Unveiling the Mechanism of Photocatalytic CO₂ Cycloaddition over Linker-Engineered Metal-Organic Frameworks

Photocatalytic CO₂ cycloaddition represents a promising route for solar-driven synthesis of value-added C₂₊ chemicals and simultaneously mitigating anthropogenic CO₂ emissions. However, the pivotal step of direct one-electron reduction of CO₂ to CO₂ ^•- requires a very high reduction potential of -1.9 V versus NHE, posing a formidable challenge. In this study, cerium-based metal-organic frameworks (MOFs) with linker-induced defects, specifically Ce-UiO-66-X (X = Me, H, and F), are investigated to elucidate the underlying mechanisms of photocatalytic CO₂ cycloaddition. Among them, Ce-UiO-66-H, which strikes an optimal balance between light absorption and charge separation, demonstrates superior catalytic performance (yield > 90%) when coupled with tetrabutylammonium bromide (TBAB) as a co-catalyst. In-situ experiments and theoretical calculations reveal that TBAB stabilizes CO₂ through the formation of [Br^-···TBA⁺]∼CO₂ adducts, which lowers the thermodynamic energy requirement for CO₂ ^•- generation from 0.66 eV (in the direct CO₂-to-CO₂ ^•- route) to -0.90 eV. This potential modulation promotes efficient photoelectron transfer from the MOFs to CO₂, substantially enhancing the overall cycloaddition efficiency.