Probing the Detailed Chemistry of Plasma-Assisted Processes: Opportunities for Mass Spectrometry

Mass spectrometry is a perfect analytical tool to study complex reaction networks as found in low-temperature plasma environments. Using this technique, unprecedentedly detailed chemical insights are generated as it allows for simultaneous and sensitive detection of all intermediates and products of the reaction network without prior knowledge of their identity. When combined with a molecular-beam sampling approach, it even allows for the detection of short-lived open-shell reactive species. In this presentation, we will present how molecular-beam mass spectrometry was used to gain chemical insights into plasma-assisted chemical looping and combustion processes. For both experiments, we coupled low-temperature plasma flow reactors via a differentially pumped molecular-beam sampling unit with a reflectron time-of-flight mass spectrometer. The spatially uniform plasmas were generated through dielectric barrier discharge (DBD), powered by a high voltage, short-pulsed power supply. We explored multi-dimensional parameter spaces for the plasma-assisted oxidation of the simple hydrocarbons methane and ethylene using CuO and NiO, and the plasma-assisted combustion of ammonia. To obtain a comprehensive understanding of these processes, we accumulated mass spectra as a function of plasma power, temperature, residence times, and reactant composition. Time-resolved mass spectra of the plasma-assisted chemical looping process show that the plasma discharge lowers the fuel oxidation temperature to 400-500°C and kinetically enhance fuel oxidation processes. Kinetic enhancements were also observed for ammonia combustion near room temperature, thus minimizing toxic thermal NOx emissions.