Classes of spinels

Eleven spinels were calcined in air at 800° and 900°C in order to reduce their surface areas. Two classes of spinels were defined (Tables I and II). Class I spinels generally had a high catalytic activity for oxidation of carbon monoxide and hydrocarbons, but they did not have or maintain a large surface area upon calcination. Class I spinels all had a surface area of less than 1 m2/g after firing at 900°C for 16 hrs. Class II spinels, on the other hand, had high surface area and stability, but they were very poor oxidation catalysts. 

A = Mg, Fe, NT, Co, and Zn and trivalent cations B = Al, Fe, Cr, and V L Hence two types of spinel structure must be distinguished normal spinels withx = 0, meaning that all the divalent cations occupy tetrahedral sites, and inverse spinels with x = 1. Of these, rods of pure magnetite (Fe OJ or its doped form" obtained by casting molten iron oxides have been used as industrial anodes since 1870. Apart from magnetite and ferrites, today other classes of spinels have been investigated such as cobaltites and chromites. Due to their better electrocatalytic properties and fewer health and safety issues, cobaltites (e.g., MCo O with M= Mg, Cu, and Zn) are now preferred and are the only ones being developed. 

Compounds that have the empirical formulas MCr02 and DCr204 where M is a monovalent and D a divalent cation, are known as chromites. These are actually mixed oxides and probably are better written as M20-Cr203 and D0-Cr203, respectively. The oxides of D are largely spinels, ie, the oxygen atoms define a close-packed cubic array having the octahedral holes occupied by the Cr(III) cation and the tetrahedral holes occupied by D (54). Chromite ore is an important member of this class of oxides. 

The number of oxide type minerals is quite large. Rostov (1956) has identified 160 specific minerals, grouped them into classes (chrysoberyl, spinel, corundum, periclase, etc.), and proposed a classification system. Only a few examples will be discussed here. 

Aromatic N-containing heterocycles (pyridine, imidazole etc.,) and their alkyl derivatives represent an important group of products and they have received considerable attention because of their various applications. For instance, methyl pyridines (picolines) and dimethyl pyridines (lutidines) are a class of industrially valuable compounds for the production of dyes and fine chemicals [108]. Sreekumar et al [109] reported pyridine methylation to 3-picolines over Zni xCoxFe204 spinel systems at reaction temperature between 325 and 425°C. The... 

The name ferrite was originally given to a class of mixed oxides having an inverse spinel structure and the formula AFe204 where A is a divalent metal ion. The term has been extended to include other oxides, not necessarily containing iron, which have similar... 

Five aspects of the preparation of solids can be distinguished (i) preparation of a series of compounds in order to investigate a specific property, as exemplified by a series of perovskite oxides to examine their electrical properties or by a series of spinel ferrites to screen their magnetic properties (ii) preparation of unknown members of a structurally related class of solids to extend (or extrapolate) structure-property relations, as exemplified by the synthesis of layered chalcogenides and their intercalates or derivatives of TTF-TCNQ to study their superconductivity (iii) synthesis of a new class of compounds (e.g. sialons, (Si, Al)3(0, N)4, or doped polyacetylenes), with novel structural properties (iv) preparation of known solids of prescribed specifications (crystallinity, shape, purity, etc.) as in the case of crystals of Si, III-V compounds and... 

Spinel oxides with a general formula AB2O4 (i.e. the so-called normal spinels) are important materials in industrial catalysis. They are thermally stable and maintain enhanced and sustained activities for a variety of industrially important reactions including decomposition of nitrous oxide [1], oxidation and dehydrogenation of hydrocarbons [2], low temperature methanol synthesis [3], oxidation of carbon monoxide and hydrocarbon [4], and oxidative dehydrogenation of butanes [5]. A major problem in the applications of this class of compound as catalyst, however, lies in their usually low specific surface area [6]. 

The spinels are a class of double oxide of general formula AB204 industrially important members of this class include aluminates (e.g. MgAl204), ferrites (e.g. MgFe204) and chromites (e.g. MgCr204). 

Spinels form a large class of compounds that are important in catalysis. The structure of normal spinels is related to that of MgAl204, the unit cell of which contains 32 oxygen atoms in a nearly perfect cubic array (30, 31). The Mg and A1 cations occupy tetrahedral and octahedral sites, respectively. 

Both Y-AI2O3 and TI-AI2O3 are members of a class of binary oxides in which the oxygen packing is of the cubic close packing type, with the cations situated partly in tetrahedral and partly in octahedral positions. These compounds are called spinels after the mineral spinel which has the composition MgAhC. In spinel Mg2+ occupies tetrahedral and Al3+ occupies octahedral positions. The unit cell of a cubic close packed structure is shown in Fig. 8.5. 

Magnetite, Fes04, is the outstanding example of this class of semi-conductor. De Boer and Verwey (7) first proposed the interpretation of its properties and Verwey and his co-workers (8) have studied the structural problem with some success. They deduce from their studies of related spinels that, at ordinary temperature, F O has all the Fe + ions and half the Fe + ions distributed statistically over one kind of cation position (octahedral) in the unit cell. The remaining Fe + are localized in tetrahedral sites. Ihe ease of electron transfer from Fe + to Fe ions is responsible for the... 

Double-salt Formation. This is an important class of solid reactions, including all the reactions of silicate chemistry, spinel formation, the formation of Ag Hgl, etc. These reactions may proceed by movement of one or more kinds of ions. In spinel formation only one kind of ion is normally mobile in the product lattice, and reaction occurs therefore at the phase boundary of the inactive oxide and the spinel. In the reaction between... 

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. 

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