Theories of Catalysis – How Do They Work?

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theories of catalysis

There are two main Theories of Catalysis. They are,

  1. The Intermediate Compound Theory
  2. The adsorption Theory

It is, however, fairly certain that no rigid division can draw between these two theories. The suppositions of which may often together in the same catalysis.

So, let’s discuss each of these theories, one by one, with their functions.

1. The Intermediate Compound Theory

This is the main theory that we discuss under theories of catalysis. In its simplest presentation, this theory takes the following form. So, suppose a reaction,

A + B → AB is slow and catalyzed by a catalyst, C. the theory supposes that C first combines with one of the reagents, A, to form the compound, AC. This compound then reacts with the reagent, B, to form the compound, AB, releasing the catalyst C. so, this may then start on another round of the catalytic cycle.

A + C → AC

AC + B → AB + C

These two reactions together are much faster than the reaction:

A + B → AB, to which, in sum, they correspond.

So, it is obvious that, in being alternately combined and released. The catalyst probably passes through varying states of oxidation. Consequently, elements of variable valency, capable of assuming varying oxidation states. Also, they are frequently found in intermediate-compound catalysts,  especially transition elements such as Mn, Co, and V.

Under Theories of Catalysis, Examples of intermediate compound catalysis quoted below. They conform to the general theory stated above, in some cases, rather more complications.

Intermediate Compound Formation Theory Examples

  1. Catalysis of the Lead Chamber process, by nitric oxide.

NO + 1/2O2 → NO2

H2O + SO2 + NO2 → H2SO4 + NO

Total result : H2O + SO2 + 1/2O2 → H2SO4

lead chamber in Intermediate Compound Formation Theory

2. Catalysis of the decomposition of a hypochlorite, by cobaltous hydroxide.

2Co(OH)2 + H2O + NaOCl → 2Co(OH)3 + NaCl

2Co(OH)3 → 2Co(OH)3 + H2O + 1/2O2

Total result : NaOCL → NaCl + 1/2O2

3. Catalysis of the decomposition of potassium chlorate, by man ganese dioxide.

2MnO2 + 2KClO3 → 2KMnO4 + Cl2 + O2

2KMnO4 → K2MnO4 + MnO2 + O2

K2MnO4 + Cl2 → 2KCl + MnO2 + O2

Total result : 2KClO3 → 2KCl + 3O2

It must be remembered. However, that definite evidence of these mechanisms is very difficult to obtain. They are usually plausible rather than proved. Even though each can show to occur as a single reaction. Also, it does not necessarily follow that the succession of reactions shown above is the actual mechanism of the catalysis. Actually, in case (c) above, the oxygen always contains a little chlorine and, if only use a relatively little manganese dioxide, it can show to contain some permanganate after the reaction. Most importantly, this gives some support to the suggested mechanism.

So, let’s head into the adsorption theory, which is also a significant part under Theories of Catalysis.

If you like to know more about Catalysts: Read 7 Types of Catalysts.

2. The Adsorption Theory

So, this theory applies mainly to catalysis of a reaction between gaseous reagents by a solid catalyst. i.e., to contact catalysis. The theory supposes that the reagents adsorb on to the surface of the catalyst. i.e., brought into some form of loose association with the outermost one or two atomic or molecular layers of the catalyst. After the reaction, the products are released (desorbed) from the catalyst surface. We are also leaving it free to absorb more of the reacting substances.

Most importantly, The factors involved in adsorption catalysis are not fully understood. The adsorption of reacting materials very close together on the catalytic surface is equivalent to a marked concentration of them and accounts partly for the catalytic effect. Also, it is known as adsorption catalysts are not equally active over their entire surfaces.

Adsorption Theory Examples

Spots of exceptional activity occur and often associate with discontinuities. For example, cracks and irregularities and crystal boundaries. It should also note that adsorption catalysts also tend to be specific in their action so that the same reagents give different products under the action of different catalysts. An example of this is Carbon monoxide and Hydrogen.

CO + H2 + Heated Cu H.CHO (Formaldehyde)

Co + 2H2 + Heated Cr2O3 CH3OH (Methyl alcohol)

However, these facts cannot explain on a theoretical basis.

The phenomena of Catalyst promotion and Catalyst poisoning are well known in connection with contact catalysts.

Catalyst Promoter in Theories of Catalysis

Under theories of catalysis, a catalyst promoter adds to the material to enhance the activity of the main catalyst. But it does not necessarily catalyze the reaction if alone. Most mixed catalysts consist of a primary catalyst and a promoter. The action of a promoter is not usually very spectacular. But it gives a useful increase in catalytic activity. For example, the activity of the main catalyst, iron, in Haber’s process for ammonia synthesis substantially promotes by alumina and potassium oxide. Also, the production of methyl alcohol from carbon monoxide catalyzes more effectively by a mixture of zinc and chromium oxides than by either oxide separately.

catalyst promoter in theories of catalysis

Moreover, there is no adequate theory of the action of catalyst promoters. Possibly some part of their activity lies in the creation of the discontinuities alluded to a little earlier.

Catalyst Poisoning

Catalyst poisoning occurs when a small proportion of impurity in the reagents gradually suppresses the activity of the catalyst in theories of catalysis. The impurity is the catalyst poison. Well, known examples are the poisoning of platinum as a catalyst in the oxidation of sulfur dioxide by arsenious oxide. Also, the poisoning of the iron catalyst in Haber’s process by sulfur compounds. For example, H2S. Also, in some cases, the poisoning occurs because the poison absorbs on to the catalyst surface and not release again. So that the surface gradually becomes unavailable for oxide appears to be of this kind. In other cases, the catalyst chemically changes by reaction with the poison. The poisoning of iron by sulfur compounds falls into this class. The chief safeguard against catalyst poisoning is a thorough purification of the reagents.

Summary of Theories of Catalysis

So, from this article, we discussed two main types of Catalytic theories. They are The Intermediate Compound Theory and The adsorption Theory. These theories explain how catalysts contribute to increasing the rate of a chemical reaction. Also, by knowing these two theories, you can easily understand the Characteristics of Catalysts as well.

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