A semiempirical quantum mechanical study of methane-graphite interaction

As the first step in the study of methane pyrolysis on the graphite surface, interaction potential of methane molecule with the graphite surface has been studied using semiempirical quantum mechanical AM1 and PM3 computations. The interaction potential energy curves for nine different orientations of methane molecule with respect to the reference plane of graphite have been calculated with approximate step size of 0.2 Å. The average closest approach of the methane molecule to the graphite surface have been calculated based on the balance between potential energy and thermal energy using Boltzmann molecular energy distribution at three temperatures 300, 1000 and 111.75 K (normal boiling point of methane). These calculations showed that different orientations of the methane molecule with respect to the graphite plane result in different curves with different well depths and positions. Distinction between different orientations is decreased significantly at high temperatures. The orientation in which methane molecule approaches the graphite surface in the graphite plane, has the strongest interaction. Results of the present study shows that tendency of graphite surface for growing from its edges, i.e. increasing surface area and retaining graphite structure, is much greater than that from its surface which requires increasing thickness and destroying graphite structure.