Synergistic effect of non-thermal plasma–catalysis hybrid system on methane complete oxidation over Pd-based catalysts

Abstract

Abstract The complete oxidation of methane was carried out in a dielectric barrier discharge (DBD) quartz tube reactor where both catalyst and plasma were hybridized into one in-plasma catalysis system. The palladium-based catalysts such as Pd/Al2O3, Pd/SiO2, and Pd/TiO2 were used as oxidation catalyst. Input voltage of the plasma–catalyst reactor varied from 2 kVp-p to 4 kVp-p to investigate which input voltage offered the best circumstance for plasma–catalyst interaction. In the absence of catalyst, methane began to be oxidized to CO and CO2 even at room temperature, and the conversion increased with the increment of temperature and the input voltage since the active radicals were generated more abundantly under those conditions. However, large amount of CO were also produced in addition to CO2, especially at low temperature below 200 °C when plasma was only used. In the presence of both plasma and catalyst, methane was oxidized to produce mostly CO2 with low CO selectivity at room temperature, indicating that the complete oxidation was successfully performed with the aid of catalyst. The role of plasma was to oxidize CH4 to produce CO, which was subsequently oxidized to CO2 over catalyst at low temperature. Hence, in most cases, the methane conversion of plasma–catalysis hybrid system was almost equal to the summation of two separate systems. Interestingly, it was found that the synergistic effect of plasma–catalysis hybrid system on methane oxidation existed substantially only under specific condition. For example, 2 wt% Pd/Al2O3 presented higher methane conversion than the summation of conversion for catalyst only reaction and plasma only reaction when 4 kVp-p was applied.