Transition-Metal Doping

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... 

Another application is in tire oxidation of vapour mixtures in a chemical vapour transport reaction, the attempt being to coat materials with a tlrin layer of solid electrolyte. For example, a gas phase mixture consisting of the iodides of zirconium and yttrium is oxidized to form a thin layer of ytnia-stabilized zirconia on the surface of an electrode such as one of the lanthanum-snontium doped transition metal perovskites Lai j.Srj.M03 7, which can transmit oxygen as ions and electrons from an isolated volume of oxygen gas. 

Figure 8.1 (a) Resistivity of a doped transition-metal oxide (schematic) and (b) an oxide... 

Negative temperature coefficient thermistors are made from impurity-doped transition-metal oxides. Donor doping, for example, Fe203 doped with Ti02, produces n-type thermistors. The favored mechanism is the formation of electrons ... 

At the heart of the sensor is a slab of doped transition-metal oxide. Its mode of operation is to detect the concentration of CO within the oxide slab, which is in direct proportion to the concentration of CO gas in the air surrounding it, according to Henry s law. 

Heterometal cubanes of general formula, [M 4 M" (LH)4](0Ac)4 (C104), have shown to be versatile singlesource precursors to prepare metal-doped transition metal oxides. The heterocubanes allow several combina-... 

The other interesting challenge from a synthetic viewpoint is the synthesis of doped QDs, particularly to study their magneto-optical properties. Doping transition metal ions in bulk as well as nanoscale semi-conductors has been an important area of study for the past couple of decades. Recent studies have further shown that for several applications, dopant associated emission from semiconductor nanocrystals may be more advantageous than the usual band-edge emission. Extensive attempts have been made to dope intentional impurities in various semiconductor nanocrystals, including those in ZnSe, ... 

In this sections, we will present the overview of the natural nano-silicaties as advanced nanoarchitectures currently employed to transform solar into chemical energy. It should be pointed out that this section will emphasize the optimization of silicate-based photocatalysts under sunlight-type excitation operation both under UV and under visible light excitations are considered. In addition, our group s work will be exemplified to discuss the metal oxide-based minerals naturally contain doped transition metal ions isomorphically substituted into the structure that can alter the structural as well as electronic properties. 

Actually, silicate minerals naturally contain doped transition metal ions isomorphically substituted into the structure that can alter the structural as well as electronic properties. In our researches, for ATP, the... 

Reinhard C, Gemer P, Valiente R, Wenger OS, Giidel HU (2001) Upconversion phenomena in the Yb doped transition metal compounds Rb2Mnd4 and CsMnBrs. J Lumin 94-95 331-335... 

The term upconversion describes an effect [1] related to the emission of anti-Stokes fluorescence in the visible spectral range following excitation of certain (doped) luminophores in the near infrared (NIR). It mainly occurs with rare-earth doped solids, but also with doped transition-metal systems and combinations of both [2, 3], and relies on the sequential absorption of two or more NIR photons by the dopants. Following its discovery [1] it has been extensively studied for bulk materials both theoretically and in context with uses in solid-state lasers, infrared quantum counters, lighting or displays, and physical sensors, for example [4, 5]. Substantial efforts also have been made to prepare nanoscale materials that show more efficient upconversion emission. Meanwhile, numerous protocols are available for making nanoparticles, nanorods, nanoplates, and nanotubes. These include thermal decomposition, co-precipitation, solvothermal synthesis, combustion, and sol-gel processes [6], synthesis in liquid-solid-solutions [7, 8], and ionothermal synthesis [9]. Nanocrystal materials include oxides of zirconium and titanium, the fluorides, oxides, phosphates, oxysulfates, and oxyfluoiides of the trivalent lanthanides (Ln ), and similar compounds that may additionally contain alkaline earth ions. Wang and Liu [6] have recently reviewed the theory of upconversion and the common materials and methods used. 

The performance of many metal-ion catalysts can be enhanced by doping with cesium compounds. This is a result both of the low ionization potential of cesium and its abiUty to stabilize high oxidation states of transition-metal oxo anions (50). Catalyst doping is one of the principal commercial uses of cesium. Cesium is a more powerflil oxidant than potassium, which it can replace. The amount of replacement is often a matter of economic benefit. Cesium-doped catalysts are used for the production of styrene monomer from ethyl benzene at metal oxide contacts or from toluene and methanol as Cs-exchanged zeofltes ethylene oxide ammonoxidation, acrolein (methacrolein) acryflc acid (methacrylic acid) methyl methacrylate monomer methanol phthahc anhydride anthraquinone various olefins chlorinations in low pressure ammonia synthesis and in the conversion of SO2 to SO in sulfuric acid production. 

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. 

In the following we will concentrate on three important cases, i.e. CO oxidation on alkali doped Pt, ethylene epoxidation on promoted Ag and synthesis gas conversion on transition metals. We will attempt to rationalize the observed kinetic behaviour on the basis of the above simple rules. 

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

Effect of the various transition metals doping on TiOj/SiOi... 

Fig 5. TCE removal efBdency with various transition metals doped on "nOj/SiCh. 

The performance of the catalysts doped with transition metals on 20 mol% NiO/Ti02 catalyst for hydrogen production at 998 K and GHSV of 2700 h" at steady state. 

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