Surface area stability of aluminas

Measurements were made of the surface area of aluminas steamed for various times up to 1000 h, at constant temperature and water partial pressure. The data were fitted to the integrated form of the equation − dS/ dt = kS n , as a smoothing operation; this accounts for the strong effect of area on the rate of area loss. The derived constants, n and k, were used to calculate the rate of area loss at any given area level. This rate is proportional to water partial pressure, with an apparent activation energy of 40.4 kcal/mol. Under a given set of conditions, the rate of area loss increases as the content of amorphous alumina in the precursor increases. Several elements (Ba, Sr, La, Sn, SiO 2, PO 4) added to alumina markedly increase area stability. Pore distribution measurements show that small pores become enlarged as area decreases by the same factor as larger pores. A model of the process of surface area loss is proposed which can explain the effects of surface area, water partial pressure, and stabilizing additives, involving condensation between hydroxyls on adjacent particles.