Molecule conversion in recombining plasmas : the role of radical-surface interactions

Abstract

The conversion of molecules proceeds commonly by activated chemical pro- cesses which require a high temperature, high pressure and often speci¯c catalytic surfaces. The use of a plasma introduces new degrees of control of the conversion processes. The feedstock gases are dissociated in the plasma, thereby eliminating one reason for activation. In particular at low pressure the association of plasma produced radicals occurs mainly at the walls of the reactor. In this thesis, the role of radical-surface interactions in a plasma reactor is investigated and details of these heterogeneous processes during exposure of the surfaces to an intense, recombining plasma are studied. To investigate the role of radical-surface interactions, the conversion of molecules in a plasma is studied in detail by means of mass spectrometry and a dedicated op- tical absorption spectroscopy system in two di®erent types of plasma reactors: an expanding thermal plasma (Eindhoven) and a microwave discharge (Greifswald). Although the reactors are fundamentally di®erent, the conversion of molecules on a global scale shows a remarkable resemblance. First of all radicals produced from feedstock gases containing N, H and O are mainly converted to N2, H2 and O2 (but also H2O). If the feedstock gases contain also C (e.g. CH4 or CO2) then CO is a major product. These results are independent of the nature of the feed- stock gases. Smaller fractions of other gases, like NO and NH3, have also been observed. Admixing O2 to plasmas created from a mixture of N2 and H2 results in both types of reactors in a signi¯cant decrease of the observed abundance of NH3. Instead, mainly H2O and NO are formed. We have studied the role of surface association in the formation of molecules by a detailed comparison of the measured composition of the studied plasmas with the results of a simulation that considers both homogeneous and heterogeneous interactions. In short, the conversion of molecules in low pressure, recombining plasma systems proceeds as follows. The injected gases are dissociated by the plasma and the plasma-produced radicals °ow through the volume towards the surfaces of the reactor where they adsorb to the surface, pick up a surface-adsorbed species or re°ect at the surface. Surface-adsorbed species also interact with each other, thereby producing new molecules that desorb from the surface. In the volume of the reactor, also gas phase interactions play a role and in°uence the composition of the plasma. The best agreement between calculations and measurements was found for low activation energies and desorption energies of the surface processes implemented in the simulation. A low surface coverage follows from the calculations. This can be interpreted as a layer of mobile species that contribute to surface association, on top of a passivating chemisorbed layer of radicals. By means of time-dependent dosing studies, the types of surface-adsorbed species during exposure of plasma produced radicals on stainless steel reactor walls are identi¯ed as: N, O, NO, H and NH2 in mixtures of nitrogen and oxygen or nitrogen and hydrogen. By comparing the results from the simulations with the measured composition of the plasma, it is shown in low pressure plasmas created from mixtures of N2 and O2 that the total rate of surface production is even in the same order as the rate of dissociation of the feedstock gases in the plasma. This indicates that surface association of plasma-produced radicals is a dominant process in the conversion of molecules in low pressure recombining plasma system