Catalysts and Catalysis

Transition metals and their compounds can act as catalysts, which are either homogeneous or heterogeneous.

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   Homogeneous catalysts are in the same phase as the reactants

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   Heterogeneous

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   A catalyst that acts in a different phase from the reactants is called heterogeneous.

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Homogeneous Catalysts

When the reaction is catalyzed 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 oxidized 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+

Heterogeneous Catalysts

A heterogeneous catalyst works differently. The reaction occurs at the surface of a transition metal, where molecules is 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.

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   Weak

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   If it adsorbs too weakly (like silver), there will not be enough of an effect to make much difference.

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   Strong

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   If adsorption (like tungsten provides), is strong, on the other hand, it is also a disadvantage as the products will not be released fast enough to have a effect on the rate.

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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 is applied. This maximizes the surface area and minimizes the cost.

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, vanadium oxide is a catalyst. The vanadium is first reduced to vanadium (IV) oxide then oxidized back to vanadium (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 their exhaust, usually palladium, platinum and rhodium. This removes hazardous chemicals resulting from fuel combustion - and one of those is sulphur dioxide', however, sulphur dioxide adsorbs very strongly to the catalyst and blocks other molecules from it. This is 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 sulphide, which smells terrible.

What are the advantages or disadvantages of using a transition metal compounds catalyst?

Same as the question. What are the advantages or disadvantages of using a transition metal compounds catalyst?

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