Oxidation dissociated

It is concluded [634] that, so far, rate measurements have not been particularly successful in the elucidation of mechanisms of oxide dissociations and that the resolution of apparent outstanding difficulties requires further work. There is evidence that reactions yielding molecular oxygen only involve initial interaction of ions within the lattice of the reactant and kinetic indications are that such reactions are not readily reversed. For those reactions in which the products contain at least some atomic oxygen, magnitudes of E, estimated from the somewhat limited quantity of data available, are generally smaller than the dissociation enthalpies. Decompositions of these oxides are not, therefore, single-step processes and the mechanisms are probably more complicated than has sometimes been supposed. 

However, reduced ceria is able, alone, to dissociate NO. Martinez-Arias et al. [85] have first investigated by electron paramagnetic resonance (EPR) and FTIR spectroscopies NO reaction on ceria pre-outgassed at different temperatures and showed the role of superoxides differentially coordinated in the formation of hyponitrites species further decomposed into NzO. Later Haneda et al. [86] have demonstrated that reduced ceria and reduced praseodymium oxide dissociate NO even though the presence of a noble metal (Pt) significantly increases the formation of N2 or N20. The main results of this study are summarized in Table 8.9. 

Schematic representation of the cross section of the surface layer of a metal oxide. , Metal ions O, oxide ions. The metal ions in the surface layer (a) have a reduced coordination number. They thus behave as Lewis acids. In the presence of water the surface metal ions may first tend to coordinate H20 molecules (b). For most of the oxides dissociative chemisorption of water molecules (c) seems energetically favored.

D. S. Bohle, C. H. Hung, Ligand-Promoted Rapid Nitric Oxide Dissociation from Ferrous Porphyrin Nitrosyls , J. Am. Chem. Soc. 1995,117, 9584-9585. 

Fabisiak, J.P., Tyuiin, V.A., Tyurina, Y.Y., Sedlov, A., Lazo, J.S., and Kagan, V.E., 2000, Nitric oxide dissociates lipid oxidation from apoptosis and phosphatidylserine externalization during oxidative stress. Biochemistry 39 127-138. 

The dissociation process is described by a free radical chain mechanism. The thermo-oxidative dissociation is initiated by the oxidation of the aliphatic moieties by a subsequent cleavage of the hydroperoxides formed. With increasing time of oxidation the temperature of the onset of degradation is lower as compared with that for a purely thermal degradation. 

Reductive and oxidative dissociative adsorption involve usage analogous to that in coordination chemistry in which one speaks of the following reaction as an oxidative addition... 

It is noticed that when Fe3+OOR is formed in aprotic diluters (under strongly basic conditions) at low temperature, degradation products are synthesized by reaction (7.4) [29], However, in hydroxide diluters oxidants dissociate heterolytically by reaction (7.3) [30, 31], which is proved by the formation of the following products epoxides from alkenes and alcohols from ROOH. 

Rhodium Sesquioxide, Rh2Oa, results when rhodium is heated in air or oxygen between 600° C. and 1000° C. The rate of oxidation of the metal increases rapidly with the temperature. Above 1150° C. the oxide dissociates, metallic rhodium being obtained. The sesquioxide is greyish black in colour.8... 

Similar to dissociation of water, all soluble acid phosphates, and soluble oxides dissociate or dissolve in water. When acid phosphates dissociate in water, they lower the pH of the solution by releasing protons (H ), while most of the oxides or hydroxides when mixed with water release hydroxyl ions (OH ) by removing protons from the solution. As a result, initially neutral water becomes richer in protons when acid phosphates are dissolved in it and the pH becomes < 7. On the other hand, for certain oxides such as those of alkaline elements (e.g., Na, K, Mg, and Ca), the pH is increased because the solution becomes deficient in protons. Thus, the pH scale is a good indicator of the extent of release of protons and hydroxyl ions and will be used throughout this book to represent the extent of acid-base reactions. 

