CO2Adsorption on Carbon Models of Organic Constituents of Gas Shale and Coal

Imperfections of the organic matrix in coal and gas shales are modeled using defective and defect-free graphene surfaces to represent the structural heterogeneity and related chemical nature of these complex systems. Based upon previous experimental investigations that have validated the stability and existence of defect sites in graphene, plane-wave electronic density functional theory (DFT) calculations have been performed to investigate the mechanisms of CO2 adsorption. The interactions of CO2 with different surfaces have been compared, and the physisorption energy of CO2 on the defective graphene adsorption site with one carbon atom missing (monovacancy) is approximately 4 times as strong as that on a perfect defect-free graphene surface, specifically, with a physisorption energy of ∼210 meV on the monovacancy site compared to ∼50 meV on a perfect graphene surface. The energy associated with the chemisorption of CO2 on the monovacancy site is substantially stronger at ∼1.72 eV. Bader charge, density of states, and vibrational frequency estimations were also carried out and the results indicate that the CO2 molecule binds to the surface becoming more stable upon physisorption onto the monovacancy site followed by the original C═O bonds weakening upon CO2 chemisorption onto the vacancy site.