Homogeneous & Heterogeneous Catalysts (CIE A Level Chemistry)

• Catalysts increase the rate of reaction by providing an alternative pathway which has a lower activation energy
• Catalysts can be either homogeneous or heterogeneous

Homogeneous catalysts

• Homogeneous catalysts are those that are in the same phase as the reaction mixture
• For example, in the esterification of ethanoic acid (CH₃COOH) with ethanol (CH₃CH₂OH) to form ethyl ethanoate (CH₃COOCH₂CH₃) under acidic conditions:

CH₃COOH (aq) + CH₃OH (aq)  CH₃COOC₂H₅ (aq) + H₂O (l)

• The H⁺ is a homogeneous catalyst and like the reactants and product it is in the aqueous phase

Heterogeneous catalysts

• Heterogeneous catalysts are those that are in a different phase to the rest of the reaction mixture
• For example, in the Born-Haber process to form ammonia (NH₃) from nitrogen (N₂) and hydrogen (H₂) an iron (Fe) catalyst is used:

N₂ (g) + 3H₂ (g)  2NH₃ (g)

• The Fe catalyst is a heterogeneous catalyst as it is in the solid phase, whereas the reactants and products are all in the gas phase

• In heterogeneous catalysis, the molecules react at the surface of a solid catalyst
• The mode of action of a heterogeneous catalyst consists of the following steps:
• Adsorption (or chemisorption) of the reactants on the catalyst surface
  • The reactants diffuse to the surface of the catalyst
  • The reactant is physically adsorbed onto the surface by weak forces
  • The reactant is chemically adsorbed onto the surface by stronger bonds
  • Chemisorption causes bond weakening between the atoms of the reactants
• Desorption of the products
  • The bonds between the products and catalyst weaken so much that the products break away from the surface
• For example, the adsorption of hydrogen molecules onto a palladium (Pd) surface

How heterogeneous catalysts generally work

The reactants are adsorbed on the catalyst surface causing bond weakening and eventually desorption of the products

Iron in the Haber process

• In the Haber process ammonia (NH₃) is produced from nitrogen (N₂) and hydrogen (H₂)
• An iron catalyst is used which speeds up the reaction by bringing the reactants close together on the metal surface
• This increases their likelihood to react with each other
• The mode of action of the iron catalyst is as follows:
  • Diffusion of the nitrogen and hydrogen gas to the iron surface
  • Adsorption of the reactant molecules onto the iron surface by forming bonds between the iron and reactant atoms
    • These bonds are so strong that they weaken the covalent bonds between the nitrogen atoms in N₂ and hydrogen atoms in H₂
    • But they are weak enough to break when the catalysis has been completed
  • The reaction takes place between the adsorbed nitrogen and hydrogen atoms which react with each other on the iron surface to form NH₃
  • Desorption occurs when the bonds between the NH₃ and iron surface are weakened and eventually broken
  • The formed NH₃ diffuses away from the iron surface

Iron as a heterogeneous catalyst in the Haber Process

Iron brings the nitrogen and hydrogen closer together so that they can react and hence increases the rate of reaction

Heterogeneous catalysts in catalytic converters

• Heterogeneous catalysts are also used in the catalytic removal of oxides of nitrogen from the exhaust gases of car engines
• The catalysts speed up the conversion of:
  • Nitrogen oxides (NO_y) into harmless nitrogen gas (N₂)
  • Carbon monoxide (CO) into carbon dioxide (CO₂)
• The catalytic converter has a honeycomb structure containing small beads coated with platinum, palladium, or rhodium metals which act as heterogeneous catalysts
• The mode of action of the catalysts is as follows:
  • Adsorption of the nitrogen oxides and CO onto the catalyst surface
  • The weakening of the covalent bonds within nitrogen oxides and CO
    • Formation of new bonds between:
    • Adjacent nitrogen atoms to form N₂ molecules
    • CO and oxygen atoms to form CO₂ molecules
  • Desorption of N₂ and CO₂ molecules which eventually diffuse away from the metal surface

Heterogeneous catalysts in car exhausts

The metals in catalytic converters speed up the conversion of nitrogen oxides and CO into N₂ and CO₂ respectively

• Homogeneous catalysis often involves redox reactions in which the ions involved in catalysis undergo changes in their oxidation number
  • As ions of transition metals can change oxidation number they are often good catalysts
• Homogeneous catalysts are used in one step and are reformed in a later step

The iodine-peroxydisulfate reaction

• This is a very slow reaction in which the peroxydisulfate (S_2,O₈^2- ) ions oxidise the iodide to iodine

S₂O₈^2- (aq) + 2I^- (aq) → 2SO₄^2- (aq) + I₂ (aq)

• Since both the S₂O₈^2- and I^- ions have a negative charge, it will require a lot of energy for the ions to overcome the repulsive forces and collide with each other
• Therefore, Fe³⁺ (aq) ions are used as a homogeneous catalyst
• The catalysis involves two redox reactions:
  • First, Fe³⁺ ions are reduced to Fe²⁺ by  I^-

2Fe³⁺ (aq) + 2I^- (aq) → 2Fe²⁺ (aq) + I₂ (aq)

• • Then, Fe²⁺ is oxidised back to Fe³⁺ by S₂O₈^2-

2Fe²⁺ (aq) + S₂O₈^2- (aq) → 2Fe³⁺ (aq) + 2SO₄^2-  (aq)

• By reacting the reactants with a positively charged Fe ion, there are no repulsive forces, and the activation energy is significantly lowered
• The order of the two reactions does not matter
  • So, Fe²⁺ can be first oxidised to Fe³⁺ followed by the reduction of Fe³⁺ to Fe²⁺ 

The reaction pathway diagram for a two-stage catalysed reaction

The catalysed reaction has two energy ‘humps’ because it is a two-stage reaction

Nitrogen oxides & acid rain

• As fossil fuels contain sulfur, burning the fuels will release sulfur dioxide which oxidises in air to sulfur trioxide, and then dilute sulfuric acid (H₂SO₄) is formed by reaction with water. The result is acidification of rain:

SO₃ (g) + H₂O (l) → H₂SO₄ (aq)

• Nitrogen oxides can act as catalysts in the formation of acid rain by catalysing the oxidation of SO₂ to SO₃

NO₂ (g) + SO₂ (g) → SO₃ (g) + NO (g)

• The formed NO gets oxidised to regenerate NO₂

NO (g) + ½ O₂ (g) → NO₂ (g)

• The regenerated NO₂ molecule can again oxidise another SO₂ molecule to SO₃ which will react with rainwater to form H₂SO₄ and so on