CO ₂ photoreduction on mixed Ti/Zr-MOF-525: bicarbonate as the active intermediate and the role of Ti substitution

The photocatalytic reduction of CO₂ in metal-organic frameworks (MOFs) offers a sustainable route to C₁ fuels and chemicals. Herein, density functional theory (DFT) calculations elucidate CO₂ reduction on mixed Ti/Zr-MOF-525 clusters bearing missing linker defects, modeled by Zr₆, Ti₁Zr₅, and Ti₂Zr₄ clusters. Two distinct mechanistic pathways are identified: the OH-passive and OH-assisted routes. In the passive case, CO₂ binds weakly at a coordinatively unsaturated Ti/Zr site and undergoes direct hydrogenation to CO and HCOOH, with desorption being thermodynamically preferred over further hydrogenation. In contrast, the OH-assisted pathway proceeds <i>via</i> a bicarbonate-mediated mechanism, where surface -OH attacks adsorbed CO₂ to form node-bound *HCO₃. This step is both thermodynamically favorable and kinetically accessible (Δ<i>G</i>^‡ < 0.5 eV). Subsequent proton-electron additions convert *HCO₃ to *OCHO and H₂O, favored by ∼1 eV over competing routes. These findings identify *HCO₃ as the true reactive precursor and reveal that Ti substitution promotes deeper hydrogenation beyond two-electron products, enhancing CH₄ formation on the Ti₂Zr₄ cluster. Overall, the results highlight the importance of node composition and surface hydroxyl groups in porphyrinic MOFs for optimizing multi-electron CO₂ reduction and controlling product selectivity by tailoring the metal node environments.