Nitrogen as a fuel of the future?

New form of nitrogen produced, which could serve as an energy store

Cubic bond MPI for chemistry
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Scientists have produced a new form of gaseous nitrogen that is extremely energetic due to its special bonding geometry. Perhaps this form could be used in the future as an energy storage and thus as an environmentally friendly fuel.

Nitrogen, the main constituent of air, usually consists of inert molecules in which two atoms are bound in triplicate. Now researchers at the Max Planck Institute for Chemistry have for the first time synthesized a polymeric cubic form in which all nitrogen atoms are bound together by means of covalent single bonds. The special cubic form of the atomic bonds has not been found in any other element. It has unique properties, such as an energy content five times greater than that of the strongest non-nuclear explosives.

For two decades scientists have predicted that there is also simply bound nitrogen. Thereafter, the molecular nitrogen at high pressure would form an atomic solid with the structure of a cubic lattice (cg-N). Since then, intensive research has been conducted on this polymeric nitrogen, at high pressure and in various temperature ranges. Although several new nitrogen phases were found, including a nonmolecular semiconductor phase, no one has been able to produce polymeric nitrogen.

This has now been achieved by scientists of the high-pressure group at the Max Plack Institute for Chemistry: They synthesized polymeric nitrogen directly from diatomic nitrogen at temperatures above 2, 000 degrees Kelvin and pressures in excess of 110 gigapascals (1.1 × 106 atm). For this purpose, the researchers used a new arrangement of a laser-heated high-pressure diamond cell. X-ray diffraction studies and Raman spectra of the transparent crystal show that in fact polymeric nitrogen had formed with the theoretically predicted cubic lattice structure. This phase with the compression modulus of 300 GigaPascals is characteristic for strongly covalent bound solids. "That's why we call them Nitrogen Diamond, " says Mikhail Eremets, one of Mainz's Max Planck researchers.

Diatomic nitrogen is a non-reactive substance because its triple bond is one of the most stable chemical bonds we know. In contrast, nitrogen atoms bound with single bonds in a polymeric network form a high energy density material. This is because nitrogen has a uniquely large energy difference between a single bond and the third of a triple bond. That's why a lot of energy is released in the transition from the simple to the triple bound form, about five times more than with the most dangerous explosives. display

Since the only end product of this conversion is ordinary, environmentally friendly nitrogen, the researchers want to investigate whether one could use polymeric nitrogen as fuel or explosive. "First of all, we have to try to maintain the connection at ambient temperature and pressure, " says Eremets.

(MPG, 04.08.2004 - NPO)