Nature et caractéristiques des réactions de l'ammoniac avec le carbone porté à très hautes températures

Laboratoire de cinétique hétérogène. Université de Nancy I, B.P. 239, 54506 Vandoeuvre Les Nancy Cedex, France.

Résumé

En présence d’ammoniac sous de basses pressions (10^-4-10^-2 torr) une surface de carbone maintenue à hautes températures (1000-2000°C) est le siège d'une gazéification du carbone suivant NH₃ + C = HCN + H₂ et d'une décomposition de l'ammoniac suivant NH₃ = 1/2 N₂ + H₂. Certaines des singularités cinétiques des réactions sont liées à la réactivité variable de la surface du carbone, phénomène connu car observé aux très hautes températures dans toutes les réactions du carbone étudiées à ce jour. D'autres sont spécifiques des interactions carbone-ammoniac.

Abstract

The reaction of various kinds of carbon with ammonia was investigated at high temperatures (1000-2000°C) and low pressures (10^-4-10^-2 torr). Mass spectrometric data indicate that a decomposition according to NH₃ = 1/2 N₂ + 3/2 H₂ takes place in addition to the carbon gasification process following NH₃ + C = HCN + H₂. Above 1500°C indications suggest the possible formation of some CN radicals.

With increasing temperatures ammonia decomposition rates go through a maximum whereas carbon gasification rates show two maxima. These variations are largely related to the variable reactivity of a given carbon sample which is strongly temperature dependent as shown earlier in many other carbon reactions at high temperatures. However, with amorphous and pyrolytic carbons – but surprinsingly not with Acheson graphites – it was found that, even by taking into account this well recognized surface property, the kinetics exhibit unusual features, especially between 1500 and 1800°C (negative temperature coefficient, order of reaction greater that one). These observations are correlated to the fact that only a fraction of the NH₃ molecules which adsorb on the carbon surface lead to carbon gasification – while the rest gives molecular nitrogen and hydrogen – and that this fraction is higly temperature and pressure dependent.