Micropore formation in heated synthetic Al-goethites from a ferrous system

Nitrogen adsorption and desorption isotherms were obtained for microcrystalline, platy crystals of synthetic Al-substituted goethites synthesized from the ferrous system and heated to 260 degrees C. Specific surface area, cumulative pore area and frequency of mesopore wall-separations of 2-15 nm in size increased as Al substitution increased. Nanometre-size micropores developed at heating temperatures between 200 and 240 degrees C and decreased during the growth of hematite crystals between 240 and 260 degrees C. Aluminium substitution had little effect on micropore development and a moderate effect on mesopore development. The increases in micropore volume and specific surface area due to heating are much smaller than is reported in the literature for large, lath-like crystals of goethite that develop abundant slit-shaped micropores.