Application of XPS to coal characterization

The use of X-ray photoelectron spectroscopy to probe the chemistry of coal surfaces is reviewed and its application to the functional group composition of bulk coals is discussed. The surface composition of a range of 19 coals (anthracite to brown coal), ground under heptane, was measured and compared with the results of bulk analysis. A good correlation was obtained for oxygen, with the bituminous and higher-rank coals showing surface enrichment in oxygen. The surface bulk correlation was less good for sulphur, nitrogen and chlorine and was poor for silicon, aluminium and iron. Silicon and aluminium are enriched at the surface while iron is surface deplected. These effects are either due to different particlesize distributions of the mineral and organic phases or to the mechanism of fracture in heptane preferentially exposing specific components of the coal. Oxidation and carbonization of a bituminous coal were also investigated. Oxidation was seen to occur initially via the exterior surface, producing a distribution of carbon—oxygen groups. Singly-bonded species predominate at all temperatures, stable carboxyl groups forming in significant proportions only at temperatures > 250 °C. Carbonization was seen to result in the formation of ether linkages by condensation of hydroxyl groups.