Heterogeneous Catalysis
A concise revision guide to transition metal catalysts: heterogeneous catalysis at a surface, adsorption and desorption, V₂O₅ in the Contact process, iron in the Haber process, support media and catalyst poisoning.
- 3.2.5.6i-a
- 3.2.5.6i-c
- 3.2.5.6ii
- 3.2.5.6iii
- 3.2.5.6iv
- 3.2.5.6v
- 3.2.5.6vi
- 3.2.5.6viii
What these spec points say
- 3.2.5.6i-a know that transition metals can act as heterogeneous catalysts
- 3.2.5.6i-c know that transition metal compounds can act as heterogeneous catalysts
- 3.2.5.6ii know that a heterogeneous catalyst is in a different phase from the reactants and the reaction occurs at active sites on the surface
- 3.2.5.6iii know the use of a support medium to maximise the surface area of a heterogeneous catalyst and minimise cost
- 3.2.5.6iv know that V2O5 acts as a heterogeneous catalyst in the Contact process and explain with equations how it does so
- 3.2.5.6v know that Fe is used as a heterogeneous catalyst in the Haber process
- 3.2.5.6vi know that heterogeneous catalysts can become poisoned by impurities that block the active sites
- 3.2.5.6viii explain the importance of variable oxidation states in catalysis
Catalysis at a Surface
Choose a step to watch carbon monoxide and nitrogen monoxide being adsorbed onto a metal surface, reacting there and leaving as products.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
Before You Start
Catalysts were first met in 3.1.5 Kinetics. This page explains why the transition metals and their compounds are such good catalysts, and how a solid catalyst works.
- A catalyst increases the rate of a reaction and is chemically unchanged at the end.
- It provides an alternative reaction route with a lower activation energy.
- More particles then have at least the activation energy, so more collisions are successful.
- A catalyst does not change the position of equilibrium or the amount of product.
GCSE Recap: What a Catalyst Changes
Three quick questions on the GCSE ideas this page builds on.
Why Transition metals Are Good Catalysts
Transition metals and their compounds are used as catalysts throughout the chemical industry. Two features of their chemistry explain this.
| Feature | How it helps |
|---|---|
| Variable oxidation states | the metal can gain electrons from one reactant and give electrons to another, then return to its original oxidation state |
| Available d orbitals | atoms at the surface of the solid use their 3d and 4s electrons to form weak bonds to reactant molecules |
- The 3d and 4s sub-shells are close in energy, so changing oxidation state needs little energy. See the properties of the transition metals.
- The catalyst takes part in the reaction, but it is regenerated, so it is not used up.
Two types of catalyst
| Type | Meaning | Typical example |
|---|---|---|
| Heterogeneous | the catalyst is in a different phase from the reactants | a solid catalyst with gaseous reactants |
| Homogeneous | the catalyst is in the same phase as the reactants | aqueous ions catalysing a reaction between other aqueous ions |
Two features make these elements good catalysts: more than one stable oxidation state, and d orbitals that bond to reactants. A catalyst is heterogeneous or homogeneous.
Remember: A phase is not quite the same as a state. Two liquids that do not mix are two phases. For most exam examples, a solid catalyst with gases is heterogeneous.
Quick Check: Would It Work as a Catalyst?
Pick the one accurate statement in each set, using metals and mixtures that are new to this page.
How a Heterogeneous Catalyst Works
A heterogeneous catalyst is usually a solid, and the reactants are gases or in solution. The reaction takes place at the surface of the catalyst, at places called active sites.
- Adsorption: reactant molecules form weak bonds to atoms at active sites on the surface.
- Bonds inside the reactant molecules are weakened. The molecules are also held close together, in a favourable position.
- Reaction: new bonds form between the particles held on the surface.
- Desorption: the product molecules leave the surface, which is then free to adsorb more reactant.
The four stages of heterogeneous catalysis: adsorption of the reactants, weakening of their bonds, reaction on the surface and desorption of the products.
Why the surface matters
- Adsorption means bonding onto a surface. It is not absorption, which means soaking into the bulk.
- Weakened bonds break more easily, so the activation energy is lower.
- Adsorbed reactants are more concentrated at the surface than in the gas, so they meet more often.
- A larger surface area gives more places for the reaction, so the rate is higher. Catalysts are used as powders, pellets, gauzes or thin coatings.
- The catalyst is unchanged at the end, so a small amount converts a large amount of reactant.
Strength of adsorption
| Adsorption | Example | Effect |
|---|---|---|
| Too weak | silver | reactants are not held long enough, or in high enough concentration, to react |
| Too strong | tungsten | products cannot leave, so the active sites stay blocked |
| About right | nickel, platinum | reactants are held and weakened, and products are released |
Check: Catalysts and Surfaces
Explain why transition metals make good catalysts and put the stages of surface catalysis in order, in questions that do not repeat the examples on this page.
Vanadium(V) Oxide in the Contact Process
The Contact process makes sulfur trioxide for sulfuric acid manufacture. The catalyst is solid vanadium(V) oxide, V₂O₅, and the reactants are gases, so the catalysis is heterogeneous.
