DEHYDRATION, REHYDRATION WITH RECONSTRUCTION OF OXIDE SURFACES EXPOSED TO HIGH-TEMPERATURES

The paper reports results relative to the effects introduced, to the conditions of the surface of ZrO2 samples, by treatments at 800-degrees-C. The interfacial electrostatic behaviour of the samples soon after preparation, shows several peculiarities, a point of zero charge at pH 8.2 and the presence of some degree of surface heterogeneity. The pattern presented, however, by the aged powder (one year), is markedly different with regular trends and a point of zero charge at pH = 6.8. It is suggested that the calcination step at 800-degrees-C determines removal, at least in part, of the water chemisorbed at the surface. During the long aging the powder undergoes re-hydration with a reconstruction of a hydrous surface layer of Zr(OH)4 which shields entirely the former ZrO2 surface and imposes on the system the behaviour of bulk Zr(OH)4 phases.