Accurate adsorption energies of small molecules on oxide surfaces: CO–MgO(001)

Abstract

A hybrid MP2:DFT + D optimization method is applied using a 3 × 3 × 2 cluster model (Mg₉O₉) embedded in a 4 × 4 × 4 slab model. The calculated CO–Mg²⁺ distance is 248 pm, and the calculated CO frequency (blue) shift is 20 cm⁻¹, 6 cm⁻¹ larger than the experimental value. For the structure obtained, MP2 calculations with basis set extrapolation on a series of cluster models of increasing size are performed. Taking into account the difference in the periodic limit at the DFT + D level, 20.9 ± 0.7 kJ mol⁻¹ is obtained as the estimate for the full periodic MP2 limit for the energy of CO desorption from the MgO(001) surface. CCSD(T) corrections are evaluated for the Mg₉O₉ cluster model using an augmented double-zeta basis set. Basis set extension effects are examined for smaller models. For a loading of Θ = 1/8, the estimated CCSD(T) value is 21.0 ± 1.0 kJ mol⁻¹, which is 0.4 ± 1.0 kJ mol⁻¹ larger than the (electronic) desorption energy derived in this study from TPD desorption barriers reported in the literature.