Binary layered oxides

The oxides and oxide-hydrates of molybdenum in its highest oxidation state display a variety of structural types involving linked MoOg octahedra. M0O3 is such a host. Of the anhydrous M0O3, the well-known orthorhombic form (a-Mo03) is the stable form in normal conditions, which possesses a layered structure as shown in Fig. 5.1 

The electrochemical Uthium insertion into the M0O3 framework can be described according the following reaction  

In M0O3, electrical conductivity comes from the hopping of electrons forming small polarons between Mo and Mo ions. Infrared absorption studies of LUM0O3 compounds revealed a transition small polaron to metallic featmes [14], 

When the amount of lithium content is increased beyond x = 1, the discharge curve exhibits a sharp potential drop followed by a region with a plateau characteristic of a two-phase domain, for which the 5- and Y-LUV2O5 phases are in 

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This chapter provides the relationships between structural and electrochemical properties of lamellar compounds the 3d-transition metal oxides currently studied as for their potential use in LiBs. First, we examine briefly three binary layered oxides, M0O3, V2O5, and LiVsOg which were proposed as intercalation compounds since at the end of 1970s. Then, the ternary layered oxides are considered. Starting from the historical and prototype compound LiCo02, which is the dominant positive electrode material employed by all Li-ion cell manufacturers so far, we state the broad family of layered oxides such as LiM Oy and their derivatives the... 

In catalysis, oxides with well defined acidic and basic properties are used in different forms that have found application in numerous catalytic applications in the gas-solid and liquid-solid heterogeneous catalysis [3, 46, 47], Among the most used oxide materials in catalysis, we And (i) bulk oxides (one component metal oxides) (ii) doped and moditied oxides (iii) supported metal oxides (dispersed active oxide component onto a support oxide component) (iv) bulk and supported binary metal oxides to quaternary metal oxides (mixed oxide compositions) (v) complex oxides (e.g., spinels, perovskites, hexa-aluminates, bulk and supported hydrotalcites, pillared clays, bulk and supported heteropolyacids, layered silicas, etc.). 

SiC has greater thermal stability than any other binary compound of Si and decomposition by loss of Si only becomes appreciable at 2700°. It resists attack by most aqueous acids (including HE but not H3PO4) and is oxidized in air only above 1000° because of the protective layer of Si02 this can be removed by molten hydroxides or carbonates and oxidation is much more rapid under these conditions, e.g. ... 

All three metals form a wide variety of binary chalcogenides which frequently differ both in stoichiometry and in structure from the oxides. Many have complex structures which are not easily described, and detailed discussion is therefore inappropriate. The various sulfide phases are listed in Table 22.4 phases approximating to the stoichiometry MS have the NiAs-type structure (p. 556) whereas MS2 have layer lattices related to M0S2 (p. 1018), Cdl2, or CdCl2 (p. 1212). Sometimes complex layer-sequences occur in which the 6-coordinate metal atom is alternatively octahedral and trigonal prismatic. Most of the phases exhibit... 

This analysis shows that if the oxides of the two components of a binary alloy are mutually insoluble, and if one of the components has a much greater affinity for oxygen than the other, then the oxide of the baser metal will be formed exclusively even though it is present in the alloy in only a small amount. It seems that the importance of beryllium as an alloying constituent can be explained in this way. It has a high affinity for oxygen [p(BeO) = 10 atm at 1000°C] and also forms a highly protective oxide layer. The... 

The second stage in the carburisation process, that of carbon ingress through the protective oxide layer, is suppressed by the development of alumina or silica layers as already discussed and in some cases protective chromia scales can also form. Diffusion and solubility of carbon in the matrix has been shown by Schnaas et to be a minimum for binary Fe-Ni alloys with a nickel content of about 80<7o, and Hall has shown that increasing the nickel content for the nickel-iron-2S<7o-chromium system resulted in lower rates of carburisation (Fig. 7.54). 

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

Miller et al. (1996) give examples of APFIM studies of the surface oxides formed on silicon, and studies of the stoichiometry of thermal oxide layers of this element, as well as a wide range of binary III-IV semiconducting materials. 

In a scheme complementary to the one just presented where thiols are removed by reductive desorption of thiols, molecules can also be removed during stripping of a UPD layer. This was demonstrated by Shimazu et al. [221] where an alkane thiol SAM was deposited onto a Au(l 11) that had been modified with Pb. Oxidative stripping of the lead also caused thiols to be removed. The empty sites were then subsequently filled with mercaptopropionic acid (MPA). A remarkable result is that the binary SAMs exhibit only one desorption peak. From this it was concluded that a well-mixed layer forms that is very different from the mixed SAM obtained by adsorption from solution containing both types of thiols. In this case the layer exhibits singlecomponent domains that are refiected by two desorption peaks. 

Most chemical properties of technetium are similar to those of rhenium. The metal exhibits several oxidation states, the most stable being the hep-tavalent, Tc +. The metal forms two oxides the black dioxide Tc02 and the heptoxide TC2O7. At ambient temperature in the presence of moisture, a thin layer of dioxide, Tc02, covers the metal surface. The metal burns in fluorine to form two fluorides, the penta- and hexafluorides, TcFs and TcFe. Binary compounds also are obtained with other nonmetaUic elements. It combines with sulfur and carbon at high temperatures forming technetium disulfide and carbide, TcS2 and TcC, respectively. 

In contrast to the aforementioned binary oxides, V2Os has a stronger oxidation power and is able to attack hydrogen attached to the aromatic nucleus. Sometimes attention is drawn to the importance of a layer structure in the catalyst or to geometric factors (e.g. Sachtler [270]). Unexpectedly, however, very effective vanadium-based catalysts exist which operate in the molten state, indicating that a fixed structure is not important. The catalytic activity of molten oxide phases seems to occur exclusively in the oxidation of aromatic hydrocarbons over V2Os-based catalysts, such systems have not been reported for the selective oxidation of olefins. 

Alloy oxidation processes are far more complex than the oxidation of metallic elements. Let us also distinguish between external and internal oxidation. In external oxidation, a layer forms by way of a heterogeneous reaction as discussed in Chapter 7. In this section, however, we are concerned with the internal oxidation of alloys. Pure metal A can only be oxidized externally. The simplest system for the study of internal oxidation is the binary metal alloy (A,B), to which we shall confine our discussion. 

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