DeterminantRole of Electrogenerated Reactive NucleophilicSpecies on Selectivity during Reduction of CO2 Catalyzedby Metalloporphyrins

摘要

This work provides insights to understand the selectivity during the reduction of CO2 with metalloporphyrin (MP) catalysts. The attack of a nucleophile on the carbon of the CO2 appears as an important event that triggers the catalytic reaction, and the nature of this nucleophile determines the selectivity between CO (or further reduced species) and HCOOH/HCOO^–. For MP, the possible electrogenerated nucleophiles are the reduced metal-center and the hydride donor species, metal-hydride and phlorin-hydride ligand. The reduced metal-center activates the CO2 with the formation of the metal–carbon bond, which then gives rise to the formation of CO. The hydride donor species trigger the CO2 reduction by the attack of the hydride on the carbon of the CO2 (formation of a C–H bond), which results in the formation of HCOOH/HCOO^– (formation of the metal-bonded formate intermediate is not involved). The MP with the metals Ni, Cu, Zn, Pd, Ag, Cd, Ga, In, and Sn are predicted to only form the phlorin-hydride intermediate and are thus suitable to produce HCOOH/HCOO^–. Thisagrees well with the available experimental results. The MP with themetals Fe, Co, and Rh can form both the reduced-metal center and thehydride donor species (metal-hydride and phlorin-hydride), and thusare able to form both CO and HCOOH/HCOO^–. The productionof CO for Fe and Co is indeed observed experimentally, but not forRh, probably due to the presence of axial ligands that may hinderthe formation of the metal-bonded intermediates and thus drive theCO2RR to HCOOH/HCOO^– via the phlorin intermediate.