A quantum-chemical cluster model has been proposed which provides an adequate description of electronic charge on differently coordinated aluminium atoms both in the bulk and on the surface of y-Al,O,.It also describes Lewis acidity of the surface aluminium atoms characterized by (a) the energy of interaction with electron donor molecules, (b) position of the local acceptor energy levels and (c) atomic charge. The model employs the semi-empirical MO CNDO/2 method in a standard parametrisation. Geometrical optimisation of surface atoms is shown to be important for the electronic modelling properties of Lewis-acid sites.Aluminium ions in the bulk of y-Al,O, crystal are known to occupy octahedral and tetrahedral interstices between the surrounding densely packed oxygen ions. Accordingly, two types of coordinatively unsaturated (cus) aluminium ions are expected to appear on the surface following crystal splitting: Al$' with coordination number 5 and AlP' with coordination number 3.' These ions are responsible for the local acceptor states in the band gap of the crystal energy scheme and, provided they are sterically accessible to the gas-phase molecules, display themselves in chemical experiments as Lewis acids.' The three main parameters usually employed to measure Lewis acidity of the surface metal ions in oxides are (a) the energy of bond formation with electron-donating molecules, (b) the position (depth) of acceptor energy level localized on the metal ion and (c) the value of the positive atomic charge. The stronger the bond with the electron-donating molecule, the deeper the local acceptor energy level will be and the greater the positive atomic charge i.e. the stronger Lewis acid is the metal ion on the solid surface.
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