Homogeneous/heterogeneous catalysis; intermediate compound theory

Homogeneous catalysis

Homogeneous catalysis: the catalyst and the reactants are in the same phase (all gases or all in one solution).

• 2SO₂(g) + O₂(g) → 2SO₃(g), catalyst NO(g) (lead chamber process; all gaseous)
• CH₃COOCH₃(l) + H₂O(l) → CH₃COOH + CH₃OH, catalyst H⁺ (acid hydrolysis of ester, all in solution)
• C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆, catalyst H⁺ (inversion of cane sugar)

Heterogeneous catalysis

Heterogeneous catalysis: the catalyst is in a different phase from the reactants; usually a solid catalyst with gaseous or liquid reactants.

• N₂(g) + 3H₂(g) → 2NH₃(g), catalyst Fe(s) (Haber's process)
• 2SO₂(g) + O₂(g) → 2SO₃(g), catalyst V₂O₅(s) or Pt (contact process)
• Vegetable oil + H₂ → vegetable ghee, catalyst Ni(s) (hydrogenation)
• 4NH₃ + 5O₂ → 4NO + 6H₂O, catalyst Pt gauze (Ostwald's process)

Intermediate compound formation theory

According to the intermediate compound formation theory, the catalyst takes part in the reaction by forming an unstable intermediate compound with one of the reactants. The intermediate has a lower activation energy path and reacts with the other reactant to give the product and regenerate the catalyst.

Uncatalysed:  A + B  ───────────→  AB        (high Ea)
Catalysed  :  A + C  →  [AC]       (fast, low Ea)
              [AC] + B → AB + C    (fast, low Ea)
              (C = catalyst, regenerated)

Examples

1. Lead chamber process (NO as catalyst): 2NO + O₂ → 2NO₂ (intermediate) NO₂ + SO₂ → SO₃ + NO (catalyst regenerated)
2. Preparation of diethyl ether from ethanol with conc. H₂SO₄: C₂H₅OH + H₂SO₄ → C₂H₅HSO₄ (ethyl hydrogen sulphate) + H₂O C₂H₅HSO₄ + C₂H₅OH → C₂H₅OC₂H₅ + H₂SO₄
3. Contact process with V₂O₅: V₂O₅ + SO₂ → V₂O₄ + SO₃, then V₂O₄ + ½O₂ → V₂O₅.

Limitation: it explains homogeneous catalysis well, but is not suitable for many heterogeneous cases where no stable intermediate is found.