Thermal stability and pore sizes of microporous zirconia gels

A series of hydrous zirconia gels was prepared under carefully controlled conditions of pH. Porosities of the gels were characterized by adsorption studies using nitrogen (volumetric) and water vapor (gravimetric) at 77 and 298 K, respectively. The amorphous gels were predominantly microporous (pore radius <2 nm). Increasing the pH of preparation enhances the pore volume and shifts the pore size distribution to higher microporous regions. These trends were observed under two different outgassing conditions (298 and 473 K), and also using two molecular probes (N₂ and H₂O). The thermal stabilities of the micropores were found to diminish with decreasing micropore sizes. The role of two precipitants (a) ammonia and (b) ammonium bicarbonate is discussed in this paper. Micropore volumes available to nitrogen [V(N)] and water [V(W)] were very similar in all the gels precipitated with ammonia. While ammonium bicarbonate product showed that V(W) was eight times greater than V(N). This has been attributed to a type of ‘‘zeolitic effect’’ in zirconia gels, preferentially permitting water with its lower molecular cross section to penetrate the micropore structure. Ligands, mainly H₂O, help stabilize the micropore structure in zirconia gels and thermal removal of ligands destabilizes the pore system.