Catalytic Converters
Chemistry: Cambridge International A Level, topic 2.5.6
Catalytic Converters are Heterogeneous Catalysis
A heterogeneous catalyst is a catalyst which is in a different phase to the reactants. Catalytic converters are heterogeneous catalysts used in cars to reduce pollution.
Heterogeneous catalysis
- Heterogeneous catalysts catalyse the reaction on their surface.
- The reactants attach to the metal in places called active sites.
- The term for the attachment to the metal active site is adsorption (i.e. the reactants adsorb to the metal surface).
- If you increase the surface area of the metal, you increase the rate of reaction because more reactants can adsorb.
Efficient catalysis
- Catalysts are expensive, so you want to get the most out of your money.
- We commonly coat a support medium in the metal so that we don’t need to buy much metal.
- We can also powder the metal to increase its surface area.
Catalytic converters
- Catalytic converters are used to convert harmful substances produced in car exhausts into less harmful substances.
- For example, they concert toxic carbon monoxide and nitrous oxide into carbon dioxide and nitrogen:
- CO + NO → CO2 + N2
- Catalytic converters are made of metals such as platinum and rhodium.
Catalyst poisoning
- Unfortunately, substances other than reactants can adsorb metal surfaces.
- Sometimes, these molecules will adsorb irreversibly, and so slowly coat the surface in impurities.
- This stops reactants from absorbing. It is named catalyst poisoning.
- Catalyst poisoning slows down reactions by reducing the number of available active sites.
- If a catalyst is heavily poisoned, it might need replacing, which costs money.
Test yourself on Catalytic Converters
What is a negative of catalysts?
If we increase the surface area, the rate of reaction increases. Why is this?