Catalysts

Full Notes

Types of catalyst have been outlined in more detail here and here.
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Catalysts increase the rate of a reaction by providing an alternative reaction pathway with a lower activation energy (E_a) barrier.

Transition metals are often used as catalysts because of their ability to form ions with different oxidation states and because of their (relatively) low reactivity.

There are two main types of catalyst, homogeneous and heterogeneous. Transition metals can be examples of both.

Heterogeneous Catalysis

Heterogeneous catalysts are in a different phase than the reactants.

The reaction occurs on the catalyst’s surface at active sites.

A cheaper solid is often coated in the catalyst and acts as a support medium to maximise surface area of the catalyst and reduce cost, such as in car catalytic converters.

Examples of Heterogeneous Catalysts:

Vanadium(V) oxide (V₂O₅) in the Contact Process

Reaction: SO₂ + ½O₂ → SO₃ (used to make H₂SO₄).

Catalytic Cycle:
V₂O₅ is reduced to V₂O₄: V₂O₅ + SO₂ → V₂O₄ + SO₃
V₂O₄ is re-oxidised by oxygen: V₂O₄ + ½O₂ → V₂O₅
Catalyst remains unchanged overall.

Iron (Fe) in the Haber Process

Reaction: N₂ + 3H₂ ⇌ 2NH₃
Iron provides a surface for nitrogen and hydrogen to react.

Catalyst Poisoning

Heterogeneous catalysts can be poisoned by impurities blocking active sites.

For Example: Sulfur poisons Fe in the Haber process and lead poisons platinum catalysts in catalytic converters. Poisoning increases costs because catalysts must be replaced.

Homogeneous Catalysis

Homogeneous catalysts are in the same phase as the reactants.

The reaction proceeds via an intermediate species and transition metals can be effective as homogeneous catalysts due to variable oxidation states.

Example of Heterogeneous Catalysis

Fe²⁺ catalysing the reaction between I⁻ and S₂O₈²⁻

Reaction:
S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂
This reaction is slow because both reactants are negatively charged.

Fe²⁺ speeds up the reaction by forming an intermediate:
Fe²⁺ is regenerated, so it remains a catalyst.

Autocatalysis (Self-Catalysis)

Autocatalysis occurs when a reaction produces its own catalyst.

Example Mn²⁺ catalysing the reaction between C₂O₄²⁻ and MnO₄⁻

Reaction:
2MnO₄⁻ + 16H⁺ + 5C₂O₄²⁻ → 2Mn²⁺ + 10CO₂ + 8H₂O

Without Mn²⁺, the reaction is slow.
As Mn²⁺ is produced, it catalyses the reaction by forming intermediates:

4Mn²⁺ + MnO₄⁻ + 8H⁺ → 5Mn³⁺ + 4H₂O
2Mn³⁺ + C₂O₄²⁻ → 2Mn²⁺ + 2CO₂
The reaction speeds up as more Mn²⁺ is produced.

Summary

• Transition metals act as effective catalysts in both heterogeneous and homogeneous systems.
• Heterogeneous catalysis occurs on active sites; catalysts may need supports and can be poisoned by impurities.
• Key heterogeneous examples: V₂O₅ (Contact Process) and Fe (Haber Process).
• Homogeneous catalysis proceeds via intermediates; Fe²⁺ catalyses S₂O₈²⁻/I⁻.
• Autocatalysis: Mn²⁺ formed in the MnO₄⁻/C₂O₄²⁻ reaction accelerates the process.