Linker Functionalization and pH Tuning Enhance Solar-Driven Catalytic CO$_2$ Reduction in MOF-5
The wide band gap of metal-organic framework (MOF) 5 constrains its use in photocatalytic carbon dioxide (CO$_2$) reduction despite its high porosity and favorable mass-transport properties. Adopting a state-of-the-art first-principles approach, we systematically investigate the effects of volumetric strain, metal-node substitution, linker functionalization, and pH control as knobs to improve the CO$_2$ photocatalytic ability of MOF-5. Strain and metal-node substitution negligibly affect the gap, whereas linker functionalization narrows it into the visible range via in-gap states while preserving reduction-side alignment at pH = 0. The resulting reduction energetics are strongly sensitive to both linker functionalization and pH. Halogenated and hydroxylated frameworks provide access primarily to HCOOH, CO, and HCHO under alkaline conditions, principally with the Mg and Zn nodes, whereas COOH functionalization offers the broadest thermodynamic accessibility across the full CO$_2$ reduction sequence. NH$_2$ retains thermodynamic feasibility for all target reduction pathways but with larger overpotentials, while NO$_2$ generally yields unfavorable reduction energetics. Crucially, within the COOH series, the choice of the metal node tunes the fundamental gap by over 1 eV with only minor changes in the reduction overpotentials, placing Sr- and Ba-based architectures as the most favorable ones for broad product selectivity with visible-light excitation. Linker functionalization substantially reduces the spatial overlap of the frontier states, promoting photoinduced charge separation. Taken together, these results establish linker functionalization and solution pH as complementary design levers for independently tuning light absorption and CO$_2$-reduction energetics in MOF-5, establishing a rational and viable route for designing efficient MOF-based photocatalysts.