This study investigates the protonation pathway for CO₂ photoreduction on TiO₂ nanoparticles using kinetic isotope effect (KIE) evidence, challenging the long-held assumption of electron-initiated activation. By employing isotopically labeled H₂O/D₂O and in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), the researchers observed H⁺/D⁺-protonated intermediates and captured their inverse decay KIE. The findings provide significant insights into the CO₂ uptake mechanism in semiconductor photocatalysts, suggesting that the protonation pathway involving the formation of O=C-OH/D intermediates is more plausible than the electron-initiated pathway. The study also confirms that this mechanism is intrinsic to TiO₂ materials, regardless of their crystal structure, exposed facets, or oxygen vacancy concentration. These results have implications for the development of more efficient and sustainable photocatalytic CO₂ reduction technologies.This study investigates the protonation pathway for CO₂ photoreduction on TiO₂ nanoparticles using kinetic isotope effect (KIE) evidence, challenging the long-held assumption of electron-initiated activation. By employing isotopically labeled H₂O/D₂O and in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), the researchers observed H⁺/D⁺-protonated intermediates and captured their inverse decay KIE. The findings provide significant insights into the CO₂ uptake mechanism in semiconductor photocatalysts, suggesting that the protonation pathway involving the formation of O=C-OH/D intermediates is more plausible than the electron-initiated pathway. The study also confirms that this mechanism is intrinsic to TiO₂ materials, regardless of their crystal structure, exposed facets, or oxygen vacancy concentration. These results have implications for the development of more efficient and sustainable photocatalytic CO₂ reduction technologies.