Homogeneous catalysis

The catalyst and reactants are in the same phase, usually liquid.

Examples are

• transition metal ions
  
  
• transition metal complexes
  
  
• inorganic acids and bases
  
  
• enzymes (these are dealt
  with in a separate section below)
  

Advantages

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 greater selectivity.

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.

Disadvantages

The catalyst needs to be separated after reaction. This separation can involve distillation.

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: gases
catalyst: solid

reactants: organic liquids or aqueous solutions
catalyst: solid

It is easier to study gas reactions.

examples

ammonia
fuel cell
catalyic convertor
hydrogenation
Friedel Crafts

Some examples of heterogeneous catalysts are

• Transition metals
  
  
• Transition metal oxides
  
  
• zeolites
  
  
• silica/alumina

• 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

Advantages

• There is little difficulty in separating and recycling the catalyst.

Disadvantages

• 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.