Horiuti and Polanyi

Juro Horiuti later became the most well-known physical chemist in Japan and held the chair of that subject at the University of Hokkaido in Sapporo. His fame was due not only to his huge ability to concentrate and produce highly charged mathematical treatments while working in a noisy laboratory but also to his eminence in sculling. 

Polanyi was what is called a polymath he ended his career having held positions in physical chemistry, medicine, and, finally, philosophy. Horiuti was a student when he collaborated with Polanyi in the paper cited. This work, published in English, shows no signs of having been influenced by the prior publication of Butler. 

It can be seen from the figure that, with the r.d.s. quoted, the stronger the H-M bonding, the smaller the heat of activation and the faster the reaction. If one makes a plot of the logarithm of the exchange current density, jQ, for hydrogen evolution, as a function of the bond strength of H to the metal, one can see that log iQ increases with an increase of M-H. This, then, is electrocatalysis in a very explicit way [10]. 

These descriptions about the rate of the mechanism of the hydrogen evolution reaction and how the mechanism changes with increase of M-H are rather simple, but they are consistent with the observations and the proposed change of r.d.s. can be regarded as probable. In fact, over the 78 years that have passed, it is still current thinking. 

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As a result of rather extensive work on the hydrogenation of olefins 139,151 mechanism originally proposed by Horiuti and Polanyi is currently accepted ... 

Before we examine the hydrogenation of each type of unsaturation, let us first take a look at the basic mechanism assumed to be operating on metal catalytic surfaces. This mechanism is variously referred to as the classic mechanism, the Horiuti-Polanyi mechanism, or the half-hydrogenated state mechanism. It certainly fits the classic definition, since it was first proposed by Horiuti and Polanyi in 193412 and is still used today. Its important surface species is a half-hydrogenated state. This mechanism was shown in Chapter 1 (Scheme 1.2) as an example of how surface reactions are sometimes written. It is shown in slightly different form in Fig. 2.1. Basically, an unsaturated molecule is pictured as adsorbing with its Tt-bond parallel to the plane of the surface atoms of the catalyst. In the original Horiuti-Polanyi formulation, the 7t-bond ruptures... 

The articles by J. R. Anderson, J. H. Sinfelt, and R. B. Moyes and P. B. Wells, on the other hand, deal with a classical field, namely hydrocarbons on metals. The pattern of modem wTork here still very much reflects the important role in the academic studies of deuterium exchange reactions and the mechanisms advanced by pioneers like Horiuti and Polanyi, the Farkas brothers, Rideal, Tw igg, H. S. Taylor, and Turkevich. Using this method, Anderson takes ultrathin metal films with their separated crystallites as idealized models for supported metal catalysts. Sinfelt is concerned with hydrogcnolysis on supported metals and relates the activity to the percentage d character of the metallic bond. Moyes and Wells deal with the modes of chemisorption of benzene, drawing on the results of physical techniques and the ideas of the organometallic chemists in their discussions. 

The present article is a review of tt complex adsorption which has recently been proposed in catalytic reaction mechanisms (2-11). The main evidence for this intermediate has been obtained from isotopic hydrogen exchange reactions with aromatic compounds where an interpretation according to classical theories has met with increasing difficulties. The limitations of the classical associative and dissociative exchange mechanisms originally proposed by Horiuti and Polanyi (12) and Farkas and Farkas (13-15) re discussed. This is followed by a... 

Horiuti and Polanyi (50) argued differently and concluded that the exchange reaction had an associative mechanism as does the addition of hydrogen to benzene. However, they assumed that in the latter the two atoms of hydrogen added consecutively. 

For a reaction adequately described by just two configurations, reactant and product, the analysis of substituents effects is straightforward and was first treated by Horiuti and Polanyi (1935) almost 50 years ago. Subsequent contributions by Bell (1936) and Evans and Polanyi (1938) have led to these general ideas being jointly termed the Bell-Evans-Polanyi principle (Dewar, 1969). The treatment of multiconfiguration reactions is analogous and is illustrated in Fig. 12. Let us discuss this in detail. 

It is seen that Horiuti and Polanyi s 1934 mechanism for hydrogenation (426) has stood up very well to detailed spectroscopic scrutiny, with the proviso that the reactive ethene surface species is to be identified as of a n rather than of a di-a type. This mechanism is now as follows ... 

Mechanism. The generally accepted mechanism for the hydrogenation of double bonds over heterogeneous catalysts was first proposed by Horiuti and Polanyi,50,51 and was later supported by results of deuteration experiments. It assumes that both hydrogen and alkene are bound to the catalyst surface. The hydrogen molecule undergoes dissociative adsorption [Eq. (11.1)], while the alkene adsorbs associa-tively [Eq. (11.2)]. Addition of hydrogen to the double bond occurs in a stepwise manner [Eqs (11.3) and (11.4)] ... 

It has been observed [23,91,92] that when an unsaturated hydrocarbon is reacted with (a) equilibrated and (b) non-equilibrated hydrogen—deuterium mixtures, the deuteroalkane distributions are identical. Such observations indicate that the direct addition of a hydrogen molecule across the olefinic bond does not occur, and provides strong evidence for the formation of a half-hydrogenated state , that is, an adsorbed alkyl radical, first suggested by Horiuti and Polanyi [81], as a relatively stable reaction intermediate. The process of hydrogenation may thus be represented as... 

