Pore Structure of Thermally Treated Goethite (a-FeOOH)

The thermal decomposition of synthetic a-FeOOH in air has becn studied by X~ray powder diffraction (XRD), IR spectroscopy. transmission electron microscopy, and nitrogen adsorption. Three stages may be differentiated during the course ofthe reaction: (i) 200-250°C; slit-shaped micropores of nearly equal width, about 1.4 nm, are formed along the elongated direction of the lath-shaped acicular mícrocrystals of the hematite producÍS, the highest surface area is obtained on a a-FeOOH decomposed at 250°C; (ii) 300-600°C; an internal sintering occurs within the microcrystals transforming the micropores into dosed spherical mesopores with the consequent significant decrease in surface area; and (iii) above 600°C; interparticle sintering takes place and no valuable porosity is detected. Particle shape effects are shown to be the reason for the differences in the IR spectra and for the nonuniform broadening of the XRD peaks observed in the hematite products.