Homogeneous vs Heterogeneous Catalysts

· Catalyst = substance that increases rate of reaction and is chemically unchanged overall at the end.
· Catalysts provide an alternative reaction pathway with a lower activation energy, Ea.
· Homogeneous catalyst = catalyst and reactants are in the same phase.
· Heterogeneous catalyst = catalyst and reactants are in different phases.
· Exam shortcut:
· Homogeneous → often aqueous catalyst + aqueous reactants or gas catalyst + gas reactants.
· Heterogeneous → often solid catalyst + gaseous reactants.
· Catalysts do not change the position of equilibrium and do not change Kc/Kp; they increase the rates of both forward and reverse reactions.

A catalyst lowers the activation energy by providing a different pathway. The reactants and products remain at the same energy levels, so the overall energy change is unchanged. Source

Heterogeneous Catalysts: Mode of Action

· Heterogeneous catalysis usually occurs on the surface of a solid catalyst.
· Key stages: adsorption → bond weakening → reaction on surface → desorption.
· Adsorption = reactant molecules bind to the catalyst surface at active sites.
· Adsorption can weaken bonds in reactant molecules, making reaction easier.
· Reactants are held close together in a suitable orientation, increasing chance of successful reaction.
· Desorption = product molecules leave the catalyst surface, freeing active sites for more reactants.
· Greater surface area usually gives more active sites and a faster catalysed reaction.
· A catalyst can be poisoned if impurities bind strongly to active sites and block adsorption.

Example: Iron in the Haber Process

· Reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g).
· Catalyst: Fe(s), so this is heterogeneous catalysis because gases react on a solid surface.
· N₂ and H₂ adsorb onto the iron surface.
· The strong N≡N bond is weakened when nitrogen adsorbs.
· Adsorbed atoms react stepwise to form NH₃ on the surface.
· NH₃ desorbs, leaving the iron surface available again.
· Exam phrase: iron provides active sites where reactants adsorb and bonds are weakened.

Example: Catalytic Converters in Car Exhausts

· Catalysts: palladium, platinum and rhodium.
· This is heterogeneous catalysis because exhaust gases react on the surface of solid metal catalysts.
· Main exam focus: catalytic removal of oxides of nitrogen, NOx, from exhaust gases.
· NOx adsorbs onto the catalyst surface.
· N–O bonds are weakened and nitrogen-containing species are converted into N₂.
· Carbon monoxide can act as a reducing agent: 2NO + 2CO → N₂ + 2CO₂.
· Products desorb, freeing active sites for further reaction.
· Environmental link: reduces NOx pollution, which contributes to photochemical smog and acid rain.

Catalytic converters use solid transition metal catalysts to convert harmful exhaust gases into less harmful products. This is a key CIE example of heterogeneous catalysis. Source

Homogeneous Catalysts: Mode of Action

· Homogeneous catalyst is in the same phase as the reactants.
· It works by being used in one step of the mechanism and reformed in a later step.
· The catalyst may change oxidation state during the reaction, then return to its original oxidation state.
· It provides an alternative route that avoids a slow direct reaction.
· Exam phrase: the catalyst is regenerated, so it is not used up overall.

A catalytic cycle shows how a homogeneous catalyst participates in one step and is regenerated later. This matches the CIE idea that a homogeneous catalyst is used and then reformed. Source

Example: Atmospheric Oxides of Nitrogen Catalysing Oxidation of SO₂

· NO and NO₂ act as homogeneous catalysts in the oxidation of atmospheric SO₂.
· All species are gases, so the catalyst and reactants are in the same phase.
· Step 1: NO₂ oxidises SO₂ to SO₃ and is reduced to NO.
· SO₂ + NO₂ → SO₃ + NO
· Step 2: NO is oxidised back to NO₂ by oxygen.
· 2NO + O₂ → 2NO₂
· Overall: SO₂ is oxidised to SO₃, and NO₂ is regenerated.
· SO₃ + H₂O → H₂SO₄, contributing to acid rain.
· Exam phrase: NOx is used in one step and reformed in another, so it acts as a homogeneous catalyst.

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

· Overall reaction: S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂.
· Without a catalyst, the reaction is slow because two negative ions repel: I⁻ and S₂O₈²⁻.
· Fe²⁺/Fe³⁺ ions act as a homogeneous catalyst because all ions are in aqueous solution.
· If Fe²⁺ is used:
· S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺
· 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂
· Fe²⁺ is regenerated, so it is a catalyst.
· The catalysed route is faster because each step involves attraction between oppositely charged ions or less repulsion than the direct reaction.

The Fe²⁺/Fe³⁺ system is a classic homogeneous catalyst example. Iron ions change oxidation state during the mechanism but are regenerated overall. ron ions as a catalyst in the reaction between persulphate ions and iodide ions

Exam Command Words and How to Answer

· Explain homogeneous catalysis → state same phase, then say catalyst is used in one step and regenerated later.
· Explain heterogeneous catalysis → state different phases, then describe adsorption, bond weakening, reaction, desorption.
· Identify a catalyst from a mechanism → look for a species that is used then reformed and does not appear in the overall equation.
· Identify an intermediate → look for a species that is formed then used up and does not appear in the overall equation.
· Use examples:
· Fe in Haber process = heterogeneous.
· Pd/Pt/Rh in catalytic converters = heterogeneous.
· NOx in SO₂ oxidation = homogeneous.
· Fe²⁺/Fe³⁺ in I⁻/S₂O₈²⁻ reaction = homogeneous.

Common Mistakes to Avoid

· Do not say a catalyst is unchanged during the reaction; it may change during steps, but is unchanged overall.
· Do not say catalysts increase yield at equilibrium; catalysts do not change equilibrium position.
· Do not confuse adsorption with absorption: adsorption happens on a surface.
· Do not call Fe in the Haber process homogeneous; Fe is solid and reactants are gases.
· Do not call Fe²⁺/Fe³⁺ intermediates; they are catalysts because they are regenerated.