6.2.9 Catalysis

• Transition metals are often used as catalysts in the elemental form or as compounds
• The ability of transition metals to form more than one stable oxidation state means that they can accept and lose electrons easily
• This enables them to catalyse certain redox reactions. They can be readily oxidised and reduced again, or reduced and then oxidised again, as a consequence of having a number of different oxidation states of similar stability
• There are two types of catalyst:
  • A heterogeneous catalyst is in a different physical state (phase) from the reactants
    • The reaction occurs at active sites on the surface of the catalyst
    • An example is the use of iron, Fe, in the Haber process for making ammonia

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

• A homogeneous catalyst is in the same physical state (phase) as the reactants