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

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.