Metal-Excess Oxides

FIGURE 5.41 Structural possibilities for binary oxides, (a) Type A oxides metal excess/anion vacancies, (i) This shows the two electrons that maintain charge neutrality, localized at the vacancy, (ii) The electrons are associated with the normal cations making them into M. (b) Type B oxides metal excess/interstitials. (i) This shows an interstitial atom, whereas in (ii) the atom has ionized to and the two liberated electrons are now associated with two normal cations, reducing them to M. (c) Type C oxides metal deficiency/interstitial anions. The charge compensation for an interstitial anion is by way of two ions, (d) Type D oxides metal deficiency/cation vacancies. The cation vacancy is compensated by two cations. 

Halides of non-metals are usually prepared by the direct combination of the elements. If the element exhibits more than one oxidation state, excess of the halogen favours the formation of the higher halide whilst excess of the element favours the formation of the lower halide (e.g. PCI5 and PCI3). 

A good summary of the behavior of steels in high temperature steam is available (45). Calculated scale thickness for 10 years of exposure of ferritic steels in 593°C and 13.8 MPa (2000 psi) superheated steam is about 0.64 mm for 5 Cr—0.5 Mo steels, and 1 mm for 2.25 Cr—1 Mo steels. Steam pressure does not seem to have much influence. The steels form duplex layer scales of a uniform thickness. Scales on austenitic steels in the same test also form two layers but were irregular. Generally, the higher the alloy content, the thinner the oxide scale. Excessively thick oxide scale can exfoHate and be prone to under-the-scale concentration of corrodents and corrosion. ExfoHated scale can cause soHd particle erosion of the downstream equipment and clogging. Thick scale on boiler tubes impairs heat transfer and causes an increase in metal temperature. 

Dissolution. Plutonium is solubilized in nitric acid solutions at Rocky Flats. The feed material consists of oxide, metal and glass, dissolution heels, incinerator ash and sand, slag, and crucible from reduction operations. The residues are contacted with 12M HNO3 containing CaF2 or HF to hasten dissolution. Following dissolution, aluminum nitrate is added to these solutions to complex the excess fluoride ion. 

Cadmium oxide, CdO, like nickel oxide, also adopts the sodium chloride structure (Fig. 1.14). However, unlike nickel oxide, this compound can be made to contain more metal than oxygen. The defects that cause this metal excess are usually considered to be interstitial Cd atoms or ions. In this case the reaction is one in which the solid formally loses oxygen. Because of the rules of equation writing, this must involve the removal of neutral oxygen atoms. Each oxygen lost results in the loss of a nonmetal site. In order to keep the site ratio correct, a metal site must also be lost, forcing the metal into interstitial sites ... 

Taking as an example an ionic oxide MO, this material can be made into a metal-excess nonstoichiometric material by the loss of oxygen. As only neutral oxygen atoms are removed from the crystal, each anion removed will leave two electrons behind, which leads to electronic conductivity. The oxygen loss can be incorporated as oxygen vacancies to give a nonstoichiometric oxide with a formula MOi v, or the structure can assimilate the loss and compensate by the introduction of cation interstitials to give a formula M1+xO. 

Metal-excess oxides can change composition by way of metal interstitials or oxygen vacancies. The formation of cation interstitials in a nonstoichiometric oxide MO can be represented by... 

Thus, the conductivity increases as the partial pressure of oxygen increases, which is the opposite behavior to that of the metal-excess oxides. 

An increase of oxygen partial pressure will cause the conductivity of a metal-excess nonstoichiometric oxide to ... 

The discussion of the defects in FeO has so far been only structural. Now we turn our attention to the balancing of the charges within the crystal. In principle the compensation for the iron deficiency can be made either by oxidation of some Fe(II) ions or by reduction of some oxide anions. It is energetically more favourable to oxidise Fe(II). For each Fe vacancy, two Fe cations must be oxidised to Fe ". In the overwhelming majority of cases, defect creation involves changes in the cation oxidation state. In the case of metal excess in simple compounds, we would usually expect to find that neighbouring cation(s) would be reduced. 

Type A oxides compensate for metal excess with anion vacancies. To maintain the overall neutrality of the crystal, two electrons have to be introduced for each anion vacancy. These can be trapped at a vacant anion site, as depicted in (a)(1). However, it is an extremely energetic process to introduce electrons into the crystal and so we are more likely to find them associated with the metal cations as shown in (a) (ii), which we can describe as reducing those cations from to M. VOi is an example of this type of system. 

Type B oxides have a metal excess which is incorporated into the lattice in interstitial positions. This is shown in (b)(1) as an interstitial atom, but it is more likely that the situation in (b)(ii) will hold, where the interstitial atom has ionised and the two electrons so released are now associated with two neighbouring ions, reducing them from to M. Cadmium oxide, CdO, has this type of structure. Oxygen is lost when zinc(II) oxide is heated, forming Zn +JD, oxide vacancies form and to compensate, Zn ions migrate to interstitial positions and are reduced to Zn ions or Zn atoms. Electron transfer can take place between the Zn and ZnTZn resulting in the yellow coloration seen when ZnO is heated. 

