Mixed oxides, structure types perovskite

The most important catalyst systems involving rare earth elements are the oxides and intermetallics. Catalytic properties of rare earth oxides are described in section 4 and those of intermetallic compounds in section 6. Reports on surface reactivities of other binary rare earth compounds are only sparse, and this is mentioned in section 5. A very interesting class of catalyst systems comprises the mixed oxides of the perovskite structure type. As catalysis on these oxides is mainly determined by the d transition metal component and the rare earth cations can be regarded essentially as spectator cations from the catalytic viewpoint, these materials have not been included in this chapter. Instead, we refer the interested reader to a review by Voorhoeve (1977). Catalytic properties of rare earth containing zeolites are, in our opinion, more adequately treated in the general context of zeolite catalysis (see e.g. Rabo, 1976 Katzer, 1977 Haynes, 1978) and have therefore been omitted here. 

It was rather surprising when Hund and Durrwachter [312] found that La20s is miscible with TI1O2 to a great extent (52 mole per cent) whilst still preserving the cubic fluorite structure. The lattice constant of the mixed oxide has an a value 5.645 A compared to 5.592 A for TI1O2. The lattice constants of some orthorhombic perovskite and cubic garnet-type europium compounds are listed in Table 22. 

As described in Section 8.2.6, along with YSZ, mixed oxygen-ion, and electron-conducting oxides with a perovskite-type structure, the so-called Aurivillius phase and pyrochlore materials are fundamentally used for the production of a variety of high-temperature electrochemical devices [50-58],... 

When prepared from mixed oxides, chemical homogeneity in these materials is hard to achieve. Under these circumstances, a variety of different n-values can be said to coincide in a crystal. The defects here are regarded as slabs of the halite type interspersed more or less at random in the perovskite-structure matrix. 

A primary characterization of perovskite-type oxides must include textural analysis and X-ray identification of the phase(s) present. For a more detailed characterization, structural analysis for establishing the lattice position of cations and surface analysis (by means of techniques such as XPS) for defining the surface concentration and oxidation states of cations are desirable. Consequently, information provided by these techniques will furnish the essential criteria for comparing the different preparation methods. For convenience, we will classify the methods used to date for the preparation of pure perovskite phases according to the scheme proposed by Courty and Marcilly (29) for the whole field of mixed oxides. Table I gives a survey of methods used as a function of the phenomena on which they are based. 

The most often found types of mixed oxides are perovskites (RBO3), K2NiF4-type oxides (R2BO4), R 1 B C>2 +1 ( = 2 or 3), lamellar perovskites, pyrochlores, spinels, and oxide solid solutions. Perovskite oxides are, by far, the most commonly used oxides. Therefore, this chapter will reflect this situation by putting more emphasis on this type of materials. Within this section the structure, preparation methods and general characteristics of the mixed oxides will be discussed. Note that R stands for rare-earth elements while A includes all types of elements. 

According to Campbell (1992), the double perovskite, LaCaMnCoOf, was of interest as a cyclic mode methane coupling catalyst due to its structure and redox properties. This mixed oxide has an ordered perovskite structure showing multiple occupations of both A (La, Ca) and B (Mn, Co) sublattices. The crystalline material presented some ordered domains, while in other areas the cations were distributed at random. For the ordered domains, the most probable structural model was an AB03 perovskite-type structure in which Mn4 and Co3+ ions occupy B positions in adjacent AB03 units while La3+ and Ca2+ ions alternate in A positions. Two ions of this structure can be reduced ... 

The Lai-xCexBOs (B= Ti, Cr, Mn, Fe, Ni, Co) mixed oxides of perovskite structure present a catalytic activity for CO oxidation and NO reduction which increases with increasing Ce content. LaCoOs and LaMnOs are the more active compounds for both CO oxidation and NO reduction. The oxidation of CO by O2 follows a suprafacial type mechanism where the adsorbed oxygen is the active species. The reduction of NO by CO is best explained by a redox mechanism. 

It was also found that the oxygen vacancy structure as well as nonstoichiometry in the mixed oxide systems plays important roles for the oxygen reduction performance. It is known that transition-metal oxides and mixed perovskite-type oxide system (Lai xA xCoi yFey03 A Sr,Ca x, y = 0.2-0.5) have high catalytic activity for the peroxide decomposition. 

For many oxides, including mixed metal oxides, of catalytic interest, preparing a high-surface-area solid with thermally stable porous structure was extremely difficult until recentiy. The hard templating procedure provides a systematic solution to this problem. Nanocasting that makes use of a mesostructured solid template is a special case. Both mesostructured silica and mesostructured carbon have been demonstrated so fer as hard templates. Rather precise replicas of these nanomolds have been obtained for a variety of oxide-type perovskites. Unprecedented specific surface areas in the 150-200 m /g were reached. 

Among mixed oxides, perovskite-type structures received a constant attention since the early 1970s when Voorhoeeve and Tejuca et al. pointed to their potential use as total oxidation catalysts [21,22]. This chapter will discuss the behavior of perovskites in total oxidation of heavy hydrocarbons and related chlorinated compoimds imder thermal and plasma activation conditions [20]. 

The formation of perovskite structures does not always have a positive effect on the soot combustion activity of mixed oxides, and a few examples of catalyst deactivation due to perovskite structure formation have been reported. The stability of Ba, Isoot combustion, was studied. This catalyst was thermally stable after 30 h at 800 °C, but above 830 °C catalyst deactivation due to BaCe03 perovskite structure formation was observed [46]. Ba/Mn-Ce catalysts were also tested for soot oxidation in the presence of NO,c> concluding that the formation of perovskite-type oxides after the high-temperature calcination caused the loss of NO storage capacity and a small increase in soot oxidation temperature, but did not seem to affect the NO oxidation activity [47]. 

Lanthanum-undoped perovskites are not the most frequendy used catalysts for CO oxidation reaction since their catalytic performance is not good enough at low temperatures. Nevertheless, the study of these types of materials represents a good opportunity to differentiate the effect of the nature of the B site ions in the catalytic activity and structural properties of the mixed oxides. 

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