Experimental And Density Functional Theory Study On Plasma Methane Conversion

The application of discharge plasma on methane conversion has recently drawn more and more attentions. A lot of experimental works have been reported on plasma methane conversion. However, the lack of basic research on this topic has greatly restricted the development of the related technology. In this work, by using density functional theory study, two fundamental issues on plasma methane conversion, the thermodynamic feasibility of plasma direct methane conversion and the principal mechanism of cold plasma preparation of the high performance catalysts for catalytic methane conversion, were systematically investigated at a molecular and electronic level for the first time. An experimental investigation has been conducted first to confirm the effectiveness of plasma prepared catalyst. Ni-Fe/Al2O3 catalyst for partial oxidation of methane was prepared by glow discharge plasma treatment. The conversion of methane and the selectivity of CO and H2 over plasma treated catalyst are 97.44, 100 and 100% at 875 ℃, while, at the same temperature, they are 90.09, 97.28 and 97.09%, respectively, over the catalyst prepared without plasma treatment. At the same methane conversion, the reaction temperature with plasma treated Ni-Fe/Al2O3 catalyst is at least 80 ℃ lower. In addition, the plasma treated Ni-Fe/Al2O3 catalyst also possesses better anti-carbon deposit ability than the catalyst prepared without plasma treatment.The previous studies in our research group showed that the acid amounts of zeolite catalysts prepared with plasma treatment is generally increased significantly. After plasma treatment, the catalytic active species can highly disperse on the support and the stability of the catalyst for methane combustion will be greatly improved. In this work, the structural and electronic properties of PdO/HZSM-5 for methane catalytic combustion and the relationship between PdO and acid sites of HZSM-5 were investigated by using the density functional theory methods. The study shows that the local structure of PdO supported on HZSM-5 is very similar to the four-coordinated square planar structure of bulk PdO. The acid sites of HZSM-5 can make PdO highly dispersed and hinder the formation of aggregated PdO, which is very necessary for keeping the high catalytic activity. In addition, the breakdown of the first C-H bond of methane molecule was also investigated by the cluster models, which is the rate-determining step of methane combustion reactions. It shows that the methane oxidation follows the redox or Mars and van Krevelen mechanism. Methane shows different adsorption and dissociation modes on PdO/HZSM-5 with the different acid amounts. And the catalytic activity and stability will improve with the increasing of acid amounts. It is concluded that the different acid amounts of HZSM-5 can lead to different structural and electronic properties of PdO and farther induce different catalytic activity. These conclusions are very consistent with the experimental results. Finally, a density functional theory study has been conducted to investigate the reaction mechanism of synthesis of oxygenates and higher hydrocarbons from methane and carbon dioxide at a molecular level for the first time. The main dissociation routes of reactants, important intermediate reaction steps and the possible reaction pathways to form the ultimate products (including alcohols, acids, higher hydrocarbons and syngas) have been investigated. This study shows the major obstacle of the synthesis is the dissociation of the reactants, CH4 and CO2. After the cold plasma has supplied the necessary energy for the dissociation of CH4 and CO2, syngas, higher hydrocarbons and oxygenates can be easily produced.