Ageing of some high surface area solids

Abstract

AbstractThe ageing (fall in surface area) of samples of aluminium hydroxide has been studied by observing the changes in surface area and porous texture revealed by nitrogen adsorption. Ageing was carried out over phosphorus pentoxide and at 33% r.h. The effect of this change in r.h. depended on the texture of the aluminium hydroxide. Ageing was faster at 33% r.h. in the amorphous form which contained discrete flocs. Pseudoboehmite behaved as a true xerogel and some samples even increased in surface area by swelling when stored at the higher humidity.Some data for calcium silicate hydrates and samples of a commercial pyrogenic silica are included for comparison. The former were prepared in powder and paste (xerogel) form. The two forms responded differently to the two humidities but in the opposite sense to the corresponding aluminium hydroxide textures: thus the surface area of pastes fell more rapidly at the higher r.h. The surface area of samples of the silica fell when they were stored normally in air tight containers. They eventually reached steady values and a reduction of a similar magnitude could be produced rapidly by agglomerating the spherical particles in hot water.Mechanisms of ageing involving simple adhesion and its reverse depending on r.h., hydroxyl group condensation, and sintering aided by dissolution or surface diffusion are discussed.