Surface properties of heat-treated chromia of narrow particle size distribution

Abstract

Spherical particles of chromium(III) hydrous oxide having a very narrow size distribution have been prepared on a multigram scale by the sulfate process of Matijevic and characterized with regard to texture, chemical composition, and particle size distribution. Also, the surface properties have been determined as a function of activation temperature (room temperature through 1000°C) under vacuum. The unheated chromia is amorphous and hydrophilic and likely has a chain-like molecular structure. Its water sorption characteristics correlate with its bulk dehydration properties. The initial product contains considerable amounts of sulfate, i.e., the chromium to sulfate ratio in unwashed samples approaches three; this sulfate can be removed readily by washing with acid or base. Neutral water, however, is much less effective. Particle size and shape are not significantly altered by washing. The surface properties of the heat-treated chromia are reproducible and depend on the sulfate content of the original sol. The BET surface area increases progressively up to an activation temperature of ca. 450°C, mainly because of the formation of micropores. Further increase in activation temperature results in the transformation of the micropores into mesopores on account of crystallization. The analysis of argon adsorption of the micro- and mesporous samples was carried out by using the α s-method of Sing and Brunauer's corrected modelless method, respectively. The estimated external area of the microporous samples agrees with the particle size estimated by electron microscopy. The volume to surface ratio of the micropores (a measure of their size) remains unchanged during their progressive growth with activation temperature (200–450°C). The mesopores formed have a very narrow size distribution. The uniformity of the micropores and the mesopores is attributed to the relatively narrow particle size distribution of the original hydrous oxide.