Oxides containing an excess of metal behave in a different way a representative of this group is zinc oxide. An excess of metal may be present in the form of zinc ions in interstitial positions, Zno or Zno", and... 

It is likely that the unusual effectiveness of the silver preparations described herein is due to the relationship between the surface properties/inner properties (i.e., oxide/ metal) of the particles and the size distribution of the particles. The smaller the average particle size, the greater the surface area and the greater the contribution of the particular surface chemistry. However, if the particles are excessively small there can be a loss of stability and/or other interactions that negatively affect the product. The sifver compositions of the instant invention are remarkabie because they are stable in essentially pure water without surfactants, etc. Also, the materials are essentially colorless while other colloidal silver preparations (particularly with larger particle sizes) usually show colors. These properties are a result of the exact manufacturing conditions as discussed above. 

Cyclization can also occur through halide displacement after initial metalation. For example, acylation of o-lithiofluorobenzenetricarbonylchromium with y-butyrolactone at 25 °C for 24 h is followed by spontaneous fluoride displacement to give complex 36. Oxidation with excess iodine liberates the lactone in 48 % overall yield (Scheme 15) [20]. 

Dehydroglaucine and dehydronuciferine readily undergo C-7 to C-7 coupling, using mercuric acetate or nitrate to form the corresponding aporphine and dehy-droaporphine dimers.39 Nuciferine, which is 1,2-dimethoxyaporphine, can be hydroxylated at C-3 by the sequence (i) bromination in trifluoroacetic acid, (ii) metallation with phenyl-lithium, and (iii) oxidation with excess nitrobenzene.40... 

Metal oxides, the products of oxidation of metals are ionic compounds with the metal ions and oxide ions arranged in arrays in the crystal lattices. When the metal oxide contains excess metal ions in interstitial positions, they are known as n-type (or negative carrier type) oxides. When the metal oxides contain vacant sites (deficient in metal ions) in the lattice the oxides are known as p-type (positive carrier type). 

On the other hand, polytellurides only seem to oxidize metals to the +1 or +11 state. Reaction of equimolar amounts of Te4 with M(CO)6 results in disubstitution of CO forming a cu-complex (CO)4MTe4 (M = Cr (45), W (47)47). If an excess of metal carbonyl is used in the presence of poly-telluride anion, multinuclear products can be isolated and metal-metal bonds can also form, leading to clusters. Careful manipulation of reaction conditions and choice of the polychalcogenide anion used makes possible partial oxidation of the metal centers and cluster formation. The reaction of iron carbonyls with polytelluride anions can lead to a wide array of cluster compounds, the identities of which are controlled by the stoichiometries and compositions of the starting telluride anions. For instance, reaction of [Fe(CO)5] with Te2 leads to the formation of [Fe3(CO)9(ju.3-Te)]2 (48),48 whereas its reaction with increasing amounts... 

A more delicate item may be the effect of alloy formation between noble metals and the metal components of the supports. In the case of Pt/Sn02, low-temperature (at 120 °C) reduction is required, which leads to both the formation of Pt-Sn alloys and the formation of surface hydroxyls at the perimeter [68]. On the other hand, in the case of Au/Ti02, vacuum evacuation or reduction dramatically suppresses the initial catalytic activity, which can be recovered gradually during CO oxidation in excess O2. The removal of oxygen species at the perimeter interface is deleterious to supported Au catalysts. 

Therefore the catalysis of the oxidation of the alkylbenzenes to the corresponding aldehydes is kept alive by the formation of an excess of Co ", formed by the oxidation of the aldehydes with oxygen. In general, oxidation intermediates like aromatic aldehydes and peroxides, which are normally more reactive than the corresponding toluenes, can regenerate highly oxidized metal species. Besides the free-radical mechanism stoichiometric and ionic reaction pathways also play an important role in the oxidation of alkylaromatic compounds. This is shown with Co " as oxidant on the left-hand side of Scheme 2. 

Schemes have been devised to substitute less toxic metals for more toxic ones. Potassium ferrate on K10 mont-morillonite clay has been used to replace potassium chromate and potassium permanganate in the oxidation of alcohols to aldehydes and ketones in 54-100% yields.153 The potassium ferrate is made by the action of sodium hypochlorite on iron(III) nitrate or by treatment of iron(III) sulfate with potassium peroxymonosulfate.154 After the oxidation, any excess oxidizing agent, and its reduced form, are easy to recover by filtration or centrifugation. In another case, manganese-containing reagents have been substituted for more toxic ones containing chromium and selenium (4.21).155 Selenium dioxide was used formerly in the first step and pyridinium chlorochromate in the second.

For large-scale preparations of the tetrachloride oxides, the reaction of the metal trioxide with sulfinyl chloride is most convenient. The procedure outlined below is similar to that of Col ten and co-workers,7 but gives details for obtaining the compounds, particularly tungsten tetrachloride oxide, in excess of 50-g quantities. The reactions may be scaled up or down several fold without deleterious effect. 

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