Catalytic converters in cars consist of tiny
platinum particles that have been
deposited onto a porous ceramic support. At high temperatures, these particles
can sinter, meaning that they fuse together with the support material and
undergo
chemical reactions. What is going on at the nanoscopic scale? Could
these processes be useful? Japanese researchers working with Hitoshi Kato have
examined more closely
platinum particles on a zeolite surface using an electron
microscope. They have made a surprising find: particles that "dig" corridors.
zeolites are
crystalline, highly porous
silicates. Because of their large
surface area and their cage-like
pores, which can take up "guest
molecules",
they are used as
ion exchangers,
molecular sieves, and
catalysts. The
researchers chose one of these
zeolites as a support for their platinum
particles and exposed them, at 800 °C, to an atmosphere equivalent to that in an
average car exhaust. After one hundred hours, they looked at the little
platinum-containing zeolite crystals under an electron microscope. Amazingly,
there were no more platinum particles to be found on the surface of the zeolite.
Where could they have gone? The surprising discovery: the tiny spheres of
precious metal had burrowed into the surface of the zeolite. In the process,
they left behind little channels with a diameter corresponding to the diameter
of the particles. The researchers noted that there was a preferred direction for
the channels within the zeolite crystals. The channels have a hexagonal
cross-section, which is in agreement with the lattice structure of the zeolite,
and the channel walls consist of facets of the crystal. Aside from a platinum
sphere at the end of each pore, they are empty and the surrounding crystal
structure is not disturbed in any way. Atoms have clearly just disappeared from
the crystal lattice. At the points of contact between the platinum particles and
the zeolite, the platinum presumably catalyzes a chemical reaction between the
silicon and
oxygen atoms of the zeolite and the components of the exhaust. The
components of the zeolite can thus leave the crystal in the form of gaseous SiO
or Si(OH)4. The platinum particles "sink" deeper and deeper into the holes
formed by the reaction.
"The observed phenomenon could be used to produce tailored
porous materials,"
hopes Kato. "The number of pores, as well as their shape and size could be
controlled by the diameter of the platinum particles, the duration of
heating,
the type of zeolite selected, and the orientation of the crystals."