Studies on thermal dehydration of hydrated chromic oxide.

Hydrated chromic oxide was prepared by the reduction of potassium dichromate in an autoclave using sucrose. The thermal dehydration of the parent gel in different atmospheres was studied comparatively using the techniques of thermal gravimetry, differential thermal analysis, X-ray diffraction analysis, FT-IR spectroscopy, and gas adsorption. The results of thermogravimetric and differential thermal analysis show that the temperature at which the glow phenomenon occurs is dependent on the gaseous atmosphere in the heat-treatment process and increases in the following order: air, nitrogen, and carbon dioxide. When heated in CO(2), the parent gel is transformed into crystalline alpha-Cr(2)O(3) going through the phase of amorphous Cr(2)O(3). In air, however, it is decomposed directly into crystalline alpha-Cr(2)O(3) without going through the amorphous Cr(2)O(3) phase. It is also found that microcrystallites of alpha-Cr(2)O(3) have been formed before the glow phenomenon occurs at 390 degrees C in air and at 590 degrees C in CO(2), respectively. The mechanism for thermal dehydration in air and CO(2) is discussed.