Catalysis

Catalysts speed reactions without being consumed. Learn homogeneous, heterogeneous and enzyme catalysis, how catalysts lower activation energy, adsorption–reaction–desorption steps, Langmuir–Hinshelwood kinetics, internal diffusion (Thiele modulus, effectiveness factor), selectivity and deactivation.

About 90 % of chemical processes use catalysts — from ammonia synthesis (iron) and petroleum cracking (zeolites) to car exhausts (platinum) and biotechnology (enzymes). A good catalyst makes reactions faster, more selective and possible at milder conditions.

Catalyst design combines chemistry (active sites), materials (supports, pores) and engineering (pellets, reactors, regeneration).

An analogy

A catalyst is like a matchmaker: it brings reactants together in the right orientation, helps them bond, then steps aside unchanged for the next pair.

Types of catalysis

Homogeneous: catalyst in the same phase (acids, organometallics). Heterogeneous: solid catalyst with gas/liquid reactants (most bulk chemicals). Biocatalysis: enzymes — highly selective, mild conditions.

Steps on a solid catalyst

External diffusion to the pellet → pore diffusion → adsorption → surface reaction → desorption → diffusion out. The slowest step controls the rate. Catalysts are often metals dispersed on high-area supports (alumina, silica, carbon, zeolites).

Pore diffusion

If reaction is fast compared with diffusion in pores, the pellet interior is starved (large Thiele modulus, low effectiveness factor). Smaller pellets or egg-shell catalysts help, at the cost of pressure drop.

Selectivity and deactivation

Catalysts steer reactions to desired products (shape-selective zeolites, chiral catalysts). They deactivate by poisoning (sulphur), coking, sintering and attrition; processes use guard beds, regeneration (coke burn-off, e.g. FCC regenerator) and periodic replacement.

Formulas

• Rate enhancement: k_cat/k_uncat = exp(ΔE_a / RT)
• Thiele modulus (slab): φ = L √(k / D_e)
• Effectiveness factor (slab): η = tanh(φ) / φ
• Langmuir–Hinshelwood (A + B): r = k K_A K_B p_A p_B / (1 + K_A p_A + K_B p_B)²

Worked example: Effect of lowering E_a

Given: Catalyst lowers E_a from 120 to 70 kJ/mol; T = 500 K

Find: Rate enhancement (same A).

• Ratio = exp(50 000/(8.314 × 500)) = exp(12.0) ≈ 1.7 × 10⁵

Answer: ≈ 170 000× faster

Worked example: Effectiveness factor

Given: Slab pellet φ = 2

Find: η.

• η = tanh 2/2 = 0.964/2 = 0.48

Answer: 48 % — half the pellet is under-used

Common mistakes

• Wrong: Thinking catalysts shift equilibrium. — Right: They speed both directions equally; equilibrium is unchanged.
• Wrong: Using large pellets for fast reactions. — Right: Check the Thiele modulus — diffusion may waste most of the catalyst.

Applications

• Fertilisers & fuels: Haber–Bosch ammonia, Fischer–Tropsch, methanol synthesis.
• Refining: FCC cracking, hydrotreating, reforming.
• Environmental: Automotive three-way catalysts, SCR for NOx.

Key takeaways

• Catalysts lower E_a; equilibrium unchanged.
• Steps: diffusion, adsorption, reaction, desorption.
• η = tanh φ/φ — check pore diffusion.
• Manage selectivity and deactivation.

Going further

• Sabatier principle: Best catalysts bind reactants neither too strongly nor too weakly (volcano plots).
• Single-atom catalysts: Maximum metal utilisation with unique selectivity.
• Operando spectroscopy: Observe catalysts under working conditions.