VCU-PSU researchers discover a strong and highly stable aluminum-hydrogen cluster

By Sathya Achia Abraham

A team of Virginia Commonwealth University and Penn State University researchers has created a highly stable aluminum-hydrogen cluster that may one day be used to advance propulsion systems that launch rockets or create new storage systems for large quantities of hydrogen fuel.

An important step toward achieving a hydrogen economy and creating alternative fuel sources is the identification of new species that can bind hydrogen and release it when needed.

The VCU-Penn State collaboration also provides a base of knowledge and insight into the factors controlling reactivity of aluminum clusters and how the addition of a few hydrogen atoms can turn the otherwise-reactive clusters into air-stable species ideal to make aluminum-particle based materials for fuels.

Through a team effort combining theoretical work by Shiv N. Khanna, Ph.D., professor in physics at VCU, and his team, and the experimental work by researchers led by A.W. Castleman Jr., Ph.D., a professor at Penn State University, the team identified Al₄H₇^- to be a highly stable species ideal for making materials. The work was published in the Proceedings of the National Academy of Sciences Online Early Edition the week of Sept. 3, and appears in the print issue Sept. 11.

“The main objective of our work is to make materials where clusters of atoms serve as the building blocks. The properties of clusters can be controlled by size and composition, which offer a novel way of synthesizing materials with desirable combination of properties,” Khanna said.

“It is important to find ways in which materials with aluminum particles can be synthesized that will not oxidize under ordinary conditions. The findings indicate that Al₄H₇^- is highly stable and has properties that should allow the synthesis of cluster materials using these units,” said Khanna.

Aluminum particles are used for high energy fuels – producing a lot of energy as they burn. For uniform burning, small particles are favored.  However, aluminum is easily oxidized and small particles are often coated with a dead oxidized layer composed of already burned aluminum.

“Finding ways in which the dead oxidized layer can be eliminated, leading to aluminum particles that will burn completely when needed is a giant step toward aluminum based fuels,” Khanna said. This work indicates that the addition of a few hydrogen atoms can provide the desired particles.

The team generated aluminum-hydrogen clusters (Al₄H_m^-) by exposing aluminum vapors into hydrogen gas. The strength and stability of the aluminum clusters was tested by exposing them to oxygen.

Researchers observed that Al₄H_m^- clusters containing an even number of hydrogen atoms reacted immediately when exposed to oxygen. However, Al₄H_m^- clusters containing an odd number of hydrogen atoms were significantly less reactive. In particular, Al₄H₇^- was found to be extremely stable and resistant to oxidation.

“Our theoretical studies uncovered the reasons for this special stability and showed that the cluster consists of a core of aluminum atoms decorated by hydrogen atoms. The theoretical studies also predicted that it has a high electron affinity, an attribute needed to make cluster materials out of cluster motifs,” Khanna said.

Khanna and Castleman collaborated with Art Reber, a postdoctoral associate in Khanna’s group in the Department of Physics at VCU; and W.H. Woodward, and P.J. Roach, both research associates in the Departments of Chemistry and Physics at Penn State University.