Hydrocarbon Adsorption on Carbonate Mineral Surfaces: A First-Principles Study with van der Waals Interactions

In this work, we study the adsorption of hydrocarbon. molecules on carbonate surfaces by means of first principles calculations based on Density Functional Theory (DFT) with and without van der Waals (vdW) corrections. Energetic, electronic, and structural properties have been determined for the adsorption of the representative hydrocarbons (hexane and benzene) on calcite (CaCO3) and dolomite [CaMg(CO3)(2)] (10-14) dry surfaces. Those hydrocarbons were selected to represent aromatics and alkanes on surfaces, and for each molecule the evaluated properties are similar for both surfaces. Due to the obtained similarities in both surfaces, we have evaluated the vdW corrections only for calcite. Our results suggest that Ca sites are the most energetically favorable for hydrocarbon adsorption on both minerals. This effect is mostly due to the electronic level ordering that leads to charge differences in the possible adsorbed sites (Ca, Mg, and CO3) in the carbonate surfaces. The vdW corrections strengthen the hydrocarbon surface bond with a corresponding reduction in the bond distance between the benzene and the surface. However, this reduction is not even for all atoms in the molecule, and the angle between the benzene aromatic ring and the surface increases. The energy barrier; for the displacement of the hydrocarbons along the calcite surface, was determined for representative surface direction, using the Nudged Elastic Band method, and adsorption energies for the most stable sites show the same order of magnitude.