15.11 - Transition Metal Catalysts

• why are transition metals good homogenous/heterogenous catalysts?

  they can change their oxidation number by ganing or losing electrons in their d-orbitals so they can transfer electrons to speed up reactions
• heterogenous catalysts

  a catalyst in a different state from the reactants

  provides a surface area for reaction to occur
• why is heterogenous catalysis useful?

  it's easier to separate catalyst from products
• why are transition metals good heterogenous catalysts?

  they use their partially filled d-orbitals to make weak bonds with reactant molecules
• how do heterogenous catalysts work?

  reactant molecules adsorb into surface of catalyst

  bonds between reactant molecules are weakened so they break more easily and reform as products

  products adsorb from surface of catalyst
• what happens if adsorption is too strong?

  reactants become too unreactive and catalyst doesn't work

  e.g.: tungsten
• what happens if desorption is too weak?

  not enough reactants adsorb onto surface for reaction to happen at a useful rate so catalyst doesn't work

  e.g.: silver
• catalyst in the harber process

  iron

  iron (III) oxide
• catalyst in reforming straight-chain alkanes into cyclic alkanes and arenes

  platinum
• catalyst in catalytic converters

  platinum/palladium/rhodium

  honeycomb structure to increase surface area
• catalyst in hydrogenation of unsaturated vegetable oil

  nickel/platinum/palladium
• catalyst in the contact process

  vanadium (V) oxide

  V2O5
• what does the contact process produce?

  sulfuric acid
• equation for first step in the contact process

  2SO2 + O2 <=> 2SO3
• why is vanadium (V) oxide effective as a catalyst?

  metal changes its oxidation number reversibly
• changes in oxidation numbers of vanadium (V) oxide during the contact process

  first it's reduced to vanadium (IV) as it oxidises SO2 to SO3

  then vanadium (IV) is reoxidised to vanadium (V) oxide by oxygen in the mixture of gases
• what do catalytic converters do?

  speed up reaction to remove NO and CO produced by internal combustion engines so reduces emissions
• equation for reaction catalytic converters catalyse

  2NO + 2CO -> N2 + 2CO2
• issues with catalytic converters

  impurities build up on its surface in form of sulfur and catalyst becomes poisoned
• homogenous catalysis

  catalyst in the same state as reactants
• how do homogenous catalysts work?

  combine with reactants forming an intermediate (which requires a lower activation energy) which then reacts to form the product and reform the catalyst
• why is homogenous catalysis not always good?

  extra step needed to separate catalyst from products
• overall equation of oxidation of iodide ions by peroxodisulfate

  2I- + S208^2- -> I2 + 2SO4^2-
• catalyst for oxidation of iodide ions by peroxodisulfate

  Fe2+
• why oxidation of iodide ions by peroxodisulfate is slow without a catalyst

  both ions are negatively charged so repel each other so they're unlikely to react (higher activation energy)
• equations (with catalyst) of oxidation of iodide ions by peroxodisulfate

  S2O8^-2 + 2Fe2+ -> 2Fe3+ + 2SO4^2-

  2Fe+3 + 2I- -> I2 + 2Fe2+
• why both Fe2+ and Fe3+ are effective

  reactions could happen in any order as Fe2+ is converted into Fe3+ and vice versa
• what is autocatalysis?

  when a reaction product acts as a catalyst for that reaction
• how does the rate of an autocatalysed reaction change?

  slow at first as there's not much catalyst

  then speeds up as catalyst is formed

  rate slows down as concentration of reactants decreases
• overall equation for reaction autocatalysed by Mn2+

  2MnO4^- + 16H+ + 5C2O4^2- -> 2Mn2+ + 8H2O + 10CO2
• steps in reaction autocatalysed by Mn2+

  MnO4^- + 4Mn2+ + 8H+ -> 5Mn+3 + 4H2O

  2Mn+3 + C2O4^-2 -> 2Mn+2 + 2CO2