Formation of Chromia from Amorphous Chromium Hydroxide

Forced hydrolysis of Cr(NO3)3 in a solution of decomposing ureawas investigated. Chromium hydroxide precipitates were amorphousfor the final pH values up to 9 to 9.5. Heating of amorphouschromium hydroxide up to 360 °C produced Cr2O3 crystallites ofthe order of 20 nm, whereas after heating up to 825 °C the crystallitesize of Cr2O3 increased to the order of 100 nm. Crystallizationof Cr2O3 was also monitored by FT-IR spectroscopy. TGA/DTA curves,recorded in air, showed thermal behavior of amorphous chromiumhydroxides that differed depending on the experimental conditionsof their preparation. A sharp exothermic peak between 410and 420 °C was recorded due to the crystallization of Cr2O3. Thispeak shifted to 600 °C when the heating of amorphous chromiumhydroxide was performed in an argon atmosphere. Accelerated crystallizationof Cr2O3 from amorphous chromium hydroxide in air isexplained by the catalytic effect of the higher oxidation states ofchromium species, which were probably restricted to the surface ofthe particles. It is suggested that higher oxidation states of chromiumwere not stable and they converted to the initial Cr(III)state. XRD did not detect any oxide phase of chromium except forCr2O3