Electron theory of catalysis

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... 

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-... 

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 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 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. 

The phenomenon of catalyst modification by impurities (promoting by poisons, poisoning by promoters) was discovered in 1940 in the laboratory of S. Z. Roginsky. A summary of the experimental data is given in (74, 5). A theoretical interpretation of the phenomenon was given in the first papers on the electron theory of catalysis (1, 37, 66, 47)- The effect of impurities on the activity of a catalyst may be regarded as the fourth consequence of the theory. 

See for instance VoT Kenshtein, F.F. (1953) The Electronic Theory of Catalysis on Semiconductors, Macmillan, New York. 

There have been many attempts to relate bulk electronic properties of semiconductor oxides with their catalytic activity. The electronic theory of catalysis of metal oxides developed by Hauffe (1966), Wolkenstein (1960) and others (Krylov, 1970) is base d on the idea that chemisorption of gases like CO and N2O on semiconductor oxides is associated with electron-transfer, which results in a change in the electron transport properties of the solid oxide. For example, during CO oxidation on ZnO a correlation between change in charge-carrier concentration and reaction rate has been found (Cohn Prater, 1966). 

Research on alloy catalysts started in the 1950s with attempts to investigate the role in catalysis of the electronic structure of metals. This research was initiated by several papers of Dowden which, measured by their response in the literature, rank among the most important papers ever written on catalysis. However, it appeared later (for reviews, see 1-5) that two basic ideas, on which the so-called electronic theory of catalysis was built up, were not correct. These ideas were as follows ... 

The most essential progress from the point of view of application of this theory in catalysis and chemisorption has actually been achieved by the very first papers (48-50), where the so-called coherent potential approximation (CPA) was developed and applied. By means of this, photoemission data were explained in a quite satisfying way and the catalytic research got full theoretical support for some of the ideas introduced in catalysis earlier on only semiempirical grounds (5) namely, individual components are distinguishable for molecules from the gas phase and the alloy atoms preserve very much of their metallic individuality also in alloys—something that was impossible according to the RBT and the early electronic theory of catalysis. 

How was this development reflected by the theory of catalysis on alloys An early and very important paper (9) discussed the selectivity and activity effects fully in terms of the old electronic theory of catalysis. Another paper (5), which appeared simultaneously with (9), turned attention to the fact that one must also consider effects other than only the changes in the electronic structure. The results on alloys should be rationalized on the basis of two aspects of alloying (5) ... 

The air oxidation of 2-methylpropene to methacrolein was investigated at atmospheric pressure and temperatures ranging between 200° and 460°C. over pumice-supported copper oxide catalyst in the presence of selenium dioxide in an integral isothermal flow reactor. The reaction products were analyzed quantitatively by gas chromatography, and the effects of several process variables on conversion and yield were determined. The experimental results are explained by the electron theory of catalysis on semiconductors, and a reaction mechanism is proposed. It is postulated that while at low selenium-copper ratios, the rate-determining step in the oxidation of 2-methylpropene to methacrolein is a p-type, it is n-type at higher ratios. 

Nature of Active Sites. There is no apparent correlation between the increase of catalytic activity and a modification of the electronic structure of nickel oxide, since the electrical properties of both catalysts are identical. It is probable that local modifications of the nickel oxide surface are responsible for the change of its activity and of the reaction mechanism. It should be possible to associate these structural modification with local modifications of the height of the Fermi level, but it would be difficult to explain the results by the electronic theory of catalysis which considers only collective electrons or holes. A discussion based only on the influence of surface defects seems, therefore, to be more straightforward. 

In their electronic theory of catalysis, configuration 1 is preferred to configuration 2 by interaction of the catalyst with the 7r-electrons of the graphite lattice. 

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