Mechanism of Heterogeneous Catalysis (5 Steps):
- Diffusion of reactants to the catalyst surface (from bulk gas/liquid to surface)
- Adsorption of reactants at active sites (chemisorption — forms surface intermediates)
- This step activates the reactant by weakening bonds
- Surface reaction — the chemisorbed reactant species interact (bond breaking + bond formation) to form product species
- Desorption of products from the surface (products leave active sites, freeing them)
- Diffusion of products away from the surface (to bulk gas/liquid)
Why Does the Catalyst Lower Ea? The catalyst provides active sites where: (a) reactant molecules are adsorbed in close proximity and correct orientation, (b) bonds in adsorbed molecules are weakened (partial activation), and (c) the transition state has lower energy than for the uncatalyzed gas-phase reaction.
Selectivity in Catalysis: Different catalysts provide different active sites with different electronic and geometric properties, stabilizing different transition states, leading to different products from the same reactants:
| Catalyst | Reaction | Product |
|---|---|---|
| ZnO- | CO + | (methanol) |
| Cu | CO + | HCHO (formaldehyde) |
| Ni | CO + 3 | + O (methanation) |
| Fe (Haber) | + 3 | (ammonia) |
| (Contact) | + |
Promoters vs. Poisons:
- Promoters: Increase catalyst activity/selectivity without being catalysts themselves (e.g., Mo in Haber process promotes Fe)
- Poisons: Block active sites by strong chemisorption, reducing activity (e.g., Pb poisons Pt in catalytic converters; CO, S poison Fe in Haber process)