Formation of chromia from amorphous chromium hydroxide

Abstract

Forced hydrolysis of Cr(NO3)(3) in a solution of decomposing urea was investigated. Chromium hydroxide precipitates were amorphous for the final pH values up to similar to 9 to 9.5. Heating of amorphous chromium hydroxide up to 360 degrees C produced Cr2O3 crystallites of the order of 20 nm, whereas after heating up to 825 degrees C the crystallite size of Cr2O3 increased to the order of 100 nm. Crystallization of Cr2O3 was also monitored by FT-IR spectroscopy. TGA/DTA curves, recorded in air, showed thermal behavior of amorphous chromium hydroxides that differed depending on the experimental conditions of their preparation. A sharp exothermic peak between 410 and 420 degrees C was recorded due to the crystallization of Cr2O3. This peak shifted to 600 degrees C when the heating of amorphous chromium hydroxide was performed in an argon atmosphere. Accelerated crystallization of Cr2O3 from amorphous chromium hydroxide in air is explained by the catalytic effect of the higher oxidation states of chromium species, which were probably restricted to the surface of the particles. It is suggested that higher oxidation states of chromium were not stable and they converted to the initial Cr(III) state. XRD did not detect any oxide phase of chromium except for Cr2O3.