Catalytic oxidation of methane under non-thermal plasma condition

Abstract

As one of greenhouse gases, methane is recognized to contribute to the major portion of global warming. The stable C-H bond in methane requires the large amount of noble metal catalyst to be oxidized completely at low temperature (i.e. below 500℃). Thus, we aim at lowering the light-off temperature by introducing the plasma-catalyst hybrid reaction system. The catalytic reaction needs activation energy to induce the reaction, so the motivation of this research is that the plasma is able to help reduce the activation energy by synthesizing the active radicals. There are two types of plasma source, one is thermal plasma and the other is non-thermal plasma. The thermal plasma had a possibility to interrupt the examination of plasma-catalyst hybrid interaction by increasing the catalyst bed temperature, thus the non-thermal plasma, especially the dielectric barrier discharge(DBD) was used in this research. In this experiment, the complete oxidation of methane was carried out in a DBD quartz tube reactor. Catalyst and plasma were hybridized into one in-plasma catalysis system. The palladium-based catalysts such as Pd/Al2O3, Pd/CeO2, Pd/Ce0.7Zr0.3O2, Pd/SiO2, and Pd/TiO2 were used as oxidation catalyst because palladium-based catalysts have shown the greatest oxidative ability of methane so far. In order to separate the catalytic effect from the plasma one, methane oxidation was evaluated over the catalyst in the presence or in the absence of plasma at the fixed plasma operating conditions including waveform, and frequency. Though, input voltage of the plasma-catalyst reactor varied from 2kVp-p to 5kVp-p to observe which input voltage offered the best circumstance for plasma-catalyst interaction. Also, to measure plasma effect, plasma power was calculated by V-Q Lissajous figure method. In the absence of catalyst, the methane was started to decompose from room temperature, and the conversion increased with the increment of temperature since dielectric constant of dielectric(quartz tube reactor) were changed along the temperature. In these reactions, not only CO2 but CO was also produced. As the input voltage increased from 2kVp-p to 4kVp-p, the methane conversion also increased sharply because of the upsurge of specific input energy. In the presence of catalyst alone, methane started to be activated above 200℃ for all the cases. However, in the presence of both plasma and catalyst, methane was oxidized at room temperature and the selectivity of CO which should not be produced was retained at zero percent. These results cannot be achieved in the cases of both catalyst only reaction and plasma only reaction. As the input voltage went higher, the plasma influence got stronger, so the catalytic performance was hardly observable. At low input voltage like 2kVp-p, all plasma-catalyst hybrid reaction did not shift the reaction-end temperature, but showed the activation of methane at room temperature and catalytic performances while they disappeared at higher input voltage. Also, Pd/Al2O3 presented slightly higher methane conversion than plasma only reaction at all input voltage conditions. In terms of energy efficiency, it was shown that using relatively low specific input energy could enhance the methane oxidation conversion even at ambient temperature. In summary, it was found that non-thermal plasma played an important role in converting methane to CO/CO2 in the temperature range where catalyst hardly worked, thus leading to facilitate methane oxidation with catalyst at lower temperature.