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January 20th, 2010

Could low-cost superatomic materials replace costlier elements?

Published on January 20th, 2010 | By: pwray@ceramics.org
Left: Graphic displays of energy peaks are similar between a nickel atom and a titanium-monoxide molecule. Right: Bright spots in the images, which correspond to the energy of the electrons emitted durng their remove from the atoms' outer shells, also appears to be similar. Credit: Castleman lab, Penn State

Left, graphic displays of energy peaks are similar between a nickel atom and a titanium-monoxide molecule. Right, bright spots in the images, which correspond to the energy of the electrons emitted during their remove from the atoms' outer shells, also appears to be similar. Credit: Castleman Lab, Penn State.

Based on the insights they are gaining from spectroscopic analysis of superatoms, a group out of Penn State thinks it might be possible to manipulate these materials to find low-cost replacements for expensive catalytic materials, such as platinum.

The concept of “superatoms” has been around since at least 1995. They are clusters of atoms or molecules that in the right number and conditions mimic some of the characteristics of a totally different element. In these clusters, the electrons fill the s and p orbitals. The combination can create a new arrangement of closed shells and free electrons that mimic the element that naturally has this electron arrangement. For example, clusters of 13 aluminum atoms can mimic an iodine atom, and the addition of one more electron to the cluster makes it behave like one of the noble elements.

Penn State researchers have been looking at superatomic behaviors for several years and a group led by A. Welford Castleman Jr. says they have a better handle on how to predict what element a superatom group will behave like. Using photoelectron imaging spectroscopy, they looked at the similarities between titanium monoxide and nickel, zirconium monoxide and palladium, and tungsten carbide and platinum.

Their spectroscopy method allowed them to measure the energy it takes to kick out electrons from various electronic states of the molecules. For example, the found that it takes the same amount of energy to remove electrons from titanium monoxide as it does nickel.

Using the spectroscopic data, Castleman says they now can know in advance how a superatom cluster will act. “It looks like we can predict which combinations of elemental atoms mimic elemental atoms,” he said in a news release. “For example, by looking at the periodic table, you can predict that titanium monoxide will be a superatom. Simply start at titanium, which has four outer-shell electrons, and move six elements to the right, because atomic oxygen possesses six outer-shell electrons. The element you ened up on is nickel, whose 10 outer-shell electrons make it isolectronic with the 10 outer-shell electron molecule resulting from the combination of titanuim and oxygen.

“We thought the finding must be a curious coincidence, so we tried it with other atoms and we found that a pattern emerged,” Castleman said.

The group says it doesn’t know to what extent the pattern will continue across the periodic table, but they are focusing their studies right now on the transition-metal atoms.

Another future step is to test to what extent superatoms chemically behave like their elemental counterparts. If similarities are found, the results could have a big payoff, according to Castleman. “Platinum is used in nearly all catalytic converters. [But] tungsten carbide, which mimics platinum, is cheap. Likewise, palladium is used in certain combustion processes, yet it is mimicked by zirconium monoxide, which is less expensive by a factor of 500,” he said.

The group’s paper is available in the online version of the Proceedings of the National Academy of Science.




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