Making hydrogen stick

As part of ongoing research to make hydrogen a mainstream source of clean, renewable energy, scientists from the U.S. Department of Energy's

have determined how titanium atoms help hydrogen atoms attach to an aluminium surface.

Their study isolates the role of titanium, which is used as a catalyst in the crucial first step to trap hydrogen within a particular class of hydrogen-storage materials. The work may also help identify and develop similar hydrogen-storage systems.

Brookhaven chemist Santanu Chaudhuri recently presented this research at the 230th national meeting of the American Chemical Society in Washington, D.C.

To be a mainstream source of fuel, hydrogen must be stored safely and efficiently. Conventional high-pressure storage tanks can be dangerous and are too big and heavy for certain applications, such as hydrogen-based fuel cells in automobiles. Hydrogen-storage materials, however, incorporate hydrogen safely and compactly, and temporarily hold large quantities of it that can be recovered easily under safe, controlled conditions.

"A hydrogen-storage material must be able to store hydrogen quickly under 'normal' conditions -- that is, without very high temperatures and pressures," said Chaudhuri. "In tiny amounts, an appropriate catalyst, such as titanium, can speed up the reaction and make the hydrogen-storage process suitable for practical applications. Our study has helped us better understand the role of these catalysts."

Through this research, Chaudhuri and his collaborator, Brookhaven chemist James Muckerman, hope to improve the performance of sodium alanate, a hydrogen-storage material composed of sodium and aluminium hydride.

Sodium alanate, a complex metal hydride, expels hydrogen gas (the fuel) and aluminium when heated, leaving a mixture of sodium hydride and metallic aluminium. But because neither aluminium nor sodium hydride absorb hydrogen well, putting the hydrogen back in -- to reform sodium alanate and allow reuse of the material -- becomes difficult.

"We found that aluminium absorbs significantly more hydrogen -- and does so more quickly and at lower temperatures -- when a small number of titanium atoms are incorporated into its surface," Chaudhuri said.

Chaudhuri and Muckerman created a computer model that provides a plausible mechanism of the reaction. Their model agrees with an experimental x-ray absorption study of sodium alanate, performed at the National Synchrotron Light Source, a facility at Brookhaven that produces x-ray, ultraviolet, and infrared light for research.

Chaudhuri and Muckerman's collaborators at Brookhaven used x-rays to "see" and thus calculate how the titanium atoms subtly changed the atomic-level structure of the aluminium, resulting in a more hydrogen-absorbent surface. Results from these two studies agree on the role of titanium atoms on an aluminium surface and mechanisms of subsequent steps in hydrogen capture.

In the future, Chaudhuri and Muckerman's group plans to study the subsequent steps in the sodium alanate hydrogen-storage process, in which aluminium and hydrogen react with sodium hydride to reform the starting material.