Axially coordinated Co-N4 sites for the electroreduction of nitrobenzene

Single-atom cobalt (Co-N-C) catalyst has been proposed as a superior catalyst, but the symmetrical structure of Co-N-4 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-N-4 site (pentafluorophenyl-porphyrin cobalt, CoTFP) and polarizable N-doped graphene (NG) can alter the spatial structure of Co-N-4 site and lead to the transformation of Co(ii) to monovalent Co(i) as the active center (defined as (CoTFP)-T-I@NG). Taking the electroreduction of nitrobenzene (NB) on the as-prepared catalyst as a model reaction, (CoTFP)-T-I@NG showed an increase of about three times the reaction kinetics and turnover frequencies (TOFs) compared with those of (CoTFP)-T-II@OG, where CoTFP was immobilized on O-doped graphene (OG) via regular pi-pi 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-N-4 sites represents an effective and feasible means for the rational ligand-field engineering of single-atom catalysts.

Automated summary

The introduction of axial coordination in the Co-N-4 site of single-atom cobalt catalyst can alter its spatial structure and transform it into an active center, leading to improved catalytic activity. The catalyst with axial coordination showed higher reaction kinetics and turnover frequencies in the electroreduction of nitrobenzene.