23.3    Catalysis

Phase and physical state

• A phase is a region of space, throughout which all physical properties of a material are essentially uniform
• Examples of phases:

Heterogeneous catalysis

• Heterogeneous catalysis involves the use of a catalyst which is in different phase than the reactants. Most often, the reactants are in gaseous state while the catalyst is in solid state

• In heterogeneous catalysis, the reactants are often adsorbed onto the surface of the active site of the catalyst

• Adsorption means ‘sticking onto the surface’. A good catalyst should adsorb reactants strong enough but not so strong that the products cannot break  away

• For example, the reaction between ethene and hydrogen is catalysed by nickel catalyst. Both ethene and hydrogen are gases while nickel is a solid
  1. Ethene molecules are adsorbed on the surface of the nickel. The double bond between the carbon atoms breaks and the electrons are used to bond it to the nickel surface
  2. Hydrogen molecules are also adsorbed on to the surface of the nickel. When this happens, the hydrogen molecules are broken into atoms. These can move around on the surface of the nickel
  3. If a hydrogen atom diffuses close to one of the bonded carbons, the bond between the carbon and the nickel is replaced by one between the carbon and hydrogen
  4. That end of the original ethene now breaks free of the surface, and eventually the same thing will happen at the other end
  5. As before, one of the hydrogen atoms forms a bond with the carbon, and that end also breaks There is now space on the surface of the nickel for new reactant molecules to go through the whole process again.
• Most heterogeneous catalysts are transition elements or their compounds. This is because in transition elements, there are empty/partially ftlled d-orbitals available for the molecules to be adsorbed to

Homogeneous catalysis

• Homogeneous catalysis involves the use of a catalyst which is in the same phase as the reactants.
• 1. For example, the reaction between persulfate ions and iodide ions:
     S2O8²⁻ + 2I⁻ → 2SO4²⁻ + I2
  2. This reaction proceeds very slowly because it involves two negative ions colliding
  3. Iron(III) ions, Fe³⁺ is used as a It oxidises the iodide ions into iodine molecules and itself is reduced to iron(II) ions, Fe²⁺.
     Fe³⁺ + 2I⁻ → Fe²⁺ + I2
  4. The persulfate ions will oxidise the iron(II) ions into iron(III) ions and itself is reduced to sulfate ions
     S2O8²⁻ + 2Fe²⁺ → 2SO4²⁻ + 2Fe³⁺