The Phenomenon of Catalysis

In the years following Berzelius, a number of further examples of catalytic action were discovered, but scientific appreciation of their mode of action had to await the arrival of experimental and theoretical techniques for the study of reaction rates. It then became possible for F.W. Ostwald to define a catalyst as a substance that increases the rate at which a chemical system approaches equilibrium, without being consumed in the process. This handy form of words encapsulates the essential truth of the catalytic effect, and has stood the test of time it carries with it a number of important implications that we should now explore. The first of these is that the position of equilibrium attained in a catalysed reaction is exactly the same as that which would ultimately be arrived at in its absence this must be so because the equilibrium constant K is determined by the Gibbs free energy of the process, and this in turn is fixed by the enthalpy and entropy changes, thus  

It is inconceivable that the same reaction could have two different sets of thermodynamic parameters, and this basic principle has been put to good use by using catalysts for determining heats of hydrogenation of alkenes at room temperature 1 this would otherwise be impossible because reactions would be inordinately slow at all reasonable temperatures. 

It is sometimes a source of confusion that different catalysts can effect different courses of reaction on the same molecule. A good example of this is the decomposition of ethanol, which when metal-catalysed undergoes 

There are a number of other qualities that catalysts possess which should be introduced at this point. It is not only different types of catalyst that afford different products a single catalyst can also do this, so for example the hydrogenation of ethyne can lead with a platinum catalyst to a mixture of ethene and ethane, and ethene once formed can reant further to ethane. The extent to which the intermediate product, which is often the desired one, is formed is measured by the selectivity S, where 

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Many reactions proceed much faster in the presence of a substance which is itself not a product of the reaction. This is the phenomenon of catalysis, and many life processes and industrial processes depend on it. Thus, the oxidation of SO, to S03 is greatly accelerated in the presence of V2O5 as a catalyst, and the commercial manufacture of sulfuric acid depends on this fact. 

Catalysts are materials that change the rate at which chemical equilibrium is reached without themselves undergoing any change. Through the phenomenon of catalysis, very small quantities of a catalytic material can facilitate several thousand transformations. In addition to the remarkable increases in activity observed in the presence of a catalyst, an additional attribute of catalysts is that there is often a selectivity toward certain reaction products. Often, this selectivity is of greater importance than activity since a highly selective process eliminates the generation of wasteful by-products. 

The phenomenon of enzyme catalysis in living organisms was known a long time before anyone really knew what an enzyme was in chemical terms. Enzymes are exceptionally efficient catalysts. So efficient in fact that chemists working in the early twentieth century could observe the phenomenon of catalysis under conditions in which the analytical techniques available at the time could not detect any protein. Proteins were there but just not detectable. Many chemists of that era considered enzymes to be carbohydrate in nature. 

Catalysis, It may be defined as the effect produced on the rate of reaction either by a small quantity of a substance(called "catalyst ), which appeares to be unchanged in the reaction products, or by some physical energy, such as radiation, electricity or magnetism. Reactions accelerated by catalysts are called "positive catalytic reactions , while those retarded are known as "negative catalytic reactions . The Phenomenon of catalysis is essentially a branch of "surface chemistry ... 

The phenomenon of catalysis is a subject of chemical kinetics, as follows from Ostwald s definition of catalysis. Therefore, the accumulation of data on the kinetics of concrete catalytic reactions favors the progress of the theory of catalysis. 

However, even with this a priori knowledge, the actual state of the system still remains somewhat vague on many essential points. For instance, is the nickel present as separate crystals, or is there a film of metal on the silica walls How narrow are the pores An answer to these questions and to many more is necessary before we can hope to gain an insight into the phenomenon of catalysis in general and of the influence of the carrier system in particular. 

The study of these catalysts may sharpen our insight into the phenomenon of catalysis itself however. We have seen that the study of electron donor/ac-ceptor complexes for the activation of molecular hydrogen has shown many correlations between physical properties of the acceptor and catalytic activity. 

