Topic 17 · page 19 of 20

V₂O₅ and catalytic converters

Vanadium(V) oxide speeds up the contact process by cycling between +5 and +4. In a catalytic converter, CO and NO stick to the surface of platinum and rhodium, react, and the products leave.

13 min read17.2817.29(i)17.29(ii)17.27

Read first: Two kinds of catalyst

The contact process 17.28

The key step in making sulfuric acid is the oxidation of sulfur dioxide:

2 SOX2(g)+OX2(g)⇌2 SOX3(g) \ce{2SO2(g) + O2(g) <=> 2SO3(g)}

The catalyst is solid vanadium(V) oxide, V₂O₅. The reactants are gases, so this is heterogeneous catalysis. It works because vanadium has a variable oxidation number.

  1. V₂O₅ oxidises SO₂ to SO₃ and is itself reduced. Vanadium goes from +5 to +4:
SOX2+VX2OX5→SOX3+VX2OX4 \ce{SO2 + V2O5 -> SO3 + V2O4}
  1. Oxygen re-oxidises the catalyst. Vanadium goes from +4 back to +5:
VX2OX4+12 OX2→VX2OX5 \ce{V2O4 + 1/2O2 -> V2O5}
  1. Add the two equations. V₂O₅ and V₂O₄ appear on both sides and cancel, leaving the overall equation SOX2+12 OX2→SOX3\ce{SO2 + 1/2O2 -> SO3} (or, doubled, the equation above).
A cycle of two boxes. At the top, V2O5 with vanadium plus 5. At the bottom, V2O4 with vanadium plus 4. An arrow on the right going down is labelled SO2 in, SO3 out. An arrow on the left going up is labelled half O2 in.
V2O5 is reduced by SO2 and then re-oxidised by oxygen. It is regenerated, so it is not used up.

The sulfur is oxidised from +4 in SO₂ to +6 in SO₃. The vanadium is first reduced and then oxidised again: it is a catalyst because it is regenerated.

Catalytic converters 17.29(i)17.29(ii)17.27

The exhaust gases of petrol engines contain poisonous carbon monoxide and nitrogen monoxide. A catalytic converter turns them into harmless carbon dioxide and nitrogen:

2 CO(g)+2 NO(g)→2 COX2(g)+NX2(g) \ce{2CO(g) + 2NO(g) -> 2CO2(g) + N2(g)}

The catalyst is a thin layer of platinum and rhodium metals on a ceramic honeycomb. The honeycomb gives a very large surface area for a small mass of expensive metal.

Four panels showing a metal surface as a row of grey circles. Panel 1: two CO and two NO molecules approach in the gas above the surface. Panel 2: the molecules are adsorbed on the surface, bonds shown dashed to indicate weakening. Panel 3: the atoms have rearranged on the surface into two CO2 molecules and one N2 molecule. Panel 4: the CO2 and N2 molecules leave the surface by desorption.
The steps at the surface of the catalyst. Two CO and two NO give two CO2 and one N2, with every atom accounted for.
Step What happens Spec term
1 CO and NO molecules bond to the surface of the metal adsorption
2 Bonds within the molecules are weakened, so they can react bond weakening and reaction
3 O atoms transfer to carbon; N atoms join to give N₂ reaction
4 CO₂ and N₂ leave the surface, freeing it for more molecules desorption

Count the atoms in the figure: two carbon, two nitrogen and four oxygen at the start and at the end.

Check yourself

  1. Explain, using oxidation numbers, how V₂O₅ catalyses the oxidation of SO₂ to SO₃. Give both equations.
  2. Why is V₂O₅ described as a heterogeneous catalyst in the contact process?
  3. Write the equation for the reaction in a catalytic converter.
  4. Describe how a catalytic converter removes CO and NO. Use the words adsorption and desorption.
  5. Explain why the catalyst is spread thinly on a ceramic honeycomb.
  6. What is the difference between adsorption and absorption?

Answers cover these until you have tried the questions

  1. SO₂ + V₂O₅ → SO₃ + V₂O₄: vanadium is reduced from +5 to +4 while sulfur is oxidised from +4 to +6. V₂O₄ + ½O₂ → V₂O₅: vanadium is oxidised from +4 to +5. The catalyst is regenerated.
  2. It is a solid and the reactants (SO₂ and O₂) are gases: a different phase, and the reaction takes place on its surface.
  3. 2 CO+2 NO→2 COX2+NX2\ce{2CO + 2NO -> 2CO2 + N2}
  4. CO and NO molecules are adsorbed onto the surface of the platinum/rhodium. The bonds within them are weakened, so they react. The products, CO₂ and N₂, are desorbed from the surface.
  5. It gives a large surface area for the reaction using only a small amount of expensive metal.
  6. Adsorption is bonding of molecules to the surface; absorption is uptake into the bulk of a substance.