# Catalysis by transition metals

> Chemistry · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9701-u21-catalysis-by-transition-metals/

This sub-topic covers why transition metals are effective catalysts, distinguishes between homogeneous and heterogeneous catalysis, and explores key industrial examples regularly assessed in CIE A-Level Chemistry exams.

**Prerequisites:** [Properties of transition elements](https://www.owlsprep.com/study/cie-9701-u21-properties-of-transition-elements/); [Reaction kinetics and activation energy](https://www.owlsprep.com/study/cie-9701-u05-reaction-kinetics/)

## Learning objectives

- Explain why transition metals are effective catalysts
- Distinguish between homogeneous and heterogeneous catalysis by transition metals
- Describe key examples of transition metal catalysis
- Explain catalytic behaviour in terms of oxidation states and adsorption

## Why Transition Metals Are Effective Catalysts

**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.

**Worked example:** 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.

## Heterogeneous Catalysis

**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:

1. Adsorption of reactant molecules onto active sites on the catalyst surface
2. Weakening of covalent bonds within reactant molecules, lowering activation energy
3. Reaction of adsorbed reactants to form product molecules
4. Desorption of product molecules, freeing active sites for further reaction

**Worked example:** 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.

## Homogeneous Catalysis

**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.

**Worked example:** Explain how Fe²⁺ ions catalyse the reaction between peroxodisulfate(VI) ($S_2O_8^{2-}$) and iodide ($I^-$) 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, $S_2O_8^{2-}$ is reduced by Fe²⁺, oxidising Fe²⁺ to Fe³⁺:
3. $$S_2O_8^{2-} + 2Fe^{2+} \rightarrow 2SO_4^{2-} + 2Fe^{3+}$$
4. Next, the newly formed Fe³⁺ oxidises I⁻ to iodine, regenerating the original Fe²⁺ catalyst:
5. $$2Fe^{3+} + 2I^- \rightarrow 2Fe^{2+} + I_2$$
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.

## Key Industrial Examples

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 ($V_2O_5$) | Heterogeneous | Sulfuric acid |
| Hydrogenation of alkenes | Nickel | Heterogeneous | Saturated fats/margarine |
| Decomposition of hydrogen peroxide | Manganese(IV) oxide ($MnO_2$) | Heterogeneous | Oxygen |

> **tip**
>
> Learn these examples by heart: they are extremely common in both multiple choice and structured papers.

## Common pitfalls

- **Wrong:** Confusing adsorption with absorption
  - Why it fails: Adsorption is binding to the catalyst surface, while absorption is uptake into the bulk of the solid. Examiners penalise the wrong term.
  - Correct: Always use adsorption when describing reactant binding to a solid heterogeneous catalyst surface.
- **Wrong:** Stating catalysts lower the activation energy of the original reaction pathway
  - Why it fails: Catalysts do not change the original pathway, they provide an entirely separate alternative pathway.
  - Correct: Always state that catalysts provide an alternative reaction pathway with lower activation energy.
- **Wrong:** Forgetting to mention the catalyst is regenerated
  - Why it fails: A key defining property of a catalyst is that it is not consumed in the reaction. Missing this point loses marks.
  - Correct: Always include the point that the catalyst is regenerated and unchanged at the end of the reaction.
- **Wrong:** Classifying V₂O₅ in the Contact process as homogeneous catalysis
  - Why it fails: V₂O₅ is solid while reactants are gaseous, so it is heterogeneous.
  - Correct: Remember that almost all industrial transition metal catalysts are heterogeneous.

## 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) |

## 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.

- [Arenes](https://www.owlsprep.com/study/cie-9701-u22-overview/)
- [Structure of benzene](https://www.owlsprep.com/study/cie-9701-u22-structure-of-benzene/)
- [Reactions of arenes](https://www.owlsprep.com/study/cie-9701-u22-reactions-of-arenes/)

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