The Mechanism of Heterogeneous Catalysis

It is generally accepted that heterogeneous catalysis represents a sequence of elementary reactions such as the adsorption of the reactant on the catalyst surface, atomic rearrangements of the adsorbed particles, and desorption of the products, the overall reaction rate being governed by the slowest step of these elementary reactions. The rate of the slowest 

The idea of n was first put forward by Horiuti (30) and designated as the stoichiometric number ( Kagaku-Ryosu ). 

For instance, if we write down the ammonia synthesis reaction from the elements as N2 + 3 H2 = 2 NHs, we can particularize the elementary reactions involved in this process to the stoichiometric numbers M = 1, 2, and 3, respectively, as  

We shall now express the stoichiometric number in the form accessible to experimental determination. The net rate of reaction v, i.e., the 

Since all the elementary reactions except the slowest are usually assumed to be in partial equilibrium, we have the relation 

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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. 

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 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. 

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. 

Of special interest for stereochemical studies of the mechanism of heterogeneous catalysis are the bisbicyclic diketones (XV), (VII), and (VIII) containing tridimensional groups of bicyclo[2.2.2]octane. Therefore, let us consider in some detail the results of the studies of Balandin et al. (306). Hexahydrodioxodibenzo- and hexahydrodioxo-tetrabenzoanthrylenetriptycenes (VII) and (VIII) obtained by us differ from the formerly known diketone (XV), obtained by Clar, (318) by their more complicated structure ... 

Optical absorption spectroscopy is one of the most versatile and informative techniques for investigation of surface chemistry, particularly as related to heterogeneous catalysis. In principle, at least, it is possible to determine in detail the chemical functionality of a surface, the structure of an adsorbed species and their interactions and interrelationships. Such information, in addition to better defining the nature of surface active sites, is particularly valuable in elucidating the mechanisms of heterogeneous catalysis by identification of the chemisorbed reactive intermediates. 

Surface science studies of thin films may be very helpful for the understanding of the mechanisms of heterogeneous catalysis on intermetallics. This was true in particular for the AES study of the Ru(0001)-Ce-H2 interface performed by Walker and Lambert (1992) in the context of ammonia synthesis or the growth of cerium films on polycrystalline rhodium (Warren et al. 1993) on top of which carbon monoxide oxidation was performed. 

G.A. Somo/jai. Directions of Theoretical and Experimental Investigations into the Mechanisms of Heterogeneous Catalysis. Catal. Lett. 9 311 (1991). 

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