  Single-atom catalysts (SACs) are rapidly developing in various application
areas, including electrocatalysis of different reactions, usually taking place
under harsh pH-electrode potential conditions. Thus, a full atomic-level
understanding of the nature of the active sites under realistic electrochemical
conditions is needed, having in mind that the state of SACs active centers
could be altered by the adsorption of spectating species. In this contribution,
Density Functional Theory is employed to conduct thermodynamic analysis of SACs
with metal atoms (Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, or Au) embedded
into N4 moiety in graphene. Various surface electrochemical processes on such
SACs are considered, their Pourbaix plots are constructed, and their activity,
selectivity, and stability under operating conditions are discussed. It is
demonstrated how adsorption of H, O and OH can cause blockage and restructuring
of the active sites and alter the electronic structure. Furthermore, when one
deals with metals with lower d-band filling, it is shown that metal center
oxidation is preferred over the oxidation of carbon lattice. The effect of the
state of the metal center on the reactivity of the carbon lattice is discussed
in the case of Fe@N4-graphene. Finally, a possible strategy for confirming the
changes in the architecture of the SACs' active site by analyzing their
vibration spectra is suggested.

Materials Science