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

Abstract

Crystalline phases have been coprecipitated from Al/Cr mixed-nitrate aqueous solutions with ammonium carbonate. These phases have been characte rized as probably layered hydroxycarbonates with the likely composition (NH ₄ ) ₂ (Al,Cr) ₆ (OH) ₁₄ (CO ₃ ) ₃ ·H ₂ O, 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.