I just did a chem exam a couple of days ago a part of which was based on catalysts. I thought I might be able to elaborate on SK's post before.
For now, without thinking about catalysts:
When you have the Nitrogen oxide and carbon monoxide turning into carbon dioxide and nitrogen and oxygen. Basically, these reactions can take place without a catalyst such as (platinum, rhodium) if the right temperature (really f&*ckin high) is used.
The reason for this, is these reactions have whats called an 'activation energy' where the molecules need x amount of energy before they can break and form new compounds (C02, N2, 02). To reach that activation energy, for these reactions alot of heat is required.
A catalyst lowers the activation energy required to break the bonds within the molecules. Eg, less heat is required to form the products (C02, N2, 02). Catalysts lower the activation energy because they are slightly attracted to the C0 and N0/N02 molecues. So this slight attraction causes the C0 and N0/N02 molecules to collide with the catalyst. Upon collision with the catalyst, the bonds between the C0 and N0/N02 molecules are weakened. Because the bonds within molecules are weak, LESS energy is required to break them.
So this means that the addition of a catalyst will make the converision of C0/N0/N02 to C02/N2/02 possible.
So what SK and lucien said before is right, without the coating (of platinum or rhodium or aluminium oxide (there was another also)) of a catalyst the exhuast gas won't be converted into the less dangerous emmisions. And emmision-wise, your exhaust will probably be as clean as one with a straight through pipe...
my 2c