Mixed oxides structure

The structure of these solid compounds is not known with certainty but an approximate formula might be NaAlOj.xHjO. Many aluminates occur in minerals, for example the spinels of general formula M (A102)2 where M may be Mg, Zn or Fe these have a mixed oxide structure, i.e. consist essentially of M AF and O ions. 

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

The spinel structure is a common mixed oxide structure, typified by spinel itself MgAl204, in which the oxide ions are in a face-centred cubic close packed array (see Section 1.6.3 and Figure 1.43). For an array of TV oxide ions, there are TVoctahedral holes, and the trivalent ions (AP" ) occupy half of the octahedral sites (Figure 9.10). 

In the case of the La2Cuo.9Pdo.i04+s catalyst, we have seen that the mixed oxide structure collapses in the presence of CO, and although the easy reduction and oxidation of the well-dispersed copper and palladium seems to be sufficient to compensate for the variations in the composition, it is difficult to invoke real oxygen storage properties for this solid. 

Some Mixed Oxide Structures. There are a vast number of oxides (and also some stoichiometrically related halides) having two or more different kinds of cation. Most of them occur in one of a few basic structural types, the names of which are derived from the first or principal compound shown to have that type of structure. Three of the most important such structures will now be described. 

There are a few catalytic applications in which other mixed oxide structures are cited. Among them are pyrochlore compounds such as La2Zr207, spinels such as AB2O4, and lanthanum beta aluminates, all of them with well-defined crystalline structures. There are also a few examples of oxide solid solutions made up of a rare-earth oxide and a transition-metal oxide. 

There are two ways of preparing a catalyst with the mixed oxide structure ... 

A number of tellurium-containing mixed-oxide structures have been described. In all but one, tellurium is in the (+4) state. In barium tellurite monohydrate, [BaTe08,Ha0], the tellurium geometry is three-co-ordinate pyramidal with Te-O distances of 1.847, 1.858, and 1.859 A. These values are smaller than the sum of the covalent radii, and imply considerable multiple bonding. The next closest oxygen atoms are at distances of 3.035 and 3.322 A. 

Fernandez-Garcia, A., Martinez-Arias, A., Guerrero-Ruiz, A., Conesa, J.C., and Soria, J. Ce-Zr-Ca ternary mixed oxides structural characteristics and oxygen handling properties, y. Catal. 2002, 211, 326-334. 

Chromates III). Mixed oxides, e.g. FeCr204, having spinel structures and prepared by solid state reactions. 

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 ). 

Decomposition of potassium ferratefVI) at 1000 K gives a ferrate V), K3Fe04, and several types of ferrate(IV), for example FeOj", Fe04 are known these ferrates(IV) have no solution chemistry and are probably best regarded as mixed oxides, since the FeOl" ion has no identifiable structure. 

The mixed oxide Fc304 (tri-iron tetroxide) is a black solid, which occurs naturally as magnetite it is formed when iron(III) oxide is strongly heated, and its structure is effectively made up of oxide (O ) and iron(II) and iron(III) ions. 

This section considers a number of extremely important structure types in which A1 combines with one or more other metals to form a mixed oxide phase. The most significant of these from both a theoretical and an industrial viewpoint are spinel (MgAl204) and related compounds, Na- -alumina (NaAlnOi ) and related phases, and tricalcium aluminate (Ca3Al20g) which is a major constituent of Portland cement. Each of these compounds raises points of fundamental importance in solid-state chemistry and each possesses properties of crucial significance to... 

S. Rossignol, C. Micheaud-Especel, and D. Duprez, Structural and catalytic properties ofZr-Ce-0 mixed oxides. Role of the anionic vacancies, Stud. Surf. Sci. Catal. 130, 3327-3332 (2000). 

The new phases were discovered by the combination of exploratory synthesis and a phase compatibility study. As commonly practised, the new studies were initially made through the chemical modification of a known phase. Inclusion of salt in some cases is incidental, and the formation of mixed-framework structures can be considered a result of phase segregation (for the lack of a better term) between chemically dissimilar covalent oxide lattices and space-filling, charge-compensating salts. Limited-phase compatibility studies were performed around the region where thermodynamically stable phases were discovered. Thus far, we have enjoyed much success in isolating new salt-inclusion solids via exploratory synthesis. 

The present work demonstrates that the mixed oxide catalyst with inhomogeneous nanocrystalline MosOu-type oxide with minor amount of M0O3- and Mo02-type material. Thermal treatment of the catalyst shows a better performance in the formation of the crystals and the catalytic activity. The structural analysis suggests that the catalytic performance of the MoVW- mixed oxide catalyst in the partial oxidation of methanol is related to the formation of the M05O14 t3 e mixed oxide. 

An interesting recent example of successful application of the SSG process combined with ensuing supercritical drying is the design of titania-silica mixed oxides for the epoxidation of bulky olefins [16-18]. This example will be used to illustrate the opportunities the combined use of SSG and SCD provide for tailoring the chemical and structural properties of mixed oxides. 

Due to the formation of Ca/Al mixed oxide on the surface, the Ca -modified alumina has a completely different structure compared to the spinel one This leads to a different type of surface Lewis acid/basic sites, rendering the catalyst 30 times less active. 

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. 

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