Heterogeneous catalysis; adsorption theory; criteria

Heterogeneous catalysis

Heterogeneous catalysis is a catalytic reaction in which the catalyst is in a different phase from the reactants. Example: N₂ + 3H₂ → 2NH₃ with solid Fe.

Adsorption theory

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).

Two criteria for an industrial catalyst

• High activity and selectivity: it must give a high yield of the desired product at a fast rate at moderate temperature and pressure.
• Stability and resistance to poisons: it must keep its activity for a long time, be mechanically strong, and be cheap and available.