The Preparation Of Highly Active And Coke Resistant Ni-based Catalysts For Methane Reforming For Hydrogen Production

Hydrogen is one of the most promising clean fuels, because of its high energy density around 120.7kJ/g, and theoretically, it combusts without emitting any environmental pollutants. Up to date, the production of hydrogen has been tried by different ways from natural gas, such as steam reforming and dry reforming etc. Among these processes, methane steam reforming has been widely industrialized for commercial hydrogen production due to the abundance of natural gas. Over recent years, dry reforming of methane has been widely studied due to its high efficiency and relatively simple requirements for the process. At the same time, this process consumes a significant amount of greenhouse gas CO2, which is of importance on environmental protection.Ni-based catalysts have been extensively investigated owing to their initial high activity and low cost. However, the major drawback of Ni-based catalysts is their quick deactivation due to severe carbon deposition and the sintering of active Ni sites during the reaction. To develop and design improved catalysts with higher efficieny, it is necessary to study the reaction mechanism, kinetic properties, carbon deposit behavior of methane reforming over Ni-based catalysts; and the interaction between Ni active species with the supports, the effect of Ni size on the reaction performance and how to intentinally control the Ni size.First, mesoporous Ni-Al2O3 catalysts were prepared facially in one pot with evaporation-induced self-assembly method and use for CH4 dry reforming. Compared with the traditional Ni/Al2O3 catalyst prepared with impregnation method, the EISA catalysts display significantly improved coke-resistance as well as activity. It is revealed by SXRD, N2 adsorption-desorption and TEM that ordered mesoporous structure has been formed in these catalysts prepared with EISA method, which is believed to be favorable for the mass transfer for the reaction. In addition,the Ni species in the reduced EISA samples has much higher dispersion, more uniform distribution and smaller crystallite size. After all, these are believed to be the major reasons accounting for the significantly improved dry reforming performance of the EISA catalysts. The modification of the catalyst by ZrO2 played an important role in improving the catalytic activity and suppressing the carbon deposition.Second, mesoporous SiO₂ support was prepared by a simple soft template synthesis method, which were used to support Ni to prepare catalysts for methane dry reforming for hydrogen production. It was revealed that the Ni/m-SiO₂ catalysts modified by a suitable amount of Ceria showed excellent performance for reaction. The promoted catalytic activity was found to be associated with the special mesoporous and large surface area, which could reduce the Ni crystallite size and improve its active metal surface area. Therefore, the activity of the CeO2-modifed catalysts was evidently improved but the coke formation was suppressed. The CeNi/m-SiO₂ catalysts were proved to be a promising catalyst for the real methane dry reforming process.Third, Y2Zr2O7 pyrochlore supports were synthesized by co-precipitation, glycine-nitrate combustion and hydrothermal methods, which were used to support 10% Ni to prepare catalysts for methane steam reforming for hydrogen production. H2-TPR demonstrates that the Ni active species has stronger interaction with Y2Zr2O7-GNC than with other two supports. Therefore, much smaller Ni crystallite sizes with higher metallic active surface area have been achieved. As a consequence, Ni/Y2Zr2O7-GNC exhibits the highest activity and stability, and the most potent coke resistance among all the catalysts, for which no evident coking and deactivation was observed even after testing at 800 oC under 20 atm for 200 hours.Last, Ni/Y2Zr2O7 catalysts prepared with impregnation method and treated by DBD plasma in different atmospheres have been investigated for methane dry reforming. It is revealed by H2-TPR that plasma treatment can enhance the interaction between NiO/Ni particles and the Y2Zr2O7 pyrochlore support. Therefore, catalysts with smaller NiO and Ni grains sizes, higher metallic Ni active surface areas can be achieved. As a consequence, the plasma-treated catalysts show significantly improved activity, stability and coke resistance...