Heterogeneity of oxides

The initial slopes obtained are different from the first loading to the third, indicating clearly that there is not one unique zirconium monohydride site on the surface of silica, thus showing the heterogeneity of oxide surface, which leads to different active sites. 

The calorimetric studies of the surface heterogeneity of oxides were initiated half a century ago, and experimental findings as well as their theoretical interpretation have been recently reviewed by Rudzinski and Everett [2]. The last two decades have brought a true Renaissance of adsorption calorimetry. A new generation of fully automatized and computerized microcalorimeters has been developed, far more accurate and easy to manipulate. This was stimulated by the still better recognized fact that calorimetric data are much more sensitive to the nature of an adsorption system than adsorption isotherm for instance. It is related to the fact that calorimetric effects are related to temperature derivatives of appropriate thermodynamic functions, and tempearture appears generally... 

Although, the view about the energetic heterogeneity of oxide surfaces is now commonly accepted, the nature of that phenomenon and its role in adsorption is not well understood yet. The structure of oxide surfaces is still often assumed to be corresponding to their ideal crystallographic structure. 

As it was mentioned earlier the surface heterogeneity of inorganic oxides, which consists of surface hydroxyls, attached impurities and surface roughness, is the main source of the energetic heterogeneity of these solids. Although the surface heterogeneity of oxides can... 

Rudzinski, W., and Charmas, R., Effects of surface heterogeneity of oxides in simple ion adsorption at oxide/electrolyte interfaces, Adsorption, 2(3), 245-260 (1996). 

In this overview we have tried to highlight some of the major advances reported in the heterogenization of oxidation catalysts emphasizing those aspects (where present) that can add extra properties to the heterogeneous system with respect to the homogeneous counterpart. The examples cited have tried to cover the major strategies that have been developed to achieve heterogenization in a stable manner... 

There has been a general updating of the material in all the chapters the treatment of films at the liquid-air and liquid-solid interfaces has been expanded, particularly in the area of contemporary techniques and that of macromolecular films. The scanning microscopies (tunneling and atomic force) now contribute more prominently. The topic of heterogeneous catalysis has been expanded to include the well-studied case of oxidation of carbon monoxide on metals, and there is now more emphasis on the flexible surface, that is, the restructuring of surfaces when adsorption occurs. New calculational methods are discussed. 

The scope of oxidation chemistry is enormous and embraces a wide range of reactions and processes. This article provides a brief introduction to the homogeneous free-radical oxidations of paraffinic and alkylaromatic hydrocarbons. Heterogeneous catalysis, biochemical and hiomimetic oxidations, oxidations of unsaturates, anodic oxidations, etc, even if used to illustrate specific points, are arbitrarily outside the purview of this article. There are, even so, many unifying features among these areas. 

Mechanistic Models. A general theory of the mechanism for the complete heterogeneous catalytic oxidation of low molecular weight vapors at trace concentrations in air does not exist. As with many catalytic reactions, however, certain observations have led to a general hypothesis (17). 

The critical properties of water are 374°C (705°F) and 218 atm (3,205 psi). Above this condition a heterogeneous mixture of water, organic-compounds, and oxygen may become homogeneous. Then the rate of oxidation may be considerably accelerated because of (1) elimination of diffusional resistances, (2) increase of oxygen concentration by rea-... 

Sample surface unrepresentative. Heterogeneity of the sample was given above as the cause of Class II deviations. The case in which heterogeneity causes one part of a surface to differ from another is clear enough it is often encountered with minerals. Here we wish to direct attention to cases where a surface, though uniform, differs in composition from the bulk of the sample. The cause may be an oxide film thick enough that composition differences between it and the bulk of the sample influence the analytical results. 

The chemical properties of oxide surfaces have been studied by several methods, including oxygen exchange. This method has been used to investigate the mechanisms of heterogeneous reactions for which oxides are active catalysts [36]. The dimerization step does not necessarily precede desorption and Malinin and Tolmachev [634], in one of the few reviews of decomposition kinetics of solid metal oxides, use this criterion to distinguish two alternative reaction mechanisms, examples being... 

Zeolites form a unique class of oxides, consisting of microporous, crystalline aluminosilicates that can either be found in nature or synthesized artificially [J.M. Thomas, R.G. Bell and C.R.A. Catlow in Handbook of Heterogeneous Catalysis (Ed. G. Ertl, H. Knbzinger and J. Weitkamp) (1997), Vol. 1, p. 206, VCH, Weinheim.]. The zeolite framework is very open and contains channels and cages where cations, water and adsorbed molecules may reside and react. The specific absorption properties of zeolites are used in detergents, toothpaste, and desiccants, whereas their acidity makes them attractive catalysts. 

Frank SN, Bard AJ (1977) Heterogeneous photocatalytic oxidation of cyanide and sulfite in aqueous solutions at semiconductor powders. J Phys Chem 81 1484—1488... 

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