Thermoanalytical and XPS surface characterization of oxidized chars

The chemical nature of the carbon-oxygen complexes on the surface of solid carbons was investigated with reference to a char from medium-rank coal (South African). Chars have been pre-oxidized in air at temperature 573 K <= T <=1073 K. The carbon oxides that populate the char surface and their evolution have then been investigated combining thermal analysis with core-level XPS Spectra, acquired at high-resolution using Synchrotron radiation. TPDs show that the amount of surface oxides is maximum after exposure to oxygen at 723 K, in correspondence to a carbon conversion degree of ? 20%. It decreases for higher temperature of oxidation and carbon burnoff. XPS show formation of epoxy functionalities at low temperature and their evolution into more stable oxides (carbonyl and ether-hydroxy) afterwords. The relative abundance of carbonyl/epoxy and ether-hydroxyl/epoxy on the carbon surface is in fact maximum for an oxidation temperature of 723 K and carbon burnoff of ? 20%. Results are analysed in the light of the semi-lumped kinetic models of carbon oxidation with a focus on the role and nature of surface oxides as intermediates in carbon gasification.