Heterogeneous Catalysis
Heterogeneous catalysis involves a catalyst in a different phase from the reactants — most commonly a solid in contact with gases or liquids. It is the most widely used form of catalysis industrially (ammonia synthesis, catalytic cracking, automotive converters).
General Mechanism: Adsorption and Surface Reaction
Reactants must first adsorb onto the catalyst surface before reacting. The cycle involves five steps:
1. Diffusion of reactants from the bulk to the surface. 2. Adsorption on active sites (physisorption or chemisorption). 3. Surface reaction between adsorbed species. 4. Desorption of products. 5. Diffusion of products back to the bulk.
The rate-determining step is often adsorption or the surface reaction itself.
Langmuir Isotherm
The Langmuir model describes equilibrium adsorption on a homogeneous surface with independent sites:
θ = KP / (1 + KP)
where θ is the surface coverage (fraction of occupied sites), K = k_ads/k_des, and P is the partial pressure (or [A] in solution).
- At low P: θ ≈ KP (first-order in reactant).
- At high P: θ → 1 (saturation — zero-order).
Surface reaction rate: v = k_r · θ_A · θ_B (Langmuir-Hinshelwood) or v = k_r · θ_A · P_B (Eley-Rideal).
| Mechanism | Description |
|---|---|
| Langmuir-Hinshelwood | Both reactants adsorb before reacting |
| Eley-Rideal | Adsorbed reactant reacts with a gas-phase species |
| Mars-van Krevelen | Reactant reacts with a lattice atom of the solid |
Supported Catalysts
To maximize specific surface area (m² g⁻¹), the active metal (Pt, Pd, Ni, Fe) is deposited as nanoparticles on a porous support (Al₂O₃, SiO₂, TiO₂, activated carbon). Typical specific surfaces reach 100–500 m² g⁻¹.
Key deactivation phenomena: - Sintering: nanoparticle agglomeration at high temperature (surface area loss). - Poisoning: irreversible adsorption of impurities (S, CO for certain metals).
Haber-Bosch Process
Ammonia synthesis is one of the most important reactions in chemical industry:
N₂ + 3 H₂ ⇌ 2 NH₃ ΔH° = −92 kJ·mol⁻¹
The catalyst is α-Iron (Fe) promoted with K₂O (electronic promoter) and Al₂O₃ (structural promoter). The rate-determining step is N₂ dissociation on the iron surface (breaking the N≡N triple bond, Ea ≈ 130 kJ·mol⁻¹ on surface vs 941 kJ·mol⁻¹ in the gas phase).
Industrial conditions: 400–500 °C, 150–300 bar. The temperature-pressure trade-off is dictated by kinetics (high T needed) and equilibrium (high P and low T favored by Le Chatelier).