Inorganic — heterogeneous catalysis
Ostwald process: NH₃ → NO over Pt gauze
How the Ostwald 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: NH₃, O₂ (air), Pt/Rh gauze. Conditions: ≈ 900 °C, 4–10 atm, contact time ≈ 1 ms. Bonds formed: N=O; O–H (H₂O); bonds broken: N–H (×3); O=O. Stereochemistry: Not applicable (surface reaction). Rate and selectivity: Very fast and selective for NO; ΔH ≈ −905 kJ per 4 NH₃.
At a glance
Step 1: N–H bonds break on Pt
Ammonia adsorbs onto a platinum-rhodium gauze catalyst; N–H bonds break sequentially at the surface, releasing hydrogen atoms that combine with surface oxygen.
Step 2: N + O combine at the surface
Surface nitrogen and oxygen atoms combine directly at the metal surface to form NO, a step that would be far slower without the catalyst's ability to hold both atoms in close, activated proximity.
Step 3: NO → NO₂ → HNO₃
NO desorbs and is further oxidized downstream to NO₂, then absorbed in water to make nitric acid — this is the industrial route from ammonia to essentially all the world's nitric acid.
Abbreviations and terms
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
How it compares with related reactions
| Ostwald process | Catalytic hydrogenation of an alkene (Pd/C, H₂) | Contact process | |
|---|---|---|---|
| Reagents | NH₃, O₂ (air), Pt/Rh gauze | H₂ (1 atm), Pd/C | SO₂, O₂, V₂O₅ catalyst |
| Conditions | ≈ 900 °C, 4–10 atm, contact time ≈ 1 ms | 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 | Very fast and selective for NO; ΔH ≈ −905 kJ per 4 NH₃ | 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 Ostwald process?
NH₃, O₂ (air), Pt/Rh gauze. Conditions: ≈ 900 °C, 4–10 atm, contact time ≈ 1 ms.
Which bonds are formed and which are broken?
Formed: N=O; O–H (H₂O). Broken: N–H (×3); O=O.
What is the stereochemical outcome?
Not applicable (surface reaction)
What controls the rate or the selectivity?
Very fast and selective for NO; ΔH ≈ −905 kJ per 4 NH₃
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam