1This involves the use of a catalyst in a different phase from the reactants. Typical examples involve a solid catalyst with the reactants as either liquids or gases.
Note: It is important that you remember the difference between the two terms heterogeneous and homogeneous.
hetero implies different (as in heterosexual). Heterogeneous catalysis has the catalyst in a different phase from the reactants.
homo implies the same (as in homosexual). Homogeneous catalysis has the catalyst in the same phase as the reactants.
How the heterogeneous catalyst works (in general terms)
Most examples of heterogeneous catalysis go through the same stages:
One or more of the reactants are adsorbed on to the surface of the catalyst at active sites.
Adsorption is where something sticks to a surface. It isn't the same as absorption where one substance is taken up within the structure of another. Be careful!
An active site is a part of the surface which is particularly good at adsorbing things and helping them to react.
There is some sort of interaction between the surface of the catalyst and the reactant molecules which makes them more reactive.
This might involve an actual reaction with the surface, or some weakening of the bonds in the attached molecules.
The reaction happens.
At this stage, both of the reactant molecules might be attached to the surface, or one might be attached and hit by the other one moving freely in the gas or liquid.
The product molecules are desorbed.
Desorption simply means that the product molecules break away. This leaves the active site available for a new set of molecules to attach to and react.
A good catalyst needs to adsorb the reactant molecules strongly enough for them to react, but not so strongly that the product molecules stick more or less permanently to the surface.
Silver, for example, isn't a good catalyst because it doesn't form strong enough attachments with reactant molecules. Tungsten, on the other hand, isn't a good catalyst because it adsorbs too strongly.
Metals like platinum and nickel make good catalysts because they adsorb strongly enough to hold and activate the reactants, but not so strongly that the products can't break away.
Examples of heterogeneous catalysis
The hydrogenation of a carbon-carbon double bond
The simplest example of this is the reaction between ethene and hydrogen in the presence of a nickel catalyst.
In practice, this is a pointless reaction, because you are converting the extremely useful ethene into the relatively useless ethane. However, the same reaction will happen with any compound containing a carbon-carbon double bond.
One important industrial use is in the hydrogenation of vegetable oils to make margarine, which also involves reacting a carbon-carbon double bond in the vegetable oil with hydrogen in the presence of a nickel catalyst.
Ethene molecules are adsorbed on the surface of the nickel. The double bond between the carbon atoms breaks and the electrons are used to bond it to the nickel surface.
Hydrogen molecules are also adsorbed on to the surface of the nickel. When this happens, the hydrogen molecules are broken into atoms. These can move around on the surface of the nickel.
If a hydrogen atom diffuses close to one of the bonded carbons, the bond between the carbon and the nickel is replaced by one between the carbon and hydrogen.
That end of the original ethene now breaks free of the surface, and eventually the same thing will happen at the other end.
As before, one of the hydrogen atoms forms a bond with the carbon, and that end also breaks free. There is now space on the surface of the nickel for new reactant molecules to go through the whole process again.
Note: Several metals, including nickel, have the ability to absorb hydrogen into their structure as well as adsorb it on to the surface. In these cases, the hydrogen molecules are also converted into atoms which can diffuse through the metal structure.
This happens with nickel if the hydrogen is under high pressures, but I haven't been able to find any information about whether it is also absorbed under the lower pressures usually used for these hydrogenation reactions. I have therefore stuck with the usual explanation in terms of adsorption.
If anyone has any firm information about this, could they contact me via the address on the about this site page.
