26.2 Homogeneous and heterogeneous catalysts
- Syllabus
- 9701–2028–2029
- Topic
- 26.2
- Level
- A2
Learning objectives
26.2.1Catalysts can be homogeneous or heterogeneous• Explain that catalysts can be homogeneous or heterogeneous26.2.2Heterogeneous catalyst action• Describe the mode of action of a heterogeneous catalyst to include adsorption of reactants, bond weakening and desorption of products, e.g.:- (a) iron in the Haber process- (b) palladium, platinum and rhodium in the catalytic removal of oxides of nitrogen from the exhaust gases of car engines26.2.3Mode of action of a homogeneous catalyst• Describe the mode of action of a homogeneous catalyst by being used in one step and reformed in a later step, e.g.:: (a) atmospheric oxides of nitrogen in the oxidation of atmospheric sulfur dioxide; (b) Fe 2+ or Fe3+ in the I–/S2O8; 2– reaction
Classify catalysts by their phase relative to reactants
| Catalyst type | Phase relationship | Example |
|---|---|---|
| homogeneous | same phase as reactants | Fe²⁺/Fe³⁺ ions with aqueous I⁻ and S₂O₈²⁻ |
| heterogeneous | different phase from reactants | solid Fe with gaseous N₂ and H₂ in the Haber process |
Both types provide an alternative route with lower activation energy and are regenerated overall. The classification depends on physical phases during reaction, not on whether the catalyst is a metal.
A catalyst may change chemically in individual steps even though it is reformed overall. 'Homogeneous' does not mean uniform appearance alone; the catalyst and reactants must share a phase.
Heterogeneous catalysis uses adsorption, bond weakening and desorption
| Stage | What happens at the solid surface | Why it speeds reaction |
|---|---|---|
| 1 adsorption | reactants diffuse to and bond at active sites | holds reacting species close and suitably oriented |
| 2 activation/reaction | adsorption weakens bonds within reactants; new bonds form | provides a lower-energy surface pathway |
| 3 desorption | product–surface bonds break and products leave | frees active sites for another cycle |
In the Haber process, N₂ and H₂ adsorb on iron. Their bonds weaken, adsorbed atoms form N–H bonds, and NH₃ desorbs, leaving the iron surface available again.
In a catalytic converter, palladium, platinum and rhodium surfaces adsorb carbon monoxide and oxides of nitrogen. Surface reaction forms products including CO₂ and N₂, which desorb from the metal.
Adsorption is attachment to the surface, not absorption into the bulk or dissolution. Products must desorb; otherwise occupied active sites would stop further catalytic cycles.
A homogeneous catalyst is used in one step and reformed later
A homogeneous catalyst reacts in one elementary step to form a temporary different species, then is regenerated in a later step. Adding the steps cancels the catalyst cycle species and gives the overall reaction.
2FeX3+(aq)+2IX−(aq)2FeX2+(aq)+IX2(aq)
2FeX2+(aq)+SX2OX8X2−(aq)2FeX3+(aq)+2SOX4X2−(aq)
Fe³⁺ is used in the first step and reformed in the second; equivalently the cycle may start from Fe²⁺. The two steps avoid the difficult direct collision between two negatively charged reactants, I⁻ and S₂O₈²⁻.
NOX2(g)+SOX2(g)SOX3(g)+NO(g)
NO(g)+21OX2(g)NOX2(g)
NO₂ is used to oxidise SO₂ and is reformed when NO reacts with O₂. Cancelling NO/NO₂ across the cycle leaves the overall oxidation of SO₂ by oxygen.
Regenerated overall does not mean chemically unchanged at every stage. Identify the catalyst by consumed-then-reformed order; a formed-then-consumed species is an intermediate.