The transition state theory was applied to the proton transfer reaction at electrodes by Horiuti and Polanyi [58] and Eyring et al. [31]. The stretching of the H+—OH2 bond gives rise to the activated complex by a gradual transition in time and space. Details of this model were discussed in Sect. 3.1. 

Accordingly, in order to understand the roles of active sites in catalysis, we should clarify not only the intermediates but also the functions of active sites in relation to elementary processes. For this reason, it may be an interesting question whether the two reactions taking place via the same kind of intermediates occur on the same or different active sites. One good example is the isomerization of olefins via alkyl intermediates and their subsequent hydrogenation. In the Horiuti and Polanyi mechanism, complete overlapping of the intermediates, as well as the reaction routes, was tacitly assumed as described in Eq. (1). In this reaction scheme, step (1) and step ( ) are the... 

If two such types of site having different catalytic abilities coexist on a catalyst surface, the isomerization and hydrogenation reactions of olefins could proceed simultaneously but on different sites. As will be discussed later, it is rather reasonable that real catalysts involve active sites with different abilities. For this reason, the Horiuti and Polanyi mechanism which assumes a priori common alkyl intermediates for the isomerization and hydrogenation reactions, appears less logical. 

An early mechanism proposed by Horiuti and Polanyi in 1934 to interpret the hydrogenation of the C=C bond85,86 over transition metals is still generally accepted and postulated in the majority of cases (equations 3-6). In its original form this mechanism... 

The general energy relationships of discharge have been discussed in terms of quantum theory by Horiuti and Polanyi,2 Horiuti and Okamoto,3 and Butler.4 Fig. 51 (redrawn from the first-named authors paper) shows... 

Studies of the mechanism of catalytic hydrogenation [42] are complicated by its heterogeneous character, for the reaction is highly sensitive to variations in the surface properties of the catalyst. Most steroid hydrogenations are performed on palladium, platinum, or Raney nickel, although other metals have been used. Interpretations of mechanism are stiU based upon the suggestion by Horiuti and Polanyi [43] 1934 that the... 

A simple mechanism that has been proposed for ethylene hydrogenation on metal catalysts is that of Horiuti and Polanyi [J. Horiuti and M. Polanyi, J. Chem. Soc., Faraday Trans., 30 (1934) 1164] ... 

The early use of deuterium in place of hydrogen in the study of catalytic hydrogenation led to the recognition that the process was not simply the addition of H2 to the double bond. Horiuti and Polanyi proposed that both H2 and alkene (1) are bound to the catalyst surface and transformed to products by a sequence of elementary steps, which they represented as shown in Scheme 1, where an asterisk ( ) represents a vacant site on the catalyst.The last step, (d), is virtually irreversible under the usual hydrogenation conditions, but can be observed in the exchange reactions of D2 with alkanes. The mechanism accounts for the isomerization of an alkene if the reversal of step (c), which involves the formation of the alkyl intermediate (3), involves the abstraction of a hydrogen atom other than the one first added, and is coupled with the desorption of the alkene, (2) - (1). At present, the bond between the alkene and the metal often is represented as a ir-complex (4), as in equation (7). ... 

The mechanisms of isomerization which have been considered fall into two categories associative , first proposed by Horiuti and Polanyi, and dissociative , advanced by Farkas et al The associative mechanism is a consequence of the reversibility of the formation of the alkyl intermediate shown in Scheme 1 and in equation (10), while the dissociative mechanism, in its current form, involves allylic species (5)-(7) shown in Scheme 2. The Horiuti-Polanyi mechanism implies that double bond isomerization and the addition of H2 proceed through a common intermediate, whereas the dissociative mechanism represents an independent path. ... 

The involvement of chemisorbed intermediates in many electrode processes has been recognized for many years. As we indicated earlier, probably the first theoretically based ideas were those of Horiuti and Polanyi (72) and Butler (79) with respect to H in the HER. Many subsequent papers treated... 

According to Horiuti and Polanyi (111) the hydrogenation reaction of ethylene in the presence of a nickel catalyst consists of the following consecutive elementary steps ... 

The term electrocatalysis was first introduced by Grubb in 1963 [1] in connection with the anodic and cathodic charge-transfer reactions in fuel cells. However, the first systematic experimental investigations of various electrocatalysts had been carried out in the 1920s by Bowden and Rideal [2] and the concept and first interpretation of electrocatalysis had been introduced in the 1930s by Horiuti and Polanyi [3]. Their theory and its extensions and improvements have been analyzed lucidly by Bockris [4-6]. Recent reviews of progress in electrocatalysis can be found in more general [6-8] or specialized books [9-11]. 

Horiuti and Polanyi [3] were the first to provide an interpretation of electrocatalysis (Fig. 2). The potential energy of a proton moving toward the electrode exhibits a maximum at the intersection of the potential energy curve of the proton in solution and that of a H atom adsorbed on the electrode. A good electrocatalyst is that... 

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