These gaseous pollutants from internal combustion engines can be removed…

The idea

An engine emits three pollutants that a converter must deal with: carbon monoxide from incomplete combustion, unburnt hydrocarbons, and nitrogen monoxide formed when nitrogen and oxygen from the air combine at the high temperature inside the cylinder. A catalytic converter is a ceramic honeycomb coated with platinum, palladium and rhodium, giving a huge surface area for a small mass of metal. The exhaust gases are adsorbed onto the metal surface, react there, and the products are desorbed. The key reaction pairs two pollutants so that both are removed at once: 2CO + 2NO → 2CO₂ + N₂. Unburnt hydrocarbons are oxidised in the same way, and carbon monoxide can also be oxidised directly: 2CO + O₂ → 2CO₂.

What earns the marks

• Naming carbon monoxide, unburnt hydrocarbons and oxides of nitrogen as the pollutants removed.
• A balanced equation such as 2CO + 2NO → 2CO₂ + N₂.
• Identifying the catalysis as heterogeneous, with adsorption onto the metal surface.
• Explaining that nitrogen monoxide forms from N₂ and O₂ at the high temperature in the engine.

Common mistakes and how to put them right

• Writing NO₂ where NO is the gas formed in the engine, or writing N₂O.

• Claiming the converter removes carbon dioxide, which it actually produces.

• Leaving an equation unbalanced for nitrogen, the atom most often miscounted.

• Describing the converter as a filter that traps the gases rather than a catalyst that converts them.

Exam tips

• Balance nitrogen last and check it explicitly, because it is the atom examiners find unbalanced most often.
• Name all three platinum group metals if asked what the honeycomb is coated with.
• Say the gases are adsorbed onto the surface, which earns the heterogeneous catalysis mark.