Study On Catalytic Roles Of Molybdenum ?Tungsten? Carbide For CH₄/CO₂ Reforming Over Ni Based Catalysts

For the energetic utilization of anthropogenic CO2,dry reforming of methane (DRM) is a possible option to convert these two greenhouse gases into syngas (CO/H2 mixtures).This reaction produces syngas that can be used to produce a wide range of products, such as higher alkanes and oxygenates by means of Fischer-Tropsch synthesis. So, carbon dioxide reforming of methane has multiple values in terms of the science, economy and environmental protection.Due to the similar fermi energy, the transition metal carbides perform similarly with the noble metals in a variety of catalytic reactions. Based on such properties, a bi-functional catalyst of nickel modified molybdenum carbide was built to catalyze the DRM reaction, where Ni is to enhance the dissociation of CH4 and ?-Mo2C is to enhance the dissociation of CO2. In the present study, the influence of preparation methods of NiMoOx precursors on the process of carbonization, the microstructure of the obtained carbides, and therefore, their catalytic properties for the DRM reaction were studied. The important roles of NiMoO4 as precursors for carbonization into Ni/?-Mo2C were addressed for the first time. On the other hand, we loaded Ni/?-Mo2C on La2O3 for a better dispersion. However, Ni/?-Mo2C/La2O3 changed its phase to Ni/La2MoO6 during the DRM reaction. The catalytic roles of ?-Mo2C in participating the oxidation and re-carbonation redox cycle was clarified. With the similar research idea, we found the formation of ?-WC over Ni17W3/SiO2 catalyst during the DRM reaction. ?-WC participated CO2 activation, leading to a less amount of carbon deposition, which contributed to the better stability of Ni17W3/SiO2 compared with Ni/SiO2. The main results were summarized as follows:(1) The influence of NiMoO4 precursors on the catalytic behaviors of Ni/?-Mo2C catalysts for CH4/CO2 reforming was investigated. The important roles of NiMoO4 as a precursor for the generation of Ni/?-Mo2C by carburization are addressed for the first time. Due to the formation of NiMoO4 (CP), molybdenum can be reduced at a temperature lower than that required for MoO3 reduction. Accordingly, with the modification of Ni, the carburization temperature for ?-Mo2C formation can be lowered. Moreover, in the form of NiMoO4, nickel is better stabilized compared to that in the form of NiO, and the sintering of metallic nickel during high-temperature carburization is lessened. With smaller Ni particles and higher CH4 dissociation rate, Ni/?-Mo2C (CP) performs better than Ni/?-Mo2C (MM) in CH4/CO2 reforming.(2) We loaded N1/?-Mo2C on La2O2 for a better dispersion and improved efficiency of Ni and ?-Mo2C. However, Ni/?-Mo2C/La203 changed its phase to Ni/La2MoO6 during the DRM reaction, leaving only a small amount of ?-Mo2C. Accordingly, Ni/La2MoO6, NiMoOx/La2O3 and Ni/La2O3 catalysts were prepared and their catalytic activities for the DRM reaction were compared with that of Ni/?-Mo2C/La2O3. It was found that the Mo-containing ones were superior to Ni/La2O3 in catalytic stability in DRM reaction. The addition of Mo to Ni/La2O3 resulted in better resistance towards carbon deposition, and therefore catalytic stability. For the first time, the better resistance to carbon deposition observed over the Mo-containing catalysts was attributed to the formation of ?-Mo2C, which acted as an important intermediate in the oxidation and re-carbonation redox cycle.(3) Ni17W3/SiO2 (Ni 10.7 wt.%, W 5.9 wt.%) and Ni/SiO2 (Ni 10.7 wt.%) catalyst were prepared and their catalytic activities for the DRM reaction were compared. It was proved that nickel was stabilized by Ni17W3 alloy, and the particle size of Ni17W3 kept almost unchanged during the DRM. However, metallic nickel aggregated seriously over the spent Ni/SiO2. A better coke resistance obtained over Ni17W3/SiO2 catalyst was related to ?-WC formed during the reaction, which could participate the CO2 activation. Therefore, Ni17W3/SiO2 catalyst exhibited a better performance compared with Ni/SiO2 for the DRM reaction. The present study provides a possible solution for the problem of sintering and coking for the nickel catalysts.