ELECTRONIC CHARGE-DISTRIBUTION AND LEWIS ACIDITY OF SURFACE ALUMINUM ATOMS IN GAMMA-AL2O3 - A QUANTUM-CHEMICAL MODEL

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 gamma-Al2O3. 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.