Applied Basic Research On Cold Plasma-catalytic Oxidation Of Methane To Methanol

With the lack of petroleum resources in the world, the development of natural gas and methane hydrate (flammable ice) utilization as the feedstock, instead of crude oil, for the in-situ production of oxygenates has been extensively studied. Cold plasma offers a unique way to excite stable methane molecule under mild conditions. High conversion and selectivity are expected to be obtained by exploiting the inherent synergetic potential of plasma through combination with heterogeneous catalysts. In our study, methane-air partial oxidation to methanol (MPOM) was investigated using plasma-catalysis hybrid system.Firstly, MPOM through a dielectric barrier discharge (DBD) plasma reaction was performed at ambient temperature and atmospheric pressure. The effects of input power, discharge frequency, discharge gap distance, residence time, and CH4/air ratio on CH4 conversion and CH3OH yield were studied, and those parameters were optimized. Moreover, the discharge intensity and the reaction efficiency were greatly enhanced with the addition of inert gases such as argon and nitrogen. It was found from optical emission spectrometry that free radical reactions were of importance for initiating methanol synthesis.Secondly, combination of heterogeneous catalysts with non-thermal plasma was operated in two configurations:post-plasma catalysis (PPC) or in-plasma catalysis (IPC). In the former case, Fe2O3-based catalyst showed the best catalytic activity and high stability in the reaction. The CH3OH selectivity of 10.66%was obtained over Fe2O3/CP at the rather low catalyst temperature (150℃), which was 35.1% higher than that of non-catalytic system. Addition of the CuO promoter to Fe2O3/CP facilitated selective methane oxidation. At the same catalyst temperature, the CH3OH selectivity achieved with the Fe2O3-CuO/CP catalyst was at 11.33%. For the IPC configuration, the average electric field in a packed-bed reactor would be enhanced compared with the non-packed plasma reactor. Furthermore, the reaction efficiency over the Fe2O3-CuO/γ-Al2O3 catalyst was improved due to the enrichment of reactants and radicals on the catalyst surface. However, the alumina based catalyst exhibited superior anti-deactivation ability and reaction stability in the PPC process compared to the IPC process. In addition, the main pathways to methanol synthesis in the different processes were investigated based on the behavior of both short-and long-lived species.Finally, the Fe203-Cu0/y-Al203 catalyst was modified by Ar plasma treatment using two methods. Modification of catalyst before calcination was preferred because it could improve the dispersion of active species and enhance the metal-support interaction, which led to high catalytic activity and excellent resistance to carbon formation.