The energy released when two H atoms and
a D2 molecule are bound to the catalytic
surface is approximately equal to the energy required to dissociate
the H2 molecule; thus panels one and
three represent nearly the same energy states.
The contribution of the catalyst
arises because the energy of the activated
complex in panel four is not nearly as high as that of the intermediate
complex in the gas phase:
The metal atoms help to hold the complex
in place and stabilize it. The activation barrier therefore is lower,
and the reaction takes place faster. Reaction energy profiles for
uncatalyzed and catalyzed reactions are shown opposite.
This particular kind of assistance by
a catalyst is known as a "rack" mechanism, because it literally
pulls molecules apart and weakens bonds, thereby making them more
susceptible to attack. The molecules that bind to a catalytic surface
and are acted upon are called "substrate"
molecules.
A catalytic surface, whether it be a clean
surface of a finely divided metal or metal oxide, or the active
site of an enzyme molecule, must be structured in such a way that
it can bind substrate
molecules from the reaction that it catalyzes, enable them to react,
and release the products
afterward.
In the words of Emil Fischer, a turn-of-the-century
enzyme chemist, a catalyst
and its substrate molecules must fit one another like a lock and
key.
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