CO2 Competitive Adsorption and Oxidation Passivation Mechanism on Bituminous Coal Surface: A Combined Experimental and DFT Study

Aiming at the unclear microscopic mechanisms underlying the prevention and control of low-temperature coal oxidation and spontaneous combustion, as well as CO2 geological sequestration, this work used LD bituminous coal as the research material. A macromolecular structure model of coal was constructed via multi-scale characterization methods including elemental analysis, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and solid-state 13C nuclear magnetic resonance. Combined with density functional theory calculations and macroscopic adsorption experiments, the adsorption and reactivity characteristics of different active sites on the coal surface toward CO2 and O2 were systematically analyzed. The results showed that the optimized molecular formula of the coal macromolecule was C205H127N3O20S, with an aromaticity of 71% and a bridge-to-peripheral carbon ratio of 0.275. All active sites on the coal surface had stronger adsorption capacity for CO2 than for O2, among which oxygen-containing functional groups and alkyl bridge bond sites showed the most significant competitive adsorption advantage. The adsorption strengths of CO2 at phenylacetaldehyde and –CH2–CH2– bridge bond sites were 47.76% and 38.64% higher than those of O2, respectively. CO2 occupied the active adsorption sites of alkyl groups and aromatic rings on the coal surface. Meanwhile, electronic structure analysis indicates that CO2 is unlikely to initiate chain oxidation reactions, such as methyl dehydrogenation and cycloalkane ring opening. It is expected to hinder the oxidation conversion pathway from alkyl groups to carbonyl and methylene groups at the source and exert an oxygen-inhibiting passivation effect.