Perovskites aluminates

The results obtained point to the real possibility of practical application of Mn REEAEE/Ce/0-Al2O3 catalysts for utilization of CH4 lean mixtures in catalytic heat generators. As to its activity and thermal stability, the catalyst developed is not inferior to well-known analogs using perovskites, aluminates, and manganese hexaluminates. 

Peter, SD Garbowski, E Perrichon, V Primet, M. NO reduction by CO over aluminate-supported perovskites. Catal. Lett., 2000, Volume 70, 21-33. 

Another class of important materials with oxygen ion conduction are perovskites (see Section 2.4.4). Perovskite phases have been, initially, considered as possible electrode materials for SOFCs. The first report of the existence of oxide conductivity in a perovskite material was made using a calcium-doped lanthanum aluminate [98], But the discovery of attractive oxygen conduction properties is rather recent [84],... 

The authors [34] proposed to use perovskites ABO3, where A are calcium cations, or a mixture of calcium and lanthanum, and B are iron, cobalt, nickel or manganese cations, or their mixtures. Besides, aluminates, silicates, aluminium sihcates, zirconates and chromates of different types are added as structure-forming components providing strength and stability to thermal shocks [34]. 

L. Vasylechko, A. Senyshyn, and U. Bismayer, Perovskite-Type Aluminates and Gallates 113... 

Lanthanide aluminates (LnAlOJ are of interest as buffer layers in controlling the overgrowth of various perovskite films and as alternative gate dielectrics to Si02. Using the singlesource heterometallic lanthanide aluminum isopropoxide precursors [LnAl(OPr )6(HOPr )]2... 

By solid-state reactions of Am02 with AI2O3 at 1250°C. in H2 no compound formation could be observed. In order not to increase the reaction temperature, a 1 1 mixed hydroxide precipitation of americium and aluminium reacted in H2 for 2-8 hrs. at 1250°C. This resulted in the quantitative formation of rose-colored AmAlOa having the hexagonal distorted perovskite structure of the aluminates of La, Pr, Nd (JJ), and Pu (17,35). 

The dominant CAIs are spinel-pyroxene inclusions, with centers that are rich in spinel, with or without accessory hibonite and perovskite, and Wark-Lovering rims consisting mainly of aluminous diopside and iron-rich phyllosilicate. These CAIs can take the form of compact objects (e.g., see figure 4 of MacPherson and Davis, 1994), porous aggregates, or even distended chains of spinel nodules (e.g., Eigure 13). In some cases, the entire CAI and its rim are encased in an outer shell of forsterite or enstatite. 

Brodholt J. P. (2000) Pressure-induced changes in the compression mechanism of aluminous perovskite in the Earth s mantle. Nature 407, 620-622. 

I. Zvereva et al., Complex aluminates RE2SrA1207 (RE = La, Nd, Sm-Ho) Cation ordering and stability of the double perovskite slab-rocksalt layer P-2/RS intergrowth. Solid State Sci. 5(2), 343-349 (2003). 

As an attempt to simulate real operating conditions of automotive converters, a laboratory bench has been designed and ageing procedures determined to reproduce simultaneous chemical and thermal modifications encountered by catalysts in the exhaust line. Characterization of commercial samples after ageing according to different temperature cycles evidences formation of both platinum/rhodium alloys and cubic perovskite-type compound, CeA103. Simultaneously with the formation of cerium aluminate, a thermal stabilization of catalysts is observed, in terms of mean noble metal particles size and concentration of rhodium in alloyed phases. An interpretation based on the crystallographic adaptation of alumina, cerium aluminate and ceria is proposed. 

In addition, thermally stable oxides, like perovskites or hexa-aluminates, which are used as active material or washcoat in catalytic combustion, can be used to manufacture high temperature supports. Consequently, promising materials that are reported here could be used as support and active phase in one and the same material or as a washcoat on a support made of another material. 

The main drawback with the use of perovskites is the poor thermal stability of the materials compared with hexa-aluminate based catalysts. Zwinkels et al. have compared the thermal stability of two different hexa-aluminates with a SrZr03-perovskite, a pyrochlore, see Section 3.2.4, and two spinels, see Section 3.2.3. The perovskite, as well as the pyrochlore and one of the spinels decreased in their surface area significantly. One of the explanations of the much lower stability of the perovskites compared with the hexa-aluminates is that the crystal growth will take place in three dimensions and thereby yield a material with a low surface area. Lowe et al. have studied several different perovskites and their thermal stability and conclude that the surface area of the perovskites is not sufficient for use in high temperature catalytic combustion. Similar results have been shown by Cristiani et al. ... 

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