Catalysis of Chemical Reactions (Part-2)

A continuation to the previous article: Catalysis of Chemical Reactions (Part-1)

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• Amino complexes of copper, iron and other metals have great catalytic power.
  In the case of the decomposition of hydrogen peroxide, the ammoniacal copper complex providing the ion [Cu(NH3)4]^2+ has a catalytic power a million times more intense than that of the copper ion; by substituting other amines for ammonia, the catalytic activity of the complex can be controlled to a large extent. It emerges from the study of these complexes that the presence of amino nitrogen and of a coordination equal to four are the factors increasing the catalytic activity. If the amine contains two amino groups, chelates are obtained which are internal complexes. The existence of an internal cycle also contributes to a reinforcement of the catalytic activity, the complexes catalyze the reactions of oxidation, hydrolysis, decarboxylation, polymerization, etc...
  The complexes can also give rise to chain oxidation reactions, playing the role of initiators. The mechanism of their action is not always well established, differing from one case to another. For example, the decomposition of hydrogen peroxide catalyzed by copper complexes probably proceeds by formation of unstable intermediate compounds and partly by a radical chain mechanism.

Catalyst Activity and Selectivity:

An important characteristic of all catalysts is the selectivity of their action, the catalysts accelerate a given reaction or a given reaction group in a particularly intensive manner. The enzymes which are the catalysts of biological phenomena act in a strictly selective and specific way with respect to well determined reactions.
The selectivity of industrial catalysts is much less marked than that of biological catalysts and it is precisely the problem of increasing the specificity of catalysts which constitutes one of the major concerns of specialists. Very often the addition of minute quantities of suitable substances makes it possible to increase the specificity of the catalytic activity.

Heterogeneous catalysis:

In heterogeneous catalysis the substances which contribute to increase the activity of the catalysts are called promoters; the quantity of promoters that is added to the catalyst is generally small and the promoter may not have any activity with respect to the catalyzed reaction. Thus, the activity of a ferrous catalyst used in the synthesis of ammonia can be increased by adding 3 percent of potassium and aluminum oxides which by themselves have no catalytic power with respect to the reaction considered.

• Decreased catalytic activity:
  The bodies decreasing the catalytic activity are qualified as catalytic poisons, their effects manifest themselves especially in heterogeneous catalysis. Thereby; very small quantities of hydrogen sulphide, acetylene or oxygen can considerably reduce the catalytic activity of platinum in the oxidation reaction of sulphurous gas. The fact that even an infinite trace of poison can suppress the activity of a large mass of catalyst shows that only part of the surface of a heterogeneous catalyst actually participates in catalysis, there are numerous experimental data testifying to this that the surface of the usual catalysts is far from being homogeneous.
  It is therefore assumed that catalysis develops only on a relatively small number of active regions of the surface called active centers.

In what follows we will only apply the concept of active center in relation to solid catalysts, because the surface of any liquid is homogeneous and liquid catalysts are much less sensitive to poisoning.

• A whole series of particularities distinguish catalysis by metals and semiconductors from that by bodies such as aluminosilicates and insulators. Very often the catalytic activity of metals is due to the thin layer of oxides (or sulphides) which are formed by interaction of the metal with the ambient environment, so it is often difficult to experimentally study the catalytic activity of the metals themselves. even. However, there can be no doubt as to the role that electrons play in the catalytic activity of metals and semiconductors. The model that is often adopted is that of a perfect metallic crystal with no defects, such a model cannot however be considered suitable because experience shows that the catalytic reaction that is generally located in a zone of crystalline irregularities.

From the point of view of the transition state theory, the increase in the rate of the reaction due to the catalytic action of the surface must be attributed to the fact that the activation energy of the activated complex adsorbed at the surface of the catalyst is lower than that of a reaction carried out in homogeneous phase.
Heterogeneous catalysis begins with an adsorption of the initial bodies, this first step requires only a low activation energy and is followed by the actual reaction which takes place via the adsorbed activated complex; this last reaction proceeds more rapidly than in the homogeneous phase; finally, the products of the reaction must be eliminated by desorption, this desorption also requiring only a small activation energy.

References:

Optimization of the Interface Between Catalyst Layer and Proton Exchange Membrane via rolled technique. Sciencegate
Catalysis by SUBHRANGSU DEY. Academia.edu
Types of catalysts. Khan Academy
Base Catalyzed Reactions of Hydrocarbons and Related Compounds
[Book: Catalysis From Principles to Applications](Edited by Matthias Belter, Albert Renken, and Rutger van Santen)