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Angew. Chem. Int. Ed. 2004, 43 (10) 12511254 No. 10/2004

Nano-mole

Platinum nanoparticles burrow into a zeolite support

and form new pores

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

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