Spin-State Control of CO₂ Photoreduction on Defective TiO₂: Decoupling Hydrogenation Selectivity and Charge...

The spin state of catalytic active sites can critically influence the activity and selectivity of photocatalytic CO₂ reduction, yet its microscopic role remains unclear. Here, using anatase TiO₂(001) as a model oxide photocatalyst, we combine first-principles calculations and nonadiabatic molecular dynamics (NA-MD) to reveal how the spin state of vacancy-adjacent Ti sites governs CO₂ activation, protonation pathways, and charge-carrier recombination. Oxygen vacancies together with electron trapping enable strong activation of adsorbed CO₂ into a bent CO₂ ^--like intermediate, and this initial activation is largely determined by defect-assisted charge localization rather than the specific Ti spin state. In contrast, the subsequent hydrogenation chemistry is strongly spin-dependent. High-spin and low-spin Ti sites favor the formation of the *COOH intermediate, whereas the nonspin-polarized Ti site selectively produces the *HCOO intermediate. This selectivity originates from spin-dependent changes in orbital hybridization, interfacial bonding, and reactant adsorption geometry. NA-MD further shows that high-spin and low-spin systems exhibit nanosecond-scale electron-hole recombination, whereas the nonspin-polarized system undergoes much faster trap-mediated recombination. These results establish a correlation among spin state, reaction selectivity, carrier lifetime, and identify spin-state engineering as a promising strategy for improving photocatalytic CO₂ reduction.