Characteristics of coal-oxygen chemisorption at the low-temperature oxidation stage: DFT and experimental study

Abstract

Oxygen chemisorption on the surface of coal plays a key role in the low-temperature oxidation of coal. Due to the complex molecular structure of coal, it is difficult to fully analyse and characterize coal-oxygen chemisorption, so its mechanism remains unclear. Based on the simple coal molecule active groups, the reaction pathway, electron transfer and heat of adsorption of oxygen adsorbed by each active group in the coal molecule were calculated by density functional theory (DFT). The characteristics of chemisorption and its relationship to the molecular coal structure at different temperatures were studied experimentally. The results showed that carbon radicals (·C radicals) were the oxygen chemisorption sites, and the activation energy of free hydroxyl radicals for hydrogen extraction to produce ·C radicals was 0.1–5.02 kJ/mol, that of methyl radicals was 18.43–26.32 kJ/mol, and that of oxygen radicals was as high as 107.56–132.17 kJ/mol. The free radicals in coal play a major role in the production of chemisorption sites at room temperature. Oxygen was chemically adsorbed on the coal surface in the form of peroxide radicals (COO·), binding to the ·C radicals with a binding energy of −0.2935 eV. Then, the chemical reaction generated gas, free radicals and new ·C radicals. The coal-oxygen chemisorption reaction sequence of different temperature stages was constructed to determine the key reaction of chemisorption. This study could improve the understanding of coal oxidation at low temperatures and guide the prevention of spontaneous coal combustion.