The work and ingenuity invested in the study of these materials may not only be of importance for a better understanding of the phenomenon of catalysis, but may also pave the way for the development of better and more selective catalysts. 

In terms of atomistic description of the phenomenon of catalysis, the border between these two approaches is determined by the rate of attenuation of the electron perturbation in a solid with the distance from an adsorbed molecule as well as by the degree of similarity between electron structures of the whole surface of a solid and of a fragment considered as a model of the adsorption or of an active site. For a long time this problem lacked an unambiguous solution. Therefore both approaches were equally widely used to describe catalytic phenomena, often without proper regard to specific features of the systems considered. Thus, active sites on metal catalyst surfaces were frequently modeled by individual metal atoms. On the contrary, catalysis on insulator oxide surfaces was sometimes discussed in terms of their cooperative electron properties. 

A catalyst is a substance that increases the rate of a reaction leading to the product(s), whereby the catalyst does not undergo any chemical change. The phenomenon of catalysis was known to chemists as far back as the 19th century. Only in the 20th century did chemists understand the behavior of a catalyst. Now it is recognized that catalysts are substances that change the rate of a chemical reaction without... 

Acid—base catalysis is caused by the formation of a reactive intermediate from substrate and catalyst which opens a low free energy pathway for the reaction. Consequently, the phenomenon of catalysis cannot be separated from problems of reaction mechanism. In this section, various possibilities of reaction mechanisms involving acid—base catalysis are discussed from a deductive point of view, with respect to structure and reactivity of substrates and intermediates [3, 4,14, 83]. 

It is now clearly recognized that elements exhibiting the phenomenon of catalysis, either in the zero-valent (t.e., metallic) or some other oxidation state, occur in or adjacent to the transition series. For all practical purposes we may confine our attention to the elements and compounds of Groups VI, VII, VIII and IB of the Periodic Table by doing so of course we ignore some technically important acidic and other oxides e.g., alumina, silica, zinc oxide) which are not within the terms of reference here. 

Along the time, different approaches have been developed in order to computationally model, the phenomenon of catalysis. Thanks to the development of high capacity (speed and memory) computers and software, nowadays, it is possible to mimic, with a fair degree of confidence, the steps involved in the cycle of a given catalyst. The number of quantum calculation studies published in journals dedicated to catalysis has rapidly increased during the last four years. For example, the Journal of Catalysis, traditionally dedicated almost exclusively to experimental catalysis (1], published around 50 density functional theory (DFT) calculation related articles during the 2(XM)-2003 period. Similarly, the Journal of Molecular Catalysis A published 74 related articles during the same time period, while... 

The recession which has hit to some degree almost every sector of manufacturing industries in the Western World has not left the chemical industry unscathed, but it has understandably led to a resurgence of interest in the phenomenon of catalysis. It is appreciated that new and better catalysts can reduce the energy input into chemical processes, improve product yields, minimise by-products, and hence lead to decreased process costs. The associated energy crisis, and the geopolitical vulnerability of much of our crude oil supply, have focused attention on alternative raw materials for the petrochemicals industry, and for liquid fuels. The scope for innovative catalysis in this area is without limit, and the contents of the present volume reflect these potentialities. 

Fittingly enough, Ostwald was among the first Europeans to discover and appreciate Gibbs s work. He translated Gibbs s papers on chemical thermodynamics into German in 1892. Ostwald proceeded to put Gibbs s theories to use almost at once in connection with the phenomenon of catalysis. 

Life as we know it would be impossible without the phenomenon of catalysis. Proteins (page 563) that catalyze biochemical reactions are called ENZYMES. Many industrial processes would also be impossible without catalysts, A CATALYST is a substance that increases the rate of a reaction but is recovered chemically unchanged at the end of the reaction. The catalyst enters into the chemical reaction, but is subsequently regenerated. Typical is the catalytic effect of nitrogen oxide on the rate... 

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