Transition Metal Catalysis

Transition metals and their compounds are exceptionally effective catalysts. Their ability to switch between oxidation states and to form complexes with reactant molecules allows them to provide alternative reaction pathways with lower activation energies. Catalysis by transition metals is one of the most industrially important topics in chemistry.

Why Transition Metals Make Good Catalysts

Two key properties of transition metals enable their catalytic behaviour:

1. Variable Oxidation States

Transition metal ions can gain or lose d electrons relatively easily, switching between oxidation states. This allows them to act as both electron donors and electron acceptors during a reaction, facilitating electron transfer between reactants.

A catalyst works by providing an alternative reaction pathway with a lower activation energy. Transition metals can participate in intermediate steps involving changes in oxidation state, enabling reactions that would otherwise be too slow.

2. Ability to Form Complexes

Transition metals can adsorb reactant molecules onto their surface (heterogeneous catalysis) or form intermediate complexes with reactants in solution (homogeneous catalysis). This brings reactant molecules into close proximity, with the correct orientation for reaction, weakening bonds within the reactant molecules and lowering the activation energy.

Heterogeneous Catalysis

A heterogeneous catalyst is in a different phase from the reactants — most commonly a solid catalyst with gaseous or liquid reactants. The reaction occurs at the surface of the catalyst.

The mechanism involves four steps:

Adsorption: Reactant molecules bind to the catalyst surface (bonds form between reactant molecules and metal atoms on the surface)
Weakening: Bonds within the adsorbed reactant molecules are weakened
Reaction: New bonds form between the adsorbed species (or with other reactant molecules)
Desorption: Product molecules leave the catalyst surface, freeing active sites for further reactions

flowchart LR
    A["Reactant molecules<br>approach surface"] --> B["Adsorption<br>Reactants bind to<br>metal surface"]
    B --> C["Bond weakening<br>Intramolecular bonds<br>are stretched"]
    C --> D["Reaction<br>New bonds form<br>between adsorbed species"]
    D --> E["Desorption<br>Products leave<br>the surface"]
    E --> F["Active sites freed<br>for next cycle"]

Example 1: Iron in the Haber Process

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Iron (Fe) is the catalyst. The N≡N triple bond is very strong (945 kJ mol⁻¹), making the uncatalysed reaction extremely slow. On the iron surface:

N₂ and H₂ molecules adsorb onto the iron surface
The N≡N and H–H bonds are weakened
N and H atoms migrate across the surface and react to form NH₃
NH₃ molecules desorb from the surface

The iron catalyst does not change the position of equilibrium — it increases the rate of both forward and reverse reactions equally, helping the system reach equilibrium faster.

Example 2: V₂O₅ in the Contact Process

2SO₂(g) + O₂(g) ⇌ 2SO₃(g)

Vanadium(V) oxide (V₂O₅) catalyses this step in the manufacture of sulfuric acid. The mechanism involves a change in the oxidation state of vanadium:

Step 1: V₂O₅ oxidises SO₂ to SO₃, and vanadium is reduced from +5 to +4: SO₂ + V₂O₅ → SO₃ + V₂O₄

Step 2: V₂O₄ is re-oxidised back to V₂O₅ by oxygen: V₂O₄ + ½O₂ → V₂O₅

The catalyst is regenerated. The overall effect is that SO₂ is oxidised to SO₃, with vanadium cycling between +5 and +4 oxidation states.

Note for exams: This is sometimes described as having characteristics of both types of catalysis — the V₂O₅ is a solid (heterogeneous) but the mechanism involves oxidation state changes typical of homogeneous catalysis. For Edexcel, it is classified as heterogeneous.

Example 3: Catalytic Converters

Catalytic converters in car exhausts use platinum (Pt), palladium (Pd), and rhodium (Rh) to convert toxic gases into less harmful products:

Reaction	Type	Products
2CO(g) + O₂(g) → 2CO₂(g)	Oxidation	Carbon dioxide
CₓHᵧ + O₂ → CO₂ + H₂O	Oxidation	Carbon dioxide + water
2NO(g) → N₂(g) + O₂(g)	Reduction	Nitrogen + oxygen

The converter has a honeycomb structure to maximise the surface area for adsorption. The transition metals adsorb the toxic molecules onto their surfaces, weaken their bonds, and facilitate the rearrangement into less harmful products.

Catalyst poisoning: Lead in petrol poisons catalytic converters because lead atoms adsorb strongly to the catalyst surface, blocking active sites permanently. This is why leaded petrol was phased out.

