The Spinel Type

FIGURE 4.41 The Spinel Network after The Chemistry of Sihcates (2003). 

The above description corresponds to the normal type of spinel where the octahedral interstices are occupied in proportion of 1/2 with the more polarized cations, vdiile the tetrahedral interstices are filled only in 1/8 proportion by the cations with lower polarization ability. 

instead, the tetrahedral interstices are occupied by the cations, and the octahedral ones are occupied by a cationic and B combination the resulting structure is said of reverse spinel. 

a reverse spinel structure appear with octahedral interstices occupied by both the cation species, in equal proportions, and statistically 

FIGURE 4.42 The direct spinel structure (see the text for the details) left the view along the 4th order axes of symmetry right the view of packed structure after Heyes (1999). 

---

Many of the spinel-type compounds mentioned above do not have the normal structure in which A are in tetrahedral sites (t) and B are in octahedral sites (o) instead they adopt the inverse spinel structure in which half the B cations occupy the tetrahedral sites whilst the other half of the B cations and all the A cations are distributed on the octahedral sites, i.e. (B)t[AB]o04. The occupancy of the octahedral sites may be random or ordered. Several factors influence whether a given spinel will adopt the normal or inverse structure, including (a) the relative sizes of A and B, (b) the Madelung constants for the normal and inverse structures, (c) ligand-field stabilization energies (p. 1131) of cations on tetrahedral and octahedral sites, and (d) polarization or covalency effects. ... 

Figure 17. Schematic drawing of the spinel-type structure of Mn304 and y - Mn20,. The structure is built up of Mn06 octahedra (white) and Mn04 tetrahedra (shaded).

The low-temperature Mossbauer spectra of the spinel type oxides, NiCr204 [14,18] (Fig. 7.6b) and NiFe204 [3, 18], have been found to exhibit combined magnetic dipole and electric quadrupole interaction (Fig. 7.7). For the evaluation of these spectra, the authors have assumed a small quadrupolar perturbation and a large magnetic interaction, as depicted in Fig. 7.3 and represented by the Hamiltonian [3]... 

The electronic structure of the spinel type compound NiCo204 has been investigated by XANES, EXAFS, and Ni Mdssbauer studies. On the basis of the derived cation valencies, the octahedral and tetrahedral site occupancies as well as the formula in standard notation for spinel compounds could be delineated [25]. 

Packings of spheres having occupied tetrahedral and octahedral interstices usually occur if atoms of two different elements are present, one of which prefers tetrahedral coordination, and the other octahedral coordination. This is a common feature among silicates (cf. Section 16.7). Another important structure type of this kind is the spinel type. Spinel is the mineral MgAl204, and generally spinels have the composition AM2X4. Most of them are oxides in addition, there exist sulfides, selenides, halides and pseudohalides. 

One of the unusual features of spinel crystal chemistry is that some transition metals form normal spinels and others inverse. The spinel-types are summarized in table 6.2. The site occupancy patterns were considered to be anomalous until they were explained by crystal field theory (McClure, 1957 Dunitz andOrgel, 1957). 

The structure of y-alumina resembles that of the spinel-type MgAl204 t/-alumina has a very similar structure. The difference lies in the concentration of stacking faults which is caused by the difference in the structures of the precursors boehmite and bayerite [45]. The concentration of stacking faults is higher in the fj-alumina. 

These effects have been observed by Margolis and co-workers in their detailed kinetic study of the catalytic oxidation of hydrocarbons (219-222). The extensive oxidation of hydrocarbons to carbon dioxide over catalysts of the spinel type has been studied by a number of investigators. It was possible for Margolis and co-workers to establish the effect of additives upon the basic kinetic constants, namely the activation energy of oxidation and the frequency factor for this reaction. 

Many of the mineralogically important transition-metal oxide phases contain more than one cation species, or more than one type of coordination site for the cations. Commonly, the cations are in more than one oxidation state. Examples include ilmenite (FeTiOj) and the family of minerals with the spinel-type crystal structure, including magnetite (Fe304), chromite... 

