Surface catalytic contributions to molecule conversion in plasmas

The contribution of surface-related processes to the formation of new types of molecules in a recombining plasma with a low electron temperature is investigated. The recombination of a highly dissociated mixture of nitrogen and oxygen is studied with a combination of tuneable diode laser absorption spectroscopy and mass spectrometry. A simulation in CHEMKIN, based on a simplified set of chemical reactions, has been developed to describe the system in detail and to determine the contributions of volume processes and surface-related processes. With a sticking coefficient of 0.1 for the radials for all studied conditions, a chance of unity for Eley-Rideal processes to be successful, a desorption energy of 0.7 eV for NO molecules and relatively low activation energies, around 0.5 eV, for the Langmuir-Hinshelwood processes gives a good agreement with the measurements. We show that NO$_{2}$ can only be formed at surfaces in our system, whereas NO and N$_{2}$O are at least for a significant fraction formed at the surfaces of the reactor. Especially at low pressure conditions and at low oxygen admixture, the role of the surfaces is pronounced.