Platinum Metals Rev., 1994, 38, (2), 59
Encapsulation of Palladium Crystallites in Carbon
The growth in carbon nanotechnology has produced carbon in the form of giant clusters, large nested fullerenes, bamboo-like structures and nanotubes. Metals and metal carbides have been successfully included into these structures, and carbon clusters in tubular form encapsulating a metal are expected to result in new technology. However, encapsulating a metal has generally been selective with respect to the metal.
Palladium is an important catalyst and is used in many chemical reactions, often supported on carbon. Now a researcher at the DuPont Company in Delaware, has succeeded in encapsulating cubic palladium crystals inside giant carbon clusters and produced worm-like carbon nanotubes (“nanoworms”) (Y. Wang, “Encapsulation of Palladium Crystallites in Carbon and the Formation of Wormlike Nanostructures”, J. Am. Chem. Soc., 1994, 116, (1), 397–398).
The carbon-encapsulated palladium was found in the cathode deposit produced from arcing between a hollow carbon anode filled with palladium oxide/graphite cement paste and a carbon cathode within a chamber filled with flowing helium at 500 Torr. The ratio of the palladium oxidexarbon, in weight per cent, was from 0.05 to 1.0. The worm-like structures were mainly observed in the core of the cathode deposit and rarely found in the shell.
Deposits formed at the tip of the cathode were analysed by transmission electron microscopy. The head region of the “worm” was examined by energy dispersive X-ray analysis and was found to consist of palladium encapsulated in carbon clusters; electron diffraction of the body of the “worm” showed that it consisted of many sections of carbon tubes 20–50 nm in diameter and several hundred nanometres long. The tubes had mosdy cone-shaped internal voids, with the tip of each void always pointing to the back of the worm.
Palladium appears to act as the seed for the growth of the “nanoworm”; the cone-shaped internal void being the result of tube closure caused by pentagonal ring formation due to the presence of palladium. Each closure is followed by new growth from the outer layer which forms the next section. This periodic tube closure and regrowth eventually produces the worm-like structure. Some worm-like bodies were without palladium in the head, but the internal cone-shaped voids indicated that palladium had once been present.
As it is already known that chemical methods and electron beam radiation can open the ends of nanotubes and nano wires it is suggested that such carbon-encapsulated palladium could become very useful. The carbon protects the palladium in a pristine form and only needs to be able to release it periodically under controllable conditions. This would allow the palladium to be introduced into a chemical reaction with regulated and timed release.