Heterogeneous catalysis: Giving reactants a better target

A heterogeneous catalyst ties one molecule to a surface, putting the molecule in an orientation that makes another reactant more likely to hit the reactive site.

C~A~B → C-A + B

Reactant C must hit the reactive site on the A end of molecule A-B in order to break the A-B bond and form the C-A bond shown in the equation. The probability of the collision occurring in the proper orientation is pretty much driven by chance. The reactants are moving around, running into each other, and sooner or later the collision may occur at the reactive site. But what would happen if you could tie the A-B molecule down with the A end exposed? It’d be much easier and more probable for C to hit A with this scenario.

The catalyst that does this tying down is called heterogeneous because it’s in a different phase than the reactants. This catalyst is commonly a finely divided solid metal or metal oxide, and the reactants are gases or in solution. This heterogeneous catalyst tends to attract one part of a reactant molecule due to rather complex interactions that aren’t fully understood. After the reaction takes place, the forces that bound the B part of the molecule to the surface of the catalyst are no longer there.

So B can drift off, and the catalyst is ready to do it again. Most people sit very close to a heterogeneous catalyst every day — the catalytic converter in an automobile. It contains finely divided platinum and/or palladium metal and speeds up the reaction that causes harmful gases from gasoline (such as carbon monoxide and unburned hydrocarbons) to decompose into mostly harmless products (such as water and carbon dioxide).