Dynamic in situ 9 GHz c.w.-e.p.r. low-temperature oxidation study of selected Alberta coals: 1. Comparison of an hv bituminous and a subbituminous coal

Abstract

This paper reports the results of a study of the changes in the character and spin concentrations of the free radical species present in subbituminous and hv bituminous Alberta coals caused by low-temperature oxidation whilst subjected to dry air flow in the temperature interval 20–250 °C. These results were obtained using a new experimental technique called dynamic in situ 9 GHz continuous wave-electron paramagnetic resonance (c.w.-e.p.r.) spectroscopy, which is described. The e.p.r. lineshape changes were analysed using radial difference a o d o -Argand diagrams. All the c.w.-e.p.r. spectral parameters and the spin concentrations exhibit maxima near 100 °C. The observations can be explained by the free radical involvement in the oxidation process. It is concluded from the temperature variation of these parameters and of the lineshape that two free radical species exist with slightly different g-factors and linewidths whose spin concentrations increase as the temperature rises and additional oxygen is coordinated such that the composite lineshape remains essentially constant and nearly Lorentzian. It is shown that the temperature dependence of the free radical evolution process differs significantly in the subbituminous and hv bituminous coal samples studied and that this evolution is determined by the quantity of dry air available.