Catalytic poisoning; adsorption theory

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.

Adsorption theory of catalysis

According to the adsorption theory (for heterogeneous catalysis), the catalyst works by adsorbing the reactant molecules on its surface. The steps are:

1. Diffusion and adsorption: reactant molecules diffuse to the catalyst surface and are adsorbed on the active sites, so their concentration at the surface becomes high.
2. Activation: adsorption weakens the bonds of the reactants (e.g. H–H breaks), forming activated, reactive species. Adsorption is exothermic, and the heat released helps to supply activation energy.
3. Surface reaction: the activated adsorbed molecules, now close together, react to form the product.
4. Desorption: the product molecules have little affinity for the surface; they leave the surface, which frees the active sites.
5. The freed sites adsorb new reactant molecules and the cycle repeats.

 H₂    C₂H₄          H–H  C=C         H H
  ↓     ↓             ↓    ↓           | |
 ▒▒▒▒▒▒▒▒▒▒▒  →  ▒▒▒▒▒▒▒▒▒▒▒  →   H–C–C–H   (leaves)
 Ni surface      adsorbed, H–H      | |
 (adsorption)    bond broken        H H

Example: hydrogenation of ethene on nickel: C₂H₄ + H₂ → C₂H₆ (Ni). H₂ is adsorbed on Ni and splits into H atoms, ethene is also adsorbed, the H atoms add to the double bond, and ethane is desorbed.

The theory explains why a finely divided catalyst is more active (larger surface), why poisons stop catalysis (they occupy the active sites) and why promoters help (they increase the active surface).