Size-selective reactivity models

Electronic structural model. The size selective reactivity of these metal clusters Ts surprisi ng. Certainly the metal cluster are... 

In heterogeneous catalysis, the catalyst often exists in clusters spread over a porous carrier. Experimentally, it is well established that reactivity and selectivity of heterogeneous reactions change enormously with cluster size. Thus, theoretical studies on clusters are particularly important to establish a basis for the determination of their optimal size and geometry. Cluster models are also important for studying the chemistry and reactivity of perfect crystal faces and the associated adsorption and desorption processes in heterogeneous catalysis (Bauschlicher et al, 1987). 

Molecular mechanics have become more and more important in the study of the correlation between cavity size and ionic radius of coordinated metal ions (84CCR1, 93CCR177). The different aspects of metal ion reactivity and ligand design for specific and selective binding can be modeled and understood in detail. 

Many of the TEM studies of catalysts reported in the literature use model catalysts with visible metal crystallites supported on a metal oxide support, which usually is selected because it offers minimal electron scattering interference. These model catalysts are studied because direct measurements regarding crystallite size and structure can be made. Extrapolation of this information to smaller metal cluster sizes usually present in commercial catalysts is anticipated to provide insight into the relationship between structure and chemical reactivity (10). 

Diffusive and convective transport processes introduce flexibility in the design of catalyst pellets and in the control of FT synthesis selectivity. Transport restrictions lead to the observed effects of pellet size, site density, bed residence time, and hydrocarbon chain size on chain growth probability and olefin content. The restricted removal of reactive olefins also allows the introduction of other intrapellet catalytic functions that convert olefins to other valuable products by exploiting high intrapellet olefin fugacities. Our proposed model also describes the catalytic behavior of more complex Fe-... 

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