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Catalytic poisoning; intermediate compound theory

Short answer

Catalytic poisoning is deactivation by trace impurities that strongly adsorb on active sites, e.g., As₂O₃ poisoning Pt in the contact process. The intermediate compound formation theory: catalyst first forms an unstable intermediate with one reactant, which reacts with the second reactant, regenerating the catalyst, e.g., NO in lead chamber process.

What is meant by catalytic poisoning? Explain intermediate compound formation theory of catalysis referring suitable example.

Catalytic poisoning

Catalytic poisoning: a small amount of some substance (the poison) greatly reduces or destroys the activity of a catalyst. Poisons do not take part in the main reaction.

  • As₂O₃ poisons platinum in the contact process (2SO₂ + O₂ → 2SO₃).
  • H₂S, CO and compounds of S or P poison iron in the Haber process.
  • CO poisons platinum in the reaction 2H₂ + O₂ → 2H₂O and in hydrogenation reactions; lead compounds poison the Pt catalyst of a car's catalytic converter.

How a poison acts: the poison is strongly adsorbed on the active sites of the catalyst surface (or forms a stable compound with the catalyst). The sites are then unavailable for the reactant molecules, so the surface area available for reaction decreases and the catalyst becomes inactive. Poisoning may be temporary (poison removed by heating/washing) or permanent. For this reason the reactants are purified before use in industry.

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.

✓ Answer checked by Claude Opus 5.5 (high effort)

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