## 3 Probing the Metal-Support Interactions at the Interface
### X-ray Absorption Spectroscopy (XAS)  
XAS measurements describe intermolecular interactions, probing the valence and local structures of investigated elements. The carbon and nitrogen K-edge and metal L-edge near-edge X-ray absorption fine structure (NEXAFS) spectra together with the extended X-ray absorption fine structure (EXAFS) data provide a detailed picture of chemical and electronic structure speciation at the metal-support interface.  
Concerning carbon K-edge spectra, the introduction of nitrogen in graphene sheet has been reported to produce a shift in the C-C \(\sigma^{\star}\) peak (286.5 eV), a shift to higher energy and a broadening of the C-C \(\pi^{\star}\) peak (292.8 eV), and the presence of an additional sharp peak in the region between the \(\pi^{\star}\) and \(\sigma^{\star}\) resonance (289.1 eV), which is related to the introduction of structural defects. Moreover, other peaks can appear due to \(\pi^{\star}\) transitions deriving from the introduction of functionalities (e.g., amine or imide groups) [89].  
Nitrogen K-edge spectra is usually dominated by \(\pi^{\star}\)-transitions at 398.5 eV, 400 eV, and 401.4 eV, which can be credited to pyridinic, amino, and pyrrolic/graphitic type N-groups. The immobilization of metal on the surface on N-doped surfaces results in the decrease of the \(\pi^{\star}\) resonance intensity in N K-edge NEXAFS spectra (Figure 7). This evidence can be explained by invoking the Stohr model: a metal-support \(\pi\) back-bonding interaction involves the N1\(\sigma\)-\(\pi^{\star}\) transition. Thus, a \(\sigma\) donor is formed by the lone pair orbital of the pyridine-like N, which is stabilized by the back-bonding of metal d\(\pi\)-p\(\pi\) orbitals with the N \(\pi^{\star}\) antibonding orbitals. Due to the enhanced bond strength, the metal d\(\pi\)-p\(\pi\) contributes in a greater extent to the \(\pi^{\star}\) orbital, thus producing the decrease of \(\pi^{\star}\) resonance intensity. In addition, a shoulder to the lower energy side of the \(\pi^{\star}\) resonance appears, indicating a charge localization on the N atom and a charge transfer between donor/acceptor sites [60].  
Figure 7: Near-edge X-ray absorption fine structure (NEXAFS) Nitrogen K-edge spectra for the N-doped surfaces. Reprinted with permission from reference [63]. Copyright 2016 American Chemical Society.  
In some cases, a new peak at \(\sim\)398.9 eV has been observed after Pd deposition, which is ascribable to the nitrogen species interacting with Pd. [63].  
Also, the K-edge and L-edge NEXAFS spectra of the metal are affected by the interaction with the heteroatoms incorporated in the support, inducing changes in electron occupancy in the valence orbital and ligand field environments. As an instance, monitoring Co K-edge XANES provided information on the oxidation state of Co and allowed unraveling strong covalent interactions between Co\({}_{3}\)O\({}_{4}\) and N-graphene, resulting in a partial reduction of Co\({}^{3+}\)[90]. A shift toward higher energy in K-edge of the X-ray absorption near-edge structure spectra (XANES) for Pd@PdO on nitrogen-doped graphene revealed a change of the metal's electronic structure and strong interface interaction between the Pd@PdO particle and nitrogen-doped graphene [80].  
In many cases, evidence of XANES is corroborated by EXAFS data, reporting lower metal-metal coordination numbers compared with bulk materials (i.e., high metal dispersion in the form of small nanoparticles), shorter metal-heteroatom and longer metal-metal distances, which are indicative of the strong character of metal-support interaction and demonstrate that these interactions are mediated by the heteroatom present in the support surface [80].