Mixed oxides, structure types phases

This section considers a number of extremely important structure types in which A1 combines with one or more other metals to form a mixed oxide phase. The most significant of these from both a theoretical and an industrial viewpoint are spinel (MgAl204) and related compounds, Na- -alumina (NaAlnOi ) and related phases, and tricalcium aluminate (Ca3Al20g) which is a major constituent of Portland cement. Each of these compounds raises points of fundamental importance in solid-state chemistry and each possesses properties of crucial significance to... 

In technological applications, mixed, doped, or multi-metal oxides play an important role, for example, Mo-V-Te-Nb oxide [15] is used for selective oxidation of propane to acrylic acid. For some complex oxides, the bulk oxide structures and distribution of phases are often unknown and there is little knowledge of the atomic surface structure and composition, extent of hydroxylation, type and density of defects, and the location of dopants (homogeneously distributed, concentrated at the surface, grain boundaries, or interfaces). 

The key parameters in the preparation of active and stable catalysts for N2O decomposition under conditions of industrial interest were examined using mixed oxides obtained form hydrotalcite-type (HT) precursors. The relationship between activity and composition of the catalysts was studied, with attention focused on the nature and amount of metal ions present in the HT precursor (Rh/Mg/Al or Rh/Co/Al). An activation procedure was developed which makes it possible to prepare catalysts, active and stable even under the severe reaction conditions of industrial interest. The relationship between active phase and composition of the bulk matrix was also investigated. Moreover, the flexible structure of the HT precursor made it possible to investigate the use of other active elements (Pd or La) to develop synergetic effects, without any interactions due to side phase segregation, or lack of homogeneity. 

Upon calcination, the crystallinity of the lamellar structure progressively decreases while a mixed oxide phase of the NiO type appears for every samples. Only file mixed oxide is observed from calcination at 623 K. 

The bulk structures of most minerals important to soil chemists are well known from x-ray and neutron diffraction studies. Exceptions to this generality would be the various amorphous oxides and mixed oxide precipitates. Methods pioneered by Catlow s group should find increasing applicability to the structural study of these amorphous soil minerals and mixed phases. Their approach has been to develop ionic-type force fields for performing static energy minimizations of general oxide and halide minerals. Since ionic force fields rei uire far fewer parameters than va-... 

A primary characterization of perovskite-type oxides must include textural analysis and X-ray identification of the phase(s) present. For a more detailed characterization, structural analysis for establishing the lattice position of cations and surface analysis (by means of techniques such as XPS) for defining the surface concentration and oxidation states of cations are desirable. Consequently, information provided by these techniques will furnish the essential criteria for comparing the different preparation methods. For convenience, we will classify the methods used to date for the preparation of pure perovskite phases according to the scheme proposed by Courty and Marcilly (29) for the whole field of mixed oxides. Table I gives a survey of methods used as a function of the phenomena on which they are based. 

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