## 2 Theoretical Studies on Heteroatom-Mediated Metal-Support Interactions
### Metal Single-Atom Adsorption on Doped Carbon Surfaces
However, it is worth demonstrating that the coordination of nitrogen species in the carbon framework can determine the interaction of metal atoms with nitrogen-doped supports. Indeed, when a nitrogen atom replaces a carbon atom of graphitic plane, it will be bonded with three carbon neighbors, thus generating a so-called graphitic nitrogen: a sp\({}^{2}\) nitrogen species. Nevertheless, graphitic nitrogen is not the unique species that can be created in an N-doped surface. The substitution of a carbon atom with nitrogen can occur according to almost three primary structural models differing in their nitrogen bonding configuration (Figure 4): pyridine-like (N\({}_{\rm pyrr}\)), pyrrole-like (N\({}_{\rm H}\)), and graphitic (N\({}_{\rm grf}\)) [21]. Pyridinic nitrogen refers to sp\({}^{2}\) two-fold coordinated nitrogen atoms contributing one p electron to the \(\pi\)-system. It could be incorporated as a vacancy (N\({}_{\rm v}\)) or as atom on an edge site (N\({}_{\rm e}\)). Nitrogen atom in pyrrole-like configuration is an sp\({}^{3}\) coordinated (usually in a five-membered ring) species sharing two p electrons in the \(\pi\) system. Doping with pyridine-like, pyrrole-like, and graphitic nitrogen species impacts differently on the electronic band structure of graphitic carbon materials, having unavoidable implications for the interactions with adsorbates. The adsorption of single atoms of Pt-group metals on graphitic surfaces containing nitrogen species with different coordination has been modeled by DFT. The results from theoretical modeling unanimously identify the following ascending order of the binding energies of noble metals: N(graphitic) > N(pyrrolic) > N(pyridinic) [60; 61; 62]. [PERSON] et al. [63] modeled the adsorption of a Pd atom on four different nitrogen forms: graphitic (N\({}_{\rm grf}\)), pyrrole (N\({}_{\rm H}\)), pyridinic at the monovacancy boundary (N\({}_{\rm v}\)), and pyridinic at the edge (N\({}_{\rm e}\)) nitrogen atoms, probing different adsorption sites in the graphene fragment. The obtained relaxed structures and binding energies revealed that the Pd atom preferentially interacts with pyridinic nitrogen. A similar behavior was observed with other Pt-group metals (Pt, Ru). In general, all Pt-group metal atoms strongly interact with N\({}_{\rm pyrr}\) sites, although the interaction with the edge N\({}_{\rm pyrr}\) sites is significantly weaker than the one with the vacancy (mono or di) N\({}_{\rm pyrr}\) atoms. This evidence can be rationalized considering the number and the stability of the bonds generated upon the interaction between metal and vacancy N\({}_{\rm pyrr}\) atoms. In particular, the highest affinity for divacancy N\({}_{\rm pyrr}\) atoms can be related to the formation of four stable bonds among the central metal atom and the adjacent heteroatom and carbon atoms. The strong interaction between metal atoms  
Figure 4: Bonding configurations for N in graphitic networks. Reprinted with permission from reference [18]. Copyright 2009 American Chemical Society. and monovacancy N\({}_{\rm pyrr}\) atoms is associated with the breaking of a C-C bond in the pentagon ring, the formation of three new bonds, and the concomitant creation of three more stable six-member ring systems. Differently, just two and one new bonds are formed when metal atoms are adsorbed on N\({}_{\rm pyrr}\) atoms on the armchair or zigzag graphene edge, respectively. As expected, the interaction with graphitic nitrogen is not energetically favorable. Pd and Pt atoms do not bind to this site, whilst they prefer to migrate to the neighboring carbon sites and locate above the center of the C-C bond, forming a bridge with two carbon atoms. Ru is the only one that is able to interact directly with the N\({}_{\rm gr}\) nitrogen, forming a bond with this atom and two neighboring carbon atoms. The sequence for the binding energy of metal atoms with a particular site was Ru > Pt > Pd, which is in agreement with [PERSON]'s results [57].