Elucidating protonation pathways in CO2 photoreduction using the kinetic isotope effect

Abstract The surge in anthropogenic CO2 emissions from fossil fuel dependence demands innovative solutions, such as artificial photosynthesis, to convert CO2 into value-added products. Unraveling the CO2 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 CO2 photoreduction on TiO2 nanoparticles, which challenges the long-held assumption of electron-initiated activation. Employing isotopically labeled H2O/D2O and in-situ diffuse reflectance infrared Fourier transform spectroscopy, we observe H+/D+-protonated intermediates on TiO2 nanoparticles and capture their inverse decay kinetic isotope effect. Our findings significantly broaden our understanding of the CO2 uptake mechanism in semiconductor photocatalysts.