Homogeneous Catalysis

A homogeneous catalyst is in the same phase as the reactants — typically all dissolved in aqueous solution. Homogeneous catalysis by transition metals works by the catalyst being involved in the reaction in one oxidation state and being regenerated in another.

Example: Fe²⁺/Fe³⁺ in the Reaction of S₂O₈²⁻ with I⁻

The reaction between persulfate ions and iodide ions is very slow without a catalyst:

S₂O₈²⁻(aq) + 2I⁻(aq) → 2SO₄²⁻(aq) + I₂(aq)

Both reactants are negatively charged anions, so there is strong electrostatic repulsion preventing them from colliding effectively. The activation energy for this anion-anion collision is very high.

Fe²⁺ ions catalyse this reaction by acting as an electron shuttle:

Step 1: Fe²⁺ is oxidised to Fe³⁺ by S₂O₈²⁻: S₂O₈²⁻(aq) + 2Fe²⁺(aq) → 2SO₄²⁻(aq) + 2Fe³⁺(aq)

Step 2: Fe³⁺ is reduced back to Fe²⁺ by I⁻: 2Fe³⁺(aq) + 2I⁻(aq) → 2Fe²⁺(aq) + I₂(aq)

The iron cycles between +2 and +3 oxidation states. Each step involves a positive ion reacting with a negative ion, so electrostatic attraction helps the reaction proceed. The iron catalyst provides a pathway that avoids the repulsive collision between two negatively charged species.

flowchart TD
    A["S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂<br>(SLOW without catalyst:<br>anion-anion repulsion)"]
    B["Step 1: S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺<br>(anion + cation: attraction)"]
    C["Step 2: 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂<br>(cation + anion: attraction)"]
    B --> C
    C -->|"Fe²⁺ regenerated"| B

Autocatalysis

Autocatalysis occurs when a product of the reaction acts as a catalyst for the same reaction, causing the reaction to accelerate as it proceeds.

Example: MnO₄⁻ with C₂O₄²⁻ (Permanganate-Oxalate Reaction)

2MnO₄⁻(aq) + 5C₂O₄²⁻(aq) + 16H⁺(aq) → 2Mn²⁺(aq) + 10CO₂(g) + 8H₂O(l)

At the start of this reaction (carried out warm, ~60 °C), the purple MnO₄⁻ decolourises very slowly. As the reaction proceeds, Mn²⁺ ions are produced. These Mn²⁺ ions catalyse the reaction.

The characteristic rate profile of an autocatalytic reaction:

Drop Number	Time to Decolourise / s	[Mn²⁺]
1st	~45	Very low
2nd	~12	Low
3rd	~3	Moderate
4th	~2	High
5th	~2	High
6th (near end)	~4	High, but reactant depleted
7th	~8	High, but reactant very low

The time first decreases (autocatalysis — Mn²⁺ accumulates) then increases again at the end (C₂O₄²⁻ is consumed, so even with plenty of catalyst, there is insufficient reactant).

This is an example of homogeneous catalysis — Mn²⁺ is in the same aqueous phase as the reactants.

Exam tip: If asked to sketch a graph of reaction rate vs time for an autocatalytic reaction, the rate starts low, increases to a maximum, then decreases. This gives a curve that rises then falls — quite different from a standard reaction where rate is highest at the start.

Comparison of Heterogeneous and Homogeneous Catalysis

Feature	Heterogeneous	Homogeneous
Phase	Different from reactants	Same as reactants
Mechanism	Surface adsorption	Oxidation state cycling
Recovery	Easy (filter/separate solid)	Difficult (dissolved in product)
Selectivity	Can be lower	Often higher
Examples	Fe (Haber), V₂O₅ (Contact), Pt/Pd/Rh (converter)	Fe²⁺/Fe³⁺ (persulfate-iodide), Mn²⁺ (autocatalysis)

Summary

Transition metals catalyse reactions because their variable oxidation states allow them to facilitate electron transfer, and their ability to form complexes brings reactants together. Heterogeneous catalysts (Fe, V₂O₅, Pt/Pd/Rh) work via surface adsorption. Homogeneous catalysts (Fe²⁺/Fe³⁺) work by cycling through oxidation states in solution. Autocatalysis (Mn²⁺ in the permanganate-oxalate reaction) is a special case where the product catalyses the reaction. A catalyst never changes the position of equilibrium — it only increases the rate at which equilibrium is reached.

