Catalysis by transition metals
Chemistry· 21.3 Catalysis by transition elements· 20 min read
1. Why Transition Metals Are Effective Catalysts★★☆☆☆⏱ 5 min
Catalyst
A substance that increases the rate of a chemical reaction without being consumed, by providing an alternative reaction pathway with lower activation energy.
Example:
Solid iron in the Haber process for ammonia synthesis
Transition metals have two key properties that make them ideal catalysts. These properties are exploited in both laboratory and industrial reactions:
Variable oxidation states allow transition metals to participate in electron transfer as intermediates, regenerating the original oxidation state at the end of the reaction.
Solid transition metals have surface active sites with available d-orbital electrons that form weak bonds with reactant molecules during adsorption, weakening existing bonds to lower activation energy.
Explain why nickel (a transition metal) can act as a catalyst, but calcium (an s-block metal) cannot.
- 1
First, compare electronic structure: Nickel is a transition metal with partially filled d-orbitals, while calcium has no partially filled d-orbitals.
- 2
Next, compare oxidation states: Nickel has multiple stable variable oxidation states (0, +2, +3) that allow it to form intermediate reaction compounds.
- 3
Calcium only has one stable oxidation state (+2), so it cannot participate in the stepwise redox reactions required for catalysis.
- 4
Finally, nickel can adsorb reactants onto its surface via weak d-bond interactions, which calcium cannot do. This makes nickel an effective catalyst, while calcium is not.
Exam tip:
Always link catalytic ability to variable oxidation states for homogeneous catalysis, or adsorption/surface area for heterogeneous catalysis to gain full marks.
2. Heterogeneous Catalysis★★☆☆☆⏱ 6 min
Heterogeneous Catalysis
Catalysis where the catalyst is in a different physical state from the reactants.
Example:
Solid vanadium(V) oxide catalyst for gaseous reactants in the Contact process
Most industrial transition metal catalysis is heterogeneous, with a solid catalyst and gaseous or liquid reactants. The mechanism follows four core steps:
Adsorption of reactant molecules onto active sites on the catalyst surface
Weakening of covalent bonds within reactant molecules, lowering activation energy
Reaction of adsorbed reactants to form product molecules
Desorption of product molecules, freeing active sites for further reaction
Describe the role of solid iron catalyst in the Haber process for ammonia synthesis.
- 1
Iron is solid, while nitrogen and hydrogen reactants are gaseous, so this is heterogeneous catalysis.
- 2
N₂ and H₂ molecules adsorb onto active sites on the iron surface. The strong triple bond in N₂ is significantly weakened when it binds to iron.
- 3
Weakened nitrogen and hydrogen atoms react on the catalyst surface to form ammonia (NH₃) molecules.
- 4
NH₃ molecules desorb from the iron surface, leaving active sites free for new reactant molecules. Iron remains unchanged at the end of the reaction.
3. Homogeneous Catalysis★★★☆☆⏱ 6 min
Homogeneous Catalysis
Catalysis where the catalyst is in the same physical state as all reactants.
Example:
Aqueous Fe²⁺ ions catalysing the reaction between aqueous peroxodisulfate and iodide ions
Homogeneous catalysis by transition metals relies entirely on their ability to change oxidation state, acting as an electron transfer intermediate between two reactants. This creates a lower activation energy pathway than the direct reaction between reactants.
Explain how Fe²⁺ ions catalyse the reaction between peroxodisulfate(VI) () and iodide () ions in aqueous solution.
- 1
All species (catalyst and both reactants) are aqueous, so this is homogeneous catalysis. The direct reaction has high activation energy because both reactant ions are negatively charged and repel each other.
- 2
First, is reduced by Fe²⁺, oxidising Fe²⁺ to Fe³⁺:
- 3
- 4
Next, the newly formed Fe³⁺ oxidises I⁻ to iodine, regenerating the original Fe²⁺ catalyst:
- 5
- 6
The overall reaction is unchanged, and Fe²⁺ is fully regenerated. Each step involves reaction between oppositely charged ions, so activation energy is much lower than the direct reaction.
Exam tip:
Always write both half-equations for the catalytic cycle and state that the catalyst is regenerated to gain full marks.
4. Key Industrial Examples★★☆☆☆⏱ 3 min
CIE exams regularly ask 1-2 mark questions to recall the transition metal catalyst for common industrial processes:
Industrial Process | Transition Metal Catalyst | Catalysis Type | Main Product |
|---|---|---|---|
Haber process | Iron | Heterogeneous | Ammonia |
Contact process | Vanadium(V) oxide () | Heterogeneous | Sulfuric acid |
Hydrogenation of alkenes | Nickel | Heterogeneous | Saturated fats/margarine |
Decomposition of hydrogen peroxide | Manganese(IV) oxide () | Heterogeneous | Oxygen |
5. Common Pitfalls
Wrong move:
Confusing adsorption with absorption
Why:
Adsorption is binding to the catalyst surface, while absorption is uptake into the bulk of the solid. Examiners penalise the wrong term.
Correct move:
Always use adsorption when describing reactant binding to a solid heterogeneous catalyst surface.
Wrong move:
Stating catalysts lower the activation energy of the original reaction pathway
Why:
Catalysts do not change the original pathway, they provide an entirely separate alternative pathway.
Correct move:
Always state that catalysts provide an alternative reaction pathway with lower activation energy.
Wrong move:
Forgetting to mention the catalyst is regenerated
Why:
A key defining property of a catalyst is that it is not consumed in the reaction. Missing this point loses marks.
Correct move:
Always include the point that the catalyst is regenerated and unchanged at the end of the reaction.
Wrong move:
Classifying V₂O₅ in the Contact process as homogeneous catalysis
Why:
V₂O₅ is solid while reactants are gaseous, so it is heterogeneous.
Correct move:
Remember that almost all industrial transition metal catalysts are heterogeneous.
6. Quick Reference Cheatsheet
Feature | Heterogeneous Catalysis | Homogeneous Catalysis |
|---|---|---|
Catalyst state | Different to reactants | Same as reactants |
Key property exploited | Adsorption on surface, d-orbitals | Variable oxidation states |
Core mechanism | Adsorption → reaction → desorption | Redox electron transfer cycle |
Common example | Iron (Haber process), V₂O₅ (Contact) | Fe²⁺ (peroxodisulfate-iodide reaction) |
7. Frequently Asked
Why do transition metals make better catalysts than other metals?
Transition metals have two unique properties: variable oxidation states that enable redox intermediates, and partially filled d-orbitals that allow weak bonding to reactants during adsorption. Most non-transition metals only have one stable oxidation state and cannot form these intermediates or adsorb reactants effectively.
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 · 22
Explain Haber process iron catalysis
- 2021 · 12
Compare homogeneous/heterogeneous catalysis
- 2023 · 21
Role of V₂O₅ in Contact process
Going deeper
What's Next
Catalysis by transition metals is a core topic that links to multiple other areas of CIE A-Level Chemistry, from reaction kinetics to organic chemistry and industrial chemistry, a common theme in paper 4. Understanding how catalysis works also helps explain the role of transition metals in biological systems, such as enzyme cofactors that contain transition metal ions. This sub-topic builds on your existing knowledge of transition element properties and variable oxidation states, and connects to the study of large-scale industrial chemical processes regularly assessed in extended response questions. Mastering this topic will also help you answer general kinetics questions about activation energy and reaction mechanisms.