Characterisation of Fe-Cr-Al mixed oxides

Several samples of iron chromium aluminium mixed oxides with different comp osition have been prepared by coprecipitation at controlled pH starting fro m the corresponding nitrate salts and following dried at 393 It and calcina tion at 673 K for 3 h and 1173 K for 3 h. The powders have been characteris ed by XRD, FT-IR and DR W-Vis spectrocopies, DTA-TG thermal analyses and me asurements of BET surface area. It has been found alumina is soluble into a lpha-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 trans ition is shifted towards higher temperatures by increasing Al content, bein g not detectable when alpha-FeCrO3 phase is the main phase. Surface chromat es species are identified by the different techniques used and their amount seem to depend directly on the specific surface area of each sample. (C) 1 999 Elsevier Science S.A. All rights reserved.