A-Level Deep Dive: Transition Metal Catalysis

Spec mapping

Edexcel 9CH0 specification Topic 15 — Transition Metals, sub-topic 15.4 covers heterogeneous catalysis (Fe in the Haber process, V₂O₅ in the Contact process, Pt/Rh/Pd in catalytic converters, Ni in hydrogenation), homogeneous catalysis (Fe²⁺/Fe³⁺ catalysing S₂O₈²⁻ + I⁻ reaction; Mn²⁺ as autocatalyst in MnO₄⁻ + C₂O₄²⁻; Co²⁺ industrial catalysts), and the role of variable oxidation state in providing alternative reaction pathways with lower activation energy (refer to the official specification document for exact wording). Examined in Paper 1 (9CH0/01) and Paper 3 (9CH0/03). Synoptic links: Topic 16 (Kinetics II) for Ea/rate, Topic 8 (Redox I) for half-equations, Topic 11 (Equilibrium I) for catalyst not affecting K, Topic 17 (Organic II) for hydrogenation and Ziegler-Natta polymerisation.

Worked example with full mark scheme

Question (8 marks):

(a) Explain why transition metals make particularly good catalysts. (2)

(b) The reaction 2I⁻(aq) + S₂O₈²⁻(aq) → I₂(aq) + 2SO₄²⁻(aq) is slow at room temperature. Explain how Fe²⁺ catalyses it, writing equations for the two-step mechanism. (4)

Solution with mark scheme:

(a) Transition metals exhibit variable oxidation states (close-spaced 3d/4s energies allow facile redox cycling) and partially filled d orbitals that can accommodate substrate binding through the formation of intermediate complexes (Lewis acid-base interaction).

M1 — variable OS named.

A1 — empty/partially filled d for substrate binding.

(b) Why slow without catalyst: the direct collision is between two negative ions (I⁻ and S₂O₈²⁻); electrostatic repulsion gives high Ea (~60 kJ/mol).

Step 1 (oxidation of Fe²⁺): 2Fe²⁺(aq) + S₂O₈²⁻(aq) → 2Fe³⁺(aq) + 2SO₄²⁻(aq)

M1 — equation correct, Fe²⁺ → Fe³⁺ noted.

Step 2 (reduction of Fe³⁺): 2Fe³⁺(aq) + 2I⁻(aq) → 2Fe²⁺(aq) + I₂(aq)

M1 — equation correct, regenerates Fe²⁺.

Net result: sum of the two steps gives the overall equation; Fe²⁺ is regenerated (not consumed). Each step involves collision between oppositely charged species (Fe²⁺ + S₂O₈²⁻; Fe³⁺ + I⁻), drastically lowering Ea.

A1 — alternative pathway with lower Ea explicitly stated; catalyst regeneration explicit.

(c) Homogeneous: catalyst and reactants in the same phase. Example: Fe²⁺/Fe³⁺ catalysing S₂O₈²⁻ + I⁻ in aqueous solution.

Heterogeneous: catalyst and reactants in different phases. Example: Fe(s) catalysing N₂(g) + 3H₂(g) → 2NH₃(g) (Haber).

B1 — distinction stated.

B1 — one valid example each.

Total: 8 marks (M3 A2 B2 + 1 description).

Specimen question modelled on the Edexcel 9CH0 Paper 1 format

Question (6 marks): The reaction MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O is autocatalysed.

(a) State and explain what autocatalysis means in this context, identifying the catalyst. (3)

(b) Sketch a graph of [MnO₄⁻] vs time during the reaction and explain its shape. (3)

Mark scheme decomposition by AO:

Mark	AO	Awarded for
(a) M1	AO1	Autocatalysis = the catalyst is a product of the reaction
(a) M1	AO2	Catalyst is Mn²⁺ (one of the products)
(a) A1	AO2	Mn²⁺ catalyses by alternating between Mn²⁺ and Mn³⁺ (Mn²⁺ + MnO₄⁻ → intermediate → Mn³⁺ + Mn³⁺ products) — variable OS provides alternative pathway
(b) M1	AO2	Initial slow region (no Mn²⁺ present, only direct uncatalysed reaction)
(b) M1	AO2	Steep middle region (Mn²⁺ accumulates → catalysed pathway dominates)
(b) A1	AO3	Final flat tail (reactants exhausted) — sigmoidal shape characteristic of autocatalysis