Nanosized Cr2O3 hydrate spherical particles prepared by the urea method

A procedure for the preparation of non-agglomerated spherical particles of narrow size distribution in the nanometer range, based on the aging at 100°C of aqueous solutions of Cr(III) nitrate in the presence of sodium sulfate, urea and polyvinyl pyrrolidone is reported. The mean particle diameter could be controlled by altering the Cr(III) concentration in the starting solutions. The so produced particles consisted of an amorphous Cr2O3 hydrate containing a certain amount of carbonate and sulfate anions. The carbonate anions decomposed when these particles were heated at 325°C which also resulted in the crystallization of α-Cr2O3. The decomposition of the sulfate groups requires a further annealing at 500°C. The surface area of the particles increased considerably as increasing heating temperature up to 325°C. However, an important decrease of this magnitude was detected after calcination at 500°C as a consequence of particle sintering.