Protecting Groups Go Rogue in Aluminum Superatoms

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Virginia Commonwealth University scientists have discovered that certain arrangements of ligands, which are atoms or functional groups that bind a central core of metal atoms, act as lightning rods that make nanoparticles highly reactive.

The addition of ligands to metal clusters may create novel catalysts that could one day be used to improve the combustion of fuels in jets and to develop new energetic materials for propulsion and other applications. 

Previously, scientists hypothesized that the addition of ligands acts as “protecting groups” that prevent the nanoparticle from reacting with its environment.

Through an elaborate series of theoretical studies, Shiv N. Khanna, Ph.D., commonwealth professor in the Department of Physics in the VCU College of Humanities and Sciences, and his team including A.C. Reber, Ph.D., research associate professor, and graduate students Marissa Baddick Abreu and Christopher Powell, examined the reactive properties of aluminum clusters containing iodine atoms as ligands with methanol. The findings were published online last week in the Journal of the American Chemical Society.

“This work offers new intuition on what criteria allow the formation of stable clusters, and how to induce these clusters into becoming effective catalysts,” said Khanna.

“This can aid in the design and synthesis of cluster assembled materials that take the exotic properties of nanoscale clusters and embed them in a functional material,” he said. Aluminum nanoparticles also are proposed as a highly energetic material for energy storage. 

The research builds on the team’s previously published work introducing superatoms - selected atomic clusters of a given element that could mimic the chemical behavior of another element. Superatoms could form a third dimension to the periodic table.

“Superatoms are so small that the wave-like nature of the electrons within the cluster results in their quantum states bunching into shells similar to what is found in atoms, and the addition of ligands may enhance their stability through the closing of these electronic shells,” said Khanna.

”What was surprising is that certain arrangements of iodine ligands would induce active sites that make these clusters quite reactive,” said Reber. “These active sites form because the ligands are electronegative and may deform the metallic sea of electrons resulting in uneven charge densities that serve as active sites for reactions or catalysis.”

Further, the team found that a ligand attached to an adatom, a defect atom sitting on top of an otherwise compact icosahedral structure, was highly reactive with methanol.

“This was only reactive in the presence of both the adatom and ligand, so that adding the ligand actually made the cluster more reactive,” said Abreu.

The research was funded by the Air Force Office of Scientific Research (FA9550-08-01-0400).

Read the research article here: http://pubs.acs.org/doi/pdf/10.1021/ja309473s.

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