Mixed Oxides Perovskite Structures Perovskites

The perovskite-type oxides have unique characteristics in response to a wide range of properties that are assigned to the cation substitution capacity in its structure, generating isostructural solid with formula Ai- AyBi-yByOsi s. These substitutions can lead to the stabilization of the stmcture with an unusual oxidation state for one of the cations and the creation of anionic and cationic vacancies. This has a significant influence on the catalytic activity of these materials compared to the typical supported materials. Another important feature is the thermal stability of these materials and mechanical and chemically stable reaction conditions [41,148]. 

The ternary oxide of perovskite type can be divided into A B +Os, A B 03, and A +B +Os. The former is of particular interest because of their ferroelectric properties, e.g., KNbOs and NaNbOs e KTaOs A B +03 probably forms the largest number of perovskite-type oxides, in which the cation may be an alkaline earth, cadmium, and lead, and B + includes Ce, Fe, Ti, Zr, Mo, and others. Finally, A +B +Oj includes several compounds such as LaCrOa, EuFeOs, LaCo03, etc. [153]. 

The ideal structure of a perovskite-type oxide is cubic, space group Pm3m-Oh, and A, a cation of large size coordinated to 12 oxygen ions, while B is a smaller cation coordinated to six oxygen ion. Schematically, Fig. 13.24 shows a unit cell 

where the A cation occupies the center of the cube cations are located at the vertices B, and oxygen anions are centered on the edges of the cube. Alternatively, the structure can be displayed with the B cation occupying the center of an octahedron formed of oxygen vertices, which in turn would be inside a cube whose vertices are the A cations. 

For measuring the deviation from ideality of the cubic structure ABO3, Galasso [153] defined the tolerance factor f, according to Eq. (1). While in the ideal structure the atoms are in touch, this factor is calculated from the interatomic distances Aq and Bo, respectively, defined as (rA + o) = and r-g, + Vq) = a/2, which 

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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]. 

Titanium IV) oxide, T1O2. See titanium dioxide. Dissolves in concentrated alkali hydroxides to give titanates. Mixed metal oxides, many of commercial importance, are formed by TiOj. CaTiOj is perovskite. BaTiOa, per-ovskite related structure, is piezoelectric and is used in transducers in ultrasonic apparatus and gramophone pickups and also as a polishing compound. Other mixed oxides have the il-menite structure (e.g. FeTiOj) and the spinel structure (e.g. MgjTiO ). 

Complex Base-Metal Oxides Complex oxide systems include the mixed oxides of some metals which have perovskite or spinel structure. Both the perovskites and the spinels exhibit catalytic activity toward cathodic oxygen reduction, but important differences exist in the behavior of these systems. 

In the early 1990s, Balachandran et al. (51,64,65) of the Argonne National Laboratory, in collaboration with Amoco (now part of BP), investigated the partial oxidation of methane using membrane materials consisting of Sr-Fe-Co-O mixed oxides with the perovskite structure, which have high oxygen permeabilities. In their experiments (51,66), the membrane tubes, which were... 

Figure 20. Electronic structure and transport in mixed conducting perovskites. (a) Band picture of electronic structure in the high-temperature metallic phase of Lai- r tCo03-(5. (Reprinted with permission from ref 109. Copyright 1995 Elsevier.) (b) Localized picture of electron/ hole transport in semimetallic Lai- 3r Fe03-(5, involving hopping of electrons and/or electron holes (depending on the oxidation state of iron).

Finally, a number of other mixed oxides that do not have the perovskite structure have also been examined. For example, niobium titanates with the rutile structure,tetragonal tungsten bronze... 

In recent years, research on catalysts for the ATR of hydrocarbons has paid considerable attention to perovskite systems of general formula ABO3. In the perovskite stmcture, both A and B ions can be partially substituted, leading to a wide variety of mixed oxides, characterized by structural and electronic defects. The oxidation activity of perovskites has been ascribed to ionic conductivity, oxygen mobility within the lattice [64], reducibility and oxygen sorption properties [65, 66]. 

Three important mixed oxide structures exist spinel, perovskite, and ilmenite... 

Barium titanate is one example of a ferroelectric material. Other oxides with the perovskite structure are also ferroelectric (e.g., lead titanate and lithium niobate). One important set of such compounds, used in many transducer applications, is the mixed oxides PZT (PbZri-Ji/Ds). These, like barium titanate, have small ions in Oe cages which are easily displaced. Other ferroelectric solids include hydrogen-bonded solids, such as KH2PO4 and Rochelle salt (NaKC4H406.4H20), salts with anions which possess dipole moments, such as NaNOz, and copolymers of poly vinylidene fluoride. It has even been proposed that ferroelectric mechanisms are involved in some biological processes such as brain memory and voltagedependent ion channels concerned with impulse conduction in nerve and muscle cells. 

A common feature of all the new ceramic superconductors is that they are cuprates, that is, they are complex copper oxides. The structure of YBCO is given in Fig. 19.3, which also shows that it is related to the perovskite structure (Fig. 4.17). Synthesis of YBCO is remarkably easy appropriate amounts of dry yttrium oxide (Y203), copper oxide (CuO), and barium carbonate (BaC03) are ground together into a fine, well-mixed... 

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. 

Alternatively, we have attempted the molecular design of mixed-oxide catalysts by using crystalline mixed oxides whose bulk structures are known and whose potential for practical use is good. Heteropoly compounds, perovskites, and zeolites are the candidate catalysts. 

Neither element shows any simple aqueous chemistry in the M(IV) state, as the oxides M02 are insoluble in water at all pH values. Reaction of Sn02 in molten KOH gives the octahedral hydroxanion [Sn(OH)6]2-, in contrast to the normal tetrahedral silicates and germinates, but in parallel with isoelectronic compounds such as Te(OH)6 also found in period 5. Other stannates are mixed oxides without discrete oxoanions (e.g. CaSn03 with the perovskite structure). 

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. 

Barium titanate, which has many novel properties, is a mixed oxide ceramic. It has the same structure as the mineral perovskite, CaTiOs (Fig. 22.13), except, of course, that Ba replaces Ca. Perovskites typically have two metal atoms for every three O atoms, giving them the general formula ABO3, where A stands for a metal atom at the center of the unit cube and B stands for an atom of a different metal at the cube corners. 

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