Catalyst action; heterogeneous catalysis; adsorption theory

How a catalyst increases the speed of a reaction

A catalyst provides a new reaction path with a lower activation energy (Ea). More reacting molecules then have energy equal to or greater than the lowered threshold at the same temperature, so the number of effective collisions rises and the reaction becomes faster (rate constant k = A e^(−Ea/RT) increases). The catalyst is regenerated at the end.

Activation energy on the path R → P

  uncatalysed : ████████████  (high Ea)
  catalysed   : ████          (low Ea)

  Catalyst lowers the barrier; R and P levels unchanged.

The energies of reactants (R) and products (P) are unchanged, so ΔH is the same; only the barrier is lowered.

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)

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).