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

Abstract

The interaction between greenhouse gases (such as CH4 and CO2) and carbonate rocks has a significant impact on carbon transfer among different geochemical reservoirs. Moreover, CH4 and CO2 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 CH4 and CO2 onto three different CaCO3 polymorphs (i.e., calcite(10.4), aragonite(011)Ca, and vaterite(010)CO3) are compared in the present work. The calculated adsorption energies (Ead) are always negative for the three substrates, which indicates that their adsorptions are exothermic processes and spontaneous in thermodynamics. The Ead of CO2 is much more negative, which suggests that the CO2 adsorption will form stronger interfacial binding compared with the CH4 adsorption. The adsorption precedence of CH4 on the three surfaces is aragonite(011)Ca > vaterite(010)CO3 > calcite(10.4), while for CO2, the sequence is vaterite(010)CO3 > 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 CH4 is physisorption and that its interfacial interaction mainly comes from the electrostatic effects between H in CH4 and O in CO32–, while the CO2 adsorption is chemisorption and the interfacial binding effect is mainly contributed by the bonds between O in CO2 and Ca2+ and the electrostatic interaction between C in CO2 and O in CO32–.