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Abstract Atoms or groups foreign to the structure of a solid are often bonded to its surface. On diamond, graphite, and even silicon dioxide, the foreign atoms are bonded covalently, whereas the bonding of the structural groups to titanium oxide and alumina is predominantly ionic. Oxides are normally covered with a monomolecular hydroxide layer. Changes in the valence of the metal atoms lead to changes in the acidity of the surface; for example, reduction of surface Ti 4+ ions gives the surface of TiO 2 an acidic nature.
H.P. Boehm (Wed,) studied this question.