AVS 53rd International Symposium
    Plasma Science and Technology Friday Sessions
       Session PS1-FrM

Paper PS1-FrM6
Investigation of the Plasma-Activated Catalytic Formation of Ammonia in N@sub 2@ - H@sub 2@ Plasma

Friday, November 17, 2006, 9:40 am, Room 2009

Session: Plasma-Surface Interactions III
Presenter: R. Engeln, Eindhoven University of Technology, The Netherlands
Authors: J.H. van Helden, Eindhoven University of Technology, The Netherlands
P.J. van den Oever, Eindhoven University of Technology, The Netherlands
W.M.M. Kessels, Eindhoven University of Technology, The Netherlands
M.C.M. Van De Sanden, Eindhoven University of Technology, The Netherlands
D.C. Schram, Eindhoven University of Technology, The Netherlands
R. Engeln, Eindhoven University of Technology, The Netherlands
Correspondent: Click to Email
In this contribution we report on the investigation of the plasma-activated catalytic formation of ammonia in N@sub 2@-H@sub 2@ containing plasmas. The formation of ammonia is generally ascribed to stepwise addition reactions from adsorbed nitrogen and hydrogen radicals at the surface, i.e. the ammonia is formed via subsequent hydrogenation of adsorbed nitrogen atoms and the intermediates NH and NH@sub 2@ at the surface. To obtain further insight in the ammonia formation mechanism, the plasma chemistry in a plasma expansion created from mixtures of nitrogen and hydrogen is studied in more detail. The ammonia density and the NH@sub x@ radical density were determined by means of cavity enhanced absorption and cavity ring-down spectroscopy, respectively. It will be shown that ammonia can be formed efficiently in plasmas generated from mixtures of hydrogen and nitrogen. At optimal conditions 11% of the total background pressure, typically in the order of 20 - 100 Pa, was measured to be ammonia. This result turned out to be independent of the position in the plasma reactor. The NH@sub x@ radical densities, however, show a decrease as function from the distance from the exit of the plasma source, i.e. along the expansion axis. The measured NH and NH@sub 2@ densities are at maximum only about 1% of the ammonia density. Also, the NH@sub x@ radicals show the temperature of the plasma expansion, i.e. about 1500 K, while the ammonia molecules show the temperature of the background gas, i.e. about 600 K. These results indicate that the NH@sub x@ radicals are produced in the plasma expansion, while the ammonia is formed at the wall of the reactor. First results of a model describing the trends of the NH@sub x@ radical densities, indicate that NH is mostly produced in reactions of hydrogen molecules with N atoms, while NH@sub 2@ is most probably formed out of NH@sub 3@.