Research iridium

A high-surface-area form of iridium has promise as a catalyst from producing hydrogen from ammonia, according for researchers at Rutgers, the State University of New Jersey. Using ammonia as a source of hydrogen for automotive fuel cells, rather than hydrogen itself, would avoid many of the engineering problems associated with hydrogen, says research director Ted Madey, of Rutgers’ physics department.

Madey’s team describes in a recent edition of the Journal of the American Society how heating a flat surface of iridium in the presence of oxygen changes the shape of the surface. At temperatures around 300°C, the oxygen atoms pull the iridium atoms into a more ordered crystalline structure, consisting of an ordered array of pyramids, whose faces can be as small as 5nm. Different temperatures create different face sizes, the researchers say.

‘The nanostructured surfaces we’re examining are model catalysts,’ says Madey. Ammonia molecules can nestle into the dips between the pyramids, with the lone pair of electrons on the nitrogen atom bound to the metal surface; this sets the molecules up to shed their hydrogen atoms efficiently.

This technology means that ammonia could be used as a source of hydrogen, Madey believes. Ammonia is easily liquefied, so could be handled much the same way as petroleum, with some additional safeguards. The extremely high pressures and low temperatures needed to store hydrogen would not be necessary. Cars would be equipped with catalytic converters, based on nanostructured catalysts similar to Madey’s iridium, to extract the hydrogen, releasing the resulting nitrogen back into the atmosphere. As ammonia contains no carbon, this would also be less likely to poison the catalyst in the fuel cell itself.

The applications are not confined to automotive fuel cells, however: Madey believes that this approach could be used to make new forms of industrial heterogeneous catalysts that are more robust and selective than current forms. ‘They also have the potential to catalyse chemical reactions for the chemical and pharmaceutical industries,’ he says.