Characterization of coprecipitated aluminium-chromium mixed hydroxides and of the products of their calcination

Crystalline phases have been coprecipitated from Al/Cr mixed-nitrate aqueous solutions with ammonium carbonate. These phases have been characterized as probably layered hydroxycarbonates with the likely composition (NH4)(2)(Al,Cr)(6)(OH)(14)(CO3)(3) . H2O, using XRD, FTIR and TG-DTA analyses. The calcination of these materials gives rise to amorphous mixed oxides where Cr is partly in the hexavalent state, while further calcination at 1173 K gives rise to polyphasic materials. Samples with Al:Cr atom ratio between 5.1 and 1.1 give rise to four phase materials with three corundum-type phases and a spinel-type phase. All these phases contain both Al and Cr.