Theories of catalysis

VoPkenshtein, F. F. Elektronic theory of catalysis on semiconductors. Moscow Fismatgiz 1960... 

The poisoning effect of hydrogen when dissolved in palladium was for the first time properly described and interpreted by Couper and Eley (29) in 1950 in their study of the fundamental importance of the development of theories of catalysis on metals. The paper and the main results relate to the catalytic effect of an alloying of gold to palladium, on the parahydrogen conversion. This system was chosen as suitable for attempting to relate catalyst activity to the nature and occupation of the electronic energy... 

The mechanism of the poisoning effect of nickel or palladium (and other metal) hydrides may be explained, generally, in terms of the electronic theory of catalysis on transition metals. Hydrogen when forming a hydride phase fills the empty energy levels in the nickel or palladium (or alloys) d band with its Is electron. In consequence the initially d transition metal transforms into an s-p metal and loses its great ability to chemisorb and properly activate catalytically the reactants involved. 

Herman Pines and Luke A. Schaap The Use of X-Ray K-Absorption Edges in the Study of Catalytically Active Solids Robert A. Van Nordstrand The Electron Theory of Catalysis on Semiconductors Th. Wolkenstein... 

The notions of the electronic theory of catalysis developed in the 1950s by... 

A clue to the understanding of the photocatalytic effect is the electronic theory of catalysis on semiconductors (1). As will be seen later, the existence and the basic regularities of the photocatalytic effect follow directly from the electronic theory of catalysis. Whereas the theory of the photoadsorp-tive effect (the influence of illumination on the adsorption capacity of a semiconductor) has received much attention in the literature, the theory of the photocatalytic effect based on the electronic theory of catalysis has almost escaped the attention of investigators. The purpose of the present work is to fill in the gap to a certain extent. We shall naturally start by recalling certain principal concepts of the electronic theory which will be needed later. 

Reaction (70) in the dark has been discussed in the literature (1) from the viewpoint of the electronic theory of catalysis. The photoreaction (70) has also been considered in the literature (8), though briefly and purely qualitatively. In the present article we shall proceed from the mechanism which has been discussed in the literature (1) as one of the possible mechanisms. Let us examine the influence of illumination on the rate of the reaction [see reference (47) ]. 

Wolkenstein, Th. Theorie electronique de la catalyse sur les semiconducteurs. Masson et Cie, Paris, 1961 F. F. Vol kenshtein (Th. Wolkenstein), The Electronic Theory of Catalysis on Semiconductors. Pergamon, Oxford, 1963 Th. Wolkenstein, Elektronentheorie der Katalyse an Halbleitern. VEB Deutscher Verlag der Wissenschaften, Berlin, 1964. 

Robert A. Van Nordstrand The Electron Theory of Catalysis on Semiconductors Th. Wolkenstein... 

The complete theory of catalysis, which would start with the isolated reaction participants, was not available until now because of the lack of adequate knowledge of the participants themselves (even the complete theory of the isolated participants, starting from the first principles, is still lacking). However, in analogy with the homogeneous chemical reactions one can expect that the quantum chemical approach, based on the semiempirical quantum mechanical methods, could be a prospective one. 

Donor-acceptor stabilizations of a TS A- B complex are intimately related to the general theory of catalysis. Reduction of the repulsive TS barrier between... 

Evidently, there does not exist a general theory of catalysis. From a survey of the voluminous literature on catalysis,8 however, it is apparent... 

The two main theories of catalysis are (i) intermediate compound formation theory and (ii) adsorption theory. 

The decrease in activation energy for the chemisorption process as a result of 77 complex adsorption is readily explained by Lennard-Jones general theory of catalysis (19). Curve I in Fig. 1 represents the van der... 

The practice of considering the catalyst as a featureless surface or a planar array of atomic centers deprives theory of an adequate concern for the geometry of the transition from reactants to products. Balandin (23) recognized the importance of the concept of a transition state to the development of a mechanistic theory of catalysis, and in his hands the multiplet theory proved fruitful. However the directional properties of binding orbitals, a subject of more recent development, apparently has not been incorporated into his theory. 

This article presents a concise account of the present state of the electron theory of catalysis on semiconductors. It aims to describe the main outlines of the electron theory primarily as it has been developed in the past ten years by the author and co-workers. It also contains a short summary of the results of a number of experimental works dealing with electronic phe-... 

Firstly and primarily, it seeks to disclose the elementary (microscopic) mechanism of the catalytic act. Every heterogeneous catalytic process, like any chemical process, is based in the final reckoning on an electronic mechanism. It is the aim of the theory to elucidate this mechanism. This is necessary if the theory of catalysis is to rise above vulgar empiricism and to show how to control the activity and selectivity of catalysts, i.e., how to vary them to the required degree and in the required direction. 

The electron theory of catalysis cannot as yet be regarded as a complete theory. It resembles a building from which the scaffolding has not yet been removed. It is being erected on the foundation of the modern theory of the solid state and thus introduces new concepts and ideas into the theory of catalysis. This does not mean, of course, that it excludes other concepts and ideas prevalent today in other theories of catalysis. On the contrary, it makes use of these while attempting to disclose their physical content. 

The electron theory of catalysis and other, mainly phenomenological, theories of catalysis are not as a rule mutually exclusive. They deal with different aspects of catalysis and thus differ from one another mainly in their approach to the problem. The electron theory is interested in the elementary (electronic) mechanism of the phenomenon and approaches the problems of catalysis from this point of view. 

The existing phenomenological theories of catalysis bear approximately the same relation to the electron theory as the theory of the chemical bond, which was prevalent in the last century and which made use of valence signs (and dealt only with these signs), bears to the modern quantum-mechanical theory of the chemical bond which has given the old valence signs physical content, thereby disclosing the physical nature of the chemical forces. 

The father of the electron theory of catalysis is L. V. Pisarzhevsky (Kiev). His work, begun in 1916, formed part of an extensive series of investigations dealing with electronic phenomena in chemistry. L. V. Pisarzhevsky was the first to attempt to relate the catalytic properties of solids to... 

In recent years the electronic trend in catalysis has also been developing rapidly abroad (Dowden in England, Boudart in the United States, Germain and Aigrain in France, and especially Hauffe and co-workers in Germany, and others). It may today be considered the main trend in the development of the theory of catalysis. 

In its present stage of development, the electron theory of catalysis deals with catalysts which by their electrical properties belong to the class of semiconductors. Catalysis on semiconductors, as is well known, is extremely widespread, far more so than might appear at first sight. This is due to the circumstance that in most cases a metal is enclosed in a semiconducting coat and the processes which apparently take place on the surface of the metal actually take place on the surface of this semiconducting coat, whereas the underlying metal frequently takes practically no part in the process. 

Here the role of the geometrical factors in chemisorption is especially vividly expressed. These factors have been analyzed in detail by A. A. Balandin and co-workers in their papers (see, for example, ref. 18) on the multiplet theory of catalysis, in which they show their prime importance in a number of cases of the catalytic process. The electronic mechanism of chemisorption does not at all exclude these factors, but just stresses their role it retains the geometrical schemes of the multiplet theory but gives them physical content. 

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