and Homogeneous Catalysis
The catalyst and reactants are in the same phase, usually liquid.
- transition metal ions
- transition metal complexes
- inorganic acids and bases
- enzymes (these are dealt
with in a separate section below)
There is good contact with reactants so a much greater effective concentration
of catalyst than with heterogeneous catalysts. This means that to achieve
the same rate milder conditions can be used and so it is possible to achieve
At the research and development stage it is often quicker and simpler to
work with homogeneous catalysts, and then find ways of making the system heterogeneous
for industrial application.
The catalyst needs to be separated after reaction. This separation can involve
In some cases this makes catalyst recovery difficult because the temperature
needed for the distillation can destroy the catalyst
Catalyst and reactants are in different phases, either
reactants: organic liquids or aqueous solutions
It is easier to study gas reactions.
Some examples of heterogeneous catalysts are
- Transition metals
- Transition metal oxides
- First the gas molecules must reach the surface, often within the pores
of catalyst particles.
- Bonds within the reactant molecules are weakened, and sometimes broken,
as the catalyst attaches the molecules. This process is called adsorption.
- Next the adsorbed molecules react on the surface, often in several steps.
- The products desorb from the surface, regenerating the active sites.
- Finally the products diffuse away from the surface of the catalyst, leaving
it free to interact with new reactant molecules
- There is little difficulty in separating and recycling the catalyst.
- There is a lower effective concentration of catalyst since the reaction
occurs only on the exposed active surface.
- To maximise the surface area catalysts are spread thinly on a cheap and
sturdy support. In the case of some metals a fine mesh is used, such as
the Pt mesh used to catalyse the oxidation of ammonia.