Catalysis – Definition, Mechanism, Types, Characteristics

The majority of natural and manmade reactions take a long time to complete. A material known as the catalyst must be added to speed up the process. As a result, catalysis is the process of accelerating a reaction. Let’s take a closer look at catalysis, catalysts, and the significance of surface chemistry in the process.

What is Catalysis?

A catalyst is a substance that can speed up and slow down a chemical reaction without being consumed at the end of the reaction, which is known as catalysis.

Catalysis is the increase in the rate of a chemical reaction caused by the inclusion of a catalyst, which is not consumed in the catalyzed reaction and can function multiple times. This effect may usually be achieved with just a little quantity of catalyst. The catalyst in catalyzed processes normally reacts to make a transient intermediate, which then regenerates the original catalyst in a cyclic process.

Mechanism of Catalysis

The total free energy from reactants to products does not vary in the presence of a catalyst since less free energy is required to achieve the transition state. A catalyst can be used in a number of different chemical reactions. The addition of inhibitors (which limit catalytic activity) or promoters (which boost catalytic activity) can change the effect of a catalyst (which increase the activity and also affect the temperature of the reaction).

At the same temperature and for the same reactant concentration, catalyzed reactions have lower activation energy (rate-limiting free energy of activation) than uncatalyzed reactions, resulting in a faster reaction rate. The rate of the reaction, like any other chemical reaction, is determined by the frequency with which the reactants come into contact in the rate-determining phase.

Typically, the catalyst is involved in the slowest stage. The rate of reaction is determined by the amount of catalyst used. Although catalysts are not consumed during the reaction, later processes can inhibit, deactivate, or destroy them.

Types of Catalysis

The following types of catalytic reactions can be roughly classified.

Homogeneous Catalysis: When the reactants and catalyst are in phase with each other (i.e. solid, liquid or gas). It is claimed that the catalysis is homogeneous. 

Some examples of homogeneous catalysis are listed below.

• In the lead chamber method, sulphur dioxide is oxidized to sulphur trioxide with oxygen in the presence of nitrogen oxides as a catalyst.

2SO₂(g)  +  O₂(g)    →   2SO₃(g)    (In presence of NO(g))

The reactants, products, and catalysts are all in the same phase or gaseous state.

• Sugar hydrolysis is catalyzed by the H+ ions produced by sulphuric acid.

C₁₂H₂₂O₁₁(l)  +  H₂O(l)   → C₆H₁₂O₆(l) + C₂H₁₂O₆(l)     (In presence of H₂SO₄(l))  

Heterogeneous Catalysis: Heterogeneous catalysis is a catalytic process in which the reactants and catalysts are in various phases.

The following are some examples of heterogeneous catalysis-

• In the contact method for the synthesis of sulphuric acid, oxidation of sulphur dioxide into sulphur trioxide with platinum metal or vanadium pentaoxide as a catalyst. While the reactants are gaseous, the catalyst is solid.

SO₂(g) + O₂(g)   →  2SO₃(g)          (In presence of Pt(s))

• In Haber’s process, nitrogen and hydrogen are combined to produce ammonia in the presence of finely split iron.

N₂(g) + 3H₂(g)   →   2NH₃(g)        (In presence of Fe(s))

• In Ostwald’s procedure, ammonia is converted to nitric oxide in the presence of platinum gauze as a catalyst.

4NH₃(g) + 5O₂(g)   →   4NO(g) + 6H₂(g)          (In presence of Pt(s))

Positive Catalysis: When a foreign substance accelerates the pace of a reaction, it is referred to as a positive catalyst, and the phenomenon is known as positive catalysts.

The following are some examples of Positive catalysis-

• Decomposition of H₂O₂ in presence of colloidal platinum.

2H₂O₂(l)   →   2H₂O(l) + O₂(g)            (In presence of Pt)

Negative Catalysis: Certain substances, when added to the reaction mixture, slow down rather than speed up the reaction rate. Negative catalysts or inhibitors are used in this process, which is known as negative catalysis.

The following are some examples of Negative catalysis-

• If some alcohol is added to chloroform, the oxidation by air is slowed.

2CHCl₃(l) +O₂(g)  →   2COCl₂(g) + 2HCl(g)         (In presence of Alcohol(l)) 

Auto-Catalysis :

One of the products functions as a catalyst in some processes. The reaction is slow in the beginning, but after the products are formed, the rate of reaction accelerates. Auto-catalysis is the term for this type of event.

Some examples of homogeneous catalysis are listed below.

• When nitric acid is poured on copper, the reaction is slow at first, but it quickly accelerates due to the creation of nitrous acid, which works as an auto-catalyst during the process.

Acid-base Catalysis:

H⁺ or OH^– ions, according to Arrhenius and Ostwald, operate as catalysts.

