Methane activation at low temperature in the presence of oxygen using plasma-catalyst hybrid system

Abstract

Natural gas is one of the most abundant fossil fuels around the world. Cleaner energy source than other fossil fuels and world-wide presence of natural gas make it an attractive energy source. Despite of these advantages, the emission of unburned natural gas makes it difficult to use since the methane is recognized as the major portion of global warming gas. It contributes 25–34 times more to global warming than CO2 at equivalent emission rate and has quite long lifetime. Therefore, complete oxidation of methane is one of the critical problems to solve for widening the use of methane without worrying about the environmental concern such as global warming. Except for the role as fuel, the utilization of methane has been limited mostly to the route of synthesis gases to produce liquid hydrocarbons and other chemical products, which is regarded as indirect methods. Such indirect process has weaknesses of high operating cost and low thermodynamic efficiency due to the swing between endothermic and exothermic reaction. Hence, if methane is directly utilized as alternate feedstock to petroleum, it will be highly desirable from the economic point of view. Thus, many efforts have been made for the direct conversion of methane into more useful products like olefins, aromatics, and alcohols by using various catalysts for decades. However, harsh reaction conditions including high temperature and pressure are required to start the catalytic reaction such as complete oxidation of methane and direct conversion of methane to value-added chemicals because methane can be hardly activated due to its stable C-H bond. The methane activation is the key step to initiate such reactions. In order to overcome these difficulties, various catalysts were investigated and applied. Nevertheless, the activation of methane is still hard to be carried out because tough reaction condition can deactivate the catalyst. An alternative way to activate methane at low temperature would be to use plasma. There are various thermal and non-thermal plasma sources such as dielectric barrier discharge (DBD), corona, gliding arc, rotating arc, spark, microwave, glow discharge and pulsed discharge. In the non-thermal plasma, high-energy electrons (1-20 eV) are produced and they can initiate the formation of other various radicals. Since electron mass is very light, non-thermal plasma gives rise to the increase in temperature by only few degrees. In this work, dielectric barrier discharge (DBD) plasma was used since it is easier to set up than other non-thermal plasma sources. Firstly, 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. Non-PGM catalysts such as Co1Ni1Ox and CoCr2O4 were used as oxidation catalyst. Input voltage of the plasma-catalyst reactor maintained to 4kVp-p to minimize the effect of plasma power 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 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, however, methane was oxidized even at room temperature mostly to CO2 with low CO selectivity over certain non-PGM catalyst like Co1Ni1Ox, 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, methane complete oxidation reaction proceeded at much lower temperature similar to PGM catalyst such as Pd/Al2O3, while maintaining low CO selectivity. Next, oxidative coupling of methane (OCM) was carried out to produce C2 or C3 hydrocarbons from methane under plasma-catalyst hybrid system. Dielectric barrier discharge (DBD) plasma was applied as plasma source to lower the reaction temperature since catalyst only reaction required high temperature above 700 °C. Plasma only reaction was performed to compare with plasma-catalyst hybrid reaction. We tried to seek appropriate support under plasma-catalyst hybrid reaction at low temperature. Among various supports, only SiO2 has shown the higher yield when combined with dielectric barrier discharge plasma than plasma only reaction. When various metals were impregnated on SiO2 to investigate the effect under plasma condition, it was found that Ag/SiO2 demonstrated the highest C2+ hydrocarbon yield of about 10% below the reaction temperature of 400 °C. In this process, oxygen was proved to play an essential role in the coupling of methane to C2+ hydrocarbons over Ag/SiO2 catalyst. However, Ag/SiO2 catalyst under plasma condition became deactivated with time-on-stream because of coking. However, during the stability test with time-on-stream, Ag/SiO2 catalyst with plasma became deactivated due to coking. Therefore, regeneration process was introduced after OCM reaction. As a result, it was found that plasma regeneration at 378 °C gave rise to the full recovery of activity while thermal regeneration did not due to partial removal of coke and sintering of Ag. Finally, the direct methanol synthesis from methane was carried out in a plasma-catalyst hybrid system. Since catalyst only reaction requires high pressure and batch reactor, dielectric barrier discharge (DBD) plasma was applied as a plasma source to overcome the difficulties. Among the transition metal oxides, Mn2O3-coated glass bead showed the highest methanol yield about 12.3% in the plasma-catalyst hybrid system. The reaction temperature was maintained below 100 °C because of low plasma input power (from 1.3 kJ/L to 4.5 kJ/L). Furthermore, the reactivity of the catalyst was maintained for 10 h without changing the selectivity. The mechanistic study indicated that the plasma-induced OH radical generated on the transition metal oxide catalyst possessed high selectivity toward methane to produce methanol.