbed CO₂^- species, followed by a second nonadiabatic charge transfer to produce CO. Our analysis provides important computational guidelines for modeling these types of processes.