Oxide type perovskites

Synthesis and Catalytic Applications of Nanocast Oxide-Type Perovskites... 

The use of oxide-type perovskites as dry reforming catalysts will be dealt with extensively in Volume II of this book. In this chapter, we report one example of such catalyst in order to highlight the potentiahty of the nanocasting preparation method as a way to control bulk and smface properties of perovskite-derived catalysts. 

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. 

Few oxide superconductors were known prior to 1985 and we shall now return to these so that we can discuss these materials in reference to their crystal structure classes. There are only three broad structural categories in which most of the oxide superconductors occur. The important structural types include sodium chloride (rocksalt, or Bl-type), perovskite (E2X), and spinel (Hlx). 

Electrodes The anodes of SOFC consist of Ni cermet, a composite of metallic Ni and YSZ, Ni provides the high electrical conductivity and catalytic activity, zirconia provides the mechanical, thermal, and chemical stability. In addition, it confers to the anode the same expansion coefficient of the electrolyte and renders compatible anode and electrolyte. The electrical conductivity of such anodes is predominantly electronic. Figure 14 shows the three-phase boundary at the interface porous anode YSZ and the reactions which take place. The cathode of the SOFC consists of mixed conductive oxides with perovskite crystalline structure. Sr doped lanthanum manganite is mostly used, it is a good /7-type conductor and can contain noble metals. 

The various processes for the catalytic reaction are similar. The factor that makes the difference is the choice of catalyst, which in turn affects the temperature regime needed to trigger the decomposition of nitrous oxide. In the literature, numerous works illustrate the several classes of catalysts appropriate for this reaction [9a, k] noble metals (Pt, Au), pure or mixed metal oxides (spinels, perovskite-types, oxides from hydrotalcites), supported systems (metal or metal oxides on alumina, silica, zirconia) and zeolites. 

XRD reflections in the 20 region of 5-70° were found to be characteristic of the pure perovskite oxide-type structure (JCPDS n° 50-298). The XRD patterns of MW samples prepared in one step and their corresponding MWhyd catalysts prepared in two steps were almost the same. 

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. 

Table 4 Catalytic activity of methane oxidation over perovskite-type oxides and Pt/alumina catalysts ...

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. 

Canted spin alignment is not restricted to oxides. The perovskites NaMnFj, NaFeFj, NaCoF and NaNiFj all show G-type antiferromagnetic arrangement of the... 

Figure 6.19 shows the temperature dependence of the 0 -ion conductivity in an oxygen-deficient brownmillerite-type perovskite Ba2ln20s that exhibits a first-order order-disorder transition at 930 °C. The Arrhenius plot of 0 -ion conductivity shows that, above T, a considerable short-range order persists nevertheless, the oxide-ion conduction in this field can be competitive with that in the commercial electrolyte yttria-stabilized zirconia (YSZ). 

Other types of oxides, non-perovskite oxides including pyrochlores and fluorites, have also been investigated. It was found that the ammonia formation rates from pyrochlore-type membrane were close to that obtained from perovskite-type membranes, although in the case of the fluorite membrane, the formation rates were slightly higher (Liu et al., 2006 Xie et al., 2004 Wang et al., 2005). More comprehensive details about characteristics of each material, fabrication, cell components, and the formation rate can be found in a review presented by Amar et al. (2011). 

The pure compounds are divided into simple perovskite-type oxides and complex perovskite-t)fpe oxides. Simple perovskite-type oxides have the chemical formula A +B +03 or A +B" +03. Complex perovskite-type oxides have chemical formulas expressed by (A +Af+)B03, A2+(B2+Bf+)03,... 

Miscellaneous perovskite-type oxides ([AC3](B4X)i2-type perovskites)... 

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