HIT found that hydrogenation pathways for selective photocatalytic CO2-to-CH4 conversion are not exclusively PCET processes

The conversion of CO₂ to CH₄ involves the transfer of eight electrons and eight protons, and thus typically exhibits low selectivity. In previous studies, the photocatalytic reduction of CO₂ to CH₄ has generally been considered to follow the Eley–Rideal (E-R) mechanism, which proceeds via proton-coupled electron transfer (PCET). However, the hydrogenation of CO₂ may also involve a Langmuir–Hinshelwood (L-H) mechanism mediated by surface-adsorbed *H species. Nevertheless, which mechanism actually governs the selectivity has remained unclear.

Recently, HIT constructed an Au₁/CeO₂ photocatalyst by anchoring isolated Au single atoms onto CeO₂ nanosheets, which reversed the hydrogenation pathway and achieved high selectivity (92%) for CH₄ production under CO₂ and H₂O atmospheres. Kinetic isotope effect (KIE) results indicated that the hydrogenation processes of CO₂-to-*CO and *CO-to-CH₄ on Au₁/CeO₂ followed the Eley–Rideal (E-R) and Langmuir–Hinshelwood (L-H) mechanisms, respectively. Owing to the lower Gibbs free energy (∆G) for *CO hydrogenation and the regulated hydrolysis behavior, the developed Au₁/CeO₂ is more favorable for *CO hydrogenation under sufficient *H supply. Combined in situ mass spectrometry, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and density functional theory (DFT) calculations further revealed the critical role of balanced *H supply in achieving selective CH₄ generation over Au₁/CeO₂. This work provides a fresh understanding of the hydrogenation pathway for CH₄ photosynthesis.