CO2 Reduction on an Iron-Porphyrin Center: A Computational Study

Carbon dioxide is a greenhouse gas and is available in abundance in the atmosphere. Because of this, many researchers have searched for opportunities to utilize CO2 and convert it into valuable materials. In this work, we study the electrochemical reduction of CO2 to CO on an iron-porphyrin center using computational modeling. We tested two types of iron-porphyrins, namely, the tetraphenylporphyrin (TPP) and meso-(ortho-2-amide-phenyl)(triphenyl)porphyrin ligands. Density functional theory calculations investigated a catalytic cycle that involves a reduction, CO2 binding, two protonations, and another reduction step. We tested several density functional theory methods, basis sets, and model structures. There is a certain degree of variation between the results obtained with different density functional methods, but the same general trends are found. The calculations show that during the reduction processes, a ligand rather than metal reduction takes place, which enables stable binding of CO2 as an [FeIII(CO22-)(TPP-center dot)](2-) complex. The subsequent proton transfer from phenol has a small barrier and is identified as a proton-coupled electron-transfer process, while the second proton transfer does not change the electronic configuration of the metal complex. Overall, the studies show that iron porphyrins are efficient CO2 reducing systems that should be able to turnover CO2 into CO efficiently. Second coordination sphere perturbations influence CO2 positioning but are not seen to have electronic or thermochemical effects on the overall reaction mechanism.