Activity and Selectivity of Catalyst: Explanation

Activity and selectivity are the two basic properties of a catalyst.

• The activity of a catalyst indicates how fast a reaction occurs in the presence of a catalyst.
• Selectivity is the ability of a catalyst to direct a reaction to produce a particular product.

The rate of a chemical reaction can be increased or decreased by the addition of a suitable substance which when itself is not consumed during the reaction.

• Such a substance is called a catalyst and the phenomenon is known as catalysis.
• Catalysis is a surface phenomenon, and the mechanism involved in the activity of the catalyst is chemisorption.
• In this process, the reactant molecules are absorbed on the surface of the catalyst.
• Most industrial syntheses and biological reactions require catalysts.
• Enzymes are a common catalyst found in nature.

Key Terms: Catalyst, Catalysis, Enzymes, Chemical reaction, Reactant, Product, Activity of catalyst, Selectivity of catalyst, Homogeneous catalyst, Heterogeneous catalyst

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What is a Catalyst

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A catalyst is a substance that boosts the rate of a reaction without being unchanged in mass and composition.

Consider the following chemical reaction:

\(2KClO_3 \:\: \longrightarrow \:\: 2KCl + 3O_2\)

In the above chemical reaction potassium chlorate is decomposed into potassium chloride and oxygen.

• This is a slow reaction that takes place in temperatures between 653 K to 873 K.
• When we add manganese dioxide to this reaction, it accelerates and occurs at a temperature range of 473-633 K.
• Therefore manganese dioxide acts as a catalyst in the above reaction.

The two most important aspects of catalysis are the activity and the selectivity of the catalyst to be used in a particular chemical reaction. The proper knowledge of the activity and selectivity of catalysts helps us to find the most suitable catalyst for the preferred result of a chemical reaction. 

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Activity of Catalyst

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The ability of a catalyst to accelerate the reaction is referred to as the activity of a catalyst.

• It is determined by the adsorption of reactants on the surface of the catalyst.
• Chemisorption is the primary factor in the activity of catalysts.
• The bond established between the catalytic surface and the reactants during adsorption must not be too strong or too weak. 
• It must be strong enough to activate the catalyst while not being so strong that the reactant molecules become immobilized on the catalytic surface, leaving no space for new reactants to be adsorbed.
• Catalytic activity for the hydrogenation process generally rises from Group 5 to Group 11 metals.
• The group 7-9 elements of the periodic table have the highest catalytic activity.

\(2H_2(g)+O_2 \:\:\xrightarrow {Pt}\:\: 2H_2O(l)\)

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Selectivity of Catalyst

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Catalysts have the ability to direct the reaction to produce a certain product.

• The same reactants but different catalysts may result in different products.
• This is referred to as catalyst selectivity.
• Catalysts in nature are very selective.
• They have the ability to boost a particular reaction while inhibiting another.
• Therefore, we can say that a certain catalyst can only catalyze one specific reaction.
• It may not be able to catalyze another reaction of the same kind.

For example, the reaction of hydrogen and carbon monoxide

• produces methane when nickel is used as the catalyst.
• produces methanol when zinc oxide and chromium oxide are used as the catalyst
• produces methanal when only copper is used as the catalyst.

\(CO(g) + 3H_2(g)\:\: \xrightarrow{Ni}\:\: CH_4(g) + H_2O(g)\)

\(CO(g) + 3H_2(g)\:\: \xrightarrow{Cu/Zno-Cr_2O_3}\:\: CH_3OH(g)\)

\(CO(g) + 3H_2(g)\:\: \xrightarrow{Cu}\:\: HCHO(g)\)

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Types of Catalysts

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Catalysts are classified into the following two categories

• Based on activity
• Based on the states of the reactant molecules and the catalyst molecules

Classification based on activity

Catalysts are classified into two broad categories depending on their activity (whether they accelerate or inhibit a particular reaction).

• Positive Catalyst
• Negative Catalyst 

Positive catalyst

When a catalyst increases the rate of a reaction, it is termed a positive catalyst and the process is known as positive catalysis. Some examples of positive catalysis are as follows. 

