## 3 Results
### Characterization of Milled and Unmilled Oxidized Coal Char Samples
It is worth noting the correspondence among the charging-induced shifts in the photoemission spectra of the same sample measured at the same photon energy (650 eV): all three core levels under consideration (O\({}_{1\text{s}}\), Si\({}_{2\text{p}}\), and Al\({}_{2\text{p}}\)) show a shift of about 9.5 eV for the 5 min milled sample and a shift of about 6.5 eV for the 30 min milled sample. This observation is consistent with the common origin of the spectral shifts, namely differential charging. Relating spectroscopic features to the milling time is not straightforward. The emergence of a pronounced peak shifted by about 9 eV toward higher energies is observed after 5 min of milling. Further milling for 30 min is associated with the appearance of a peak of comparable intensity but with a smaller shift, nearly 6 eV, in the resulting spectrum. It may be speculated that early milling discloses ash inclusions of fairly coarse sizes, whereas prolonged milling reduces the size of disclosed mineral inclusions so that the related shift is attenuated. Figure 4a-c report the C\({}_{1\text{s}}\) spectra of the three coal char samples acquired at 400 eV photon energy. The spectral patterns associated with differential charging are still detected, but they are less pronounced than in spectral analysis performed at a 650 eV photon energy. This finding may be attributed to the larger kinetic energy for the C\({}_{1\text{s}}\) photoelectrons (about 370 eV kinetic energy at 650 eV vs. 120 eV kinetic energy for 400 eV photons) that allows us to probe a deeper fraction of the insulating particles, thereby increasing the amount of signal shifted to higher binding energy. On the other hand, the largest part of the C\({}_{1\text{s}}\) signal is still due to the conductive carbonaceous matrix of the char and does not experience charging-related shifts: only a small fraction of carbon is localized in ash and gives rise to the structure at high binding energy, which is much smaller compared to what is observed in the O\({}_{1\text{s}}\) spectra. Deconvolution of the C\({}_{1\text{s}}\) spectra measured at 400 eV is also reported in Figure 4a-c. The unmilled sample shows a main component peak at about 284.54 eV. This main component represents \(sp^{2}\) carbons in C=C bonds in aromatic domains of unoxidized carbon rings and in aliphatic chains [7; 9; 15]. Other components are identified at other binding energy values: 284.98 eV related to C \(sp^{3}\)[7; 13; 15], 283.94 related to carbon vacancies [7; 9; 15], 285.46 eV related to ether [7; 9; 13; 15], and 286.26 eV related to epoxides [7; 9; 13; 15]. The C\({}_{1\text{s}}\) signal of milled samples shows a main peak ascribed to \(sp^{2}\) carbons (284.31 eV), a much smaller contribution of the \(sp^{3}\) component (284.92 eV), and an even smaller contribution peak at 283.69 eV related to carbon vacancies [7; 9; 15]. Oxidized carbons are identified on the high-binding-energy side: at 285.61 eV related to the ether component [7; 9; 13; 15], and at 286.19 eV related to the epoxy-hydroxyl component [7; 9; 13; 15]. The effect of milling on spectral patterns is remarkable, with a pronounced reduction in the sp\({}^{3}\) component and an increase in the vacancy component as compared with the \(sp^{2}\) component (see also Appendix B). This finding may be interpreted in light of the mechanochemical effects documented in previous work [54], where a remarkable increase in the intrinsic combustion reactivity of residual carbon in fly ash was observed upon milling. The mechanochemical effect in [54] was assessed by non-isothermal thermogravimetric analysis and temperature programmed desorption [54], even though it was observed that milling did not induce relevant changes in the porosity of the samples, nor in the ash content and composition. It was in fact postulated that mechanochemical activation was due to the increase in strong electron-acceptor sites that could be identified with carbon atoms with unsaturated bonds located at the edges of carbon anisotropic layered structures.