Accurate quantum chemical energies for the interaction of hydrocarbons with oxide surfaces: CH4/MgO(001)

Abstract

We examine the adsorption of CH₄ on the MgO(001) surface by a hybrid approach. It combines MP2 calculations with extrapolation to the complete basis set limit for the adsorption site and the CH₄–CH₄ pair interactions in the adsorbate layer, with DFT+dispersion calculations under periodic boundary conditions for the whole system. To the total binding energy of 10.7 kJ mol⁻¹, the DFT+D(ispersion) correction contributes 0.7 kJ mol⁻¹ only, showing that the Mg₉O₉ two-layer surface model is an excellent choice and that the interaction between the CH₄ molecules in the adsorbate layer is dominated by pair interactions. Contributions due to relaxation of the atom positions of 0.6 kJ mol⁻¹ (evaluated at DFT+dispersion) and of higher order correlation effects of 2.0 kJ mol⁻¹ (evaluated by CCSD(T)) yield a final estimate of 13.3 kJ mol⁻¹. To this total adsorption energy, the lateral interactions between the CH₄ molecules in the adsorbate layer contribute substantially, 4.1 kJ mol⁻¹.

“Observed” desorption energies of 15.3 and 16.0 kJ mol⁻¹ have been derived from the observed Arrhenius desorption barriers (12.6 and 13.1 kJ mol⁻¹) using thermal enthalpy contributions and a substantial zero-point energy (4.2 kJ mol⁻¹) calculated from DFT+D vibrational frequencies. The comparison shows that our final hybrid MP2 : PBE+D+ΔCCSD(T) estimate has reached chemical accuracy. It misses 2–3 kJ mol⁻¹ of binding only, which is most likely due to missing higher order correlation effects.

PBE+D(ispersion) itself yields an adsorption energy that agrees within 1 kJ mol⁻¹ with our final hybrid MP2 : PBE+D+ΔCCSD(T) estimate.