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Dinitrogen tetroxide

Nitrogen tetroxide (dinitrogen tetroxide or nitrogen peroxide) is the chemical compound N₂O₄. It is a powerful oxidizer, highly toxic and corrosive. N₂O₄ is hypergolic with various forms of hydrazine, i.e., they burn on contact without a separate ignition source, making them popular bipropellant rocket fuels. It is a useful reagent in chemical synthesis.

Structure and properties

The molecule is planar with an N-N bond distance of 1.78 Å and N-O distances of 1.19 Å. Unlike NO₂, N₂O₄ is diamagnetic. It is also colorless but can appear brownish yellow liquid due to the presence of NO₂ according to the following equilibrium:

N₂O₄ ⇌ 2NO₂

Higher temperatures push the equilibrium towards nitrogen dioxide. Inevitably, some nitrogen tetroxide is a component of smog containing nitrogen dioxide.

Production

Nitrogen dioxide is made by the catalytic oxidation of ammonia: steam is used as a diluent to reduce the combustion temperature. Most of the water is condensed out, and the gases are further cooled; the nitric oxide that was produced is oxidised to nitrogen dioxide, and the remainder of the water is removed as nitric acid. The gas is essentially pure nitrogen tetroxide, which is condensed in a brine-cooled liquefier.

Use as a rocket propellant

Dinitrogen tetroxide is one of the most important rocket propellants ever developed, much like the German developed hydrogen peroxide-based T-Stoff oxidizer used in their World War II rocket propelled combat aircraft designs, like the Messerschmitt Me 163 Komet, and by the late 1950s it became the storable oxidizer of choice for rockets in both the USA and USSR. It is a hypergolic propellant often used in combination with a hydrazine-based rocket fuel. One of the earliest uses of this combination was on the Titan rockets used originally as ICBM's and then as launch-vehicles for many spacecraft. Used on the U.S. Gemini and Apollo spacecraft, it continues to be used on the Space Shuttle, most geo-stationary satellites, and many deep-space probes. It now seems likely that NASA will continue to use this oxidiser in the next-generation 'crew-vehicles' which will replace the shuttle. It is also the primary oxidizer for Russia's Proton rocket and China's Long March rockets.

When used as a propellant, dinitrogen tetroxide is usually referred to simply as 'Nitrogen Tetroxide' and the abbreviation 'NTO' is extensively used. Additionally, NTO is often used with the addition of a small percentage of nitric oxide, which inhibits stress-corrosion cracking of titanium alloys, and in this form, propellant-grade NTO is referred to as "Mixed Oxides of Nitrogen" or "MON". Most spacecraft now use MON instead of NTO, for example, the Space Shuttle reaction control system uses MON3 (NTO containing 3wt%NO).

Power generation using N₂O₄

The tendency of N₂O₄ to reversibly break into NO₂ has led to research into its use in advanced power generation systems as a so-called dissociating gas. "Cool" nitrogen tetroxide is compressed and heated, causing it to dissociate into nitrogen dioxide at half the molecular weight. This hot nitrogen dioxide is expanded through a turbine, cooling it and lowering the pressure, and then cooled further in a heat sink, causing it to recombine into nitrogen tetroxide at the original molecular weight. It is then much easier to compress to start the entire cycle again. Such dissociative gas Brayton cycles have the potential to considerably increase efficiencies of power conversion equipment.