Preparation Of Novel Silicon Carbide Supports For Carbon Dioxide Reforming Of Methane

A large number of emissions of two major greenhouse gases ?CH₄ and CO₂? causes global warming and brings about a series of detrimental effects. Therefore, effective control and utilization of greenhouse gases has been urgent. The progress of CO₂ reforming of methane to syngas can alleviate the greenhouse effect and simultaneously obtain the alternative energy. Therefore, it is significant for the promotion of clean energy production and environmental protection.Ni-based catalysts in CO₂ reforming of methane require high temperature reaction conditions, suggesting high energy consumption. Moreover, Ni-based catalysts are susceptible to deactivation due to metal sintering and carbon deposition. Due to the above obstacles, the industrial application for CO₂ reforming of methane is still not realized. Silicon carbide exhibits properties such as good thermal conductivity, high chemical stability, high mechanical strength and high resistance towards oxidation, which exhibits excellent catalytic performance as support in particular reaction systems. Hence, we employ SiC to develop effecient catalysts for CO₂ reforming of methane.In this thesis, we adopted the method of halogen extraction to treat the commercial SiC with low specific surface area, which yielded the high specific area carbon layer on the surface of SiC, namely C-SiC composite. Then C-SiC was functionalized via doping N atoms or adding CeO₂ and La2O3 promoters. Ni catalysts supported on various SiC nanocomposits were then tested in CO₂ reforming of methane. The catalysts were systematically analyzed by scanning electron microscope ?SEM?, N2 adsorption, X-ray diffraction ?XRD?, transmission electron microscope ?TEM?, thermalgravimetric/differential thermal analysis ?TG/DTA?, H2 temperature-programmed reduction ?H2-TPR? and X-ray photoelectron spectroscopy ?XPS? characterization methods. The main conclusions of this thesis are as follows:1. C-SiC and N-C-SiC were obtained by surface treatment of the commercial SiC, which had higher specific area. The optimum carbonization time of SiC was 1 h. The Ni/N-C-SiC-1-0.375:1 catalyst with 1 h of N doping and 0.375:1 of NH3/CCl4 ratio exhibited the best catalytic performance in CO₂ reforming of methane.2. The activity followed the sequence of Ni/N-C-SiC-1-0.375:1> Ni/C-SiC-1> Ni/SiC in CO₂ reforming of methane, which can be explained by the enhanced dispersion of Ni on the C-SiC-1 and N-C-SiC-1-0.375:1 supports according to XRD and TEM results. The stability of Ni/C-SiC-1 and Ni/N-C-SiC-1-0.375:1 catalysts was relatively worse than that of Ni/SiC catalyst. XRD, TEM and TG characterizations showed that the deactivation was mainly caused by metal sintering and carbon deposition.3. The CeO₂ modified Ni/C-SiC-1 catalysts were prepared by co-impregnation method. Compared with Ni/C-SiC-1 catalyst, the activity of Ni/20CeO₂-C-SiC-1 catalyst increased significantly, and the stability was improved. XRD, TEM and XPS results revealed that the improved catalytic activity of Ni/20CeO₂-C-SiC-1 catalyst was due to the enhanced dispersion of Ni. Moreover, XRD, TEM and TG/DTA characterizations documented that the carbon deposition of Ni/20CeO₂-C-SiC-1 catalyst decreased dramatically, corresponding to the improved stability. H2-TPR results showed that the metal-support interaction was strengthened in the Ni/20CeC>2-C-SiC-1 catalyst. This may result in the enhanced Ni dispersion and anti-coking property on the Ni/20CeO₂-C-SiC-1 catalyst.4. Compared with Ni/C-SiC-1 catalyst, the catalytic activity of Ni/5La2O3-C-SiC-1 catalyst prepared by the co-impregnation method improved remarkably. XRD and TEM results showed that it was due to the enhanced dispersion of Ni.