Perovskites and pyrochlores

Layered perovskites with the formula A2BO4 (e.g. the K2Nip4 structure) have a well-defined bulk structure and high thermal stability. Examples include 

La2Cu04, Sr2Cu04. As we show in chapter 6, when a perovskite forms a composite or intergrowth with other structures, new compounds of interest in catalysis can be formed (such as in high-temperature superconducting copper oxides) and EM is used to determine the structures and properties of these complex compounds. The merits of using perovskites in steam reforming, membrane catalysis and fuel cells are discussed in chapter 6. 

Inorganic ceramic oxide supports for catalysts, such as alumina and silica, are used extensively in the catalysis industry because they are strong, they can have a range of shapes for different engineering needs and they are economical. They provide high surface areas for catalysis, ranging from 50 to 500 m g and can have pore sizes ranging from 2 to 20 nm in diameter. 

Silica is of particular importance because of its use as a stable catalyst support with low acidity and its relationship to zeolite catalysts, which will be discussed in chapter 4. Silicon is an abundant material in the earth s crust and occurs in various forms including silica. Silica is also polymorphous with the main forms being quartz, cristobalite and trydimite. The stable room temperature form is quartz (Si02). Recently, a new family of stable silica-based ceramics from chemically stabilized cristobalites has been described using electron microscopy (Gai et al 1993). We describe the synthesis and microstructures of these ceramic supports in chapters 3 and 5. 

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Metal oxides are usually prepared by calcinations of suitable precursors such as hydroxides, nitrates, carbonates, carboxylates, etc. This process usually gives oxides with pseudomorphs of the starting materials. When large amounts of thermal energy are applied for the decomposition of the precursors, it facilities sintering of the product particles and therefore aggregated particles are obtained. When mixed oxides such as spinel, perovskite, and pyrochlore are the desired products, heat treatment at higher temperatures is required. 

Other oxygen ion conductors that have potential use as solid electrolytes in electrochemical devices are stabilized bismuth and cerium oxides and oxide compounds with the perovskite and pyrochlore crystal structures. The ionic conductivity and related properties of these compounds in comparison with those of the standard yttria-stabilized zirconia (YSZ) electrolyte are briefly described in this section. Many of the powder preparation and ceramic fabrication techniques described above for zirconia-based electrolytes can be adapted to these alternative conductors and are not discussed further. 

Alternate Oxygen Ion Conducting Electrolytes 17.3.7.4.3. Perovskite and Pyrochlore Oxides. 

On the other hand, nonferroelectric pyrochlores may serve as technologically useful dielectrics in other applications, such as temperature-stable dielectrics or microwave dielectrics. It is important to understand the structural relationship between the perovskite and pyrochlore phases in the Pb-based systems and to elucidate the thermodynamic and kinetic factors that may influence the phase equilibrium. Understanding these will allow a more systematic method of tailoring dielectric compositions to applications. 

Noble metals (e.g., platinum, gold, and silver), metal oxides, mixed oxides such as spinels, perovskites, and pyrochlores have been investigated, but by far the best catalytic material is highly dispersed platinum. The high cost of this metal stimulated research to look for less expensive alternative materials... 

Interestingly, extended stability limit for thin films opens new perspectives including the possibility of growing new phases such as (Lni Ln ,)2Ti207 or Ln2(Tii ,Zr ,)207 with layered perovskite structure and with potential ferroelectric properties. This new limit between the layered perovskite and pyrochlore structures was illustrated in a structural diagram (Figure 11.9) proposed by Shao et al. [98]. 

According to Ferreira-Aparicio et al. [190], the supply of surface oxygen species from the hydroxyls of the acidic supports can aid the formation of methoxo (CH ) species. Based on FTIR spectroscopy analysis of methane adsorption on alumina, Li et al. [191] observed the presence of two hydroxyl signals at 3750 and 3 665 cm which shifted to 3707 and 3640 cm upon adsorption of methane. Their results indicate the possibility of weak interaction between methane and surface hydroxyls, a phenomenon also observed with Ir catalysts during methane decomposition [192]. Similarly, on perovskite- and pyrochlore-type catalysts, the lattice oxygen species on the surface were found to assist the methane activatiOTi [167, 174, 193]. 

