engleski

Formation of chromia from amorphous chromium hydroxide

Forced hydrolysis of Cr(NO3)3 in the solution of decomposing urea was investigated. Chromium hydroxide precipitates were amorphous for the final pH values up to ~9 to 9.5. Heating of amorphous chromium hydroxide up to 360 oC produced Cr2O3 crystallites of the order of 20 nm, whereas after heating up to 825 oC 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 different thermal behavior of amorphous chromium hydroxides in dependence on the experimental conditions of their preparation. A sharp exothermic peak between 410 and 420 oC was recorded due to crystallization of Cr2O3. This peak was shifted to 600 oC. when the heating of amorphous chromium hydroxide was in argon atmosphere. Accelerated crystallization of Cr2O3 from amorphous chromium hydroxide in air was explained by the catalytic effect of higher oxidation states of chromium species, which were probably restricted to the surface of the particles. Higher oxidation states of chromium were not stable and converted to the initial Cr(III) state. XRD did not detect any other oxide phase of chromium except Cr2O3.

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