Molecular Adsorption on MgO

The (100) face of MgO crystals has been the subject of many studies and is energetically the most stable face in rock salt crystals (Coluccia et al., 1979 Colburn and Mackrodt, 1983 Moodie and Warble, 1971). The reason for its stability is that it is electroneutral (nonpolar), containing equal numbers of cations and anions. MgO cleaves extremely well along the (100) plane, yielding a relatively flat and defect-free surface. The anions and cations on the (100) surface are pentacoordinated, four in the surface plane and one directly below in the second atomic plane. The missing bond lies directly above the surface ion. 

Two simple types of structural defects that have been studied in MgO are the O vacancy, or F center, and the cation vacancy, or V center. A neutral O vacancy has two electrons remaining, which may be trapped by the unbalanced Coulombic potential associated with the vacancy. If one of these two electrons is removed, this results in the formation of an F+ center. Both F and F+ centers can be characterized spectroscopically by their optical absorption bands at 5.01 and 4.95 eV, respectively. 

The V center is an Mg2+ vacancy, with one associated electronic hole. The hole may also be regarded as an O- ion. Studies of the V- center show that the electronic hole is not delocalized over the oxide ions surrounding the vacancy, but is trapped at a single O- site (Hayes and Stoneham, 1985). This configuration is stabilized by a polarization and distortion of the lattice. 

Chemisorption on an MgO surface will be primarily an acid/base interaction. Cation sites are Lewis acids and may interact with donor molecules such as H2O through a combination of electrostatics (ion-dipole attraction) and orbital overlap. Oxide ions also act as basic sites and can interact with acceptors such as H+. In fact one of the most common dissociative reactions is the deprotonation of an adsorbate to produce surface hydroxyl groups. 

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