An unprecedent hydride transfer pathway for selective photocatalytic reduction of CO2 to formic acid on TiO2

Herein we report that hydrogenated TiO2 can act as hydride donor to realize the selective photocatalytic CO2 reduction to formic acid under UV illumination. In situ FT-IR studies indicated that the adsorption modes of CO2 were divergent on Ar-treated and H-2-treated TiO2, and consequently, CO was generated on Ar-treated TiO2 whereas the exclusive reduction product on hydrogenated TiO2 was formic acid during the photocatalytic CO2 reduction. EPR and XPS spectroscopy further demonstrated that EPR-active O-vacancy was dominant on Ar-treated sample, while the hydrogenation introduced H on the surface to form EPR-silent hydride-like Ti-H-Ti site. The 2D H-1-C-13 solid-state NMR experiments corroborated the specific adsorption of CO2 on this H site. Accordingly, the preferential generation of formic acid should originate from the hydride transfer to the adsorbed CO2. Our results first demonstrate that hydrogenation can produce hydride-like sites and enable TiO2 to act as a hydride reagent during the photocatalytic reactions.