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

- Effect of oxygen-containing groups on CO2-coal interaction has been investigated.
- The forming of hydrogen bonds significantly enhances CO2 sorption capacity.
- The induced HOMOs delocalization is advantageous for CO2-coal interaction.
- Oxygen-containing groups induced charge distributions enhance CO2 adsorption.
- Steric hindrance effect forces CO2 to adsorb on the less stable sites.

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.