Metal Chromium III Oxides Chromites

Owing to the slow rates of diffusion of the cations, the direct solid-state reaction of the oxides Cr2 03 and MO at an elevated temperature is not a good preparation of divalent metal chromium(III) oxides. They can be prepared by more elaborate methods, such as controlled reduction of dichromates MCr207,1 reaction of dichromium tungsten oxide Cr2W06 with a molten divalent metal fluoride2 at 1400°, pyrolysis of complexes,3 and pulverization of slurries containing Cr2 03 and a divalent metal salt.4 

Metal chromium(III) oxides are often used as catalysts, especially dehydrogenation catalysts hence it is important to prepare them as powders.5 The synthesis described here involves the double-decomposition reaction between lithium chromium(III) oxide and molten divalent metal chlorides, and produces finely divided powders. 

Because of the low reactivity of chromium(III) oxide, lithium chromium(III) oxide is prepared by pyrolysis of the oxalate Li[Cr(H20)2(C204)2], which is prepared by the reduction of chromium(VI) oxide.3 

TABLE I X-ray Powder Diffraction Data for Lithium Chromium Oxide, LiCrO20 

Lithium diaquabis(oxalato)chromate(III) is placed in a platinum crucible and pyrolysis is carried out in a hydrogen stream. The temperature is raised to 1000° at a rate of 150°/hr and is held for 5 hours. The crucible is then cooled to room temperature. Anal. Calcd. for LiCr02 Cr, 57.2. Found 57.3. X-tsy data are shown in Table I. 

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