Influence of thermal decomposition on paramagnetic centres in vitrinite

Abstract

Paramagnetic centres of thermally decomposed vitrinite from medium-rank Polish coal (85.6 wt.% C) were studied using X-band (9.3 GHz) electron paramagnetic resonance (EPR) spectroscopy. The aim of this work was to compare concentrations and properties of paramagnetic centres in vitrinite heated at different temperatures. The vitrinite sample (92 vol.% vitrinite) obtained from vitrain was heated at 350 and 550 °C. The EPR measurements were performed for the evacuated (10 −4 Torr) samples. The line shape of the EPR spectra was numerically analysed. Amplitudes, g-factors, and line widths Δ B pp of the EPR components were evaluated. The concentration of paramagnetic centres in the samples was determined. Three different groups of paramagnetic centres exist in the studied vitrinite. EPR spectra of vitrinite were superposition of one broad Gaussian (Δ B pp: 0.80 mT, g: 2.0030), and broad (Δ B pp: 0.56 mT, g: 2.0029) and narrow Lorentzian (Δ B pp: 0.13 mT, g: 2.0028) lines. The most active during thermal decomposition were paramagnetic centres with broad Lorentzian lines. The concentrations of the all types of paramagnetic centres in vitrinite increase with temperature of heating. Spin-lattice relaxation times in vitrinite decrease with increasing of temperature of thermal decomposition.