Heterogeneous Catalysts

1.0Introduction

Catalysts that exist in a different physical state than the reactants or products involved in the reaction are heterogeneous catalysts. Solid heterogeneous catalysts are most commonly used to promote reactions between gaseous reactants.

For example, in the Haber process, ammonia (NH₃) is synthesized from nitrogen (N₂) and hydrogen (H₂) using an iron (Fe) catalyst:

N₂ (g) + 3H₂ (g) → 2NH₃ (g)

Here, the iron catalyst is solid, while the reactants and products are gases, making it a classic example of heterogeneous catalysis.

2.0Mechanism of Heterogeneous Catalysis

According to the surface adsorption theory, heterogeneous catalysis reactions occur on a solid catalyst's surface. The process involves several steps:

Reactant Diffusion to Catalyst Surface

Reactant molecules first move toward the catalyst's surface. Their diffusion rate depends on their concentration in the bulk phase and the thickness of the boundary layer surrounding the catalyst.

Adsorption: 

Reactant molecules are adsorbed onto the catalyst surface. This can occur through physical adsorption (physisorption) or chemical adsorption (chemisorption), where weak or strong forces bind the reactants.

• Physisorption: This occurs when reactant molecules are attracted to the catalyst surface via weak Van der Waals forces, such as dipole-dipole interactions or London dispersion forces. No chemical bonds are formed in physisorption, and the adsorbed molecules can either stay on the catalyst surface or desorb back into the gas phase.
• Chemisorption: In chemisorption, the adsorbate molecules are close enough to the catalyst surface that their electron clouds overlap, forming new chemical bonds between the reactants and the catalyst. This type of adsorption often involves the sharing of electrons and can occur through:
• Molecular adsorption: The adsorbate retains its molecular structure.
• Dissociative adsorption: The adsorbate's molecular structure is altered as bonds between its atoms break, forming new bonds with the catalyst

Surface Reaction

Once adsorbed, the reactants react with each other on the catalyst's surface. Chemisorption weakens the bonds within the reactant molecules, facilitating their transformation into products.

Desorption of Products

After the reaction, the products are desorbed from the catalyst surface and released into the environment. The product molecules weaken their bonds with the catalyst, breaking free.

Product Diffusion Away from Surface
Finally, the product molecules diffuse away from the catalyst, making the surface available for new reactant molecules.

3.0Examples of Heterogeneous Catalysis

• Haber Process:
  Nitrogen + Hydrogen → Ammonia (Catalyst: Iron oxide on alumina)
• Contact Process:

The Contact Process is the primary method for producing sulfuric acid in the high concentrations required for industrial applications. Initially, platinum was used as the catalyst. 

However, due to its tendency to react with arsenic impurities present in the sulfur feedstock, it has been replaced by vanadium(V) oxide (V₂O₅), which is more stable and cost-effective.

In this process, vanadium(V) oxide (V₂O₅), a solid catalyst, facilitates the reaction between the gaseous reactants sulfur dioxide (SO₂) and oxygen (O₂).

Simplified reaction:

2SO₂ (g) + O₂ (g) ⇌ 2SO₃(g)                                

Catalyst: V₂O₅ (solid)

Detailed mechanism:

1. Reduction of vanadium(V) oxide:
   2 V₂O₅ (s) + 2SO₂(g) ⇌ 2SO₃ (g) + 2 V₂O₄  (s)
2. Re-oxidation of vanadium(IV) oxide:
   2 V₂O₄ (s) + O₂ (g) ⇌ 2V₂O₅ (s)

Overall, reaction is:

2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)

• Ostwald Process:
  Ammonia + Oxygen → Nitric acid (Catalyst: Platinum-rhodium gauze)
• Steam Reforming:
  Methane + Water → Hydrogen (Catalyst: Nickel)
• Ethylene Oxide Production: Ethylene + Oxygen → Ethylene oxide  (Catalyst: Silver or alumina)

4.0Iron in the Haber Process

In the Haber process, iron acts as a catalyst to produce ammonia (NH₃) from nitrogen (N₂) and hydrogen (H₂). The process works as follows:

1. Diffusion: Nitrogen and hydrogen gases diffuse to the surface of the iron catalyst.
2. Adsorption: The reactant molecules are adsorbed onto the iron surface, where bonds are formed between the iron and the reactant molecules.
3. Bond Weakening: These bonds weaken the nitrogen-nitrogen and hydrogen-hydrogen bonds, facilitating their reaction.
4. Reaction: The nitrogen and hydrogen atoms react on the catalyst's surface to form ammonia (NH₃).
5. Desorption: Ammonia is desorbed from the catalyst surface and diffuses away.

The iron catalyst effectively brings the nitrogen and hydrogen molecules close together, increasing the chances of reaction and speeding up the overall process.

5.0Catalytic Converters

Heterogeneous catalysts are also crucial in catalytic converters used in car exhaust systems. These devices reduce harmful emissions by converting nitrogen oxides (NOₓ) and carbon monoxide (CO) into less harmful substances:

• Nitrogen oxides (NOₓ) are converted into nitrogen (N₂).
• Carbon monoxide (CO) is converted into carbon dioxide (CO₂).

Catalytic converters contain a honeycomb structure coated with metals such as platinum, palladium, or rhodium, which act as catalysts. The process involves:

1. Adsorption: Nitrogen oxides and CO are adsorbed onto the catalyst surface.
2. Bond Weakening: The catalyst weakens the covalent bonds in the nitrogen oxides and CO.
3. Reaction: Nitrogen atoms combine to form N₂, and carbon monoxide reacts with oxygen to form CO₂.
4. Desorption: The nitrogen and carbon dioxide molecules are desorbed from the catalyst surface and released.