▎ 摘　　要

Single Atom Catalysts (SAC) in graphene have been recently gaining more and more attention. They are usually non-noble transition metal (TM) adatoms getting trapped at the carbon vacancies during the fabrication of the graphene layer, which then act as active centers for catalysis and adsorption. In this work we present a systematic and comparative investigation, by means of dispersionecorrected density functional theory (DFT) calculations, of Fe, Co, Ni, and Cu as possible SACs when they become trapped at graphene C vacancies. The stability of these TM atoms is further increased by introducing pyridinic nitrogen (N) atoms and transforming graphene into a giant porphyrin-like macrocyclic ligand. The structural, electronic and energetics properties of these systems, even under the effect of a metal substrate (weakly interacting Cu (111) or strongly interacting Ni (111)), are comparatively examined in great detail by means of crystal/ligand field theories and through ad-hoc energy decomposition analysis to highlight trends and peculiar behaviors. The position of the TM d-orbitals with respect to the Fermi level of the whole system is of considerable importance for designing prospective device applications in catalysis, electrocatalysis and sensors. To this purpose, we also examine how the reactivity of the SACs in graphene towards the hydrogen evolution reaction (HER) can be tuned with N-doping and with different substrates. (C) 2020 Elsevier Ltd. All rights reserved.