Modification des processus hors équilibre au cours de l'interaction entre un plasma basse pression et une surface. I. Cas d'un plasma de NH3. Application à la catalyse chimique

Abstract

2014 Dans cette étude nous avons mis en évidence les relations existant entre les processus de catalyse ou d'inhibition, le caractère hors équilibre du plasma et les transferts d'énergie et de matière, au cours de l'interaction entre un plasma NH3 basse pression et une surface. Nous avons établi que l'activité catalytique ou inhibitrice d'un matériau est liée à son aptitude à modifier la répartition de l'énergie vibrationnelle du plasma dans la couche limite. L'ensemble de nos résultats expérimentaux, les analyses spectroscopiques et de surface, ont mis en évidence une réaction de nitruration des surfaces reliée à la chimisorption dissociative des molécules. Tout matériau qui augmente le caractère hors équilibre du plasma, favorise un processus catalytique et facilite l'étape de nitruration. Tandis que tout matériau qui thermalise le plasma, inhibe la réaction de décomposition et défavorise l'étape de nitruration. Abstract 2014 The experimental study of the interaction between a low pressure NH3 plasma and a surface has pointed out the modification of the non equilibrium processes. In this way, we have established the correlation between the heat and mass transfers in the boundary layer, the non equilibrium character of the plasma and the catalysis or inhibition process. The main results have been obtained by using a spectroscopic analysis of the boundary layer, in order to measure the vibrational and rotational levels in function of the nature of the material. These results point out that the nitriding reaction of the solid surface (ESCA measurements) is correlated with the vibrational level of the N2 molecule and the temperature of the solid. This conclusion is in agreement with the following result : the NH3 decomposition rate increases with the vibrational N2 temperature and the nitriding of the surface. So we can conclude that the catalytic or inhibitor effect during the interaction plasma-surface is controlled at first, by the energy balance between the boundary layer and the surface of the material, and in second by the dissociative chemisorption of the reactive species during the mass transfer. Any material which increases the non equilibrium character of the plasma favours a catalytic process and makes easy the nitriding step.

Direct Amination of Benzene with Ammonia by Flow Plasma Chemistry

“…Emission peaks could be observed in the range 200-1000 nm, which allowed to identify excited species contained in the gas phase (Figure 2a). The different identified peaks are located at 315 nm (N 2 * C 3 Q u !B 3 Q g 1-0), [22] 336 nm and 337 nm (NH°biradicals and N 2 * C 3 Q u !B 3 Q g 0-0, respectively), [22][23][24][25][26] at 357 nm (N 2 * C 3 Q !B 3 Q 0-1) [22,23,25,26] and 380 nm (N 2 * C 3 Q u !B 3 Q g 0-2). [22] This analysis confirmed the dissociation of ammonia in the plasma with the formation of NH°and excited neutrals N 2 *.…”

“…The different identified peaks are located at 315 nm (N 2 * C 3 Q u !B 3 Q g 1-0), [22] 336 nm and 337 nm (NH°biradicals and N 2 * C 3 Q u !B 3 Q g 0-0, respectively), [22][23][24][25][26] at 357 nm (N 2 * C 3 Q !B 3 Q 0-1) [22,23,25,26] and 380 nm (N 2 * C 3 Q u !B 3 Q g 0-2). [22] This analysis confirmed the dissociation of ammonia in the plasma with the formation of NH°and excited neutrals N 2 *. Although it cannot be observed by OES, NH 2 °is expected to be the major radical formed by ammonia dissociation.…”

Corona Corrosion of Aluminum in Air

“…The large amounts of dissociated and excited nitrogen and oxygen in coronas were not directly measured, but are inferred from calorimetric studies (19,30). Furthermore, it follows from different gas discharge studies (34)(35)(36) that the material of the electrodes can have a significant influence on the gas chemistry.…”

Mécanismes de décomposition catalytique dans un plasma d'ammoniac basse pression. Etude cinétique en présence d'argon comme actinomètre

“…Des travaux antérieurs effectués au laboratoire (Réfs. [2,5,16]) ont permis d'établir que les lois de Polanyi sont applicables aux milieux complexes tels que les plasmas hors équilibre. Nous avons pu ainsi montrer que la décomposition de NH3 en phase gazeuse est contrôlée par une étape endothermique, de type NH3 (v ) + e---+ NHx (J ) + H3 _ x + e-, puisqu'une diminution de pression de 20 à 5 mbars accentue le caractère hors équilibre du plasma (Tvib augmente de 3 000 à 3 300 K et Trot diminue de 1 207 à 817 K) ; simultanément, le taux de décomposition de NH3 augmente de 34 à 42 %.…”