Tuning the Product Selectivity of Single-Atom Catalysts for CO2 Reduction Beyond CO Formation by Orbital Engineering

Abstract

Electrochemical CO2 reduction (CO2R) is one of the promising strategies for developing sustainable energy resources. Single-atom catalysts (SACs) have emerged as efficient catalysts for CO2R. However, the efficiency of SACs for the formation of reduction products beyond the two-step CO formation is low due to the lower binding strength of the CO intermediate. In this study, we show an orbital engineering strategy based upon the density functional theory calculations and fragment molecular orbital approach to tune the product selectivity for CO2R reaction on macrocyclic-based molecular catalysts (porphyrin and phthalocyanine) and extended SACs (graphene and covalent organic framework) with Fe, Co, and Ni dopants. The introduction of neutral axial ligands such as imidazole, pyridine, and trimethyl phosphine to the metal dopants enhances the binding affinity of CO intermediate. The stability of the catalysts is investigated by the thermodynamic binding energy of the axial ligands and ab initio molecular dynamics simulations (AIMD). The grand canonical potential method is used to determine the reaction free energy values. Using a unified activity volcano plot based upon the reaction free energy values, we have investigated the catalytic activity, and product selectivity at an applied potential of -0.8 V vs SHE and pH of 6.8. We have found that with the imidazole and pyridine axial ligands, the selectivity of Fe-doped SACs towards the formation of methanol product is improved. The activity volcano plot for these SACs shows a similar activity as the Cu (211) surface. The catalytic activity is found to be directly proportional to the sigma-donating ability of the axial ligands.