Ab-initio modeling of methane adsorption at Ca impurities on a carbon nanotube side-wall

Methane adsorption at Ca decorated single wall carbon nanotubes has been studied by ab-initio total energy calculation based on the density functional theory. The adsorption configurations have been studied using various exchange correlation energy functionals including also two possible long-range interaction correction schemes. Our calculations show that methane adsorption at Ca decorated carbon nanotubes is markedly enhanced when impurity atoms are considered as individual adsorption sites. We demonstrate that up to six CH4 molecules can bind at a single Ca impurity at room temperature. The phenomenon responsible of the measured adsorption energy is recognized as a Kubas-type interaction that involves the orbital overlap between the Ca d state and the methane σ molecular orbitals. The adsorption values obtained with the different energy functionals used are discussed showing that LDA, often employed in the recent literature for similar adsorption problems, is affected by severe limitations when orbital overlap and physisorption co-exist. Lastly the well known problem of Ca clustering is studied and discussed showing that, differently to the hydrogen case, it does not induce any molecular dissociation.