Gravimetric studies of ageing in microporous zirconia gels

The work reported in this paper demonstrates the use of the gravimetric techniques to characterize the irreversible textural changes in a hydrous oxide system. A series of microporous zirconia gels were prepared under strictly controlled conditions in the aqueous medium. The porosities of the gels have been characterized by the studies of the nitrogen and water adsorption isotherms. The water sorption isotherms indicated an ageing phenomenon demostrated by the negative low pressure hysteresis.Further, the hydrous gels which were initially outgassed at 298K (at 10⁻⁴ torr for 48 h) showed, on subsequent exposure to water vapour, an enhanced desorption ( >30% in excess of the total amount sorbed.The mechanism of this ageing process is not obvious, but it is suggested that the loss in weight induced by almost saturated water vapour arises from the withdrawal of the molecular water present within the elementary particles of the hydrous oxide gels. The removal of the sorbed water results in the closure of micropores.