Two teams of US chemists have unveiled findings highlighting the importance of catalysis in hydrogen storage.

Two teams of US chemists have unveiled findings highlighting the importance of catalysis in hydrogen storage.

The launch in 2003 by the US Department of Energy (DOE) of a ’Grand Challenge’ for hydrogen storage has spurred much research in this area. The first fruits of this research were recently made public at the American Chemical Society’s national meeting in Washington, DC.

One team from the DOE’s Brookhaven National Laboratory, New York, led by Santanu Chaudhuri, investigated the role of titanium in the storage of hydrogen by sodium alanate. This metal hydride releases hydrogen in a two-step process when heated, to leave a mixture of sodium hydride and metallic aluminium. The problem has always been how to re-create sodium alanate from these waste compounds, because neither sodium hydride nor aluminium is able to absorb hydrogen efficiently.

Previous research showed that adding small amounts of titanium greatly increases the hydrogen storage ability of aluminium, but no one knew how the process worked. Chaudhuri and his team analysed the storage reaction with x-ray spectroscopy, using the results to create a computer model, and discovered that the specific location of the titanium atoms was the crucial factor.

’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,’ explained Chaudhuri. The titanium atoms subtly alter the atomic-level structure of the aluminium surface, allowing it to absorb a greater amount of hydrogen.

Sodium alanate is a well-established hydrogen storage material, but a more novel compound is ammonia borane. This was first investigated as a storage material by a group of chemists from DOE’s Pacific Northwest National Laboratory, Richland, led by Tom Autrey. They found that ammonia borane readily releases hydrogen at fairly low temperatures when in the presence of a rhodium catalyst.

To find out how the catalyst worked, Autrey and his team used various spectroscopy techniques, including x-ray and infrared, to analyse the reaction. They discovered that the rhodium compound produces its catalytic effect by altering its structure during the reaction process, and this allowed them to work out what happens to ammonia borane as the hydrogen is released.

Autrey and his team plan to use these results to develop other, cheaper catalysts that are able to work with ammonia borane. Chaudhuri and his team now plan to study how the aluminium and hydrogen subsequently react with sodium hydride to produce sodium alanate. Jon Evans