INFLUENCE OF SURFACE-AREA AND ADDITIVES ON THE THERMAL-STABILITY OF TRANSITION ALUMINA CATALYST SUPPORTS .2. KINETIC-MODEL AND INTERPRETATION

The common features of the gamma, delta and theta forms of transition aluminas found in their structural and chemical properties allow them to be described by a unique formula. This formula satisfies the conditions of electrical neutrality of the crystal, accounts for the required ratio of the number of sites in the structure and describes the range of stoichiometries from boehmite (v equals 2) to alpha alumina (v equals 0). A theoretical model for spherical particles based on the annihilation of vacancies as the rate-determining step leads to a kinetic law involving the initial specific surface area, the vacancy of the foreign cation, and a parameter which depends on its ionic radius.