Catalyst

I INTRODUCTION

Catalyst, a substance that alters the rate (usually speeding up) of a chemical reaction without itself being used up or permanently changed.

Catalysts increase the rate of a reaction by providing an alternative route for the reaction that has a lower activation energy than that of the uncatalysed reaction. Because the activation energy along the catalysed route is smaller, a greater proportion of the colliding molecules have the minimum energy needed to react, and so the rate of product formation is increased.

II HOMOGENEOUS CATALYSTS

A catalyst in a solution with or in the same phase as the reactants is called a homogeneous catalyst. The catalyst combines with one of the reactants to form an intermediate compound that reacts more readily with the other reactant. The catalyst, however, does not influence the equilibrium of the reaction, because the decomposition of the products into the reactants is speeded up to a similar degree. An example of homogeneous catalysis is the formation of sulphur trioxide by the reaction of sulphur dioxide with oxygen, in which nitric oxide serves as a catalyst. The reaction temporarily forms the intermediate compound nitrogen dioxide, which then reacts with oxygen to form the sulphur oxide. The same amount of nitric oxide exists at the end as at the start of the reaction. Another example is the use of hydrogen ions (H+) or hydroxide ions (OH-) used to catalyse the esterification of carboxylic acids.

III HETEROGENEOUS OR CONTACT CATALYSTS

A catalyst that is in a separate phase from the reactants is said to be a heterogeneous or contact catalyst. Contact catalysts are materials with the capability of adsorbing molecules of gases or liquids onto their surfaces. An example of heterogeneous catalysis is the use of finely divided platinum to catalyse the reaction of carbon monoxide with oxygen to form carbon dioxide. This reaction is used in catalytic converters mounted in cars to eliminate carbon monoxide from the exhaust gases. The iron used as a catalyst in the Haber-Bosch process to produce ammonia is another example of a heterogeneous catalyst.

A Transition Metals as Heterogeneous Catalysts

Heterogeneous catalysts are frequently transition metals, for example, iron, platinum, and nickel. Because the reacting molecules are usually adsorbed onto the surface of the catalyst it is given a very large surface area to maximize its effect on the reaction rate; a large ingot of iron would have little effect compared to the same mass in finely divided form. Inorganic catalysts such as these are used to catalyse a wide range of different reactions.

IV CATALYST PROMOTERS

Some substances, called promoters, do not have the catalytic ability by themselves but increase the effectiveness of a catalyst. For example, if alumina (aluminium oxide) is added to finely divided iron, it increases the ability of the iron to catalyse the formation of ammonia from a mixture of nitrogen and hydrogen.

V CATALYST POISONS

Although catalysts are not permanently altered during the reactions they catalyse, they can be poisoned by some impurities and will not work again. For example, the catalysts in the catalytic converter of a car are poisoned by the lead compounds in leaded petrol. Just one mistake at the petrol station and the catalytic converter is permanently poisoned. To prevent this mistake the filler tube to the tank of a vehicle fitted with a catalytic converter is too small to take the nozzle of a pump dispensing leaded petrol, which is larger than the nozzle of an unleaded pump.

VI USES

Catalysts are of immense importance to the chemical industries. The catalytic cracking of oil provides the raw materials for an enormous range of other products, and processes such as the production of ammonia in the Haber-Bosch process and the synthesis of sulphuric acid by the contact process would not be possible without catalysts, as the operating temperatures and pressures needed would not only require extreme feats of engineering but maintaining the conditions would be prohibitively expensive. It has been estimated that about 20 percent of the gross national product of the United States is generated through the use of catalytic processes.

A Enzymes as Industrial Catalysts

Enzymes, which are among the most powerful catalysts, play an essential role in living organisms, where they accelerate reactions that otherwise would require temperatures that would destroy most of the organic matter. One current area of active research in catalysis is that of enzymes. Natural enzymes have long been used by a few industries, such as the leather industry and some areas of the food industry, but technological advances like genetic engineering have made it possible to produce a much wider range of enzymes in industrial amounts. Biotechnologists are constantly seeking ways in which to expand this resource and also to develop semisynthetic enzymes for highly specific tasks.