Study of the catalysts for fuel combustion reactions: XX. Thermal stability and other properties of the CeO 2 -MgO-Al 2
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Study of the Catalysts for Fuel Combustion Reactions: XX. Thermal Stability and Other Properties of the CeO2-MgO-AI203 System N. A. Koryabkina, R. A. Shkrabina, V. A. Ushakov, and Z. R. Ismagilov Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia ReceivedFebruary 17, 1999 Abstract--The effect of simultaneous addition of magnesium and cerium cations on the structure formation and properties of aluminum oxide obtained from hydroxides with different phase compositions by thermal treatment at 773-1573 K is studied. At temperatures lower than 1273 K, the effect of specific surface area stabilization is observed for the samples. It is found that spherical aluminum oxide modified with Mg and Ce cations exhibits high mechanical strength upon calcination at T > 1373 K. Modified samples calcined at 1573 K are catalytically active in the CH4 oxidation reaction. INTRODUCTION Cerium compounds are widely used for modifying three-functional catalysts for the neutralization of car exhaust gases, because they can enhance the dispersion of an active component on the support surface and inhibit phase transformations in ~/-AI203. Moreover, CeO 2 is an oxygen accumulator; it can also produce new, or modify existing, active sites. The above properties of cerium compounds were studied earlier [1-4]. As a rule, compositions based on aluminum oxide promoted with only cerium ions and catalysts prepared from these compositions served as test materials in such studies. The properties ofA1203 modified simultaneously with several cations are poorly known. This work continues the studies on the optimization of the composition of alumina-based supports, which are characterized by an increased thermal stability and improved mechanical properties. The results of studies on the La203-AI203 and CeO2-AI203 systems were described elsewhere [5-7]. An X-ray diffraction study of the genesis of phase composition in the La203MgO-AI203 system in the temperature range 8231673 K was reported in [8]. In this work, we considered the conditions for the formation of spherical supports based on aluminum oxide modified by successively adding magnesium and cerium ions and changes in their properties upon thermal treatment at 823-1573 K. EXPERIMENTAL The following aluminum hydroxide samples, which differ in phase composition, were used in this study: (1) Commercial aluminum hydroxide with a pseudoboehmite structure with a pseudoboehmite concentration of 100% (series 1);
(2) Aluminum hydroxide PT prepared by the treatment of amorphous aluminum hydroxide using a thermal decomposition technology [9]; this contained 85% (series 2) or 65% pseudoboehmite (series 3); (3) Aluminum oxide prepared by the calcination of gibbsite at 773 K with a boehmite concentration lower than 10% (series 4). Spherical aluminum oxide (d = 1.4-2.0 mm) of series 1-3 was prepared by hydrocarbon-ammonia molding of aluminum hydroxides [9]. Samples of series 4 were used as a powder, because gibbsite is unsuitable for hydrocarbon-ammonia moldin
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