• Oxidation of ammonia is accelerated in the presence of platinum gauze. Here, Pt acts as a positive catalyst.

2N₂ + 3H₂ (Pt) → 2NH₃

• Oxidation of sulphur dioxide into sulphur trioxide is enhanced in the presence of vanadium pentoxide.

2SO₂ + O₂ (V₂0₅) → 2SO₃

• The decomposition of KClO₃ is facilitated in the presence of MnO₂.

KClO₃ (MnO₂) → 2KCI +3O₂

Negative catalysis

When a catalyst decreases the rate of a reaction, it is termed a negative catalyst and the process is called negative catalysis. Some examples of negative catalysis are as follows.

• The oxidation of chloroform by air is retarded in the presence of ethyl alcohol. Here, ethyl alcohol acts as a negative catalyst. 

CHCl₃ + 1/2O₂ → COCl₂ + HCl (retarded in the presence of C₂H₅OH)

• The decomposition of hydrogen peroxide gets retarded when some glycerine is added to it.

2H₂O₂ → 2H₂O + O₂ (retarded in the presence of glycerine)

Classification based on the states of the reactant molecules and the catalyst molecules: 

Depending upon the physical states of reactants and the catalyst, catalysis can broadly be classified into the following two categories

• Homogeneous Catalyst
• Heterogeneous Catalyst

Homogeneous Catalyst

When the catalyst mixes homogeneously with the reactant(s) to form a single phase, it is said to be a homogeneous catalyst and this type of catalysis is known as homogeneous catalysis.

In homogeneous catalysis, the catalyst is present in the same phase as the reactants. Therefore, they mix up thoroughly and form a single phase. Some common examples of homogeneous catalysis are as follows.

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

2SO₂(g) + O₂(g) [NO(g)] → 2SO₃ (g)

• Hydrolysis of ethyl acetate in the presence of dilute sulphuric acid as a catalyst

CH₃COOC₂H ₅ (l) + H₂O (l) → (H₂SO₄) CH₃COOH(l) + C₂H₅OH(l)

Heterogeneous Catalyst

When the catalyst does not mix up with the reactants and forms a separate phase, it is said to be known as a Heterogeneous catalyst and the process is called Heterogeneous catalysis.

• In Heterogeneous catalysis, the catalyst is usually in the solid state while the reactants are usually in the gaseous state.
• This is why the reactants do not mix up with catalysts and both are in separate phases.
• The reactants get temporarily attached to the surface of the catalyst to initiate the reaction.
• Heterogeneous catalysis is also known as surface catalysis.
• Heterogeneous catalysis is of great importance in Industry.
• A large number of technologically important chemical reactions make use of solid catalysts.

Some important examples of heterogeneous catalytic reactions are as follows.

• Manufacture of ammonia from N₂ and H₂ by Haber’s process using iron as a catalyst:

N₂(g) + 3H₂(g) → 2NH ₃ (g) 

• Oxidation of sulphur dioxide to sulphur trioxide using platinized asbestos or V₂0₅ as a catalyst:

2SO₂(g) + O₂(g) → SO ₃ (g)

This reaction forms the basis of the manufacture of sulphuric acid by the contact process.

• Synthesis of methyl alcohol from carbon monoxide and hydrogen using a mixture of ZnO and CuO as a catalyst:

[ZnO(s) + CuO(s) ]

CO (g) + 2H₂ → CH₃OH 

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Things to Remember

• A catalyst is a substance that boosts the rate of a reaction without being unchanged in mass and composition.
• When a catalyst increases the rate of a reaction, it is termed a positive catalyst.
• When a catalyst decreases the rate of a reaction, it is termed a negative catalyst.
• A homogeneous catalyst combines homogeneously with the reactant(s) to form a single phase.
• Heterogeneous catalysts occur when the catalyst does not mix with the reactants and creates a separate phase.
• In heterogeneous catalysis, the reactant molecules and the catalyst do not mix well together as these are in different physical states.