Transition-metal oxides properties

Several kinds of conduction mechanisms are operative in ceramic thermistors, resistors, varistors, and chemical sensors. Negative temperature coefficient (NTC) thermistors make use of the semiconducting properties of heavily doped transition metal oxides such as n-ty e Ti O andp-ty e... 

A thin layer deposited between the electrode and the charge transport material can be used to modify the injection process. Some of these arc (relatively poor) conductors and should be viewed as electrode materials in their own right, for example the polymers polyaniline (PAni) [81-83] and polyethylenedioxythiophene (PEDT or PEDOT) [83, 841 heavily doped with anions to be intrinsically conducting. They have work functions of approximately 5.0 cV [75] and therefore are used as anode materials, typically on top of 1TO, which is present to provide lateral conductivity. Thin layers of transition metal oxide on ITO have also been shown [74J to have better injection properties than ITO itself. Again these materials (oxides of ruthenium, molybdenum or vanadium) have high work functions, but because of their low conductivity cannot be used alone as the electrode. 

Preparation and properties of high valent first row transition metal oxides and halides. C. Rosen-blum and S. L. Holt, Transition Met. Chem. (N.Y.), 1972, 7,87-182 (303). 

Electrochemical properties of various transition metal oxides for energy storage... 

To finish with another trend for NO removal consisting in NO direct decomposition, we would like to depict the infrared study of NO adsorption and decomposition over basic lanthanum oxide La203 [78], In this case, the basic oxygens are proposed to lead to N02 and N03 spectator species, whereas the active sites for effective NO decomposition are described as anion vacancies, which are often present in transition metal oxides. This last work makes the transition with the study of DeNO, catalysts from the point of view of their ability to transfer electrons, i.e. their redox properties. 

Transition metal oxides, rare earth oxides and various metal complexes deposited on their surface are typical phases of DeNO catalysts that lead to redox properties. For each of these phases, complementary tools exist for a proper characterization of the metal coordination number, oxidation state or nuclearity. Among all the techniques such as EPR [80], UV-vis [81] and IR, Raman, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and NMR, recently reviewed [82] for their application in the study of supported molecular metal complexes, Raman and IR spectroscopies are the only ones we will focus on. The major advantages offered by these spectroscopic techniques are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as crystalline nanophases and (2) they are able to collect information under various environmental conditions [83], We will describe their contributions to the study of both the support (oxide) and the deposited phase (metal complex). 

After the review of literature, we report here the results of the degradation of phenol, carried out in our laboratory in the presence of ultrasound, Ti02, rare earths and transition metal ions to highlight our interpretation of the mechanism. Various transition metal salts are known for their catalytic properties due to partly filled d-orbital of the metal atom. Mesoporous transition metal oxides are used not... 

Cox PA (1992) Transition metal oxides An introduction to their electronic structure and properties. Clarendon Press, Oxford... 

Reactions involving the creation, destruction, and elimination of defects can appear mysterious. In such cases it is useful to break the reaction down into hypothetical steps that can be represented by partial equations, rather akin to the half-reactions used to simplify redox reactions in chemistry. The complete defect formation equation is found by adding the partial equations together. The mles described above can be interpreted more flexibly in these partial equations but must be rigorously obeyed in the final equation. Finally, it is necessary to mention that a defect formation equation can often be written in terms of just structural (i.e., ionic) defects such as interstitials and vacancies or in terms of just electronic defects, electrons, and holes. Which of these alternatives is preferred will depend upon the physical properties of the solid. An insulator such as MgO is likely to utilize structural defects to compensate for the changes taking place, whereas a semiconducting transition-metal oxide with several easily accessible valence states is likely to prefer electronic compensation. 

We shall briefly discuss the electrical properties of the metal oxides. Thermal conductivity, electrical conductivity, the Seebeck effect, and the Hall effect are some of the electron transport properties of solids that characterize the nature of the charge carriers. On the basis of electrical properties, the solid materials may be classified into metals, semiconductors, and insulators as shown in Figure 2.1. The range of electronic structures of oxides is very wide and hence they can be classified into two categories, nontransition metal oxides and transition metal oxides. In nontransition metal oxides, the cation valence orbitals are of s or p type, whereas the cation valence orbitals are of d type in transition metal oxides. A useful starting point in describing the structures of the metal oxides is the ionic model.5 Ionic crystals are formed between highly electropositive... 

Table 2.1 Electrical and Magnetic Properties of Binary Transition Metal Oxides...

Dunitz J. D. and Orgel L. E. (1957). Electronic properties of transition metal oxides, II Cation distribution amongst octahedral and tetrahedral sites. J. Phys. Chem. Solids, 3 318-323. 

Influence of covalent bonding on some magnetic properties of transition metal oxides. Bull. Soc. Chim. France 1212 (1965). 

From the previous paragraphes it follows that a substantial amount of experimental data exist that illustrate the oxygenation properties of zeolite catalysts. In very general terms zeolites are used to heterogenize transition metal ions in ion exchange or lattice positions, to stabilize transition metal oxide dispersions and to prepare ship-in-bottle complexes. 

Vanadium oxide dispersed on supporting oxides (Si02 Al Oo, Ti02, etc.) are frequently employed as catalysts in reactions like partial oxidation and ammoxidation of hydrocarbons, and NO reduction. The modifications induced on the reactive properties of transition metal oxides like V20 when they are supported on an oxide carrier has been the subject matter of recent study. There is much evidence showing that the properties of a thin layer of a transition metal oxide interacting with the support are strongly modified as compared to the properties of the bulk oxide (1-3). In the recent past, increasing attention has been focussed... 

Non-stoichiometry is a very important property of actinide dioxides. Small departures from stoichiometric compositions, are due to point-defects in anion sublattice (vacancies for AnOa-x and interstitials for An02+x )- A lattice defect is a point perturbation of the periodicity of the perfect solid and, in an ionic picture, it constitutes a point charge with respect to the lattice, since it is a point of accumulation of electrons or electron holes. This point charge must be compensated, in order to preserve electroneutrality of the total lattice. Actinide ions having usually two or more oxidation states within a narrow range of stability, the neutralization of the point charges is achieved through a Redox process, i.e. oxidation or reduction of the cation. This is in fact the main reason for the existence of non-stoichiometry. In this respect, actinide compounds are similar to transition metals oxides and to some lanthanide dioxides. 

Carbon monoxide in the product is not inert and may change the stoichiometry and properties of transition metal oxide pigment or magnetic ferrites, such as Fe203. 

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