A3.2 Heterogeneous Catalysts
Catalysts used in industrial process save costs by lowering operating temperatures and speeding up the production rate of the products.
However, the catalysts are usually expensive in the first place so they need to be recovered and reused. It helps if the catalyst is easily separated from the products of the reaction. For this reason, most industrial catalysts are solids which are easily separated from the gases or liquids involved in the process.
Heterogeneous means “different states” so a solid catalyst working with a liquid or gas is a heterogeneous catalyst.
The catalytic activity takes place on the surface of a heterogeneous catalyst. Reactants adsorb onto the surface by forming chemical bonds to the catalyst.
Metals are extremely good at adsorbing gas molecules so are often used as heterogeneous catalysts
(Fe in the Haber process, Pt/Pd in catalytic converters etc.)
The adsorption has two effects on the reactant molecules:
  1. 1.
    It brings them closer to each other.
  2. 2.
    It weakens the chemical bonds inside the molecules themselves.
Because the bonds are weaker, the activation energy for the catalysed reaction is lower and the reaction rate is faster.
Once the reaction has taken place the product molecules leave the surface of the catalyst (are desorbed).
Since the surface is vital in this process, having large surface area is extremely important for, for example, a fast reaction. In this case catalysts are made with small porous particles which have an enormous surface area.
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