Inorganic — heterogeneous catalysis
Haber process: N₂ + H₂ → NH₃ on an iron surface
How the Haber process works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: N₂, H₂, promoted Fe catalyst (K₂O, Al₂O₃). Conditions: 400–500 °C, 150–300 atm. Bonds formed: N–H (×3 per NH₃); bonds broken: N≡N; H–H. Stereochemistry: Not applicable (surface reaction). Rate and selectivity: Rate-limiting: dissociative chemisorption of N₂; ΔH = −92 kJ/mol.
At a glance
Step 1: N₂ dissociates on Fe
N₂'s famously strong triple bond dissociates on the iron catalyst's surface — dissociative chemisorption — because the metal surface can donate electron density into N₂'s antibonding orbitals far more effectively than any single molecule could.
Step 2: H₂ dissociates; stepwise hydrogenation
H₂ dissociates similarly on the surface, and the resulting surface-bound N and H atoms combine stepwise: N + H → NH, NH + H → NH₂, NH₂ + H → NH₃, each step a surface-mediated insertion.
Step 3: NH₃ desorbs
Ammonia desorbs from the catalyst surface, freeing the active site to repeat the cycle — this single heterogeneous catalytic process fixes the nitrogen that feeds roughly half the world's population.
Abbreviations and terms
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- π*
- Pi antibonding orbital. The empty orbital of a C=O or C=C that accepts nucleophiles.
How it compares with related reactions
| Haber process | Catalytic hydrogenation of an alkene (Pd/C, H₂) | Contact process | |
|---|---|---|---|
| Reagents | N₂, H₂, promoted Fe catalyst (K₂O, Al₂O₃) | H₂ (1 atm), Pd/C | SO₂, O₂, V₂O₅ catalyst |
| Conditions | 400–500 °C, 150–300 atm | EtOH or EtOAc, 25 °C | 400–450 °C, 1–2 atm |
| Stereochemistry | Not applicable (surface reaction) | Syn addition from the face bound to Pd | Not applicable (surface reaction) |
| Rate / selectivity | Rate-limiting: dissociative chemisorption of N₂; ΔH = −92 kJ/mol | Fast; limited by H₂ and substrate access to the surface | Mars–van Krevelen: re-oxidation of V(IV) is rate-limiting; ΔH = −99 kJ/mol |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Haber process?
N₂, H₂, promoted Fe catalyst (K₂O, Al₂O₃). Conditions: 400–500 °C, 150–300 atm.
Which bonds are formed and which are broken?
Formed: N–H (×3 per NH₃). Broken: N≡N; H–H.
What is the stereochemical outcome?
Not applicable (surface reaction)
What controls the rate or the selectivity?
Rate-limiting: dissociative chemisorption of N₂; ΔH = −92 kJ/mol
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam