Elucidating protonation pathways in CO 2 photoreduction using the kinetic isotope effect.

The surge in anthropogenic CO 2 emissions from fossil fuel dependence demands innovative solutions, such as artificial photosynthesis, to convert CO 2 into value-added products. Unraveling the CO 2 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 2 photoreduction on TiO 2 nanoparticles, which challenges the long-held assumption of electron-initiated activation. Employing isotopically labeled H 2 O/D 2 O and in-situ diffuse reflectance infrared Fourier transform spectroscopy, we observe H + /D + -protonated intermediates on TiO 2 nanoparticles and capture their inverse decay kinetic isotope effect. Our findings significantly broaden our understanding of the CO 2 uptake mechanism in semiconductor photocatalysts.