reactive H atoms then combine more rapidly with other molecules that
collide with the surface. A particularly simple reaction involving
H2 is isotope exchange with D2.
The probable series of events for the reaction
is shown in the panels opposite(fig 15).
In the first panel, an H2
molecule is pulled apart as it binds to two metal atoms on the catalytic
surface. The energy that is required to dissociate the H2
molecule is gained from the energy of the two H-metal bonds that
In panel two, a D2
molecule approaches the catalytic surface, and in panel three, one
end of the molecule binds to another site on the surface. By forming
a weak D-metal bond, the molecule weakens its own internal D-D bond,
thereby making it more susceptible to attack by the nearby bound
In panel four the central D atom is shared
equally between H and D in a configuration that is analogous to
complex in the uncatalyzed reaction. This activated complex
could decompose equally well in two ways: on to panels five and
six, or back through panels three and two.
In about half the cases it will continue
on as in panel five, breaking the D-D bond entirely and leading
in panel six to a released H-D molecule and a deuterium atom bound
to the surface.