Similar behavior has been observed with different modifications of Mn(IV) oxides. The cryptomelane type of hydrous manganese dioxide prepared by the reaction of Mn(II) and KMn04 in sulfuric acid solution exhibits an extremely high selectivity for potassium ions [120]. The spinel type of manganese oxide prepared by the introduction of lithium ion has exhibited an extremely high selectivity for lithium ion [121, 122]. These aspects are considered later. 

The reaction between the active compound and the washcoat can be avoided following two approaches. The first approach is to stabilize the active phase by applying it as a complex oxide. The spinel-type oxides, mentioned above, are relatively inactive. On the other hand, perovksites, often AMO3 (A = rare earth metal, e.g.. La, Sr M — transition metal, e.g., Co, Cr, Fe), exhibit promising behavior, which has attracted much attention [55,90,91]. Perovskites may also be used as unsupported oxides, but their surface area is small and not particularly thermostable. The activity of perovskites is dependent mostly on the M cation, but the A cation also has a significant effect [91]. Partial substitution... 

Most HTSC oxides have the structure of perovskite (though some of them have the spinel-type structure), which pertains to more than 35 structural classes [14], and includes more than a hundred typical unit cells [15]. Along with cuprates, certain bismuthates also exhibit HTSC properties. For fundamental studies of superconductivity in oxides, both the absolute Tc values and the variety of properties and structures are essential. Therefore, the titanium compounds with relatively low Tc values are also actively studied. 

The present article deals primarily with the elucidation of the surface nature of common metallic and oxidic catalysts, and with statistical-mechanical investigations of the chemisorption equilibrium on these catalysts. The surface areas of these catalysts as determined by the Brunauer-Emmett-Teller method have been taken into consideration. It was shown that a number of certain metallic catalysts such as nickel, cobalt, and platinum and also oxide catalysts of the spinel type act as an array of homogeneous active sites. There is no reason to believe that a few limited regions of the surfaces of these catalysts, such as corners, edges, lattice defects, etc. are particularly important for their catalytic activity. This conclusion is in accordance with the poisoning experiments of Maxted et al. There is some evidence that the surfaces of these catalysts... 

The XRD profile and the TEM of CAT A were shown in Figure 2 and Figure 4, respectively. The structure of CAT A before reaction was the same as that of FejO which has a spinel type structure. The small peaks assigned to CuO and ZnO were observed. The existences of metallic Cu and Zn could not be ascertained by XRD analysis. On the TEM observation, the CAT A before reaction consisted of the particles of uniform size, 10-20 nm diameters. The compositions of particles were uniform also. These results suggest that Cu and Zn components are dissolved into the spinel type structure. 

We make clear the following facts in this study. The K/Cu-Zn-Fe oxides catalyst has a spinel type structure. The K/Cu-Zn-Fe oxides catalyst is deactivated by the segregation of catalyst components to FeCOj, ZnO, and Cu during the reaction. The segregation is prevented by the addition of Cr component to the catalyst. The long life of K/Cu-Zn-Fe-Cr oxides catalyst can be explained by its slow segregation rate. It is considered that the Cr component stabilizes thermally the spinel type structure of the catalyst. 

The 5 and 6 phases formed at intermediate temperatures are both highly crystalline phases, the former with a superstructure of the spinel type and the latter with the 3-Ga203 structure mentioned in the section on Oxides M2O3 . 

Once the spinel-type oxide layer is formed, the stabilized current vs. potential profile at 1.3 V obeys... 

However, for samples 2-5, the amounts of CuO detected by quantitative XRD analysis (18) approach a maximum of 50% of the value calculated on the basis of a phase composition CuO + stoichiometric spinels, showing the existence of a consistent fraction of copper ions which escape XRD detection, probably present inside the spinel-type phase or strongly interacting with it (19). 

Figure 2. Composition of the spinel-type phases obtained eifter heating the Cu/Cd/Cr precipitate at 753K in air (A) or in vacuum (B), determined hy linear interpolation on the basis of the a values ( ) [CuCr204 cubic ICDD 26-509 and CdCr204 ICDD 2-1000] or by quantitative XRD ( ) or chemical (O) analyses.

---