Gas phase ammonolysis

Gas phase ammonolysis means the conversion of ammonia ( chemical reaction ) in the gas phase, i.e. all starting materials are converted into the corresponding products with ammonia in a reactor in the gas phase. The temperature range of this variant is thus determined by the boiling points of the starting materials and the stability of the products at the different temperatures.

mechanism

As a rule, chemically reactive metal compounds are used. Chlorides are often used as the starting material for the reaction with ammonia . If possible, this is readily available and can be introduced precisely into the reactor without major problems. In the case of titanium nitride , titanium tetrachloride is fed into the reactor as a liquid reagent using a hose pump. The ammonia is added as a gas. Liquid titanium tetrachloride changes into the gas phase and can be mixed with ammonia. In this way a homogeneous mixture of powdery products is achieved, which are ideally pure. Different reaction products arise depending on the temperature of the gas phase ammonolysis carried out.

The chlorine on the titanium is the active center for the reaction. Chloride as an electronegative partner retains all of its 8 outer electrons and forms a bond with the acidic hydrogen from ammonia. This creates hydrogen chloride, which can be seen as the driving force behind the reaction. In this example, titanium tetrachloride is used as the starting material.

${\displaystyle \mathrm {TiCl_{4}+NH_{3}\longrightarrow (TiCl_{3})^{*}+HCl+(NH_{2})^{*}} }$

Titanium is reduced from the 4-valent positive state to the 3-valent positive state during the reaction. The corresponding oxidation comes from the nitrogen in the ammonia. This is in the trivalent negative state in ammonia and is oxidized to nitrogen, which is neither positively nor negatively charged. In this respect, at least 2 ammonia molecules must react with one titanium tetrachloride molecule. The electrons released in this way not only reduce the ammonia, but also the hydrogen from the ammonia molecule. In this respect, the gas phase reaction is a redox reaction .

Partial reactions:

${\displaystyle \mathrm {2Ti^{4+}+2e^{-}\longrightarrow 2Ti^{3+}} }$

${\displaystyle \mathrm {NH_{3}\longrightarrow {\tfrac {1}{2}}N_{2}+3H^{+}+3e^{-}} }$

${\displaystyle \mathrm {H^{+}+e^{-}\longrightarrow {\tfrac {1}{2}}H_{2}} }$

In summary, this results in the following overall reaction:

${\displaystyle \mathrm {2TiCl_{4}+3NH_{3}\longrightarrow 2TiN+8HCl+{\tfrac {1}{2}}N_{2}+{\tfrac {1}{2}}H_{2}} }$

Depending on the temperature range, different products are obtained which, in addition to nitrogen, contain the positive (semi) metal, chloride and hydrogen.

However, this is only one example of the numerous possibilities for converting ammonia in the gas phase. The nitrides of the (semi) metals aluminum , boron , molybdenum , silicon , tungsten and others are also important reaction products . In addition, there are a number of other variants with regard to the modification of the reaction mixtures. For example, oxygen or water can be added to the system in order to obtain oxide nitrides. In this respect, gas-phase ammonolysis is far from being an end to the wealth of variants.

literature

[1] Šingliar, U .: Dissertation, TU Bergakademie Freiberg, 2002

[2] Pätzold, C .: Dissertation, TU Bergakademie Freiberg, 2001