QUANTUM-CHEMICAL INVESTIGATION OF THE GEOMETRIC STRUCTURE OF THE LEWIS-ACID SITES OF THE ALPHA-AL2O3 FACES (0001), (11(2)OVER-BAR-0), (22(4)OVER-BAR-0)

The quantum-chemical investigation of the Lewis acid site (LAS) geomet ric structure for perfect dehydroxylated alpha-Al2O3 faces (0001), (11 $$($) over bar 20) and (22 $$($) over bar 40) produced by oxide chopp ing along the chosen crystallographic direction was performed by the s emiempirical approximation SCF MO LCAO MINDO/3 in the frame of the clu ster scheme. The relaxation the geometric position of the Al atoms tur ns out to be maximum for the (0001) face, the principal possibility of the existence was shown for the surface Al atoms being in the three-c oordinated state. The characteristic features of the 'single' and 'pai red' LAS geometric structure for the (22 $$($) over bar 40) face have been investigated, with the latter containing the aluminum atoms in th e five-coordinated state. The influence of the mutual repulsion of the two incompletely coordinated atoms on the geometric characteristics o f such sites is described. In addition, the process of the LAS-H2O mol ecule interaction according to the associative and dissociative adsorp tion mechanisms has been studied.