## 2 Theoretical Studies on Heteroatom-Mediated Metal-Support Interactions
### Metal Single-Atom Adsorption on Doped Carbon Surfaces
Conflicting conclusions were deduced regarding the influence of the delocalized \(\pi\)-electron system of the graphene surface on the electron density redistribution in the case of Pt and Pd atoms. The comparison of the electron density surfaces for N-doped graphene fragments before and after the interaction with the Pd atom suggests that the interaction with Pd induces a more positive charging of N atoms compared to the initial states. Otherwise, looking at natural bond orbital (NBO) analysis, the interaction of the Pt atom with N\({}_{\rm pyrr}\) sites imparts a partial positive charge to the metal center (except for the multiple interactions with two N\({}_{\rm pyrr}\) atoms at the zigzag edge). Conversely, electron density is transferred from the support to Pt, when the latter is bonded close to N\({}_{\rm gr}\). It is obvious that also the electronic structure of the metal plays a central role. According to [PERSON] and [PERSON] [64], the interaction of the transition metals tends to increase from right to left, while it decreases as one progresses down (from top to bottom) a group on the periodic table. The number of D electrons, and then, the position of D-band center related to the Fermi energy, play a crucial role. This appears more evident when the study is extended to other transition metal atoms. [PERSON] et al. [59] used density functional theory (DFT) calculations to investigate the adsorption of 11 transition metal atoms (i.e., Au, Pt, Ru, Pd, Ag, Fe, Co, Ni, Cu, W, and Ti) on the sidewalls of boron and nitrogen-doped single-wall carbon nanotubes (B-SWCNTs and N-SWCNTs). In general, the highest binding energies were obtained for Sc (3d\({}^{1}\)4s\({}^{2}\)), Ti (3d\({}^{2}\)4s\({}^{2}\)), and Fe(3d\({}^{6}\)4s\({}^{2}\)), which was likely due to their capability to easily lose one or two electrons for reaching the full or half-filled steady states [65]. Again, an enhanced binding energy was observed for boron-doped surfaces, whereas N-doping produces a slighter increase in the interaction energies. In addition, a quenching to zero of the spin-magnetic moment (\(\upmu\)) of the individual metal atoms was predicted for almost all of the species (except Fe and W) upon their bonding on doped carbon nanotubes [59]. Mulliken population analysis [65] indicated the occurrence of charge transfer ranging from 1.32 e (for Sc) to 0.40 e (for Cu and Pt). Ab initio molecular dynamics (AIMD) simulations allowed investigating the short-time mobility of Pt and Ru clusters on pristine and boron-doped carbon over a range of different temperatures for a very short period. The evaluation of the mean squared displacement (MSD) provided useful details on the stability of metal atoms at increasing temperature. As expected, metal atoms tend to migrate easier on an undoped surface compared to the boron-doped one. However, the differences in MSD over the two surfaces became even less evident as the temperature increases (up to 873 K). In any case, AIMD operates in a restricted simulation time and size of the system, which are not sufficient to obtain results with general validity. Classical molecular dynamics simulations could guarantee a longer observation time. However, a proper analytical potential should be selected to adequately take into account the metal-support interactions. Besides tuning the interaction with the adsorbed metal species, the doping can also address the insertion of oxygen functionalities. The adsorption and the stability of epoxide and hydroxyl groups on undoped and B-doped graphene surfaces has been considered [57]. Epoxide functionalities are created under the adsorption of oxygen atoms on the bridge site connecting the boron and carbon atoms. The preferential site for hydroxyl group adsorption was the boron on-top site. Furthermore, the B-doping positively affects the adsorption of oxygen functionalities.