Mixed metal oxides discussion

In the heterogeneous catalytic system, the reacting molecules, and the metal-surface atoms, can be quite mobile. This leads to locally ordered structures when synchronization or self organization phenomena are present however, disorder in the surface layer prevails when these phenomena are absent. For example, it has been proposed that the catalytic oxidation over mixed metal oxides, discussed in Section 2.3.5, actually occurs in the disordered overlayer that forms at the surface under reaction conditions. 

Mixed metal oxide pigments containing iron oxide are also used (see Section 3.1.3). Magnetic iron oxide pigments are discussed in Sections 5.1.1 and 5.1.2. Transparent iron oxide pigments are described in Section 5.4.1. Methods of analysis and specifications of iron oxide pigments are listed in the standards given in Table 1. 

The vast majority of metal complexes in the solid state are insulators and do not exhibit any interesting electrical conduction properties because the metal atoms are surrounded by insulating ligands which prevent the passage of carriers from one site to another. This review will be limited to a discussion of the electrical conduction properties of coordination compounds, and will not include simple inorganic compounds with high electrical conductivity such as mixed metal oxides, (3-alumina and TaSe3. 

The best known anhydrous oxides are listed in Table 18-E-2 the tetraoxides of Ru and Os are discussed later (Section 18-F-l). The oxides, generally rather inert to aqueous acids, are reduced to the metal by hydrogen, and dissociate on heating. There are mixed metal oxides, e.g., BaRu03, and platinum and palladium bronzes of formula MlPt304 (x = 0-1). Some oxides like MnPt306 have Pt—Pt bonds. Mixed oxides are used for electrodes in H2—02 fuel cells and in the chloralkali process. 

The deposition of mixed metal oxides is based on the deposition techniques and precursors used for the formation of metal oxides discussed earlier in this chapter. The most important publications in the held of high-temperature superconducting materials produced from metal enolates since the date of release of Rees book in 1996 are summarized in Table 8. 

In the present chapter, which deals with theoretical concepts applied to vanadium and molybdenum oxide surfaces, we will restrict the discussion to binary oxide systems. So far, mixed metal oxide systems have not been studied by quantitative theory. Theoretical methods that have been used to study oxide surfaces can be classified according to the approximations made in the system geometry where two different concepts are applied at present, local cluster and repeated slab models. Local cluster models are based on the assumption that the physical/chemical behavior at selected surface sites can be described by finite sections cut out from the oxide surface. These sections (surface clusters) are treated as fictitious molecules with or without additional boundary conditions to take the effect of environmental coupling into account. Therefore, their electro-... 

The acid-base properties of amorphous mixed metal oxides can be varied by choosing different metal oxide constituents at diflerent concentrations and by changing the treatment of the sample (44). Thus, it appears that, by properly choosing the aforementioned variables, mixed oxides could be used to develop new catalysts with desired acid-base properties. The use of micro-calorimetric adsorption measurements to quantify the acid-base properties of metal oxides and mixed metal oxides has been limited, to date, to a few systems. However, for some of these solids, for example, silica, alumina, and silica-alumina, several investigations have led to a satisfactory description of their acidity and acid strength. We present here a compendium of those measurements and discuss some of the important properties observed. 

Numerous ceramics are deposited via chemical vapor deposition. Oxide, carbide, nitride, and boride films can all be produced from gas phase precursors. This section gives details on the production-scale reactions for materials that are widely produced. In addition, a survey of the latest research including novel precursors and chemical reactions is provided. The discussion begins with the mature technologies of silicon dioxide, aluminum oxide, and silicon nitride CVD. Then the focus turns to the deposition of thin films having characteristics that are attractive for future applications in microelectronics, micromachinery, and hard coatings for tools and parts. These materials include aluminum nitride, boron nitride, titanium nitride, titanium dioxide, silicon carbide, and mixed-metal oxides such as those of the perovskite structure and those used as high To superconductors. 

Interactions between the precious metal and support influence the performance of the catalyst. Beil (1987) has defined metal-support interaction as depending on contact between the metal particle and the support which can be a dissolution of the dispersed metal in the lattice. The interaction could also depend on the formation of a mixed metal oxide, or the decoration of the metal particle surface with oxidic moieties derived from the support. It is possible that in this study, the differences in catalytic performance of the same active material supported on different washcoats can be attributed to any of these phenomena. Another explanation could be that the support materials exhibit different acid-base properties. According to the Bronsted and Lewis definitions, a solid acid shows a tendency to donate a proton or to accept an electron pair, whereas a solid base tends to accept a proton or to donate an electron pair. The tendency of an oxide to become positively or negatively charged is thus a function of its composition, which is affected by the preparation method and the precursors used. Refer to the section Catalyst characterization for further discussion on the influence of support material on catalyst performance. To thoroughly examine the influence of the support... 

