Variable valence oxides

The transition elements are often said to exhibit variable valency. Because they so readily form complex compounds, it is better to use the term variety of oxidation states . The states usually found for the elements Sc-Zn are ... 

Transition elements, for which variable valency is energetically feasible, frequently show non-stoichiometric behaviour (variable composition) in their oxides, sulfides and related binary compounds. For small deviations from stoichiometry a thermodynamic approach is instructive, but for larger deviations structural considerations supervene, and the possibility of thermodynamically unstable but kinetically isolable phases must be considered. These ideas will be expanded in the following paragraphs but more detailed treatment must be sought elsewhere. " ... 

Transition-metal oxides are particularly effective decomposition and burning-rate catalysts. The metal elements can demonstrate variable valence or oxidation states. 

When a d-metal atom loses electrons to form a cation, it first loses its outer s-electrons. However, most transition metals form ions with different oxidation Variable valence is discussed further states, because the ( -electrons have similar energies and a variable number can be... 

Many metallic elements in the p and d blocks, have atoms that can lose a variable number of electrons. As we saw in Section 1.19, the inert-pair effect implies that the elements listed in Fig. 1.57 can lose either their valence p-electrons alone or all their valence p- and s-electrons. These elements and the d-block metals can form different compounds, such as tin(II) oxide, SnO, and tin(IV) oxide, Sn02, for tin. The ability of an element to form ions with different charges is called variable valence. 

The exhibition of variable valency is indeed a characteristic of transition metals. Main group metal ions such as those of groups 1 or 2 exhibit a single valence state. Other main group metals may show a number of valencies (usually two) which are related by a change in oxidation state of two units. This is typified by the occurrence of lead(iv) and lead(ii) or thallium(iii) and thallium(i). However, all the transition metals exhibit a range of valencies that is generally not limited in this manner. 

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

When the temperature of a carbonate reservoir that is saturated with high-viscosity oil and water increases to 200° C or more, chemical reactions occur in the formation, resulting in the formation of considerable amounts of CO2. The generation of CO2 during thermal stimulation of a carbonate reservoir results from the dealkylation of aromatic hydrocarbons in the presence of water vapor, catalytic conversion of hydrocarbons by water vapor, and oxidation of organic materials. Clay material and metals of variable valence (e.g., nickel, cobalt, iron) in the carbonate rock can serve as the catalyst. An optimal amount of CO2 exists for which maximal oil recovery is achieved [1538]. The performance of a steamflooding process can be improved by the addition of CO2 or methane [1216]. 

Cyclic chain termination by antioxidants. Oxidation of some substances, such as alcohols or aliphatic amines, gives rise to peroxyl radicals of multiple (oxidative and reductive) activity (see Chapters 7 and 9). In the systems containing such substances, antioxidants are regenerated in the reactions of chain termination. In other words, chain termination occurs as a catalytic cyclic process. The number of chain termination events depends on the proportion between the rates of inhibitor consumption and regeneration reactions. Multiple chain termination may take place, for instance, in polymers. Inhibitors of multiple chain termination are aromatic amines, nitroxyl radicals, and variable-valence metal compounds. 

When variable-valence metals are used as catalysts in the oxidation of hydrocarbons, the chain termination via such reactions manifests itself later in the process. This case has specially been studied in relation to the oxidation of paraffins to fatty acids in the presence of the K Mn catalyst [57], which ensures a high oxidation rate and a high selectivity of formation of the target product (carboxylic acids). As the reaction occurs, alcohols are accumulated in the reaction mixture, and their oxidation is accompanied by the formation of hydroxyperoxyl radicals. The more extensively the oxidation occurs, the higher the concentration of alcohols in the oxidized paraffin, and, hence, the higher is the kinetic... 

Clearly then, in glasses coloured by metal ions, the co-ordination chemistry of the transition metal ion has a major influence on the colour. The other major influence is the oxidation state of the metal ion, since variable valency is another characteristic of the transition metals. All other things being equal, for example, iron in the Fe11 form will give a pale blue colour, whereas Fem gives... 

Electronegativity (x) is an empirical measure of the tendency of an atom in a molecule to attract electrons. The noble gases, therefore, do not have electronegativity values because they do not easily form molecules. The electronegativity value depends primarily on the element, but also on the oxidation state, i.e., the electronegativity of elements with variable valency can be different for each valency thus that of Fe2+ is different from that of... 

Metal ions of transition and other elements of variable valency, e.g. Ce, Co, Fe, V, Mn, etc., are known to oxidize polysaccharides rather selectively, producing macroradicals as intermediates which are capable of adding vinyl monomers and form graft copolymers. These initiators are redox systems which differ from those previously described by not producing free radicals of low molecular weight. Only macroradicals on the substrate are formed in the redox reaction. Some homopolymer may still be formed in the process, e.g. due to oxidation of monomer or other side reactions. ... 

