ACIDO-BASIC PROPERTIES OF SIMPLE OXIDE SURFACES .1. MAGNESIUM-OXIDE

Thanks to a self-consistent tight-binding approach, applied to clusters of about 80 atoms embedded in a Madelung field created by an infinite array of charges at the lattice sites, we have modelized the acido-basic properties of various sites at the surfaces of MgO. We have studied H+ and OH- adsorption, on the perfect (100) and (110) surfaces and on a roughness on the (100) face. The adsorption of a proton was found to belong to the strong coupling limit of the chemisorption process, while the adsorption of an hydroxyl group belongs to the weak coupling limit. The (100) flat surface of MgO thus adsorbs more easily a proton than an hydroxyl group, revealing, by this fact, its basic character, known by the value of 12 pH units of its zero charge point (ZCP). In addition we have shown that this acido-basic character is a function of the surface orientation and of the adsorbing site: e.g., the (110) surface is less basic than the (100) because the adsorption on bridge of an hydroxyl group on (110) is easier. The details of the adsorption processes bring strong indications that covalent effects play an important role.