One of two mechanisms of catalysis, the other being the Intermediate Product Theory. The Surface Adsorbtion Theory
is used to explain most examples of heterogenous (as opposed to homogenous) catalysis: where the catalyst and the reactants are in different phases (ie. solid catalyst, gaseous reactants).
A good example of this theory in action is the the reaction of hydrogen and oxygen to form water. At room temperature the two gases do not react but, if some finely divided platinum is added to the mixture, the reaction proceeds rapidly.
The first step in the reaction occurs when the hydrogen and oxygen molecules are adsorped onto the surface of finely divided platinum. The increasesd concentration of the molecules on the surface of the catalyst makes it more likely that the molecules will collide with each other and a chemical reaction will occur. The oxygen and hydrogen molecules are held on the surface of the platinum by temporary bonds formed between the molecules and the platinum. Platinum, being a transition metal, has vacant d orbitals that help the molecules of hydrogen and oxygen to form temporary bonds with it. This is the reason that transition metals tend to be good catalysts - by definition they form at least one ion with a partially filled d sublevel. The oxygen and hydrogen molecules then react to form water molecules on the surface of the platinum. Once the product forms - in this case water, they are desorped] from the surface of the catalysis and the reaction is free to reoccur with other hydrogen and oxygen molecules.
As you can see there are three stages involved in this process:
1.Adsorption - Reactants accumulate on surface of catayst
2.Reaction - Reactants react with another to form product(s)
3.Desorption - Product(s) leaves surface of catalyst
A practical appliance of this theory is the fitting of catalytic converters to motor vehicles. Exhaust fumes containing poisonous gases such as carbon monoxide (co) and nitrogen monoxide (no) are catalysed to form relatively harmless gases: co and no react together to product carbon dioxide and nitrogen gas. A thin coating of platinum, palladium and rhodium (the catalysts) on a ceramic or metal honeycomb structure is used to catalyse a number of these reactions. The honeycomb structure maximises surface area, maximising the probability of catalysis occurring. One of the reason that lead additives are no longer added to petrol is that they act as catalyst poisons - they become permanently adsorbed to the surface of the catalytic convertor, thus rendering it useless. However, all catalytic converters gradually become poisoned and degraded with use and should be replaced every 50,000 miles. Since 1993, all new cars in EU countries must be fitted with catalytic converters.