Elucidating protonation pathways in CO photoreduction using the kinetic isotope effect.

The surge in anthropogenic CO emissions from fossil fuel dependence demands innovative solutions, such as artificial photosynthesis, to convert CO into value-added products. Unraveling the CO photoreduction mechanism at the molecular level is vital for developing high-performance photocatalysts. Here we show kinetic isotope effect evidence for the contested protonation pathway for CO photoreduction on TiO nanoparticles, which challenges the long-held assumption of electron-initiated activation. Employing isotopically labeled HO/DO and in-situ diffuse reflectance infrared Fourier transform spectroscopy, we observe H/D-protonated intermediates on TiO nanoparticles and capture their inverse decay kinetic isotope effect. Our findings significantly broaden our understanding of the CO uptake mechanism in semiconductor photocatalysts.