138 0022-4596/99 $30.00 Copyright ( 1999 by Academic Press All rights of reproduction in any form reserved. Journal of Solid State Chemistry 142, 138 — 145 (1999) Article ID jssc.1998.8002, available online at http://www.idealibrary.com on Synthesis of AFeO 2.5 1 x (04x40.5; A 5 Sr, Ca) Mixed Oxides from the Oxidative Thermal Decomposition of A[Fe(CN) 5 NO]· 4H 2 O Marı´a Ine´s Go´mez*, Juana A. de Mora´n*, Rau´l E. Carbonio,-,1,2 and Pedro J. Aymonino‡,2 * Instituto de Quı ´ mica Inorga & nica, Facultad de Bioquı ´ mica, Quı ´ mica y Farmacia. Universidad Nacional de Tucuma & n, Ayacucho 491, 4000 S. M. de Tucuma & n, Argentina; - INFIQC, Departamento de Fı ´ sico Quı ´ mica, Facultad de Ciencias Quı ´ micas, Universidad Nacional de Co & rdoba, Ciudad Universitaria 5000 Co & rdoba, Argentina; ‡ CEQUINOR (CONICET, UNLP) and LANAIS EFO (CONICET, UNLP), Departamento de Quı ´ mica, Facultad de Ciencias, Exactas, Universidad Nacional de La Plata, CC 962, 1900 La Plata, Argentina Received September 11, 1997; in revised form August 10, 1998; accepted August 11, 1998 The low-temperature formation of Sr and Ca ferrates by thermal oxidative decomposition of tetrahydrates of Sr and Ca nitroprussides (pentacyanonitrosilferrates(II)) is reported. IR spectroscopy, X-ray diffraction (including Rietveld analysis), and thermal analysis were used to follow the process. The amount of active oxygen (concentration of Fe(IV)) in the products was chemically determined. For the Sr compound, the final product was an oxygen-deficient perovskite that could be produced with a very low content of carbonate in the temperature range 750–850°C. For the calcium compound, only Ca 2 Fe 2 O 5 with the brownmillerite type structure was obtained. ( 1999 Academic Press INTRODUCTION The synthesis of mixed oxides, particularly perovskites and related oxides, from the decomposition of inorganic coordination compounds allows one to lower the reaction temperature and to obtain more homogeneous and finely divided powders (1—5). Iron(IV)-containing perovskites are extensively used in catalysis (6—12) and electrocatalysis (13—19). Their applica- tions are largely related to oxidations and electrooxidations. These properties should be related to the high oxidizing power of Fe(IV), which, for example, has been specifically recognized to play a significant role in the catalytic oxida- tive dehydrogenation of ethane on titanium-containing per- ovskites (11). Fe(III)-containing mixed oxides were reported to have good catalytic activity for NO decomposition (20). In the synthesis of compounds of the system AFeO 2.5‘x (where A is Ca2‘, Sr2‘, or Ba2‘), if a perovskite-type structure is formed, oxygen vacancies are usually present (21—24) due to the difficulty in generating Fe(IV), and in the extreme case when x"0, a brownmillerite-type oxide is 1 To whom correspondence should be addressed. 2 Member of the Research Career of the National Scientific and Techno- logical Research Council, Argentina. formed (A 2 Fe 2 O 5 ) with all of the iron in the # 3 oxidation state (22, 25, 26). Brownmillerite-type Ca 2 Fe 2 O 5 , with inter- stitial O, has been obtained at high O 2 pressures (27, 28). Stoichiometric AFeO 3 is formed only at extremely high oxygen pressures, 89 MPa for 1 week at 628 K for SrFeO 3 (24, 29) and 2 GPa at 1273 K for CaFeO 3 (24, 30). AFeO 2.5‘x is a model system to study catalytic reactions under oxidizing or reducing conditions, because by chang- ing the oxygenation (the x value), one can tailor the relation Fe(IV)/Fe(III) to specific needs. The generation of oxygen vacancies in the perovskite structure is increased in these systems as the synthesis tem- perature increases. Temperatures as high as 1400°C are sometimes used to produce well-defined structures. In such cases, oxygenation at lower temperatures is needed to de- crease the number of oxygen vacancies (21). In addition, high temperatures favor the sintering of materials. Thus, for catalytic purposes, low temperatures are needed to produce high surface area catalysts with a high content of Fe(IV) (6—15, 17, 18). For these reasons, low-temperature synthetic methods are crucial in these systems. In the present work we report the low-temperature syn- thesis of Sr and Ca ferrates by thermal oxidative decomposi- tion of Sr and Ca nitroprussides, which produces high surface area powders at lower temperatures than the con- ventional syntheses. EXPERIMENTAL Sr[Fe(CN) 5 NO] ) 4H 2 O and Ca[Fe(CN) 5 NO] ) 4H 2 O were prepared by double decomposition reaction between silver nitroprusside and the chlorides or sulfates of the alkaline-earth metals, as already described (31). AFeO 2.5‘x (A"Sr, Ca) mixed oxides were obtained by the oxidative thermal decomposition of Sr[Fe(CN) 5 NO] ) 4H 2 O and Ca[Fe(CN) 5 NO] ) 4H 2 O in air or pure oxygen as follows. Heat treatment A: samples were introduced into the furnace at room temperature and then heated to the desired