Paramagnetic properties of the pyrolyzed nanostructured carbon material (coal)

Coal is a nanostructured natural material, where the physical-chemical properties depend from the stage of metamorphism. Metamorphism is a natural transformation of structure the coal under pressure and temperature.The stage of metamorphism is estimated by a variety amount volatile substance (V ^daf). The coal structure is considered to consist of the nanoclusters with the ordered distribution of carbon atoms (aromatic area) and the amorphous bridges between carbon nanoclusters containing hydrocarbon CH_n chains (aliphatic area). The process of pyrolysis demonstrates the partial destruction of organic bonds in the aromatic and aliphatic areas with changing physical-chemical properties, including paramagnetic. In this paper, it was researched influence paramagnetic oxygen O₂ on the paramagnetic properties a gaseous type of coal with different amount of volatile substance (V ^daf = 39 - 46 wt. %.) after vacuum annealing at T = 650 - 700 ℃. EPR spectra of coal after vacuum annealing consist of a two resonance lines L1, L2, which have Lorentz shape. The broad line L1 and narrow line L2, which corresponds to the non-compensated electron spins within two areas in the coal structure: the carbon nanoclusters (aromatic area) and long one-dimensional hydrocarbon molecules of CH_n type (aliphatic area). The weakly increasing width of EPR line after vacuum annealing related to dipole-dipole interaction with paramagnetic oxygen O₂. At first, time was registered untypical behavior paramagnetic centres of coal with an interaction of oxygen molecule O₂ after vacuum annealing. The anomalous increase of the width (ΔH_pp ∼ 250 G) of L1 line related to interaction unpaired electrons of coal with paramagnetic O₂ after vacuum annealing. This behavior of width L1 line was explained by a large amount open pores of coal, which were created by vacuum annealing. During of time the intensity and width of an L1 line of coal with contacting of oxygen after vacuum annealing slowly decreased. The intensity of L2 was increased but the width did not change. The decrease of paramagnetic concentration of centres in the aromatic area of coal after vacuum annealing related to creating weakly chemical bonds with oxygen. The increase of paramagnetic concentrations of centres in the multi-ring aromatic hydrocarbon and aliphatic area by structure relaxation of bound with chemical reactions and diffusion free radicals. Finally, we investigated the same behavior of paramagnetic centres of coal with interaction of vapor of water. The interaction paramagnetic centres of coal with a vapor of water proceeds faster that with oxygen. (author)