The methane oxidation process has attracted great research attention in recent years because of its vital role in chemical synthesis and hydrocarbon fuels combustion. Here we demonstrate three kinds of methane plasma oxidative pathways including selective oxidation to H2O2 and organic oxygenates, oxidative coupling of methane to C-2 hydrocarbons, partial oxidation of methane to synthesis gas, respectively, can be strengthened by altering the contribution of oxygen species. Based on the diagnosis results of optical emission spectra and the kinetic data of combustion modeling, it can be deduced that the reaction of O(D-1) + CH4 -> OH + CH3 plays a key role for plasma promoted methane conversion, on the other hand, the ground state of O leads to synthesis gas production through the reaction of O with CH3 and CH2. We believe our discovery may be of a general nature for methane plasma oxidation and can enlighten for understanding the plasma oxidation and combustion chemistry.

Conversion of CO2 to CO by H2 has been recognized as one of the important processes for CO2 utilization, but it is difficult to obtain high CO yield by catalytic method. We discussed the role of plasma activation in the direct gas phase reduction of CO2 to CO. The plasma was obtained by dielectric barrier discharge to the mixture of H2 and CO2 at room temperature and atmospheric pressure. Moreover, we investigated the effects of main parameters on CO2 conversion, CO selectivity and energy efficiency of CO2 conversion , such as discharge power,the structure of plasma reactors, the ratio of H2 to CO2 as well as space between electrodes. It is observed that, at room temperature and atmospheric pressure, H2 and CO2 can be directly converted into CO only by plasma activation. The proper adjustment of above parameters can increase CO2 conversion. When the CO2 flow rate of the feed gas, the ratio of H2 to CO2, the discharge power, and the discharge frequency are fixed at 120 mL/min, 80 W and 10 kHz, respectively, the CO2 conversion efficiency and CO selectivity are 88.2% and 100%, respectively.

Hydrogen peroxide is an important green oxidizing agent. The conventional process for hydrogen peroxide production is the indirect anthraquinone process, which employs multiple unit operations, generates considerable waste, and requires significant energy input. Hence, the development of a simple and highly efficient process for the synthesis of hydrogen peroxide 3] is of great scientific and practical importance. The direct synthesis of H2O2 from H2 and O2 is a much greener route, and supported Pd and Au–Pd alloy catalysts are known to be effective. However, an inherent hazard of this direct route are the very wide flammability limits of H2/O2 mixtures (4– 94 mol %). Hence, safe working practices stipulate a H2 concentration below 4 mol %, and this limit greatly reduces the H2O2 formation rate. Natural gas, of which CH4 is the main component, is an inexpensive and abundant resource with a low environmental impact. Considerable efforts to develop processes for converting CH4 into more valuable products have been made. The most extensively studied processes are oxidative coupling of CH4 ; [11, 12] partial oxidation of CH4 to synthesis gas; [13] and the formation of oxygenated compounds, including methanol, formaldehyde, and formic acid. 20] To the best of our knowledge, H2O2 has not been effectively produced by oxidation of CH4, although H2 can be manufactured from CH4 and H2O2 can be produced by oxidation of H2. Spectroscopic evidence of the formation of H2O2 in a microwave discharge plasma of CH4/ O2 has been reported. [21] We have shown that the structure of the discharge reactor plays an important role in the direct synthesis of H2O2 with the plasma method. With a specially designed plasma reactor, an O2 conversion of 57.8 % with a H2O2 selectivity of 56.2 % can be obtained in the gaseous plasma of a H2/O2 mixture. Taking all these observations into account, the effective formation of H2O2 might be achieved by CH4/O2 discharge with a proper plasma reactor. Herein, we report that in a double dielectric (DD) plasma reactor a satisfactory yield of H2O2 can be achieved under ambient temperature and atmospheric pressure from a stoichiometric (1:1) feed of CH4 and O2 by using the plasma method. The oxidation of CH4 to H2O2 offers considerable advantages over the oxidation of H2 to H2O2 because valuable organic oxygenates (methanol, formaldehyde, and formic acid) can be effectively produced at the same time. Furthermore, a wide range of CH4 concentrations can be used without any explosion hazards. The DD plasma reactor was prepared according to our previous work; one modification is the use of a nonmetal composite high-voltage electrode. We call this reactor a DD plasma reactor because it uses two dielectrics. As shown in Table 1, when a CH4/O2 mixture containing 50 mol % O2 is fed into the DD plasma reactor at a total flow rate of 50 mL min 1 (CH4+O2), the reactor converts 90.8 % of the O2 and generates