Heterogeneous Catalysis
A concise revision guide to transition metal catalysts: heterogeneous catalysis at a surface, adsorption and desorption, V₂O₅ in the Contact process and catalytic converters.
- 17.26-a
- 17.26-b
- 17.26-c
- 17.26-d
- 17.27-a
- 17.27-b
- 17.28
- 17.29i-a
- 17.29i-b
- 17.29i-c
- 17.29ii-a
- 17.29ii-b
What these spec points say
- 17.26-a know that transition metals can act as heterogeneous catalysts
- 17.26-b know that transition metals can act as homogeneous catalysts
- 17.26-c know that transition metal compounds can act as heterogeneous catalysts
- 17.26-d know that transition metal compounds can act as homogeneous catalysts
- 17.27-a know that a heterogeneous catalyst is in a different phase from the reactants
- 17.27-b know that reaction occurs at the surface of a heterogeneous catalyst
- 17.28 understand, in terms of oxidation number, how V2O5 acts as a catalyst in the contact process
- 17.29i-a understand how catalytic converters decrease carbon monoxide emissions by adsorption of CO onto the catalyst surface
- 17.29i-b understand how catalytic converters decrease nitrogen monoxide emissions by adsorption of NO onto the catalyst surface
- 17.29i-c understand that adsorption weakens bonds and enables chemical reaction on the catalyst surface
- 17.29ii-a understand how catalytic converters decrease emissions by desorption of CO2 product molecules
- 17.29ii-b understand how catalytic converters decrease emissions by desorption of N2 product molecules
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 Topic 11 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 numbers | the metal can gain electrons from one reactant and give electrons to another, then return to its original oxidation number |
| 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 number 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 number, 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.
- Adsorption: reactant molecules form weak bonds to atoms 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.
- 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.
Key idea: The bonds to the surface must be strong enough to hold and weaken the reactants, but weak enough to let the products leave.
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 number and then change back.
| Step | Equation | Oxidation number 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 number of sulfur rises from +4 in SO₂ to +6 in SO₃.
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 number 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.
Catalytic Converters
Petrol engines produce toxic carbon monoxide, CO, and nitrogen monoxide, NO. A catalytic converter in the exhaust system changes them into carbon dioxide and nitrogen.
2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g)
- The catalyst is a mixture of platinum, rhodium and palladium.
- It is spread as a very thin layer over a ceramic honeycomb. This gives a very large surface area from a small mass of expensive metal.
The three stages
- Adsorption: CO and NO molecules are adsorbed onto the surface of the catalyst.
- Adsorption weakens the bonds in CO and NO, and the reaction takes place on the surface.
- Desorption: the CO₂ and N₂ product molecules leave the surface, so more CO and NO can be adsorbed.
A catalytic converter: CO and NO are adsorbed onto platinum, rhodium and palladium on a ceramic honeycomb, react on the surface, and leave as CO₂ and N₂.
Exam wording: Name all three stages and the molecules in each: “CO and NO are adsorbed”, “bonds are weakened and the reaction occurs”, “CO₂ and N₂ are desorbed”.
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.”
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.
- Catalytic converter answers need the reactant molecules, CO and NO, and the product molecules, CO₂ and N₂, by name or formula.
- 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 numbers, 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 number from +5 to +4 and back to +5. The V₂O₅ is regenerated, so it is a catalyst.
How does a catalytic converter work?
CO and NO molecules are adsorbed onto a thin layer of platinum, rhodium and palladium on a ceramic honeycomb. Their bonds are weakened and they react: 2CO + 2NO → 2CO₂ + N₂. The CO₂ and N₂ molecules are then desorbed.
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.
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