CO2 Photoreduction on Mixed Ti/Zr-MOF-525: Bicarbonate as the Active Intermediate and the Role of Ti Substitution

Abstract

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 OHpassive 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 via a bicarbonate-mediated mechanism, where surface -OH attacks adsorbed CO2 to form node-bound *HCO3. This step is both thermodynamically favorable and kinetically accessible (ΔG ‡ < 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.