Vanadium is a silvery whitish-gray metal that is somewhat heavier than aluminum, but lighter than iron. It is ductile and can be worked into various shapes. It is like other transition metals in the way that some electrons from the next-to-outermost shell can bond with other elements. Vanadium forms many complicated compounds as a result of variable valences. This attribute is responsible for the four oxidation states of its ions that enable it to combine with most nonmetals and to at times even act as a nonmetal. Vanadiums melting point is 1890°C, its boiling point is 3380°C, and its density is 6.11 glam . 

The layer-type structures and chemical nature of the constituents of the bismuth and thallium-based cuprate superconductors - notably the lone-pair stereochemistry of Bis+, variable valence of copper, and considerable exchange among some of the cation sites - combine to make structural non-ideality, nonstoichiometry, and phase intergrowth the rule rather that the exception in these families of materials. These features, as well as the probable metastability of the phases (and possibly all high-temperature oxide superconductors), also contribute to the difficulties typically encountered in preparing single-phase samples with reproducible properties and compositions. 

Oxides and metal salts with variable valency iron (III) oxide, copper (II) oxide, copper chromite, copper chromate etc. 

Nonstoichiometric metal oxides are effective catalysts for a variety of oxidation-reduction reactions (as might be expected) since the variable valence of the constituent ions enables the oxide to act as a sort of electron bank. Nonstoichiometric metal oxides resemble metals in that they can also catalyze hydrogenation and alkene isomerization reactions. However, on zinc oxide, for instance, these two processes are independent, whereas hydrogen must be present for isomerization to occur on metals. 

The allylic oxidation of propene is catalyzed by (compound) metal oxides, which essentially contain metal ions of variable valency. It is commonly accepted that a redox mechanism is operative in such a way that the catalyst acts as the oxidizer and that lattice oxygen is incorporated in the oxidation products. The assumptions have been proved for several catalysts by the analysis of cation valency changes and by experiments with labelled oxygen. 

AHt can be calculated, in principle, from thermochemical data. It is then necessary to take into account the variable valency of most metals and to fix the different oxidation states which occur during stationary or non-stationary reaction conditions. Some difficulties with this method are th scarcity of data for mixed oxides, the difference in conditions between those on the surface of the catalyst and those in the bulk and the inaccuracy of a number of data obtained by measuring differences in AH. 

Finally, it may be noted that, although variable valency in binary oxides is important, it is not a sufficient requirement, as can be concluded from the fact that even in the systems Bi—Mo—O and U—Sb—O not every crystal phase is active and selective [294], What matters is the configuration of the ions at the active site. Apparently, the character of the typical Me—O bands is a function of the situation of oxygen in the lattice. 

The variable valency of sulfur allows the formation of S-oxides (11) which unlike thiabenzene 1-oxides are non-aromatic and exist in half-boat conformations (78CC197). On the other hand, acid treatment of 6-hydroxy-6H- 1,2-oxazines (12) yields cations (13) which are analogous to pyrylium salts (74ZOR1513). 

The (/-block elements tend to lose their valence s-electrons when they form compounds. Most of them can also lose a variable number of d-electrons and show variable valence. The only elements of the block that do not use their (/-electrons in compound formation are the members of Group 12 (zinc, cadmium, and mercury). The ability to exist in different oxidation states is responsible for many of the special properties of these elements and plays a role in the action of many vital biomolecules (Box 16.1). 

Reactions of methane and ethane with HF recyclable metal fluorides to give fluorocarbons have been reported briefly in the patent and journal literature [16-18], Reaction of methane with hydrogen fluoride in the presence of oxygen and the salt or oxide of a variable valency metal as catalyst yielded small amounts of fluoromethane and difluoromethane at temperatures above 500°C. Olsen et al. [17] reacted copper(ll) fluoride with methane at high temperatures (>600°C) and found products that always included copper metal, hydrogen fluoride, fluoromethane and carbon. Although activity was first detected around... 

The effects of deliberately added donors, such as titanium, and acceptors, such as iron and magnesium, on electrical conductivity have been studied. Doping with aliovalent ions affects the concentration of intrinsic defects and, in consequence, the diffusivity of A1 and O. In the case of variable-valency dopants, changes in p0l change the fraction of dopants in the aliovalent state and the nature and concentration of the defects. For example, the dopant Ti substitutes for A1 and, in the fully oxidized state, produces the defect TiA1, compensated by Va", so that... 

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