Facet engineering of MOF supports regulates product selectivity in CO₂ photoreduction by modulating electron and proton supply to COF shells

Selective photoreduction of CO₂ with H₂O to hydrocarbons is challenged by inadequate and uncontrollable electron and proton feeding. Herein, this limitation is overcome by integrating H₂O dissociation, CO₂ reduction, and O₂ evolution catalysts into a dual S-scheme heterojunction and regulating exposed facets of the heterojunction supports. In this design, H⁺ and OH^- species generated by H₂O dissociation on the NH₂-MIL-125 support transfer to the T-COF shell and Fe₂O₃ insert for CO₂ reduction and O₂ evolution, respectively. Mechanistic investigations reveal that increasing NH₂-MIL-125{001} facet exposure promotes proton spillover, while simultaneously causing more active electrons to accumulate on the T-COF instead of NH₂-MIL-125. This suppresses H₂ evolution on the NH₂-MIL-125 core, directing more protons to the T-COF shell for CO₂ reduction. Consequently, the *CO intermediate becomes more prone to hydrogenation to CH₄ rather than desorption to CO or C-C coupling to form C₂ products, thereby progressively increasing CH₄ production while decreasing H₂, CO, C₂H₄, and C₂H₆ evolution. The three-in-one heterojunction with the highest proportion of NH₂-MIL-125{001} facets achieves a remarkable CH₄ productivity of 154.3 μmol g_cat ^-1 h^-1 with a selectivity of 87.4%. This work highlights the synergistic advantages of heterojunction construction and facet engineering in concurrently optimizing electron and proton supply for CO₂ hydrogenation.