Mixed oxide materials

Outside of catalyst preparation, reaction of sucrose with metal nitrates has been used to prepare nanocomposite mixed oxide materials. Wu et al. [46] reported the synthesis of Mg0-Al203 and Y203-Zr02 mixed oxides by reaction of nitrate precursors with sucrose. The resulting powders had smaller particles than those prepared without sucrose. Das [47] used a similar method in the presence of poly vinylalcohol to produce nanocrystalline lead zirconium titanate and metal ferrierites (MFe204, M = Co, Ni, or Zn). The materials prepared using sucrose had smaller crystallites than those made without. Both authors observed an exothermic decomposition of the precursors during calcination. 

Hence the partial pressure of oxygen and the temperature determine whether the solid will exhibit n-type, p-type or ionic conduction. Although the concentration of defects is important it is also necessary to consider the mobilities of the individual defects higher ionic mobilities will result in a larger domain for electrolytic conduction. Figure l4 shows the dominant mode of conduction in some mixed oxide materials, exhibiting solid electrolyte behaviour, as a function of temperature and oxygen partial pressure. 

These are important mixed oxide materials with octahedral Fe3+ ions used in a variety of electronic appliances ranging from magnetic recording devices to cores of power transformers. Examples are the spinels MnFe204, BaFei2019, and Ba2Mn Fei2C>22. However, some discrete ions are known. 

Attempts to stabilize the interactions between the noble metals and ceria were only partly successful. True success came in the form of a fundamentally different approach to oxygen storage, still employing ceria but in solid solution with other metal oxides - most notably zirconia. Development of these mixed oxide materials began in the late 1980s and ultimately achieved widespread incorporation... 

Most likely, the higher the degree of homogeneity in the washcoat material, the higher the resistance to sintering. Sol-gel synthesis provides a method to produce mixed oxide materials with homogeneity on the atomic scale [87] and should thus be explored further, especially with respect to novel compositions. 

Mixed Uranium-Plutonium Dioxide in Equimolar Sodium-Potassium Nitrate. The behavior of two compositions of mixed uranium-plutonium dioxide has been investigated thus far at PNL. The first composition, designated material A, consists of 5.44% Pu02/94.56% UO2. The second composition, designated material B, consists of 27.56% Pu02/72.44% UO2. Both materials were acquired as pellets that had been sintered at 1700°C. The behavior of both mixed-oxide materials was studied under the same conditions used in the previous experiments. 

Lopez. T Gomez. R Llanos, ME Garcia-Figueroa. E Navarreie, J Lopez-Salinas, E. On the surface basic properties of sulfated magnesia-silica sol-gel mixed oxides. Materials Letters 1999 39, 51-57. 

Although most product-removal applications feature H2 removal, some decomposition reactions in which O2 is removed are of environmental interest. These use dense mixed-oxide materials (see Section 2.3) to remove O2 only. Early work involved NO and CO2 decompositions using zirco-nias. ° Itoh et al. have recently reconsidered the CO2 decomposition reaction to try to run it with reasonable conversion at lower temperatures using a PBMR. Dixon et al. considered the impact of new, high O2 flux perovskite materials on both CO2 decomposition and NO decomposition. It appears that further advances in materials will be needed before these applications become viable. 

Jung, M. 2001. NMR characterization on the preparationof sol—gel derived mixed oxide materials./n/././norg. 

Rare earth oxides have been used for catalytic applications in the bulk form and supported on various different host materials. The inclusion of small amoimts of Ln20j into oxidic supports following calcination results in mixed oxide materials. The basic properties of the mixed oxide material are dependent on the type of mixed oxides formed. The support should be crafted in order to minimize the amoimt of Ln20j needed while maximizing the basic sites in the supported material. Examples of different support... 

In fact, to safeguard reprocessing plants or plants where mixed oxide material with uranium or plutonium is produced would be much more difficult, whereas, because of the no more than slight danger, of little interest are the temporary deposits of irradiated fuel, where the deposited elements maintain high levels of radioactivity—so high, in fact, that any clandestine extraction of plutonium would be practically unfeasible. 

Methane chemical looping combustion (CLC) over Ni-based mixed oxide materials. ... 

ViUa, R., Cristiani, C., Groppi, G., et al. (2003). Ni Based Mixed Oxide Materials for CH4 Oxidation under Redox Cycle Conditions, J. Mol. Catal. A Chem., 204-205, pp. 637-646. Sanfilippo, D., Paggini, A., Piccoli, V., et al. (2001). Process for the Production of Hydrogen, US Patent US687541. Snamprogetti S.p.A. Assignee. 

Actually, the major success in propane ammoxidation into acrylonitrile has been to discover MoVTeNb mixed oxide materials presenting a crystalline phase with a so-called Ml structure, since the results obtained with this type of catalyst are remarkably better than those from the other catalytic systems reported for propane ammoxidation. In fact, the complex structure of the... 

Depending on the desired materials properties and structure, these approaches have been explored for sol-gel processing of alkoxide-based precursors and further developed in combination with many experimental parameters that can also be adjusted to control the sol composition and particle size, including alkoxide/ water ratio, concentration of the precursors, reaction temperature, reaction time, drying methods, choice and concentration of the catalyst, and the solvent used [42]. Besides the well-known hydrolytic sol-gel processes, even nonhydro-lytic approaches have been reported as a powerful method in the synthesis of mixed oxide materials and will be discussed in more detail subsequently [43]. Another processing option to tailor the material properties is the application of soft or hard templates [44]. In addition to the chemical parameters, drying and thermal treatments or firing processes determine the final architecture, chemical... 

An additional advantage of this synthetic approach is that the mesopores of silica restrict the size of the TiO species in the mixed oxide materials. Furthermore, Liu et al. were able to control the homogeneity of the mixed oxide materials by changing the pH value of the initial sol-gel mixture. pH values lower than the isoelectric point of sihca (2.0) lead to uniformly dispersed titania domains, whereas at higher pH values phase separation is observed due to the instability of the titanium-citric acid complexes. A similar approach has been reported by Oki et al. [56] for the synthesis of Ti02—Si02 mixed oxides by addition of various alk)d amines such as hexylamine, dodecylamine, and octadecylamine or even... 

Single-source precursor approaches are becoming more and more attractive in the sol-gel synthesis of mixed oxide materials. Well-mixed Si/Ti binary oxides have been synthesized by Miller et al. They chose to use a diethoxysiloxane-ethyltitanate copolymer as a network-forming precursor since the Ti—O—Si linkage is already present in the precursor. This synthetic route avoids the problem associated with unequal hydrolysis rates of metal alkoxide precursors commonly used to prepare mixed oxide catalysts [60]. A more detailed discussion can be found in Section 25.4. 

Figure 25.11 Model of the formation of mesoporous mixed oxide materials with block copolymers acting as structure-directing agent and complexing ligand. (Adapted with permission from Ref. [87]. Copyright 2004, American Chemical Society.)...

Over the past few years there has been a renewed experimental search for high-temperature superconductors. This effort has been augmented by the unexpected discovery of a superconducting copper-oxide based ceramic by Bednorz and Muller. This flurry of activity as shown in Fig. 3.26 has culminated in the development of a completely new class of mixed-oxide materials that is superconducting to temperatures above the temperature of... 

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