Inorganic — industrial heterogeneous catalysis
Water-gas shift reaction
How the Water-gas shift reaction 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: CO, H₂O (steam); Fe₃O₄/Cr₂O₃ (HTS) or Cu/ZnO (LTS) catalyst. Conditions: HTS 350–450 °C, LTS 190–250 °C, 10–30 bar. Bonds formed: CO₂; H₂ (C=O, H–H); bonds broken: C≡O (one π); O–H of water. Stereochemistry: Not applicable (heterogeneous surface). Rate and selectivity: Mildly exothermic (ΔH = –41 kJ/mol); equilibrium favours products at low T.
At a glance
Step 1: CO adsorbs; water dissociates
CO adsorbs onto a metal oxide catalyst surface (commonly Fe₃O₄ or a Cu/ZnO catalyst) while water dissociatively adsorbs nearby, splitting into surface hydroxyl and hydrogen species.
Step 2: Surface OH attacks adsorbed CO
A surface hydroxyl attacks the adsorbed CO's electrophilic carbon — a surface-mediated nucleophilic addition, mechanistically related to any carbonyl addition covered throughout this library, just occurring between two species both bound to the same metal surface — forming a surface-bound carboxyl (M–COOH) intermediate.
Step 3: CO₂ leaves; H₂ desorbs
This surface carboxyl decomposes, releasing CO₂ and transferring hydrogen to another surface site, which combines with a second hydrogen to desorb as H₂ gas — this reaction, run at massive industrial scale, is the primary source of the hydrogen gas that feeds directly into the Haber process for ammonia synthesis.
Abbreviations and terms
- M
- Metal. A generic metal centre.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- CO
- Carbon monoxide (carbonyl ligand). σ-donor / strong π-acceptor ligand; also a C1 building block.
How it compares with related reactions
| Water-gas shift reaction | Catalytic hydrogenation of an alkene (Pd/C, H₂) | Contact process | |
|---|---|---|---|
| Reagents | CO, H₂O (steam); Fe₃O₄/Cr₂O₃ (HTS) or Cu/ZnO (LTS) catalyst | H₂ (1 atm), Pd/C | SO₂, O₂, V₂O₅ catalyst |
| Conditions | HTS 350–450 °C, LTS 190–250 °C, 10–30 bar | EtOH or EtOAc, 25 °C | 400–450 °C, 1–2 atm |
| Stereochemistry | Not applicable (heterogeneous surface) | Syn addition from the face bound to Pd | Not applicable (surface reaction) |
| Rate / selectivity | Mildly exothermic (ΔH = –41 kJ/mol); equilibrium favours products at low T | 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 Water-gas shift reaction?
CO, H₂O (steam); Fe₃O₄/Cr₂O₃ (HTS) or Cu/ZnO (LTS) catalyst. Conditions: HTS 350–450 °C, LTS 190–250 °C, 10–30 bar.
Which bonds are formed and which are broken?
Formed: CO₂; H₂ (C=O, H–H). Broken: C≡O (one π); O–H of water.
What is the stereochemical outcome?
Not applicable (heterogeneous surface)
What controls the rate or the selectivity?
Mildly exothermic (ΔH = –41 kJ/mol); equilibrium favours products at low T
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam