Heterogeneous Catalysis
- A heterogeneous catalyst is in a different physical state (phase) from the reactants
- Heterogeneous catalysts are usually solids whereas the reactants are gaseous or in solution
- The reaction occurs at active sites on the surface of the catalyst
Surface adsorption theory
- This theory can be used to explain how a heterogeneous catalyst works
- Adsorption, in which one or more reactants becomes attached to the surface of the catalyst
- Reaction, following the weakening of the bonds in the adsorbed reactants
- Desorption, in which the reaction product becomes detached from the surface of the catalyst
- Adsorption of reactants at active sites on the surface may lead to catalytic action
- The active site is the place where the reactants adsorb onto the surface of the catalyst
- This can result in the bonds within the reactant molecules becoming weaker, or the molecules being held in a more reactive configuration
- There will also be a higher concentration of reactants at the solid surface so leading to a higher collision frequency
Strength of adsorption
- The strength of adsorption helps to determine the effectiveness of the catalytic activity
- Some metals e.g. W have too strong adsorption and so the products cannot be released
- Some metals e.g. Ag have too weak adsorption, and the reactants do not adsorb in high enough concentration
- Ni and Pt have about the right strength and are most useful as catalysts
Surface area
- Increasing the surface area of a solid catalyst will improve its effectiveness.
- A support medium is often used to maximise the surface area and minimise the cost (e.g. Rh on a ceramic support in catalytic converters)
Advantages of heterogeneous catalysts
- Heterogeneous catalysts can be filtered off and are easy to separate from any liquid or gaseous products
- They are also suited to continuous processes rather than batch processes
Impurities
- The effectiveness of a heterogenous catalyst can be decreased by the presence of impurities in the gas involved in the reaction.
- The impurities can:
- Adsorb onto catalyst surface and occupy active sites
- Prevent bond weakening in the reactants
- 'Take up' the surface area of catalyst by forming strong bonds to surface of catalyst so are less likely to desorb from surface of catalyst