Quantum mechanical approach to the chemisorption of molecular hydrogen on defect magnesium oxide surfaces

Quantum mechanical theoretical calculations have been performed on the linear atomic chain in order to simulate the interaction of molecular hydrogen with the defects present at the surface of activated MgO. The total energy of the system, the relative energy of the various molecular orbitals, and the electronic charge distribution have been computed for various lattice parameters (d_O-O = 4.0–4.8 Å) as a function of the H-H (or O-H) separation. A symmetrical motion of the hydrogen nuclei with respect to the central Mg²⁺ vacancy was assumed. It is shown that chemisorption of hydrogen on surface O^–ions sites results in the formation of pseudo-hydroxyl groups. For a small lattice parameter (4.0 Å), no stable state of molecular hydrogen has been found while an increase in the lattice parameter results in a uniform increase of the calculated activation energy for the molecular hydrogen activation process. A mechanism is proposed which is not so different from that put forward for the hydrogen activation by transition metal complexes. Molecular hydrogen is found to act as an electron donor.