Catalysis: Definition, Classification and Sample questions

A catalyst is a material that accelerates the rate of a reaction without being consumed in the process. This process of accelerating the rate of reaction is known as catalysis. 

Key Terms: Catalyst, promoters, Homogeneous catalysts, Heterogeneous catalytic reactions, Rate of reaction, Equilibrium

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Some Important Facts based on Catalysis

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• A catalyst may go through intermediate physical changes and even create transient chemical connections with the reactants, but at the end of the reaction, it returns to its former state. 
• A catalyst accelerates the process but does not change the equilibrium position. The presence of a catalyst lowers the activation energy and lowers the height of the barrier by giving an alternate channel for the reaction. However, the activation energy for both the forward and backward reactions is reduced to the same level. 
• As a result, the rate of forwarding and backward reactions increases at the same pace, and the equilibrium position remains unchanged. However, if the two rates are increased, the equilibrium is reached sooner.
• Even though two reactions are highly similar, a catalyst that can catalyse one process may not affect the other.
• The reactions are unaffected by the catalyst. The addition of a catalyst does not affect the energy of the reactants (Er) and products (Ep), resulting in the same (Ep- Er). 

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Promoters 

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Promoters are chemical substances that increase the catalyst's activity.

For example, molybdenum is employed as a promoter for iron, which works as a catalyst, in Haber's ammonia manufacturing process.

N₂(g) + 3H₂ (g) <=> 2 NH₃ (g)

Fe (catalyst), Mo (Promoter)

Catalytic poisons are chemicals that impair the activity of the catalyst.

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Catalyst Classification

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Catalysts can be of two different types:

1. Homogeneous catalysts
2. Heterogeneous catalysts 

Homogeneous catalysts

Homogeneous catalysts are those that are present in the same phase as the reactants and products.

Homogeneous catalytic reactions are those in which the catalysts, reactants, and products are all in the same phase.

For example:

1. In the presence of nitric oxide as a catalyst, SO₂ is oxidized to SO₃ in the lead chamber process:

2 SO₂ (g) + 2 O₂ (g)  2 SO₃ (g)

Sulfur dioxide and oxygen, as well as the catalyst nitric oxide, are all in the same phase.

2. In the gas phase, chlorine atoms catalyse the breakdown of ozone:

O₃ + O → 2O₂

3. In the presence of nitric oxide (NO) as a catalyst, carbon monoxide is oxidised by oxygen (O₂): 

2CO(g) + O₂ (g) → 2 CO₂ (g)

4. In the presence of dil. HCI or H₂SO₄, sucrose hydrolysis is catalysed:

C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ (aq) ) + C₆H₁₂O₆ (aq)

5. At 413 K, conc. H₂SO₄ is used to make diethyl ether from ethyl alcohol:

2 CH₃CH₂OH → CH₃CH₂OCH₂CH₃ + H₂O

Homogeneous Catalytic Reactions Mechanism

An intermediate is formed when the catalyst reacts with one of the reactants. The catalyst is renewed when the intermediate compound decomposes or mixes with the other reactant to make the product.

For example, combining SO₂ with O₂ to generate SO₃ is a time-consuming procedure. The process becomes faster in the presence of NO (catalyst). 

2 SO₂(g) + O₂(g) → 2 SO₃(g)

Nitric oxide reacts with one of the reactants to generate an intermediate molecule in this reaction (NO₂). The catalyst NO is replenished in the last stage after this intermediate (NO₂) readily interacts with SO₂ to make SO₃.

2NO+ O₂ → 2 NO₂ (fast)

NO₂ + SO₂ → SO₃ + No (fast)

2 SO₂ + O₂ → 2 SO₃ (fast)

Heterogeneous Catalysts

Heterogeneous catalysts exist when the catalyst is in a different phase than the reactants. Heterogeneous catalytic reactions are the name for these types of reactions. 

The catalyst in heterogeneous catalysis is usually solid, and the reactants are usually gases. Because the reaction begins at the surface of the solid catalyst, it is also known as surface catalysis. These catalysts have a huge contact surface area, ranging from 1 to 500 m² per gram. 

As a result, despite the large surface area, once the reactant gas molecules have covered the surface, the rate does not increase as the reactant concentrations rise.

For example:

• Haber's technique, which uses a finely divided iron catalyst, produces NH₃ from H₂ and N₂: 

3H₂(g) + N₂ (g) → 2 NH₃ (g)

Here, the reactants are in a gaseous form, while the catalyst is solid.

• Using platinized asbestos or V₂O₅ as a catalyst, manufacture SO₃ from SO₂ in the Contact process: 

2 SO₂+ O₂ → 2 SO₃

• Methanol is made from CO and H₂ using a catalyst comprising copper, zinc oxide, and Cr₂O₃ and a promoter of Cr₂O₃:

CO (g) + 2 H₂ (g) → CH₃OH (I)

• In Ostwald's procedure, ammonia is converted to nitric oxide in the presence of platinum gauze: 

4NH₃ (g) + 5 O₂ → 4 NO (g) + 6 H₂O (g)

• Ethanol dehydrogenation with a nickel catalyst:

CH₃CH₂OH → CH₃CHO + H₂

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