CO2 adsorption-assisted CH4 desorption on carbon models of coal surface: A DFT study

Abstract Injection of CO 2 into coal is known to improve the yields of coal-bed methane gas. However, the technology of CO 2 injection-enhanced coal-bed methane (CO 2 –ECBM) recovery is still in its infancy with an unclear mechanism. Density functional theory (DFT) calculations were performed to elucidate the mechanism of CO 2 adsorption-assisted CH 4 desorption (AAD). To simulate coal surfaces, different six-ring aromatic clusters (2 × 2, 3 × 3, 4 × 4, 5 × 5, 6 × 6, and 7 × 7) were used as simplified graphene (Gr) carbon models. The adsorption and desorption of CH 4 and/or CO 2 on these carbon models were assessed. The results showed that a six-ring aromatic cluster model (4 × 4) can simulate the coal surface with limited approximation. The adsorption of CO 2 onto these carbon models was more stable than that in the case of CH 4 . Further, the adsorption energies of single CH 4 and CO 2 in the more stable site were −15.58 and −18.16 kJ/mol, respectively. When two molecules (CO 2 and CH 4 ) interact with the surface, CO 2 compels CH 4 to adsorb onto the less stable site, with a resulting significant decrease in the adsorption energy of CH 4 onto the surface of the carbon model with pre-adsorbed CO 2 . The Mulliken charges and electrostatic potentials of CH 4 and CO 2 adsorbed onto the surface of the carbon model were compared to determine their respective adsorption activities and changes. At the molecular level, our results showed that the adsorption of the injected CO 2 promoted the desorption of CH 4 , the underlying mechanism of CO 2 –ECBM.