Axially coordinated Co–N4 sites for the electroreduction of nitrobenzene

Abstract

Single-atom cobalt (Co–N–C) catalyst has been proposed as a superior catalyst, but the symmetrical structure of Co–N₄ shows unfavorable energetics, leading to much debate on its chemical nature responsible for its high activity. We found that the axial coordination between the well-defined Co–N₄ site (pentafluorophenyl-porphyrin cobalt, CoTFP) and polarizable N-doped graphene (NG) can alter the spatial structure of Co–N₄ site and lead to the transformation of Co(II) to monovalent Co(I) as the active center (defined as Co^ITFP@NG). Taking the electroreduction of nitrobenzene (NB) on the as-prepared catalyst as a model reaction, Co^ITFP@NG showed an increase of about three times the reaction kinetics and turnover frequencies (TOFs) compared with those of Co^IITFP@OG, where CoTFP was immobilized on O-doped graphene (OG) via regular π–π stacking interactions. Augmented by density functional theory (DFT) calculations, axial bonds can serve as communication channels for electron transfer, deriving a decrease in the redox potential and then contributing to the catalytic reduction of NB. Intelligently prefabricating an axial coordination for Co–N₄ sites represents an effective and feasible means for the rational ligand-field engineering of single-atom catalysts.