• Conversion of acetone into diacetone alcohol,

CH₃COCH₃(l) +CH₃COCH₃(l)   →  CH₃COCH₂.C(CH₃)₂OH(l)             (In presence of OH^–)

• Decomposition of nitramide-

NH₂NO₂(l)   →  N₂O(g) + H₂O(l)                   (In presence of OH^–)

Characteristics of Catalysis

The following are the properties that most catalytic reactions share.

1. At the end of the reaction, the mass and chemical makeup of a catalyst remains unchanged.
2. The reaction cannot be started by the catalyst.
3. In most cases, the catalyst is quite specific.
4. The catalyst has no ability to alter the state of equilibrium.
5. Catalytic poisons.
6. Catalytic promoters.
7. Changes in temperature affect the pace of catalytic reactions, just as they do in the absence of a catalyst.

Important Features of Solid Catalysis 

• Activity: Catalyst activity refers to how quickly they may speed up chemical reactions. In some reactions, the rate of acceleration can reach 10¹⁰ times. In the presence of platinum as a catalyst, the reaction between H₂ and O₂ to generate H₂O, for example, occurs with explosive violence. H₂ and O₂ can be held indefinitely without undergoing any reactions in the absence of a catalyst.
• Selectivity: Is it possible for catalysts to direct reactions to produce certain products (excluding others).

Theories of Catalysis

• Collision Theory of Homogeneous Catalysis: A reaction happens as a result of effective collisions between the reacting molecules, according to the collision theory. The molecules must have a minimum level of energy known as activation energy (E_a) in order to collide effectively. Following the collision, the molecules form an activated complex, which then dissociates to reveal the product molecules. The catalyst creates a novel pathway that requires less activation energy.
• Adsorption Theory of Heterogeneous Catalysis: Adsorption of reactants on the surface of the catalyst is usually how heterogeneous catalytic reactions work. The diffusion theory of catalysis can be used to describe the mechanism of such surface reactions. For gaseous processes on a solid surface, this theory proposes the following sequence.
  1. The reactants diffuse to the surface through diffusion.
  2. Surface adsorption of reactant molecules.
  3. On the surface, there is a chemical reaction.
  4. The products are desorption from the surface.
  5. The products diffuse away from the surface. Step 3] determines the reaction rate in general. Steps 2] and 4], on the other hand, maybe rate-determining.

Advantages of modes adsorption theory: Because the catalyst is regenerated repeatedly, a small amount of the catalyst is sufficient. The catalyst participates in the process but is created at the end of it unchanged in mass and chemical composition.

Zeolites (shape-selective catalysis)

Zeolites are aluminosilicates of the general formula, M_n[AlO₂]_x.(SiO₂)_y.mH₂O, where, M may be a simple cation like Na, K or Ca, n is the charge on the simple cation, m is the number of molecules of water of crystallization.

They have a β cage, which looks like a honeycomb. The shape and size of the reactant and product molecules influence the processes catalyzed by zeolites. This is why these reactions are referred to as shape-selective catalysis.

In the petrochemical industry, zeolites are commonly utilized as catalysts for the cracking and isomerization of hydrocarbons. The ZSM-5 zeolite catalyst is a widely used zeolite catalyst in the petroleum sector (zeolite sieve of molecular porosity 5). It transforms alcohols straight to gasoline by dehydrating them first, resulting in a mixture of hydrocarbons. In the softening of hard water, hydrated zeolites are used as ‘ion-exchangers.’

Sample Questions 

Question 1: Define Catalysis.

Answer :

“A catalyst is a substance that can speed up and slow down a chemical reaction without being consumed at the end of the reaction, which is known as catalysis.”

Question 2: Explain the Characteristics of Catalysis.

Answer :

The following are the properties that most catalytic reactions share.

1. At the end of the reaction, the mass and chemical makeup of a catalyst remain unchanged.
2. The reaction cannot be started by the catalyst.
3. In most cases, the catalyst is quite specific.
4. The catalyst has no ability to alter the state of equilibrium.
5. Catalytic poisons.
6. Catalytic promoters.
7. Changes in temperature affect the pace of catalytic reactions, just as they do in the absence of a catalyst.

Question 3: Explain Zeolites (shape-selective catalysis).

Answer :

Zeolites are alumino – silicates of the general formula, Mn[AlO₂]_x.(SiO₂)_y.mH₂O, where, M may be simple cation like Na, K or Ca, n is the charge on the simple cation, m is the number of molecules of water of crystallization.

They have a β cage, which looks like a honeycomb. The shape and size of the reactant and product molecules influence the processes catalyzed by zeolites. This is why these reactions are referred to as shape selective catalysis.

Question 4: Explain Auto-Catalysis.

Answer :

One of the products functions as a catalyst in some processes. The reaction is slow in the beginning, but after the products are formed, the rate of reaction accelerates. Auto-catalysis is the term for this type of event.

Some examples of homogeneous catalysis are listed below.

• When nitric acid is poured on copper, the reaction is slow at first, but it quickly accelerates due to the creation of nitrous acid, which works as an auto-catalyst during the process.