Study On Ni-Mo Catalysts For Dry Deforming Of Methane

Dry reforming of methane?DRM? is considered a promising process to convert two major greenhouse gases?CH4 and CO₂? into hydrogen and synthesis gas for chemical and fuel production. The ratio of H2/CO produced in DRM is close to 1/1, which is favorable for Fischer-Tropsch and acetic acid synthesis. Meanwhile, the endothermic reaction of DRM can be used for chemical energy storage. Within the reforming process, the reverse water gas shift reaction?RWGS, CO₂+H2=CO+H2O?, the methane decomposition reaction?CH4=C+2H2? and the Boudouard reaction?2CO=C+CO₂? are also involved. The RWGS reaction consumes CO₂ and H2 and produces CO, in which the overall CO₂ conversion is higher than that of CH4; meanwhile, coke may be produced via the methane decomposition and Boudouard reaction, and covers the surface of catalysts and blocks channels in catalysts, resulting in deactivation during DRM. In order to address the carbon deposition in dry reforming process, Ni-Mo2 C bimetallic and Ni-Mo/ZrO₂-MgO catalysts were prepared and applied in DRM, and the mechanism of carburization-oxidation cycle was discussed as well.In the current work, Ni Mo-Al₂O₃ samples were prepared by combustion synthesis via one-pot process using urea and starch as fuel, and then treated in a flow of CH4/CO₂ at 800 °C to in-situ synthesize the Ni-Mo₂C/Al₂O₃ catalyst, which was then tested in CH4/CO₂ for DRM. The results indicate that the Ni-Mo₂C/Al₂O₃ catalyst was successfully formed, and there was a strong interaction of Ni0-Mo2 C via in-situ carburization; within DRM, CH4 can be converted by Ni0 into intermediate species of C*, and was then further transformed into CO by the adjacent Mo2 C species in a catalytic cycle. Meanwhile, there were improved specific surface area and resistance to oxidation and sintering, as suggested by BET, XRD, XPS and SEM. As a result, the conversion of CO₂ and mole ratio of H2/CO maintained stable near 94.0% and 0.99, respectively, and the conversion of CH4 remained stable near 89.4% in a 15-h DRM test under condition of 800 °C, 12000 ml/? gcat·h? and 1 atm.The catalysts of Ni-Mo/ZrO₂-MgO were prepared by sol-gel method, and tested in DRM at 700 °C and 1 atm. Besides, XRD, H2-TPR, CO₂-TPD, BET, SEM-EDX, CH4-TPD and TG/DTA were employed to analysis structure-reactivity relationship of the Ni-Mo catalysts. The results show that the Ni-based catalyst with doping of Mo and Zr?Ni0.1Mo0.01/0.10 ZrO₂-MgO? exhibited a better performance: the conversions of CH4 and CO₂ remained stable at 60.0% and 71.2%, respectively, and the H2/CO ratio reached 0.93, while the selectivity of H2 reached 64.9% in 10-h test at 700 °C, 1 atm and 12000 ml/?gcat·h?. Characterization results show that the addition of Mo improves the performance of catalyst by means of 1) Ni-Mo strong interaction, 2) enhancement of strong alkaline sites and 3) improvement of the chemical adsorption of methane. Meanwhile, resistance to oxidation and sintering was found in Ni0.1-Mo0.01/MgO catalyst. In the mean time, with promotion of ZrO₂, there was 1) increased specific surface area, 2) improvement of the high-temperature strong alkaline, and 3) resistance to coking.