Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2000, 39 (14), 2509 - 2512
Needles of Nitrogen
A new method
of nitrogen fixation
Our air consists of 78% nitrogen. Despite this abundance, nitrogen from the air can’t be used just like that. Even plants have to get necessary nitrogen compounds from the soil, or depend on the farmer to turn up with some liquid manure or artificial fertilizer.
On a large scale, nitrogen is usually bound into ammonia by the Haber-Bosch process. Most of the ammonia is then converted to fertilizers; the rest is used by the chemical industry as a starting material for nitrogen-containing products.
The Haber-Bosch process is expensive, because it requires very high pressures and temperatures. However, a Japanese research group working with Katsuyoshi Hoshino has developed a new, distinctly unconventional method of nitrogen fixation that works under mild conditions - room temperature and atmospheric pressure. A titanium electrode is electrochemically coated with a layer of titanium dioxide, onto which is then deposited a further layer of electrically conductive polymer. The layers are infused with perchlorate ions. This array of layers is then set out in a moist nitrogen atmosphere and irradiated with white light. The astonishing result: depending on the strength of the irradiation, needle-shaped crystals grow out of the polymer matrix after a few days or weeks. The needles were identified as ammonium perchlorate, a nitrogen compound already known as a rocket propellant.
"The mechanism of needle formation and nitrogen fixation is not yet understood in detail," explains Hoshino. This much seems clear: the light produces a charge at the interface between the titanium dioxide and polymer layers, which makes possible the reaction of nitrogen with adsorbed water to form ammonia. At the same time hydrogen ions are formed, which react with perchlorate ions from the polymer matrix to form perchloric acid. An acid-base reaction between the perchloric acid and ammonia results in a salt - the observed ammonium perchlorate needles.
"The current process of nitrogen fixation requires harsh conditions. Our mild method could develop into an interesting alternative," Hoshino says optimistically.