2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
The V₂O₅ works because vanadium can change its oxidation state and then change back.
| Step | Equation | Oxidation state of vanadium | What happens |
|---|---|---|---|
| 1 | V₂O₅ + SO₂ → V₂O₄ + SO₃ | +5 to +4 | V₂O₅ oxidises SO₂ to SO₃ and is itself reduced |
| 2 | V₂O₄ + ½O₂ → V₂O₅ | +4 to +5 | oxygen oxidises V₂O₄ back to V₂O₅ |
- Adding the two steps gives the overall equation: SO₂ + ½O₂ → SO₃.
- V₂O₄, vanadium(IV) oxide, is formed in step 1 and used up in step 2.
- V₂O₅ is used in step 1 and regenerated in step 2, so it is a catalyst.
- The oxidation state of sulfur rises from +4 in SO₂ to +6 in SO₃.
- Step 2 may also be written as 2V₂O₄ + O₂ → 2V₂O₅.
The catalytic cycle in the Contact process: V₂O₅ is reduced to V₂O₄ as it oxidises SO₂, and oxygen then oxidises the V₂O₄ back to V₂O₅.
Exam wording: “Vanadium changes oxidation state from +5 to +4 when V₂O₅ oxidises SO₂, and back to +5 when V₂O₄ reacts with oxygen. The V₂O₅ is regenerated.”
Quick Check: Follow a New Catalytic Cycle
Follow the electrons round a surface catalyst that is new to this page.
Iron in the Haber Process, Support Media and Poisoning
Solid iron is the heterogeneous catalyst in the Haber process. Nitrogen and hydrogen are adsorbed at active sites on the iron, where their bonds are weakened.
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
Support media
- Only the atoms at the surface of a catalyst are used. Atoms inside a lump are wasted.
- The catalyst is spread as a thin layer on an inert support medium, such as a ceramic honeycomb or mesh.
- This maximises the surface area and minimises the cost, because little of the expensive metal is needed.
- Example: rhodium and platinum on a ceramic honeycomb in a catalytic converter.
Catalyst poisoning
- Impurities in the reactants can be adsorbed strongly onto the surface.
- They block the active sites, so the reactants can no longer be adsorbed.
- The catalyst becomes less efficient: it is poisoned.
| Catalyst | Poison | Source of the poison |
|---|---|---|
| Iron in the Haber process | sulfur compounds | impurities in the natural gas used to make the hydrogen |
| Platinum and rhodium in catalytic converters | lead | leaded petrol |
Iron in the Haber process, a ceramic support medium that carries a thin layer of catalyst, and poisoning, in which impurities block the active sites.
Exam focus: Poisoning has a cost. The catalyst loses efficiency and may have to be replaced, so the reactants are purified before they reach it.
Common Exam Points
Say
- “A heterogeneous catalyst is in a different phase from the reactants, and the reaction occurs at its surface.”
- “Reactants are adsorbed, bonds are weakened, the reaction occurs and the products are desorbed.”
- “V₂O₅ is reduced to V₂O₄ by SO₂ and then oxidised back to V₂O₅ by oxygen.”
- “A poison is adsorbed strongly and blocks the active sites.”
Do not say
- “The reactants are absorbed” (the word is adsorbed: onto the surface, not into the solid).
- “The catalyst does not take part in the reaction” (it takes part and is regenerated).
- “The catalyst gives the particles more energy” (it lowers the activation energy).
- “The catalyst increases the yield” (the position of equilibrium does not change).
Watch for
- State symbols: they show whether the catalyst is in a different phase from the reactants.
- The two reasons for a support medium: maximum surface area and minimum cost.
- The half in ½O₂: it is needed to balance step 2 for one V₂O₄.
- Homogeneous catalysts work in a different way: see homogeneous catalysis and autocatalysis.
Check: Heterogeneous Catalysis
Bring the page together: phases, adsorption and desorption, surface area and the role of the catalyst, in contexts that are not used on this page.
FAQs
Use these quick answers to check why transition metals are good catalysts and how a heterogeneous catalyst works.
What is a heterogeneous catalyst?
It is a catalyst in a different phase from the reactants, usually a solid with gaseous reactants. The reaction takes place at the surface of the solid. A homogeneous catalyst is in the same phase as the reactants.
Why are transition metals good catalysts?
They have variable oxidation states, so they can gain electrons from one reactant and give electrons to another. Atoms at the surface of the solid also use their d orbitals to form weak bonds to reactant molecules.
What is the difference between adsorption and absorption?
Adsorption means bonding onto the surface of the catalyst. Absorption means soaking into the bulk of a material. In surface catalysis the reactants are adsorbed, their bonds are weakened, they react, and the products are then desorbed.
How does V₂O₅ act as a catalyst in the Contact process?
Step 1 is V₂O₅ + SO₂ → V₂O₄ + SO₃. Step 2 is V₂O₄ + ½O₂ → V₂O₅. Vanadium changes oxidation state from +5 to +4 and back to +5. The V₂O₅ is regenerated, so it is a catalyst.
What is catalyst poisoning?
Impurities are adsorbed strongly onto the surface and block the active sites, so the reactants cannot be adsorbed. Sulfur compounds poison the iron in the Haber process, and lead poisons catalytic converters. A poisoned catalyst is less efficient and costly to replace.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
Keep this note — free
Save your progress across every AQA topic. A free account remembers which topics you have covered, saves your question scores, and syncs across your phone and laptop.
- Track every topic you have finished
- Keep your practice-question scores
- No payment, no card, free forever
Already registered? Log in