Insight into photocatalytic CO₂ reduction on TiO₂-supported Cu nanorods: a DFT study on the reaction mechanism and selectivity

Photoreduction of CO₂ into hydrocarbons is a potential strategy for reducing atmospheric CO₂ and effectively utilizing carbon resources. Cu-deposited TiO₂ photocatalysts stand out in this area due to their good photocatalytic activity and potential methanol selectivity. However, the underlying mechanism and factors controlling product selectivity remain less understood. Using first-principles calculations, this study systematically investigates the possible reaction network for CO₂ photocatalytic reduction on TiO₂ supported Cu-nanorods (nr-Cu/TiO₂), driven by the surface-bound *H species generated via a Volmer-like process (H⁺ + e^- + * → *H). Our results reveal that the initial hydrogenation of CO₂ on nr-Cu/TiO₂ is energetically more favorable via the formate (HCOO) pathway than the carboxyl (COOH) route. Notably, HCOO undergoes further hydrogenation for effective C-O bond cleavage, with H₂COOH identified as the key intermediate. Both CO (CO₂ → HCOO → H₂COOH → H₂CO → CO) and CH₃OH (CO₂ → HCOO → H₂COOH → H₂CO → CH₃OH) production share the H₂CO intermediate, with CO formation proceeding via an unexpected "forth-back" mechanism. Energy profiles suggest that CH₃OH formation is more favorable than CO formation. Additionally, excess photogenerated electrons were found to enhance CO₂ activation and C-O bond cleavage to some extent but have minimal impact on other reaction steps. This study provides atomic-level insights into the CO₂ photoreduction mechanism, offering potential guidance for improving product selectivity.