The influence of temperature on the nature and stability of surface-oxides formed by oxidation of char

A coal char has been oxidized isothermally at temperatures comprised between 300 and 1073 K. The pre-oxidized chars have been subjected to Temperature Programmed Desorption (TPD) and to core-level high-resolution X-ray photoelectron spectroscopy (XPS) analysis using Synchrotron radiation to infer the nature of the carbon oxides that populate the surface and their evolution throughout thermochemical processing. For low oxygen coverages and mild oxidation temperatures the prevailing carbon-oxygen moieties are epoxy. Raising the oxidation temperature up to ~723K the edge carbon oxygen complexes (ether-hydroxyl and carbonyl-carboxyl) increase. The amounts of CO + CO2 desorbed during TPD also increase with temperature and duration of oxidation for relatively mild oxidative treatments (temperature below ~723K). Upon further increase of the oxidation temperature the amount of CO + CO2 decrease and the ratio of CO/CO2 increases remarkably. Altogether, results suggest the existence of a strong link between a remarkable shift of surface oxides from epoxy to ether/carbonyl and the desorption of CO and CO2. Moreover, the CO/CO2 ratio during desorption can be well correlated with the relative abundance and stability of epoxy moieties with respect to the “edge” oxides. Results are analyzed in the frame of a semi-lumped kinetic model of carbon oxidation with a focus on the role and nature of surface oxides as intermediates in carbon gasification reactions.