Characterisation of Fe-Cr-Al mixed oxides

Several samples of iron chromium aluminium mixed oxides with different composition have been prepared by coprecipitation at controlled pH starting from the corresponding nitrate salts and following dried at 393 It and calcination at 673 K for 3 h and 1173 K for 3 h. The powders have been characterised by XRD, FT-IR and DR W-Vis spectrocopies, DTA-TG thermal analyses and measurements of BET surface area. It has been found alumina is soluble into alpha-FeCrO3 phase up to near 20%. These samples are stable at 1243 K with a relative high specific surface area. The gamma,theta --> alpha phase transition is shifted towards higher temperatures by increasing Al content, being not detectable when alpha-FeCrO3 phase is the main phase. Surface chromates species are identified by the different techniques used and their amount seem to depend directly on the specific surface area of each sample. (C) 1999 Elsevier Science S.A. All rights reserved