The Wright Stuff

Transition Metal Catalysts

The transition metals are the group of metals in the middle section of the periodic table. They are divided into three groups - the first row transition metals, the second row transition metals and, guess what, the third row transition metals. Many of the transition metals behave as catalysts, either as the metal itself or as a compound. Below are some of the common reactions catalysed by transition metals.

The important thing about catalysts is that they are recovered at the end of the reaction in their entirety, they are not used up during the reaction. They often undergo a temporary change during the reaction, but are turned back into the original chemical at the end of the reaction.

Catalysts change the rate of a reaction, but do not alter the position of equilibrium.
Although the yield within a short time seems more, the overall yield from the slow reaction would eventually be the same, but as it can take ages to get that far we usually don't wait that long.

Transition metal catalysts are either homogeneous catalysts or heterogeneous catalysts.
Homogeneous catalysts are those in the same physical phase as the other reactants e.g. all in solution together in the same solvent, or all are gases.
Heterogeneous catalysts are those in a different physical phase to the rest of the reactants. E.G. two different liquids in contact with catalysis occurring at the interface between them, or more commonly a solid catalyst and a solution, or a solid and gases.

Examples of Catalysts

The following reactions are the bare minimum examples of transition metals acting as catalysts that everyone studying chemistry and general science should know. Catalysts are written over the reaction arrow, sometimes with (cat) after the formula, to distinguish them from reactants and products.

The Haber Process

This is one of the best known reactions involving a transition metal catalyst. It is the formation of ammonia from nitrogen and hydrogen using iron as the catalyst. This is a heterogeneous system.

 
Fe
N2(g) + 3H2(g)
--->
 2NH3(g)
 

 

Contact Process

This reaction is part of the production of sulphuric acid. The reaction between sulphur dioxide and oxygen to form sulphur trioxide is catalysed by vanadium (V) oxide, V2O5. This is a heterogeneous system.

 
V2O5
2SO2(g) + O2(g)
--->
 2SO3(g)
 

 

Hydrogenation (Reduction)

Raney nickel, which is very finely divided nickel powder, is a catalyst for the addition of hydrogen across C=C and C=C bonds. Finely divided platinum, and finely divided paladium also catalyse this sort of reaction, but are a bit expensive. This is a heterogeneous system.

 
Raney Ni
  
2CH2=CH2(g) + H2(g)
--->
 2CH3-CH3(g)
 

The margarine industy uses the above type of reaction to convert unsaturated vegetable oils into higher metal point saturated fats - i.e. margarine.

Decomposition

Manganese dioxide catalyses the decomposition of hydrogen peroxide. Lots of other things catalyse this reaction too, but MnO2 is one of the best. This is a heterogeneous system.

 
MnO2
  
2H2O2(l)
--->
 2H2O(l) +O2(g)
 

 

Advanced Examples of Catalysts

On more advanced courses, you may come across the following examples of transition metals acting as catalysts.

Catalytic Convertors

The pollutants NO and CO are removed from car exhaust fumes by passing the exhaust gases over platinum and rhodium to produce nitrogen and carbon dioxide. This is a heterogeneous system.

 
Pt / Rh
  
NO(g) + CO(g)
--->
 N2(g) +CO2(g)
 

Decomposition

Cobalt (II) catalyses the decompositon of the chlorate(I) anion, in two steps, as it is easily converted to cobalt (III). This is a heterogeneous system.

 
4Co(OH)2(s) + 2ClO-(aq) + 2H20
--->
 4Co(OH)3(s) + 2Cl-(aq)
     
4Co(OH)3(s)
--->
 4Co(OH)2(s) + 2H20 + O2(g)
 

 

Ziegler-Natta Polymerisation

Titanium tetrachloride in the presence of triethylaluminium polymerises ethene, CH2=CH2. This is a heterogeneous system. A similar system using vanadyl halides exists in the co-polymerisation of styrene, butadiene and either dicyclopentadiene or 1,4-hexadiene to form synthetic rubber.

Many other examples of transiton metals and their compounds acting as catalysts can be found in text books. Yes, text books. Do try reading them sometimes as they are very helpful.

This page gives more information on the uses of transition metals.
And try this one for more on the general properties of transition metals.