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:
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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.
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