## 2 Materials and Methods  
Experiments were carried out on (a) a synthetic carbon (commercial name Carboxen), (b) char from a bituminous coal from South Africa, and (c) fly ash from suspension firing of the same coal. The synthetic carbon was supplied by Sigma-Aldrich, under the commercial brand of Carboxen 1000. This is a spherically shaped 180-250 \(\upmu\)m carbon molecular sieve. It is synthesized by the pyrolysis of an organic polymeric precursor. The properties of Carboxen are reported in Table 1.  
The South African coal is a medium-rank bituminous coal with a high content of inertinite macarels. Chars were produced in a lab-scale fluidized bed reactor (ID = 10 cm) at 1123 K under a nitrogen flow. The proximate and ultimate analysis results are provided in Table 2. The microstructure of South African char has been investigated by a combination of techniques, including porosimetric analysis, XRD, Raman, and EPR, reported in [53]. This sample can be considered a \"turbostatic\" carbon, with a structural order intermediate between that of amorphous carbon and crystalline graphite. The South African char was ground by means of a laboratory-scale planetary centrifugal ball mill (RETSCH PM 100). The milling balls and drum lining were composed of agate. Approximately 60 g of char samples was loaded into the drum before each milling session; the milling duration ranged from 5 to 30 min, with a rotational speed of 500 rpm. Additional details on the milling process can be found in [54]. Fly ash samples came from a 240 MWe front-fired PC boiler with low-NOx burners burning pulverized South African coal. The ash samples were collected from the hoppers below the first and second rows of electrostatic precipitators and then mechanically sieved using mesh sizes of 75 and 150 um. The results of the ash analysis are presented in Table 2. Additional details regarding their features can be found in an earlier study [30]. Oxidation treatments were carried out by heating the samples in an electric oven with an airflow. The specific time and temperature conditions for the oxidation treatment of each sample are provided in Table 3. \begin{table}
\begin{tabular}{c c c c c} \hline
**Ash a** & **Volatiles a** & **C a** & **H a** & **N a** \\ \hline (wt\%) & (wt\%) & (wt\%) & (wt\%) & (wt\%) & (wt\%) \\ \hline
3.0 & 5.0 & 92.0 & 0.7 & 0.1 \\ \hline \end{tabular} a dry basis.  
\end{table} 
Table 1: Proximate and ultimate analysis of the Carboxen samples [15]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline  & **Ash a** & **Volatiles a** & **C\({}_{\text{fx}}\) a** & **C a** & **H a** & **N a** & **S a** & **Vitriine Rr** \\ \hline  & (wt\%) & (wt\%) & (wt\%) & (wt\%) & (wt\%) & (wt\%) & (wt\%) & (wt\%) & (\%) \\ \hline Coal & 15.7 & 23.1 & 61.2 & 68.0 & 3.8 & 1.2 & 0.6 & 0.72 \\ \hline Char & 20.4 & 4.6 & 75.0 & 75.4 & 1.2 & 1.8 & n.d. & n.d. \\ \hline \multirow{4}{*}{Ash} & SiO\({}_{2}\) & 44.1 & Al\({}_{2}\)O\({}_{2}\) & 34.0 & CaO & 8.1 & MgO & 2.2 \\ \cline{2-8}  & K\({}_{2}\)O & 0.62 & Na\({}_{2}\)O & 0.15 & FeO & 1.53 & MnO & 0.01 \\ \cline{2-8}  & TiO\({}_{2}\) & 1.41 & P\({}_{2}\)O\({}_{5}\) & 2.35 & SO\({}_{3}\) & 2.08 & Others & 3.45 \\ \hline \end{tabular} a dry basis.  
\end{table} 
Table 2: Properties of South African coal, char, and ash. \begin{table}
\begin{tabular}{c c} \hline
**Sample** & **Heat Treatment Air** \\ \hline Fly ash & None \\ \hline Carboxen & 773 K 30 min \\ \hline SA unmilled & 573 K 2 h \\ \hline SA milled 5 min & 573 K 2 h \\ \hline SA milled 30 min & 573 K 2 h \\ \hline \end{tabular}
\end{table} 
Table 3: Carbon samples and their respective thermal treatments. SEM/EDX results on char particles and raw and milled fly ash are reported in Appendix A. The XPS experiments were performed in the ultra-high vacuum chamber (UHV) of the SuperESCA beamline at the Elettra synchrotron radiation facility (Trieste, Italy). A thin layer of powdered sample was deposited on circular adhesive foils (6 mm in diameter).