Catalysts and Catalysis
Transition metals and their compounds can act as catalysts, which are either homogeneous or herterogeneous.
Homogeneous catalysts are in the same phase as the reactants. A catalyst that acts in a different phase from the reactants is called heterogeneous.
When the reaction is catalysed homogeneously - an intermediate species is formed where the oxidation state of the transition metal changes. In the following reaction.
S2O82- + 2I- ® I2 + 2SO42-
Fe2+ is a catalyst. First it is oxidised to Fe3+ when it reacts with S2O82- and this can then react with 2I- to form I2.
Another catalytic phenomenon that occurs is autocatalysis where a product of a reaction catalyses it. This happens in the reaction of manganate (VII) with acidified ethanedioate ions; C2O42-.
2MnO4- + 16H+ + 5C2O42- ® 2Mn2+ + 10CO2 + 8H2O
Initially it is slow but speeds up as Mn2+ is produced. These react with manganate and ethanedioate as follows.
4Mn2+ + MnO4- + 8H+ ® 5Mn3+ + 4H2O
2Mn3+ + C2O42- ® 2CO2 + Mn2+
A heterogeneous catalyst works differently, the reacation occurs at the surface of a transition metal where molecules are adsorbed - this is where they 'stick' to the surface and makes reactions more likely.
The strength of the adsorption determines how efficient a catalyst can be in a reaction. If it adsorbs too weakly (like silver) there will not be enough of an effect to make much difference. Strongly adsorption (like tungsten provides) on the other hand is also a disadvantage as the products will not be released fast enough to have a effect on the rate.
Since only the surface of the metal acts as the catalyst, we only need a thin layer and anything extra makes no difference. So a support medium is used and a layer of the metal catalyst applied; this maximises surface area and minimises cost.
Applications of Catalysts
Catalysts are extremely important in industry because they speed up reactions and improve yields which makes production more profitable.
In the contact process - used to manufacture sulphuric acid, vandium oxide is a catalyst. The vandium is first reduced to vandium (IV) oxide then oxidised back to vandium (V) oxide.
SO2 + V2O5 ® SO3 + V2O4
2V2O4 + O2 ® 2V2O5
In the haber process a catalyst of iron is used.
Many cars have a catalytic converter in ther exhast, usually palladium, platinum and rhodium. This removes hazardous chemicals resulting from fuel combustion - one of those is sulphur dioxide.
However, sulphur dioxide adsorbs very strongly to the catalyst and blocks other molecules from it, this ia catalyst poisoning. To overcome this, bands of aluminium oxide 'stores' the sulphur dioxide under normal running conditions, when accelerating the sulphur is reduced - this removes it as hydrogen sulpide, which smells.