Inorganic — heterogeneous catalysis
Contact process: SO₂ → SO₃ over V₂O₅
How the Contact 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: SO₂, O₂, V₂O₅ catalyst. Conditions: 400–450 °C, 1–2 atm. Bonds formed: S=O (SO₃); bonds broken: V=O (reversible). Stereochemistry: Not applicable (surface reaction). Rate and selectivity: Mars–van Krevelen: re-oxidation of V(IV) is rate-limiting; ΔH = −99 kJ/mol.
At a glance
Step 1: O transfer from V₂O₅ to SO₂
SO₂ coordinates to a surface vanadium(V) oxide site; the catalyst's oxygen is transferred directly to sulfur, oxidizing V(V) down to V(IV) in the process.
Step 2: O₂ re-oxidises V(IV)
O₂ from the gas phase re-oxidizes the reduced vanadium center back to V(V), regenerating the active catalytic oxide and completing an internal redox cycle at the surface.
Step 3: SO₃ absorbed in H₂SO₄
SO₃ desorbs and is absorbed into sulfuric acid downstream — industrially, this catalytic cycle produces the majority of the world's sulfuric acid.
Abbreviations and terms
- H2SO4
- Sulfuric acid. Strong acid and dehydrating agent.
- RE
- Reductive elimination. Two groups on a metal couple and leave, lowering the oxidation state by two.
How it compares with related reactions
| Contact process | Catalytic hydrogenation of an alkene (Pd/C, H₂) | Haber process | |
|---|---|---|---|
| Reagents | SO₂, O₂, V₂O₅ catalyst | H₂ (1 atm), Pd/C | N₂, H₂, promoted Fe catalyst (K₂O, Al₂O₃) |
| Conditions | 400–450 °C, 1–2 atm | EtOH or EtOAc, 25 °C | 400–500 °C, 150–300 atm |
| Stereochemistry | Not applicable (surface reaction) | Syn addition from the face bound to Pd | Not applicable (surface reaction) |
| Rate / selectivity | Mars–van Krevelen: re-oxidation of V(IV) is rate-limiting; ΔH = −99 kJ/mol | Fast; limited by H₂ and substrate access to the surface | Rate-limiting: dissociative chemisorption of N₂; ΔH = −92 kJ/mol |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Contact process?
SO₂, O₂, V₂O₅ catalyst. Conditions: 400–450 °C, 1–2 atm.
Which bonds are formed and which are broken?
Formed: S=O (SO₃). Broken: V=O (reversible).
What is the stereochemical outcome?
Not applicable (surface reaction)
What controls the rate or the selectivity?
Mars–van Krevelen: re-oxidation of V(IV) is rate-limiting; ΔH = −99 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