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

Selective photoreduction of CO with HO to hydrocarbons is challenged by inadequate and uncontrollable electron and proton feeding. Herein, this limitation is overcome by integrating HO dissociation, CO reduction, and O 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 HO dissociation on the NH-MIL-125 support transfer to the T-COF shell and FeO insert for CO reduction and O evolution, respectively. Mechanistic investigations reveal that increasing NH-MIL-125{001} facet exposure promotes proton spillover, while simultaneously causing more active electrons to accumulate on the T-COF instead of NH-MIL-125. This suppresses H evolution on the NH-MIL-125 core, directing more protons to the T-COF shell for CO reduction. Consequently, the *CO intermediate becomes more prone to hydrogenation to CH rather than desorption to CO or C-C coupling to form C products, thereby progressively increasing CH production while decreasing H, CO, CH, and CH evolution. The three-in-one heterojunction with the highest proportion of NH-MIL-125{001} facets achieves a remarkable CH productivity of 154.3 μmol g h 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 CO hydrogenation.