Metal oxides are widely used as catalyst supports but can also be catalytically active and useful in their own right. Alumina, for example, is used to manufacture ethene from ethanol by dehydration. Very many mixed metal oxide catalysts are now used in commercial processes. The best understood and most interesting of these are zeolites that offer the particular advantage of shape selectivity resulting from their narrow microporous pore structure. Zeolites are now used in a number of large-scale catalytic processes. Their use in fine chemical synthesis is discussed in Chapter 2. 

Mixed metal oxides derived from Fe203 and of general formula M Fe 2C>4 or M Fe - 02 are commonly known as ferrites despite the absence of discrete oxoanions. They include compounds of commercial importance by virtue of their magnetic properties, e.g. electromagnetic devices for information storage for discussion of the magnetic properties of mixed metal oxides, see Chapter 27. Spinel and... 

This method has been developed largely by Klemm et al. and is discussed in a recent review,7 which gives details on catalysts, procedure, and possible mechanisms Examples include formation of the peri-condensed compounds 35 (29-39%), from bridging at positions 4 and 5 of the phenanthrene molecule 149 at ca. 630°C in the presence of various sulfided mixed metallic oxide catalysts,6,36 and 80 (18%), from similar bridging in the triphenylene molecule 150 at 500°C.37 Since the sulfur bridge forms by substitution at the sterically most hindered positions of these hydrocarbon substrates, method... 

The white oxide Y2O3 is insoluble in water but dissolves in acids. It is used in ceramics, optical glasses and refractory materials (see also Section 23.2). The mixed metal oxide YBa2Cu307 is a member of a family of materials that become superconducting upon cooling. These so-called high-temperature superconductors are discussed further in Section 28.4. Yttrium(III) hydroxide is a colourless solid, in which each Y " " ion is in a tricapped trigonal prismatic YO9 environment. The hydroxide is water-insoluble and exclusively basic. 

In this chapter, we first discuss mixed metal oxide semiconductors used in both fields. We then discuss photoelectrochemical properties of BiV04 semiconductors. Finally, we discuss our high-throughput screening system for mixed metal oxide semiconductors. 

The main OC families are based on the use of nickel, iron, copper, manganese as metal or some mixed metal oxides. The metal oxides are usually mixed with some material support to reduce the diffusion limitations inside the particles (by increasing the particle porosity), to improve the material stability and also increase the heat capacity. The most often used OCs are discussed briefly here. More information can be found in Adanez et al. [9], Hossain etal. [10] andLyngfelt etal. [11]. The main properties ofthe most often used OCs are listed in Table 5.1 [9, 11, 12]. 

Another propylene ammoxidation catalyst that was used commercially was U-Sb-0. This catalyst system was discovered and patented by SOHIO in the mid-1960s (26,27). Optimum yield of acrylonitrile from propylene required sufficient antimony in the formulation in order to ensure the presence of the USbaOio phase rather than the alternative uranium antimonate compound USbOs (28-30). The need for high antimony content was understood to stem from the necessity to isolate the uranium cations on the surface, which were presumed to be the sites for partial oxidation of propylene. Isolation by the relatively inactive antimony cation prevented complete oxidation of propylene to CO2. Later publications and patents showed that the activity of the U-Sb-0 catalyst is increased by more than an order of magnitude by the substitution of a tetravalent cation, tin, titanium, and zirconium (31). Titanium was found to be especially effective. The promoting effect results in the formation of a solid solution by isomorphous substitution of the tetravalent cation for Sb + within the catalytically active USbaOio- phase. This substitution produces o gen vacancies in the lattice and thus increases the facility for diffusion of lattice o gen in the solid structure. As is discussed below, the enhanced diffusion of o gen is directly linked to increased activity of selective (amm)oxidation catalysts based on mixed metal oxides. 

In heterogeneous catalysis, there are many examples where addition of a second component can change the overall catalytic reactivity in the system by changing its solid-state chemistry. An example of this includes the addition of Co + to the M0S2 and NiS2 systems discussed in Chapter 5. The mixed metal sulfides offer significantly increased activity due to changes in the chemical reactivity of the sulfide surface. We introduce here the solid-state chemistry of oxide catalysts (see also reference 3). A more detailed discussion on mixed metal oxides is presented in Chapter 5. 

The inorganic chemistry of a multi-component heterogeneous catalyst is often very complex, as it is quite difficult to obtain structural information at the molecular level to help establish the fundamental processes. As an example, we discuss the chemistry of the complex mixed metal oxide catalyst Mo7,5Vi,5NbTe029 shown in Fig. 2.25, which is known to catalyze the ammoxidation of propane to acrylonitrile. The active centers in this system are multifunctional metal oxide assemblies that are spatially isolated from one another owing to their unique crystal structures. 

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