PASS Scripta Varia 21 - Pontifical Academy of Sciences

SELF-PRESENTATION OF THE NEW ACADEMICIANS
One example of heterogeneous catalysis is the car exhaust catalyst where
toxic molecules like carbon monoxide or nitric oxides or hydrocarbons are
transformed into less harmful ones. In this case, carbon monoxide is oxidised
to carbon dioxide and this is through interaction of the molecules from the
gas phase with the catalyst’s surface. The carbon monoxide is bonded to the
surface through the carbon atom, while the oxygen molecule is dissociatively
adsorbed, where the oxygen-oxygen bond is breaking and then the adsorbed
species diffuse across the surface and form the new CO 2 molecule.
Where do we know this information This is obviously chemistry restricted
to two dimensions, so we need new techniques also to look at the
processes involved in these catalytic reactions on the surfaces of the small
catalyst particles involved in such a reaction. Small particles because the reactivity,
of course, depends on the overall magnitude of the surface area and
the more finely divided the particles, the higher the surface area. So the size
of these small particles is of the order of one nanometer. Catalysis has been
nanotechnology long before this term was invented. And this also shows
the problem: to investigate the chemistry of these particles we need tools
which are able to analyse the topmost atomic layer on a very, very microscopic
level. One way to do this is by just looking at separate crystal planes
of these catalytically active species. This can be done on the atomic scale,
for example, with the scanning tunnelling microscope.
If a platinum surface interacts with molecules from the gas phase, bonds
may be formed, chemisorption bonds, and we have seen one of the essential
points is the breaking of bonds, dissociation of a molecule. If we expose
such a surface to a diatomic molecule, there will be interaction between
the atoms of the molecule with the surface and the bond between the atoms
will be weakened and will be eventually dissociated. With our platinum catalyst,
in the car exhaust catalyst, at a temperature of about -100°C the oxygen
molecule is dissociated, and the atoms formed are separated by about
5 to 8 angstroms because they have to release their energy to the solid and
this takes some time, around 300 femtoseconds. At these low temperatures
the oxygen atoms stay where they have been formed. If we increase the
temperature they can jump from one site to the next site and their residence
time becomes shorter. These oxygen atoms move across the surface, they
randomly jump across the surface: whenever they come close to each other
their lifetime becomes a little bit longer because the oxygen atoms interact
with each other, they attract each other weakly. As a consequence, with increasing
surface concentration there is no longer a perfect random distribution
of the atoms on the surface but they form two new phases: a more
condensed phase, like a two-dimensional crystal, and a gaseous phase, like
46 The Scientific Legacy of the 20 th Century