Investigation of oxygen-containing group promotion effect on CO2–coal interaction by density functional theory

Abstract

Density functional theory including dispersion correction (DFT-D) calculations were carried out to investigate the adsorption mechanisms of CO2 on low rank coal, which had higher oxygen/carbon ratio and more surface oxygen-containing groups. Four typical oxygen-containing groups of –COOH, –CHO, –OH and –OCH3 were embedded in graphite surface as four coal models. For comparison, two original coal models of Perfect Graphite and Graphite–H were constructed. The formation of hydrogen bonds between functional groups and CO2 molecules significantly enhanced the CO2–coal interaction. The coal surface functional groups promoted the delocalization of electronic distributions and changed the highest occupied molecular orbitals (HOMOs), both of which were advantageous to CO2–coal interaction. When the effect of steric hindrance dominated the adsorption, CO2 molecules had to be forced to adsorb on less stable sites, leading to the decrease of CO2 adsorption.