Mechanism of Reaction on Heterogeneous Catalysts

Introduction

Heterogeneous catalysis occurs when the catalyst is in a different phase than the reactants, typically solid catalysts with gaseous or liquid reactants. The reaction proceeds through several distinct steps on the catalyst surface.

Mechanism

Step-1. Diffusion

Reactant molecules diffuse from the bulk phase to the surface of the solid catalyst.

Step-2. Adsorption

Reactants are adsorbed onto the catalyst surface. This can be:

• Physisorption – weak van der Waals forces
• Chemisorption – strong chemical bonding with surface atoms

Step-3. Reaction on Surface

Adsorbed molecules undergo bond breaking and bond formation facilitated by the catalyst surface.

A (adsorbed) + B (adsorbed) → AB (adsorbed)

Step-4. Desorption

The product molecules detach from the catalyst surface and return to the bulk phase.

Step-5. Diffusion of Products

Products diffuse away from the catalyst surface into the bulk phase.

Conclusion

The efficiency of heterogeneous catalysis depends on the surface area, nature of adsorption, and the ability of the catalyst to lower activation energy. This stepwise mechanism explains how solid catalysts accelerate reactions without being consumed.

Example: The Haber process (N₂ + 3H₂ → 2NH₃) uses iron as a heterogeneous catalyst.