Probing the chemical nature of surface oxides during coal char oxidation by high-resolution XPS

Abstract

Coal chars, like most solid carbons, have a pronounced tendency to chemisorb oxygen at low and moderate temperatures. Characterization by XPS of surface oxides on carbon has been accomplished with the aim of providing a better atomistic insight into: (a) reactions involved in molecular oxygen adsorption on coals and (b) the relations between the nanostructure of solid carbons and the chemistry of oxidation. High-resolution C 1s and O 1s core level and valence band XPS spectra effectively reflected the oxidative functionalization of different types of coals and synthetic carbons upon oxidation in air at moderate temperatures (300 and 500°C). More specifically, analysis of C 1s and valence band spectra could be directed to monitor the structural evolution of the carbons in terms of extension of sp2 versus sp3 conjugation, carbon vacancies and oxidized carbon. Comparison of the O 1s spectra, on the other hand, provided a tool to characterize the nature of oxygen bonding on carbon and to determine the relative abundance of carbon–oxygen species. Results underline the important role of epoxy groups in the early stages of oxidation, providing a mechanistic framework for the identification of the stable and metastable intermediates in the heterogeneous oxidation of coal by molecular oxygen.