Mixed oxide systems

Defects in the structure of ceria may be intrinsic or extrinsic. The intrinsic defects are related to thermal disorder of the material or can be created by the reaction between the solid and the surrounding atmosphere (redox processes). Extrinsic defects, in turn, are formed by the presence of impurities or by the introduction of dopants in the structure [1]. 

Perovskites and Related Mixed Oxides Concepts and Applications, First Edition. 

Edited by Pascal Granger, Vasile I. Parvulescu, Serge Kaliaguine, and Wilfrid Prellier. 

Ce02 is reduced at high temperatures and low oxygen pressures to form non-stoichiometric oxygen-deficient oxides, which after cooling organize themselves into highly ordered superstructures, often with complex stoichiometry. 

The open structure of the fluorite space group tolerates a high level of atomic disorder that can be introduced by reduction or doping. In the reduction process, electron vacancies are involved, giving rise to a large electronic contribution to the conductivity. When the bivalent solute is formed, the 

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It appears that the presence of U6+, for which there is no counterpart in the Pu/O system, is responsible for preventing the oxygen pressures from increasing as rapidly in the mixed-oxide system as in the plutonia system. These results (39J indicate that the oxygen partial pressures in equilibrium with the mixed oxide will not differ drastically from those in equilibrium with urania. 

It is well known that dense ceramic membranes made of the mixture of ionic and electron conductors are permeable to oxygen at elevated temperatures. For example, perovskite-type oxides (e.g., La-Sr-Fe-Co, Sr-Fe-Co, and Ba-Sr-Co-Fe-based mixed oxide systems) are good oxygen-permeable ceramics. Figure 2.11 depicts a conceptual design of an oxygen membrane reactor equipped with an OPM. A detail of the ceramic membrane wall... 

X-Ray diffraction studies have identified a distorted octahedral environment about zirconium in KZr2(P04)3, and the structures of zirconium phosphates have been discussed in terms of their utility as chemical sieves.Some preparative and structural aspects of the normal sulphates of zirconium and hafnium have been reviewed and X-ray studies have shown that the sulphates Zr20(S04)3,5H20 and Hf20(S04)3,5H20 are isostructural and are more accurately represented as M2(OH)2(S04)3,4H20. Other studies of zirconates and hafnates and related mixed oxide systems are summarized in Table 2. 

Table 2 Zirconates, hafnates, and related mixed oxide systems...

To obtain and verify such data as tliese and facilitate the extension to the broad range of complex mixed oxide systems of interest, we require ... 

The fluorescence spectra of several mixed oxide systems have been extensively studied by Bril and Wanmaker [623], who reported the following decay times (in milliseconds). 

Figure 1. Effect of mixed oxide formation on AG° of M-0 dissociation (298 K). Lines 1)—3) show the mixed oxide systems based on CoO, ZnO, and Ti02, respectively, a) - d) are - aGr for the following reactions.

Much work has been done to characterize surface and catalytic properties of Si02—A1203 and other mixed-oxide systems. It is well known from these studies that Si02—A1203 possesses Lewis as well as Br nsted acid sites that are interconvertible. Different models for the site structures have been proposed. Tanabe 20) has recently reviewed these attempts to characterize the surfaces of silica-alumina mixed oxides as well as those of crystalline silica-aluminas. 

It was demonstrated that water electrolysis generates mixed oxidant systems. Oxidation-reduction potential (Hsu and Kao 2004) is not the best parameter for system characterisation. In general, the online analysis of generated species is an unsolved problem. Probably, not all electrolysis products are known. This can be seen in analysing the active and total chlorine concentration. The standard method uses DPD but the effect of other chemicals on the DPD method is one reason that active chlorine is sometimes incorrectly measured. Amperometric analysis and... 

Many commercial catalysts employ mixtures of several metal oxides. The advantages of these mixed oxide systems varies from thermal stability to enhanced acidic properties and/or support metal interactions. The dedicated STEM does an excellent job of identifying the individual mixed oxide components and determining location of the catalytic metals. 

On fusion of Mo03 or W03 with alkali or alkaline earth oxides, mixed oxide systems are obtained. These consist, so far as is known, of infinite chains or rings of M06 octahedra, and are unrelated to the aqueous molybdates and tungstates. 

The solid state chemistry of mixed oxide systems can be directly probed by O NMR. The formation of nanocrystalline Ti02 has been studied by doping the system... 

A possible approach to combine the advantages of both solids is the preparation of mixed oxide systems. Therefore, several studies were devoted recently to the synthesis of Ti02-Al203 mixed oxides with different Ti02/Al203 ratios [3, 4]. Particular attention was paid to the sol-gel method [5, 6], since this method allows a good control of the composition and characteristics of the resulting material [7, 8]. 

O-Donor Ugands. Table 2 summarizes the results of studies of zirconates, hafnates, and related mixed-oxide systems reported this year. 

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