3.2.5.6: Catalysts

• Define: Heterogeneous catalyst

  Catalysts in a different state/phase as reactants
• Define: Homogeneous catalysts

  Catalysts in the same state/phase as reactants
• Heterogeneous catalysts catalyse reactions by:

  • providing a catalytic surface
  • …where reactants collide with & bind to the active sites on the catalytic surface (ADSORPTION)
  • products desorb as reaction progresses
• Homogeneous catalysts catalyse reactions by:

  • Through formation of an intermediate species (by reactants combining with catalyst) which react to form products
  • Catalyst is reformed
• Catalysis

  • Transition metals have variable oxidation states -> make good catalysts -> receiving & losing electrons in d-oribitals to speed up reactions
• Efficiency of (heterogeneous) catalysis

  • Increase SA -> increase in efficiency
  • Spreading catalysts over inert support mediums -> increase in efficiency
  • catalytic poisoning from impurities -> decrease in efficency
  • catalyst lost from support medium -> decrease in efficiency
• The Contact Process

  • catalyst: V2O5 (heterogeneous)
  • 1st STEP: Oxidation of SO2, reduction of V2O5

  V2O5 + SO2 -> SO3 + V2O4

  • 2nd STEP: Regeneration of V2O5 via oxidation

  V2O4 + 1/2 O2 -> V2O5
• Oxidation of I- by S2O8 2-

  • uncatalysed reaction is slow -> -vely charged ions electrostatically repel -> ∴ high Ea

  S2O8 2- + 2I- -> I2 + 2SO4 2-
• Role of Fe2+ in S2O8- & I-

  • 2Fe2+ + S2O8- + 2e- -> 2SO4 2- + 2Fe3+
  • 2Fe3+ + 2I- -> I2 + 2Fe2+
• Autocatalysis of MnO4 - with C2O4 - by Mn2+

  • C2O4 2- -> 2CO2 + 2e- (OX)
  • MnO4 - + 5e- + 8H+ -> Mn2+ + 4H2O (RED)
  • 5C2O4 2- + 2MnO4 - + 16H+ -> 2Mn2+ + 10CO2 + 8H2O (BALANCED EQ.)
  • 4Mn2+ + MnO4 - + 8H+ -> 5Mn3+ + 4H2O (AUTOCATALYSIS)
  • 2Mn3+ + C2O4 2- -> 2CO2 + 2Mn2+ (REGENERATION OF Mn 2+)