Catalysis

Catalysis

Catalyst is a substance which can change the speed of a chemical reaction without being used up in that reaction and the phenomenon is known as catalysis. If a catalyst increases speed of a reaction, it is called a positive catalyst and the phenomenon is called positive catalysis. While if a catalyst decreases the speed of a reaction, it is called a negative catalyst and the phenomenon is called negative catalysis. For example, oxidation of SO₂ to SO₃ in presence of NO (lead chamber process) or in presence of V₂O₅ (contact process) are examples of positive catalysis, decomposition of H₂O₂ in presence of phosphoric acid and oxidation of chloroform in presence of alcohol are examples of negative catalysis.

A catalyst lowers the activation energy for the forward reaction as well as for the backward reaction. As a result, the reaction follows an alternate path and the rate of forward reaction as well as that of the backward reaction are accelerated to the same extent. Hence, equilibrium constant of the reaction remains unaffected. Similarly, it may be noted that the enthalpy change of the reaction also remains unaffected. In the presence of catalyst, the equilibrium is however attained quickly.

Types of Catalysis

Catalysis can be broadly classified into two types.

1. Homogeneous catalysis: If the catalyst is present in the same phase as the reactants, it is called a homogeneous catalyst and this type of catalysis is called homogenous catalysis. Two common examples of homogenous of catalysis are

(i) Oxidation of sulphur dioxide to sulphur trioxide in presence of nitric oxide as catalyst (in lead chamber process for manufacture of H₂SO₄).

$2S{O}_2\left(g\right)+O_2\left(g\right)\stackrel{NO\left(g\right)}{\to }2S{O}_3\left(g\right)$

Here, all substances are present in the gaseous phase. Similarly, oxidation of CO by O₂ takes place in presence of NO as catalyst.

$2CO\left(g\right)+O_2\left(g\right)\stackrel{NO\left(g\right)}{\to }2C{O}_2\left(g\right)$

(ii) Decomposition of ozone in presence of Cl atoms acting as catalyst.

$O_3+O\stackrel{Cl}{\to }2O_2$

2. Heterogeneous catalysis: If the catalyst is present in a different phase than that of the reactant, it is called a heterogeneous catalyst and this type of catalysis is called heterogeneous catalysis.

The catalyst in heterogeneous catalysis is generally solid and the reactants are mostly gases and sometimes liquids. In heterogeneous catalysis the reaction starts at the surface of the solid catalyst that is why it is known as surface catalysis.

Some examples of heterogeneous catalysis are:

(i) Manufacture of ammonia from N₂ and H₂ by Haber's process using iron as catalyst.

(ii) Synthesis of methyl alcohol (CH₃OH) from CO and H₂ using a mixture of Cu, ZnO and Cr₂O₃ as catalyst.

(iii) Manufacture of sulphuric acid by the oxidation of SO₂ to SO₃ using V₂O₅ as catalyst.

Illustration 1. Write down the heterogeneous catalyst involved in polymerization of ethylene.

Solution: $n\left(C{H}_2=C{H}_2\right)\to -C{H}_2-C{H}_2-$

Heterogenous catalyst: Ziegler – Nata catalyst or (R₃Al + TiCl₄)]

Nature of solid catalysts

Solid catalysts may be metals, metal oxides, metal sulphides, clays etc. There materials may be used in their pure form or in the form of their mixture. Further they may be crystalline, microcrystalline (in the form of fine particles) or amorphous.

Important Features of Solid Catalysts

(i) Activity: Activity of the catalyst is its capacity to increase the speed of the chemical reaction. It may be increased upto 10¹⁰ times.

Combination of H₂ and O₂ in the presence of platinum (catalyst) to form water with explosive violence is an example of catalytic activity.

In the absence of the catalyst platinum, H₂ and O₂ do not combine and can be stored as such for an indefinite period.

$2H_2+O_2\stackrel{Pt}{\to }2H_{2}O$

The activity depends upon the extent of chemisorption. The adsorption should be reasonably strong but not so strong that the absorbed molecules become immobile and no space is available for other reactants to get adsorbed.

Illustration 2. Why it is advantage to use a catalyst for a reaction having endothermic nature?

Solution: For an endothermic reaction ($\Delta$H = +ve) and thus heat is required to get better yield which raised the cost appreciably. In presence of catalyst, the heat of reaction is lowered.

(ii) Selectivity: By selectivity of a catalyst we mean its ability to direct the reaction to form particular products excluding others. For example, CO and H₂ react to form different products in presence of different catalysts as follows,

$CO\left(g\right)+H_2\left(g\right)\stackrel{Cu}{\to }HCHO\left(g\right)$

$CO\left(g\right)+2H_2\left(g\right)\stackrel{Cu/ZnO-C{r}_2{O}_3}{\to }C{H}_{3}OH\left(g\right)$

$CO\left(g\right)+3H_2\left(g\right)\stackrel{Ni}{\to }C{H}_4\left(g\right)+H_{2}O\left(g\right)$

Action of a catalyst is highly selective in nature, i.e. a given substance can act as a catalyst only in a particular reaction and not for all the reactions.

(iii) Shape selective catalysis by zeolites: The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape selective catalysis. Zeolites are good shape selective catalysts because of their honeycomb-like structures. The reactions taking place in zeolites depend upon the size and shape of reactant and product molecule as well as upon the pores and cavities of the zeolites. That is why these types of reactions are called shape selective catalysis reactions. An important zeolite catalyst used in petroleum industry is ZSM -5. It converts alcohol directly into gasoline (petrol), by dehydration of alcohols, a mixture of hydrocarbons is formed.

Illustration 3. A little amount of HCl or H₂O increase the activity of AlCl₃ in the isomerisation of paraffins; why?

Solution: HCl or H₂O acts as promoters.

Enzyme catalysis: All biological reactions are catalysed by special catalysts called enzymes. Enzymes are proteins with high molar mass. They increase rates by 10⁸ to 10²⁰ times. Enzymes are also extremely specific. Each reaction is generally catalysed by a particular enzyme. Urease for example, catalyses only the hydrolysis of urea and none of the several thousand other enzymes present in the cell catalyses that reaction.

$N{H}_{2}CON{H}_2+H_{2}O\stackrel{Urease}{\to }2N{H}_3+2C{O}_2$

Illustration 4. The rate of fruit fermentation increases with time.

Solution: In fermented fruit an increase in concentration of enzymes with time speeds up enzyme catalysed reactions.

Mechanism of Enzyme Action

The two most accepted mechanisms are given below

(i) Lock and key model

The specificity of the enzymes is due to the presence of some specific regions, called the active sites which are associated with some functional groups, which form weak bond such as H – bonds, van der Waal's attraction, with the substrate (reactant) molecules. The shape of the active site of any given enzyme is such that only a specific substrate can fit into it, in the same way as one key can open a particular lock. Once the proper orientation has been achieved, substrate molecules react to form the products in two steps as shown in the figure.

(ii) Induced fit model:

According to this model, the enzyme can change its shape when the substrate comes in contact with the active site so that there is a perfect fit rather than rigidly shaped lock and key.

Kinetics of enzyme catalysis

Enzyme catalysed reactions take place in two steps as follows:

Step I:

Formation of enzyme – substrate complex.

Step II:

Dissociation of enzyme – substrate complex to form the products.

Illustration 5. The colour of KMnO₄ discharge slowly in the beginning during its reaction with oxalic acid but fastens after some time. Why?

Solution: Mn²⁺ ions formed during the reaction acts as auto catalyst.