The Effects of Adsorption on the Electronic Structure of Cobalt Porphyrins: Implications for CO\(_{2}\) Reduction

Abstract

Cobalt Porphyrins are promising catalysts for the electrochemical reduction of CO({2}) to CO. However, theoretical studies to date consistently overlook the fact that the catalyst molecules, in the most successful experiments, are adsorbed onto activated carbon fiber surfaces. In this study, we employ Density Functional Theory to explore the electronic structure and reactivity of the cobalt porphyrin, first as an isolated homogeneous catalyst, wherein we confirm that only the doubly-reduced species is active toward CO({2}), and moreover as a catalyst adsorbed to the electrode surface in a variety of configurations. We model porphyrin physisorption onto an activated carbon fiber surface by considering interactions with both pristine graphene and oxidized graphene with a divacancy site. We show that in the former case, the electronic structure of the porphyin is largely unchanged; but in the latter case, significantly altered such that the porphyrin is deactivated toward CO({2}). Next we consider two chemisorption schemes from the literature - namely a condensation reaction which forms a peptide linkage, and the "click" reaction. We compare the thermodynamics of porphyrin attachment, the electronic structures, and CO({2}) binding energies of the two systems. While the peptide-linked system has a slightly stronger binding affinity toward CO({2}), we find the "click" system to be more metallic, with a stronger and more viable mechanism of attachment. After all this, we conclude that CO2 binding requires the donation of 2 electrons from dz(^{2}) orbitals that must be around the same energy, and sufficiently close in energy to the empty orbitals of CO({2}).