Reaction 5.1 forms the basis for dissolution of an oxide for CBPC formation. It represents dissociation of metal oxides in which cations and anions are formed in an aqueous solution. In general, divalent metal oxides dissociate more easily than trivalent oxides, and quadrivalent oxides dissolve less easily than trivalent oxides, though some exceptions may be found to this general trend. The actual rate of dissolution will be discussed in detail as we develop a thermodynamic basis for these transformations. 

Cupric oxide dissociates into cuprous oxide and oxygen. Two independent components, cuprous oxide and oxygen, form the system, which] bdow a certain temperature is divided among three phases, solid cupric oxide, solid cuprous oxide, oxygen gas. The system is monovariant, admitting a curve of dissociation tensions. 

Suppose now that mercury oxide dissociates in an enclosure which is at first a vacuum, and that the mercury resulting from this decomposition ranains entirely in a state of vapor it will follow necessarily, denoting by the partial pressure of oxygen in this case, that... 

When, therdbre, mercuric oxide dissociates in an endosum at first a vacuum, the partial pressure of the oxygen must, if the preceding theory is correct, be represented by formula (20), the constants fi% /, having the values ven in (22). 

Cerium-based catalysts have been successfully used in several processes. For example, ceria (Ce02) is used as an additive [ 1,2] in modem automotive exhaust catalysts. Ceria acts as an excellent oxygen store [3-5] in the catalyst, which is thus rendered a very effective catalyst for combustion [6]. Moreover, addition of ceria to the automotive exhaust catalysts minimises the thermally induced sintering of the alumina support and stabilises the noble metal dispersion [7]. Ceria also enhances nitric oxide dissociation when added to various supported metal catalysts [8], which is another important function of the automotive exhaust catalyst. Recent investigations by Harrison et al have shown that ceria doped with certain lanthanides and promoted with copper and chromium have catalytic activities comparable to that of the noble metal catalysts [9]... 

Oxidation dissociates AMG tetramers into dysfunctional dimers. Thus as is the case for AAT and other serpins, inhibitory activity is reduced or eradicated by increased levels of oxidants (such as those from cigarette smoke or from neutrophils) or decreased levels of physiological antioxidants. 

A wealth of information concerning the identities, mobilities, concentrations and properties of defects in many oxides is available through measurements of mass transfer made in association with metal oxidation studies [8,9]. The influences of crystal structure [10], temperatine and oxygen partial pressine on cation and anion migration have also been investigated. Information on the reactivities of oxides, their polymorphism and the properties of the imperfections present, is often useful in the formulation of the mechanisms of oxide dissociation. 

In addition to the many investigations of the bulk properties of crystalline oxides, there has also been considerable interest in the surface chemistry of these compounds. Mechanistic studies in this field often include discussion of heterogeneous catalytic-type processes and intermediates, similar to or identical with, those postulated above as occurring during oxide dissociation. Some reference is made below to relevant aspects of the surface chemistry of oxides. 

Nakamori et al. [42] studied the isothermal decomposition of AgjO in vacuum (683 to 703 K), in ethene (409 to 418 K), in hydrogen (338 to 348 K) and in CO (265 to 283 K). All ar-time curves were sigmoid and a linear fi-ee energy relationship was found between values of ii, (403, 161, 103 and 68 kJ mol, respectively) and the enthalpy changes. The substantial influences of reactant gases on reaction temperatures unambiguously confirm the interactions of these additives with adsorbed intermediates in the oxide dissociation. 

Cobalt oxides Dissociation of C03O4 yields [2] some atomic oxygen as product. The activation energy for the dissociation in argon is reported [67] as 293 kJ mol (600 to 1200 K). Ichimura and Komatsu [68] reported a value of 366 kJ mol and identified a contracting interface process (970 to 1070 K) for dissociation of the pure oxide, or in the presence of AI2O3 or 10303 additives. 

In a more general comparative study of oxide dissociations, including mass spectrometric analysis of the gases released, Kazenas et al. [75] conclude that the oxides of Fe, Ni, Cu, Zn and Pb usually give oxygen and lower oxide (+ metal), the oxides of Re are rearranged, and Mo and W oxides yield volatile polymeric species. 

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