Mechanism of heterogeneous catalysis

The study of catalytic polymerization of olefins performed up to the present time is certain to hold a particular influence over the progress of the concepts of the coordination mechanism of heterogeneous catalysis. With such an approach the elementary acts of catalytic reaction are considered to proceed in the coordination sphere of one ion of the transition element and, to a first approximation, the collective features of solids are not taken into account. It is not surprising that polymerization by Ziegler-Natta catalysts is often considered together with the processes of homogeneous catalysis. 

Out of the above four steps, steps (i) and (iii) are rapid and normally do not play any part in the overall rate determination of reaction. Mechanism of heterogeneous catalysis is discussed in Section 6.9. 

Heterogeneous catalysis is primarily a molecular phenomenon since chemical bonds are created and/or broken (between the molecule and the surface) this implies that surface organometallic fragments are intermediates in any catalytic reaction on a surface. If one can design and synthesize surface organometallic fragments and study their reachvity, especially elementary steps, then one possesses in principle a crihcal tool to better understand the mechanisms of heterogeneous catalysis. 

Scheme 7.8 Mechanism of heterogeneous catalysis of hydrosilylation by a surface rhodium (diene) siloxide complex.

Weyl (9) has also outlined a picture of the mechanism of heterogeneous catalysis, which is similar to the schemes proposed by the above authors. His suggestions, based on the quanticule theory of Fajans (10), also result in a qualitative description. 

This and similar instruments (3,4) that allow one to study reaction rates and product distributions on small-area crystal and catalyst surfaces have been used in our studies of the mechanism of heterogeneous catalysis and the nature of active sites. These studies, which concentrated primarily on hydrocarbon reaction as catalyzed by platinum crystal surfaces, will be reviewed in the next section. 

Although theoretical and computational advances now afford powerful insights into the mechanisms of heterogeneous catalysis, especially on acidic, zeolitic solids (6a-d), experimental studies (7, 8) still hold sway. This we hope to demonstrate here by reference to the wide range of techniques—spectroscopic, kinetic, and analytical—that we have brought to bear in our studies of the catalytic dehydration of butyl alcohols. 

The mechanism of heterogeneous catalysis is often complex and not well understood. Important steps, however, involve (1) attachment of reactants to the surface of the catalyst, a process called adsorption, (2) conversion of reactants to products on the surface, and (3) desorption of products from the surface. The adsorption step is thought to involve chemical bonding of reactants to the highly reactive metal atoms on the surface with accompanying breaking, or at least weakening, of bonds in the reactants. 

The innate complexity of practical catalytic systems has lead to trial and error procedures as the common approach for the design of new and more proficient catalysts. Unfortunately, this approach is far from being efficient and does not permit to reach a deep insight into the chemical nature of the catalytic processes. The consequence of this difficulty is a rather limited knowledge about the molecular mechanisms of heterogeneous catalysis. To provide information about catalysis on a molecular scale, surface science experiments on extremely well controlled conditions have been designed and resulted in a new research field in its own. However, even under these extremely controlled conditions it is still very difficult, almost impossible, to obtain precise information about the molecular mechanisms that underlie catalytic processes without an unbiased theoretical guide. The development of new and sophisticated experimental techniques that enable resolution at... 

The cluster model approach and the methods of analysis of the surface chemical bond have been presented and complemented with a series of examples that cover a wide variety of problems both in surface science and heterogeneous catalysis. In has been show that the cluster model approach permits to obtain qualitative trends and quantitative structural parameters and energetics of problems related to surface chemistry and more important, provide useful, unbiased information that is necessary to interpret experiments. In this way, the methods and models discussed in the present chapter are thought to be an ideal complement to experiment leading to a complete and detailed description of the mechanism of heterogeneous catalysis. 

The rapid development of catalysis by soluble transition metal complexes and the results of surface science studies afford useful insights into the mechanisms of heterogeneous catalysis. - The surface of a metallic particle presents an array of atoms differing in coordination number which depends on the atom s location on a terrace, step or kink. The spectroscopy of adsorbed reactants and intermediates have properties which suggest structural relationships to organometallic compounds and, presumably, their reactions are analogous. - ... 

It was accordingly demonstrated that the formation of electron donor-acceptor complexes is associated with the appearance of new catalytic activity and the further extension of the concept of electron donor-acceptor would possibly lead to a fuller insight into the mechanism of heterogeneous catalysis. 

J. W. E. Coenan, The Mechanism of the Selective Hydrogenation of Fatty Oils, in J. H. deBoer (ed.), The Mechanism of Heterogeneous Catalysis, p. 126, Elsevier Publishing Company, New York, 1960. 

Much of the understanding of the solid state mechanism of heterogeneous catalysis stems from fundamental studies of single phase model compounds (1-5). In many cases, the role of a metal component in a catalytic process has been discerned through its incorporation into solid solutions of relatively inert host matrices (O. In the case of the selective oxidation and... 

The mechanism of heterogeneous catalysis is still under discussion and the same is for tribocatalysis. Usually, the decreased activation energy is presented as an apparent activation energy, Eapp. However, there is no clear theory which explains the mechanism of material s influence on the ratio of chemical reactions. 

J.H. de Boer, The Mechanisms of Heterogeneous Catalysis, Elsevier, Amsterdam 1960. 

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