Many industrially important reactions in the gas phase use a heterogeneous solid catalyst, for example in the Haber process. Only the catalyst surface is in contact with the reaction mixture and, as the catalyst increases the activated fraction, the collisions between reactant molecules must require less energy than normal. An example of the general mechanism of heterogeneous catalysis is when a catalytic converter removes carbon monoxide and nitrogen monoxide emissions from internal combustion engines, as a result reducing the atmospheric pollution produced by car exhaust fumes. It is now a legal requirement for the exhaust of all new cars sold in the UK to be fitted with a catalytic converter.
In the first step in catalytic converters reactant molecules (carbon monoxide and nitrogen monoxide) are absorbed onto the catalyst surface. In this process reactant molecules stick to the catalyst by forming weak chemical bonds. This chemisorption weakens the bonds within the reactant molecules. The absorbed reactant molecules are now much more susceptible to reaction. They are still able to move over the catalyst surface and the products are formed when they collide. In addition, the absorption process effectively concentrates the reactants molecules on the catalyst surface, increasing the frequency of collisions. The final step is desorption, where the product molecules (carbon dioxide and nitrogen) leave the catalyst surface. If this does not happen the catalyst surface will not be able to absorb fresh reactant molecules. The reactions that take place in this process are the oxidation of carbon monoxide to carbon dioxide, the reduction of nitrogen monoxide back to nitrogen and the oxidation of hydrocarbons to water and oxygen.
The efficiency of such a catalyst will depend upon how strongly it absorbs reactant molecules. Up to a certain point catalytic activity increases as the strength of absorption increase, since the bonds within the reactant molecule are further weakened. However, it the reactant molecules are absorbed too strongly this prevents free movement across the surface and reduces the overall collision frequency.
A good example of a reaction that involves homogeneous catalysis is the loss of ozone from the stratosphere as a result of the use of CFC’s. In this gas phase reaction, chlorine free radicals catalyse the decomposition of ozone into oxygen. When CFC’s, reach the stratosphere, ultraviolet light breaks the carbon-chlorine bonds, generating chlorine free radicals. The reaction is:
2O g 3O g
The two steps in the mechanism involving the chlorine free radicals are:
- Cl· + O ClO· + O
- ClO· + O Cl· + O
The chlorine free radicals regenerated in the second step are available for further reaction with the ozone molecules. The reaction rate is fast and a few chlorine free molecules rapidly destroy many ozone molecules.
The oxygen free radicals are formed continuously in the stratosphere. Ultraviolet light produces oxygen free radicals from oxygen molecules or ozone molecules.
