Sesquioxides mechanical

Mechanism (1) is particularly important in peat and other organic-rich sediments, where clay and metal complexes are present in very low amounts in relation to the humus component. A typical example of humic substances bound by polyvalent complexes (item 2) is the Spodosol. These soils have developed under climatic and biologic conditions that have resulted in the mobilization and transport of considerable amounts of iron, aluminum, and organic matter into the B horizon. This illuvial horizon is a rich source of fulvic acids, which are readily separated from the sesquioxides by mild extractants. 

Before proceeding to consider kinetic equations and implied reaction mechanisms, we may note some other pertinent features of these reactions. (A) Benzene hydrogenation was subject to the influence of the Strong Metal-Support Interaction (Section 3.35) when titania and vanadium sesquioxide were used as supports for rhodium, platinum and iridium - even Pt/Si02 and Ni/Si02 when heated... 

The strong chemical interaction alumina-rare earth sesquioxide is considered to play a key role in the textural stabilization of the transition aluminas [142,145,170,171,184-187]. In the presence of the supported rare earth oxide, the y-AI2O3 a-AkOs phase transition, and inherently the sintering mechanism, is inhibited. 

The other oxides listed in Table 9.1 are included because their mechanical properties have received some attention (especially quartz). Quartz is by no means close-packed because of the stability of the [Si04] tetrahedron. Yttria and the rare-earth sesquioxides and the mixed oxide garnet structures are compact, in spite of their complexity, but cannot be described as close-packed in the traditional sense. In fact, the C-type M2O3 structure of yttria can be thought of as having the fluorite structure of urania with one-quarter of the anion sites vacant. 

These two distinct processes lead to the formation of secondary minerals mainly phyl-losilicates such as clays, of soluble products (e.g., carbonates or silica) lixiviated by percolating waters and of colloids usually iron and aluminum sesquioxides complexed by humic acids. While physical degradation involves mechanical (e.g., abrasion, impact) or thermal (e.g., thermal shock) processes, alteration involves only chemical reactions such as hydrolysis influenced by pH conditions and/or the oxidation of primary materials depending on the Eh (redox potential) conditions. Whatever the type of underlying rock, the end product is always a clay except when silica is totally absent from the bedrock, the composition of the clay depending on the type of climate and the time over which the evolution process takes place. These conditions are summarized in Table 14.1. 

Nearly all of the information available on the kinetics of heterogeneous reactions with lanthanide oxides concerns the C-type sesquioxides or the fluorite-related higher oxides. As stated in the section above, in these materials oxygen mobility is very high, whereas, metal-atom movement is extremely limited below 1200°C. Table 19 suggests the type of experiments that were done and the phenomenological mechanisms proposed before 1980 (Eyring 1979). 

Mann A.W., Structural relationships and mechanisms for the stoichiometry change from MX3 (YF3-type) throughMX2 (fluorite-type) to M2X3 (c-type sesquioxide). J. Solid State Chem. 1974 11 94-105... 

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