## 3 Probing the Metal-Support Interactions at the Interface
### X-ray Photoelectron Spectroscopy (XPS)  
XPS is a powerful tool that is crucial for detecting the surface layer element composition, chemical state, and electronic structure of metal and carbon materials. Successively, it has been widely used to study the electronic structure change and the doping effect in supported metal nanostructures on doped carbon materials. Significant shifts in binding energy values are symptomatic of electronic structure alterations resulting from metal-support interactions. However, a core level binding energy shift can be ascribed to three different factors (changes in the local electronic structure, changes in the relaxation process, and cluster charging), and it is often difficult to discriminate among them. In some cases, the interaction can produce a new electronic state that is detectable as an additional peak (or a peak component in the case of complex signals) in the XPS spectrum. Among transition metals, noble metals (Pd, Pt, Au) are the most studied systems, due to their large use as catalysts in the form of supported metal nanoparticles. The strong interaction between Pd species and nitrogen functionalities embedded in carbon-based support has been reported in several papers. In most cases, an increase in the Pd\({}^{2+}\)/Pd\({}^{0}\) ratio and a shift to the higher energy of the Pd \(3d\) signal have been observed and ascribed to a change in the electronic structure or in the coordination environment [76]. The simultaneous analysis of N \(1s\) signals allowed unraveling a prevalent interaction of Pd species with pyridinic and pyrrolic nitrogen [68]. This interaction leads to a charge transfer between the metal and the carbon support, resulting in the stabilization of electron-depleted palladium atoms (Pd\({}^{8+}\)), whose signal has been detected in XPS spectra [67; 69]. In some cases, a very strong bond between Pd\({}^{2+}\) precursor ions and pyridinic nitrogenis created before and during the synthesis, resulting in the low reducibility of palladium species, and thus in the stabilization of isolated Pd\({}^{2+}\) ions in the final catalyst [63]. [PERSON] et al. [60; 77] proposed a convincing rationalization of the Pd _3d_ XPS signal of Pd NPs supported on nitrogen-doped carbon nanotubes. Four components were separated under the broad and complex Pd _3d_ signal (Figure 6). The signal at lower binding energy (Pd1, 335 eV) was attributed to bulk-like metallic palladium, and was associated with large Pd particles formed under drastic annealing conditions. The Pd2 signal (335.55\(-\)335.7 eV) accounted for metallic Pd nanoparticles electronically interacting with the carbon support. These species were predominant in the samples after calcination and reduction. The Pd3 signal (336.9 eV) was related to partially positive charged Pd atoms located at the interface, while the Pd4 component (337.8 eV) is generated by divalent Pd species, namely atomically dispersed Pd. The latter were stabilized by pyridinic nitrogen and possessed low reducibility. After annealing at 300 \({}^{\circ}\)C, Pd3 and Pd4 species evolved toward the formation of Pd2 species. The interpretation of XPS spectra is not trivial, and often, opposing results can appear in the literature. Specifically, in the case of Pd-N interactions, the presence of reduced Pd species [78] or negatively charged Pd has been reported because of an increased electron density by the charge transferred from N to the Pd species. The fact that these species have been revealed when prepared by the pyrolysis of chitosan and melamine or dicyandiamide [79] and Ketjen black carbon [80] suggests a possible role of the synthetic routes in addressing metal-support interactions. Similar results were obtained in the case of Pt-N and Au-N [81] interactions. The Pt 4f spectra usually present three transitions corresponding to the different oxidation states of Pt (Pt\({}^{0}\), Pt\({}^{2+}\), and Pt\({}^{4+}\)).