Synthetic catalysts generally fall into two separate groups: heterogenous and homogenous.

Heterogenous catalyst

A heterogenous catalyst is a catalyst which is in a different phase to the reactants. The most common form is a solid that catalyses a reaction of either a liquid or gas that adsorbs to the surface. For example, the surface of iron (a solid) is used in the Haber process to catalyse the formation of ammonia (NH₃).

One of the main benefits of using a heterogeneous catalyst is that it can be straightforward to separate it from a reaction mixture, e.g. via filtration. Therefore expensive catalysts can be easily and effectively recovered, an important consideration especially for industrial scale manufacturing processes.

A limitation of heterogeneous catalysis is the availability of the surface area on the catalyst. When the catalysts surface is completely saturated with reactant molecules (i.e. no more reactants can fit on surface), the reaction cycle cannot continue until some of the product molecules leave the surface allowing some space to open up again for a new reactant molecules. It is because of this that the adsorption step in a heterogeneously catalysed reaction is often the rate-limiting step. 

Homogenous catalyst

A homogenous catalyst is where the catalyst is in the same phase as the reactants. The most common form of homogenous catalysts involve metal complexes in solution catalysing a reagent which is also in solution. For example, Wilkinson’s catalyst ((PPh₃)₃RhCl) is used in alkene hydrogenation. Learn more about the catalytic cycle for Wilkinson’s catalyst on the ChemBAM page – what is a catalyst?

An advantage of homogeneous catalysis is there is a very high degree of interaction between the catalyst and reactant molecules due to both being in the same phase (as opposed to heterogeneous catalysis). A disadvantage is the homogeneous catalyst is often irrecoverable after the reaction has run to completion. Thus it could be concluded that using homogeneous catalysis for industrial scale manufacturing processes is not as efficient or economically viable. 

Learn more about the chemistry of catalysis here.

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