CATALYSTS

• OXIDATION OF TRANSITION METALS?

  • high oxidation state metals reduced in acidic solution
  • low oxidation state metals oxidised in alkaline solution (tendency to form negative ions) so stored in acid
  • M(H2O)6 (2+), M(H2O)4(OH)2, M(H2O)2(OH)4 (-2)
  • to oxidise to high oxidation state add alkali then oxidising agent
• HETEROGENOUS CATALYSTS?

  • different phase to reactants
  • usually solid
  • catalytic action occurs on solid surface which remains in place so catalyst not lost or separated from products
• HOW TO MAKE HETEROGENOUS CATALYSTS MORE EFFICIENT?

  • increase SA
  • spread on inert support medium
• EXAMPLES OF HETEROGENOUS CATALYST USES?

  1. HABER PROCESS

  • N2 (g) + 3H2 (g) ⇋ 2NH3 (g)
  • Fe catalyst
  • lasts 5 years before poisoning

  1. CONTACT PROCESS

  • produces sulfuric acid
  • 2SO2 + O2 ⇋ 2SO3
  • V2O5 catalyst
  • SO2 + V2O5 -> SO3 + V2O4
  • 2V2O4 + O2 -> 2V2O5
• CATALYST POISONING?

  when the surface of catalysts becomes covered with unwanted impurities

  -> or the finely divided catalyst may also be lost from the support medium
• HOMOGENOUS CATALYSTS?

  • same phase as reactant
  • intermediate forms
• EXAMPLES OF HOMOGENOUS CATALYSTS?

  S2O8 (2-) (aq) + 2I- (aq) -> 2SO4 (2-) (aq) + I2 (aq)

  • Fe (2+) catalyst
  • 1: S2O8 (aq) + 2Fe (2+) (aq) -> 2SO4 (2-) (aq) + 2Fe (3+) (aq)
  • 2: 2Fe (3+) (aq) + 2I- (aq) -> 2Fe (2+) (aq) + I2 (aq)
  • uncatalysed is slow as the negative ions repel
• AUTOCATALYSIS?

  when one of the products of the reaction is a catalyst
• EXAMPLE OF AUTOCATALYSIS?

  2MnO4- (aq) + 16H+ (aq) + 5C2O4 (2-) (aq) -> 2Mn (2+) (aq) + 8H2O (l) + 10CO2 (g)

  • slight heating produces catalyst (Mn (2+))
  • 1: 4Mn (2+) (aq) + MnO4- (aq) + 8H+ (aq) -> 5Mn (3+) (aq) + 4H2O (l)
  • 2Mn (3+) (aq) + C2O4 (2-) (aq) -> 2CO2 (g) + 2Mn (2+) (aq)
  • MnO4- (aq) is purple so rate of reaction can be measured using a colorimeter
• HOW DO HETEROGENOUS CATALYSTS WORK?

  1. Reactants adsorbed to surface at active sites
  2. Molecules can move about
  3. Adsorption increases reaction by...

  • concentrating reactants (closer together)
  • weaken bonds in molecule
  • position molecule in favourable orientation

  cannot be TOO WEAK (not many adsorbed) or TOO STRONG (not able to move around, less likely to meet other reactant)
• EXAMPLE HETEROGENOUS CATALYSTS?

  TOO WEAK: Ag

  TOO STRONG: W

  IDEAL STRENGTH: Ni and Pt
• EXAMPLES OF CATALYST POISONING?

  CATALYTIC CONVERTORS

  • lead poisoning in cars
  • Rh and Ph catalysts poisoned by lead from petrol

  HABER PROCESS

  • sulfur poisoning
  • hydrogen from natural gas contaminated by sulfur
  • poison Fe catalyst
• TYPES OF HOMOGENOUS CATALYSTS?

  • acid
  • transition metal
• ACID HOMOGENOUS CATALYSTS?

  • protonates reactants
  • species produced reacts with other reactant
  • creates products and reforms catalyst faster
  • e.g. esterification
• TRANSITION METAL HOMOGENOUS CATALYST?

  • vary oxidation states
  • e.g. reaction between iodide ions and peroxodisulfate ions using an Fe (2+) catalyst