Layered Oxides of Transition Metals

Layered Oxides oj Transition Metals 495 Table 3. Photodecomposition of water over K4Nb60 7 loaded with various metal oxides [14. ... 

The catalysts were prepared by thermal or chemical deposition of active cover based on Pt and Pd (using chloride precursors) or complex oxides of transition metals (using nitrate precursors) on the foam-metal or foam-ceramics support from aqueous solutions of the metal salts. The foam metal (Nichrome or steel) was coated with the layer of y-Al203 intermediate support in order to increase the catalyst surface area. Content of Pt or Pd in the catalysts accounted for 1 wt.%, concentration of the transition metal oxides amounted to 10-12 wt.%. 

Oxides of transition metals, such as niobium, tantalum, or titanium, are suitable as waveguiding layers on optical (grating coupler) waveguides due to their high refractive index and transparency. Therefore, oxide-coated chips are used as sensing devices in technology based on the interaction of a laser-induced evanescent field with analyte constituents adsorbed on or immobilized at the waveguide/ fluid interface. ... 

Gas sensors based on the MOS and FET structures is a new trend in ISSP of University of Latvia It is supported by cooperation with the Laboratory of Applied Physics of Linkoping University Our attention is given to the use of non-stoichiometric oxides of transition metals with mixed electronic-ionic conductivity (WOx, MoOx, IrOx, NiOx) instead of layers of metal and/or oxide in MOS structures... 

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. 

To dissociate molecules in an adsorbed layer of oxide, a spillover (photospillover) phenomenon can be used with prior activation of the surface of zinc oxide by particles (clusters) of Pt, Pd, Ni, etc. In the course of adsorption of molecular gases (especially H2, O2) or more complex molecules these particles emit (generate) active particles on the surface of substrate [12], which are capable, as we have already noted, to affect considerably the impurity conductivity even at minor concentrations. Thus, the semiconductor oxide activated by cluster particles of transition metals plays a double role of both activator and analyzer (sensor). The latter conclusion is proved by a large number of papers discussed in detail in review [13]. The papers cited maintain that the particles formed during the process of activation are fairly active as to their influence on the electrical properties of sensors made of semiconductor oxides in the form of thin sintered films. 

Regions characterized by large anodic overpotentials. Under such conditions, complete passivation and severe oxidation of most metal surfaces occurs. A breakdown of passive oxide layers and pitting corrosion is observed for transition-metal model systems. In this section are considered also the surfaces of electropositive metals such as aluminum. 

A number of transition metal oxides can also be intercalated by lithium. One of the best known examples is VsOi3. The VgOu structure, shown in Fig. 11.19, consists of alternate double and single layers of V2OS ribbons. The layers are connected by vertex sharing of octahedral sites, and this leads to a relatively open framework structure (Wilhelmi, Waltersson and Kihlburg, 1971). Insertion of lithium into the oxide matrix... 

Figure 1. Schematic description of a (lithium ion) rocking-chair cell that employs graphitic carbon as anode and transition metal oxide as cathode. The undergoing electrochemical process is lithium ion deintercalation from the graphene structure of the anode and simultaneous intercalation into the layered structure of the metal oxide cathode. For the cell, this process is discharge, since the reaction is spontaneous.

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... 

Layered structures are also found for many oxides and sulfides of transition metals. They can be intercalated with alkali metals (Li, Na, K) to give superconducting solids and conducting solids that are useful for solid state battery materials. 

Oxides are always present on the surface of transition metals in alkaline solution. At open circuit they are intermediates in the mechanism of corrosion. The resistance of Ni towards corrosion in base is better than Fe or mild steel, especially at high caustic concentration and high temperature [23, 24]. The role of surface oxides in the cathodic range of potentials depends on the conditions of their formation. Thus, a reducible layer of hydroxide Ni(OH)2 or even oxohydroxide NiOOH has been found [385] to be beneficial for the electrocatalytic activity. It has even been claimed [386] that some good performances are specifically due to the formation of oxide layers during the preparation (Fig. 19). An activation of the Ni surface by the application of anodic current pulses has been reported [387] to be beneficial owing to the formation of Ni(OH)2 layers. This has been confirmed by impedance studies of the mechanism [388]. 

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