## 3 Results
### Characterization of Milled and Unmilled Oxidized Coal Char Samples  
In this section, the results obtained from milled (5 and 30 min) and unmilled coal char samples oxidized at 573 K for 2 h are reported. From the O\({}_{\rm 1s}\) high-resolution XPS spectra shown in Figure 3a, it is clear that, while the O\({}_{\rm 1s}\) core level is broadly comparable with the one detected for Carboxen for the unmilled char (even if the detailed features and contribution of spectral components are locally different), the O\({}_{\rm 1s}\) spectra of the milled chars show very intense extra features shifted at \(\sim\)538 eV for the 30 min milled sample, and at \(\sim\)542 eV for the 5 min milled sample. A similar, although less pronounced, result is found when C\({}_{\rm 1s}\) spectra are analyzed. It is remarkable that even though all the three samples studied in this section underwent the same oxidative treatment, only the C\({}_{\rm 1s}\) spectra of the two milled samples exhibit higher-binding-energy carbon components. In particular, for the 5 min milled sample, a clearly distinguishable peak emerges at around 288.31 eV, while for the 30 min milled sample, two distinguishable peaks emerge at around 288.19 eV and 289.39 eV. The identification of this last component is indeed rather controversial in the literature:  
* Some authors reported that higher-binding-energy carbon components should reflect more oxidized forms of surface carbon [44; 56];
* According to other authors, higher-binding-energy peaks might originate from the inhomogeneous charging artefacts that occur in dielectric compounds [41; 45; 50]. We believe that such large shifts and associated energy differences cannot be explained on the basis of the changing nature of surface oxides of carbon, as binding energy chemical shifts related to organic oxygen moieties lie mostly in the 527-538 eV range [57]. Instead, we interpret these strong shifts as the perturbation of O\({}_{\rm 1s}\) electrons photoemitted by oxides due to differential charging effects induced by the dielectric nature of mineral components of the char. Char milling causes fragmentation and an increase in the surface area, and favours the emergence of ash inclusions on the surface (see Appendix A) where they are exposed to and detected by XPS. Within this picture, the peak centred at around 532 eV is due to the oxygen signal from the conductive carbonaceous matrix of the char, which is not affected (or only moderately affected) by the charging of the neighbouring mineral particles. A confirmation of our interpretation comes from the analysis of the survey spectra, acquired at the same photon energy of 650 eV. As shown in Figure 3b, photoemission intensities related to the Si\({}_{\rm 2p}\) and Al\({}_{\rm 2p}\) core levels are detected, in agreement with the expectations as the inorganic fraction of the coal char is mainly composed of silica and alumina (see Table 2). Although the XPS signal is fairly low and noisy and the disorder of the system under investigation prevents thorough resolution of the spin-orbit splitting, for both core levels, we can recognize two features:Figure 3: Photoemission spectra acquired at 650 eV photon energy for the South African char oxidized at 573 K in air. (**a**) High-resolution spectrum of the O\({}_{18}\) core level of milled and unmilled samples. (**b**) Si\({}_{2p}\) and Al\({}_{2p}\) core levels of the milled samples. (**c**) C\({}_{18}\) core levels of the milled samples. * A weaker intensity at the binding energy at which the signals from silica and alumina are expected (about 103-104 eV for Si\({}_{2\text{p}}\) and about 74-75 eV for Al\({}_{2\text{p}}\)[57]) that can be assigned to small mineral particles dispersed within the organic carbonaceous matrix that experiences weak or no charging effects. * A more intense signal shifted by 6-10 eV towards higher binding energy that can be due to larger inorganic grains where photo-induced charging effects are stronger.