The predicted structures obtained by optimizing the bond valences of the ions have BVSs close to their formal valences. The degrees of freedom in the spinel structure enable the simultaneous optimization of the BVS for each of the A-, B-, and X-site ions. Within calculation limitations, a zero global instability index (G) is fotmd for each of the predicted structures. The experimentally determined structures have a larger G, ranging from about 0.04 to 0.27 v.u. with an average of 0.15 v.u. for the examined structures. Unlike the cubic perovskite and pyrochlore structure, the spinel is not a strained structure. Regardless of the sizes of the A and B... 

The major minerals that contain REOE include apatite, phosphates, sfen, perovskite, eudialite, pyrochlore and ortit, some of which contain significant quantities of REOE. 

The hosts for ACT and REE immobilization are phases with a fluorite-derived structure (cubic zirconia-based solid solutions, pyrochlore, zirco-nolite, murataite), and zircon. The REEs and minor ACTs may be incorporated in perovskite, monazite, apatite-britholite, and titanite. Perovskite and titanite are also hosts for Sr, whereas hollandite is a host phase for Cs and corrosion products. None of these ceramics is truly a single-phase material, and other phases such as silicates (pyroxene, nepheliiie, plagioclase), oxides (spinel, hibonite/loveringite, crichtonite), or phosphates may be present and incorporate some radionuclides and process contaminants. A brief description of the most important phases suitable for immobilization of ACTs and REEs is given below. 

Fig. 8. Plots showing the dependence of the elemental release rate on pH for U in brannerite, pyrochlore, and zirconolite at 75 °C (after Zhang et al. 200 b). Also shown are data for Ca in perovskite and Ba in hollandite (after McGlinn et al. 1995 Carter et al. 2002). Linear fits are shown for perovskite and hollandite, but the other curves are weighted fits used to illustrate the trends.

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],... 

Corrosion of the Pu-doped pyrochlore-based ceramics is by incongruent dissolution and segregation of secondary phases [40], Leach rates in 1-year PCT-B tests were found to be (in g/(m xday) Ca - 10, Gd - 10, Pu - 10, Zr - <10. The leach rate of Pu is reduced by one order of magnitude - from lO" g/(m day) in short tests (1 to 7 days) to 10 g/(m xday) after 112-324 days of interaction with water [105], Introduction of 8 to 15 wt.% of oxides of typical contaminants (F, Cl, Na, Mg, K, Na, Si, Al, Ga, Mo, W, etc.) yielded extra glass, perovskite, and Ca-Al-Ti phase instead of brannerite [117]. This does not affect or even improve the chemical durability with respect to actinides. 

Perovskite (ABO3 in which A is divalent and B is tetravalent) and pyrochlore (AaBaOi in which A is trivalent and B is tetravalent) oxide compounds have been proposed as oxygen ion conducting electrolytes for electrochemical devices. Some of the perovskite structures (e.g., BaCeO and SrCeOs) are generating interest because of... 

Figure 3. Temperature dependence of the amorphization dose for (A) pyrochlore, zircon, monazite and perovskite and for (B) fayalite, forsterite, zirconolite, and hnttonite. The lines represent a least-squares fit to Equation (15).

Additional attempts have been presented to render hosts with the fluorite and the related pyrochlore structure electronically conductive by doping with mixed-valence and/or shallow dopants. The list of dopant materials examined includes oxides of elements of, for example, Ti, Cr, Mn, Fe, Zn, Fe, Sn, Ce, Pr, Gd, Tb and U. In general, however, the extent of mixed conductivity that can be obtained in fluorite-type ceramics is rather limited, by comparison with the corresponding values found in some of the perovskite and perovskite-related oxides considered in the next section. 

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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