Comparison of CH4 and CO2 Adsorptions onto Calcite(10.4), Aragonite(011)Ca, and Vaterite(010)CO3 Surfaces: An MD and DFT Investigation

The interaction between greenhouse gases (such as CH 4 and CO 2 ) and carbonate rocks has a significant impact on carbon transfer among different geochemical reservoirs. Moreover, CH 4 and CO 2 gases usually associate with oil and natural gas reserves, and their adsorption onto sedimentary rocks may influence the exploitation of fossil fuels. By employing the molecular dynamics (MD) and density functional theory (DFT) methods, the adsorptions of CH 4 and CO 2 onto three different CaCO 3 polymorphs (i.e., calcite(10.4), aragonite(011)Ca, and vaterite(010)CO 3 ) are compared in the present work. The calculated adsorption energies ( E ad ) are always negative for the three substrates, which indicates that their adsorptions are exothermic processes and spontaneous in thermodynamics. The E ad of CO 2 is much more negative, which suggests that the CO 2 adsorption will form stronger interfacial binding compared with the CH 4 adsorption. The adsorption precedence of CH 4 on the three surfaces is aragonite(011)Ca > vaterite(010)CO 3 > calcite(10.4), while for CO 2 , the sequence is vaterite(010)CO 3 > aragonite(011)Ca > calcite(10.4). Combining with the interfacial atomic configuration analysis, the Mulliken atomic charge distribution and overlap bond population are discussed. The results demonstrate that the adsorption of CH 4 is physisorption and that its interfacial interaction mainly comes from the electrostatic effects between H in CH 4 and O in CO 3 2- , while the CO 2 adsorption is chemisorption and the interfacial binding effect is mainly contributed by the bonds between O in CO 2 and Ca 2+ and the electrostatic interaction between C in CO 2 and O in CO 3 2- .