A flash fix for nitrogen

	Katsuyoshi Hoshino

A flash of light on a plastic plate turns nitrogen from the air into solid rocket fuel, according to Japanese scientists. The discovery could lead to a new way of making nitrogen-containing compounds.

While the Haber-Bosch process for ammonia production is not expensive and, despite its high temperatures and pressures, is very safe, nature clearly does things very differently and chemists have been curious to find alternatives for many years.

Nature, of course, beat the chemists to nitrogen fixation aeons ago. Leguminous plants such as clover evolved in parallel with legions of bacteria that live in their root nodules. These microbes do the job of a whole chemical plant with little more than a selection of enzymes and metal ions and operate at everyday temperatures and atmospheric pressure.

An accidental discovery by Katsuyoshi Hoshino and his team at Chiba University may have taken chemists a step closer to an alternative nitrogen fixation method without having to copy nature or use high temperatures and pressures.

One of Hoshino's students was working on the electrical properties of composite plastic materials containing titanium dioxide and perchlorate. He had left a sample on a bench at the back of the lab by mistake and forgotten about it. "One day I happened to find the sample and realised its surface glittered slightly," explains Hoshino, "so I asked my student to do a chemical analysis, which showed the glitter was due to ammonium perchlorate crystals, solid rocket fuel!"

The nitrogen could only have come from the air but how did it get fixed into the crystals? The team prepared a new sample by coating a titanium plate with a layer of titanium dioxide, or titania, familiar as the whiter than white pigment in household paints. They then covered this with a layer of electrically conducting polymer. Reactive "perchlorate" ions were then infused into this coating. The plate was sealed in a box moist nitrogen fed in.

Nothing happened at first but when they suddenly realised that the reaction had been driven by the overhead fluorescent light, which is left on twenty-four hours a day, they gave the plate a blast of light and sure enough glittering crystals began to form.

"We are not sure exactly how the nitrogen needles form," explains Hoshino, "but we believe the light produces a charge at the interface between the titanium dioxide and polymer layers, which allows the nitrogen to react with the moisture to form ammonia." Simultaneously, hydrogen ions are produced, which react with the perchlorate ions making perchloric acid. The perchloric acid then neutralises the alkali ammonia forming crystals of ammonium perchlorate salt. The longer they exposed the plate to light the bigger the needles got.

"The current process of nitrogen fixation requires harsh conditions," explains Hoshino, "our mild method could develop into an interesting alternative." This, he adds, is the first time a solid material has been produced synthetically using nitrogen fixation.

Reference:
Angew. Chem. Int. Ed., 2000, 39, 2509.