SITES HETEROGENEITY

Heterogeneous Ziegler-Natta catalysts used to polymerize olefins exhibit phenomena characteristic of active site heterogeneity (1- 5). Complex kinetic models which account for this likelihood have been developed and used only in simulation studies (6-7). 

Duggan, MJ and Stephenson, FA (1988) Benzodiazepine binding site heterogeneity in the purified GABAa receptor. Eur. J. Pharmacol. 154 293-298. 

The above results indicate the great progress being made in understanding the electron-transfer behavior in these sensitizer/semiconduc-tor assemblies. They also highlight the need for further work to establish the full rate laws for iodide oxidation, to obtain better resolution of site heterogeneity in the recombination process, and to define better the nature of the sites for the injected electrons. 

General Observations About x. its Relationship to the Overall Partitioning Coefficient and to the Concept of Surface-Site Heterogeneity. One approach to metal/particle surface interactions which has been developed, historically, in a variety of forms, is a conceptual model that assumes only two conditions for surface sites occupied by an adsorbate or unoccupied. In applying this approach to the solid/aqueous solution interface, the adsorption... 

In their description of metal ion adsorption, Benjamin and Leckie used an apparent adsorption reaction which included a generic relationship between the removal of a metal ion from solution and the release of protons. The macroscopic proton coefficient was given a constant value, suggesting that x was uniform for all site types and all intensities of metal ion/oxide surface site interaction. Because the numerical value of x is a fundamental part of the determination of K, discussions of surface site heterogeneity, which are formulated in terms similar to Equation 4, cannot be decoupled from observations of the response of x to pH and adsorption density. As will be discussed later, It is not the general concept of surface-site heterogeneity which is affected by what is known of x> instead, it is the specific details of the relationship between K, pH and T which is altered. 

The observation that the macroscopic proton coefficient is a function of adsorption density and pH has several implications for macroscopic modeling of cation and anion adsorption. The dependency of x on pH and T affects 1) the relationship of the macroscopic partitioning coefficient to pH and adsorption density, 2) the notion of metal ion preferences for a particular surface in systems with multiple solid phases, 3) the accuracy of predictive models when used over a range of adsorption density and pH values, and 4) conclusions about site heterogeneity based upon partitioning expressions which use constant proton coefficients. 

Macroscopic Coefficients and Surface-Site Heterogeneity. Beniamin and heckles model (5) of heterogeneous metal oxide surface sites includes two observations of metal ion/surface site Interactions. 

Cs-Ca selectivity modelling is well suited to detect surface heterogeneity in an extremely small fraction of the CEC, which is difficult to measure by differential calorimetry. However, differential calorimetry detects site heterogeneity covering the whole range of CEC. 

Raja, R., Thomas, J.M., Greenhill-Hooper, M., Ley, S.V., and Almeida Paz, F.A. (2008) Fadle, one-step production of niadn (Vitamin B3) and other nittogen-containing pharmaceutical chemicals with a single-site heterogeneous catalyst. Chem. Eur. J., 14, 2430-2438. 

Single Metal Site Heterogeneous Catalysts and the Design of New Catalysis 17... 

Building Block Approaches to Nanostructured, Single Site, Heterogeneous Catalysts... 

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