Ultraviolet-enhanced oxidative activation of pre-oxidized coal during storage: radical evolution and mechanistic insights into low-temperature oxidation

Abstract

Coal exposed to air and solar radiation during open storage and transportation undergoes progressive low-temperature oxidation; however, the molecular mechanism by which ultraviolet (UV) irradiation activates the radical network of pre-oxidized coal remains unclear. In this study, Fourier transform infrared spectroscopy, electron spin resonance, DMPO spin trapping, and DFT/TD-DFT calculations were integrated to elucidate the UV-induced generation and transformation of reactive radicals in pre-oxidized coal. Pre-oxidation reorganized oxygen-containing functionalities and aliphatic side chains, creating structurally labile sites that subsequently served as radical precursors under UV excitation. Upon irradiation, the concentration of persistent radicals increased markedly after approximately 5 min, with pre-oxidized coal exhibiting a substantially stronger radical response than weakly oxidized coal. DMPO spin trapping further identified the sequential evolution of three key reactive species. Carbon-centered radicals (R ), which were scarcely detectable before irradiation, formed rapidly under UV exposure, indicating photoinduced cleavage of activated C–H and C–C bonds. The generated R subsequently reacted with O 2 to form peroxyl radicals (ROO ), followed by hydrogen abstraction, rearrangement, and decomposition to produce alkoxyl radicals (RO ). Among these species, RO showed the most pronounced response to UV irradiation, suggesting a dominant role in sustaining radical-chain propagation. DFT/TD-DFT calculations further demonstrated that UV excitation promoted carbonyl-containing structures to the S 1 and T 1 excited states, weakened carbonyl-adjacent C–H and C–C bonds, and facilitated R formation and subsequent O 2 -involved radical reactions. The corresponding reductions in the energy barriers for O 2 addition, hydrogen abstraction, and bond cleavage provide an energetic explanation for the experimentally observed R → ROO → RO sequence. These findings establish a sequential UV-activated radical mechanism in pre-oxidized coal, in which photoinduced R initiation, ROO formation, and RO -mediated propagation collectively accelerate low-temperature oxidation and increase the self-heating reactivity of coal.