Understanding Temperature Inhibition of Methane Conversion in DBD Plasma Systems

Low-temperature non-thermal plasmas (LTPs) are known to produce highly reactive chemical environments. The combination of these reactive species with a catalyst can help drive thermodynamically unfavorable reactions and produce a synergistic conversion effect. There is appreciable promise in the direct coupling of methane (CH₄) with nitrogen (N₂) to produce value-added chemicals using plasma catalysis. In order to create effective plasma catalytic systems, it is important to understand the fundamentals of plasma-phase chemistry alone. Much research has gone into understanding how certain operating conditions affect the plasma, but there is limited knowledge of how bulk reaction temperature affects the plasma and ensuing plasma chemistry. In this work, we use a dielectric barrier discharge (DBD) to investigate the effects of operating conditions, specifically temperature, on the chemical transformation of methane species and correlate these to the plasma’s electrical and optical properties in various methane-gas mixtures. Results show an increase in temperature leads to a reduction in the conversion of methane. This can be attributed to two possible causes: an increase in the conductivity of the gas prior to plasma ignition affecting plasma electrical properties and changes to the thermal chemical reaction kinetics. Both situations then lead to a plasma environment where methane conversion is limited. We also observe a positive correlation between key electrical plasma properties (average charge and lifetime per filament) and conversion at various operating conditions, which provide insight into a relationship between plasma properties and chemical transformations.