What a car's catalytic converter does
Catalytic reduction of nitric oxide by carbon monoxide · Redox · Heterogeneous catalysis
A few grams of platinum, palladium and rhodium under a car turn two poisons into the gases you breathe out and breathe in.
Reactants approach
Drag to rotate
ΔH = -746 kJ · exothermic, releases heat
ΔH per 2 mol CO.
Meet the molecules
- Reactantgas
CO
carbon monoxide
- Reactantgas
NO
nitric oxide
- Productgas
CO2
carbon dioxide
- Productgas
N2
nitrogen
What you notice
A petrol engine's exhaust contains carbon monoxide, unburned fuel and nitrogen oxides, all pollutants. Since the 1970s, cars have been fitted with catalytic converters: a ceramic honeycomb in the exhaust pipe coated with a few grams of platinum, palladium and rhodium. Thousands of narrow channels give the exhaust a large metal surface to touch. Once hot, at roughly 400–800 °C, the metals convert over 90% of these three pollutants into carbon dioxide, nitrogen and water. A cold converter barely works, which is why much of a short trip's pollution comes out in the first minute or two.
Safety: Catalytic converters run hotter than 400 °C, so cars should not be parked over dry grass or leaves.
Step by step
- 1
Molecules land on metal
CO and NO molecules adsorb onto rhodium and platinum atoms on the honeycomb surface.
- 2
NO comes apart
On rhodium, the N–O bond weakens and breaks, leaving separate N and O atoms bound to the metal.
- 3
New pairs form
Two N atoms meet and leave as N2. A CO molecule picks up an O atom and leaves as CO2.
- 4
The surface is free again
As products leave, the metal sites are freed. The catalyst is not used up, though lead or sulfur in fuel can poison it.