Dry Reforming Of Methane Over Nickel Catalysts Supported On Complex Metal Oxides

The energy and environment issues are two main subjects for sustainable economic development. Dry reforming of methane, which offers several advantages in the utilization of natural gas and CO₂, the reduction of greenhouse gases emission and the transmission of chemical energy, has received considerable attention in recent years. In this thesis, two series of complex metal oxides (viz. La-based peroskites and magnesium aluminate spinels) were used either as precursors or as supports of Ni catalysts to investigate the main effect of which on the performance of the catalysts in dry reforming of methane. The preparation methodology, surface properties, reaction and coking mechanism were systematically investigated. Based on these experimental results, the relationship between catalyst structure and its properties were also discussed. Several conclusions were drawn from these investigations.The catalysts with well-dispersed and stable nickel particles on La₂O₃ support were successfully prepared by incorporating Ni ions into the crystal structure of perovskites precursors. It was found that La₂NiO₄ catalyst offers promising catalytic activity and stability in dry refotming conditions, which could be due to agglomerate-preventing effect of Ni by LaO_x species after reduction since the NiO₆ octahedron in the La₂NiO₄ structure is separated by a La-O rock-salt layer along the c-axis. The well-crystallized perovskites with a proper reducibility of Ni²⁺ in the structure giving the advantage of the formation of highly dispersed and stable catalyst with strong metal-support interactions. This provided a new route for improving the homogeneity in the dispersion of the metal surface and preventing the metal sintering mostly found in conventionally prepared supported metal catalysts.A promising catalyst, 5%Ni/MgAl₂O₄, was developed for the dry reforming of methane. The conversion of CH₄ and CO₂ reached 83.4% and 87.7% respectively with a H₂ and CO selectivity of 97.5% and 98.5% at 750℃ and uniform CH₄/CO₂ ratio, closing to the thermodynamic equilibration. During the stability test around 55h, no deactivation was observed due to its high metal dispersion, lower coking rate and sintering-resistant properties.A new strategy of preparing out-layer MgAl₂O₄ over γ-Al₂O₃ by impregnation method wasproposed. Alternative to the traditional technique of impregnation method, the γ-Al₂O₃ wasco-impregnated with both Mg³⁺ and Al³⁺ nitrate solution and then formed well-crystallized MgAl₂O₄layers over γ-Al₂O₃ particles under lower temperature. As a result, the introduction of out-layerMgAl2O4 over Y-A12O3 suppressed the deep dehydrogenation of CH4 and stabilized Ni metal from sintering in dry reforming conditions. Thus, we proposed that the introduction of MgO into the catalysts not only act as an active center for CO2 adjacent to Ni metal particles or acidic attenuation reagent, but also act as a stabilizer of the catalyst structure by forming a thin layer of MgAl2O4 spinels between Ni and the support.The formation and transformation of reactive intermediates were studied by isotopic tracer, pulse surface reaction and in-situ DRIFT spectroscopy. Some direct evidence was gained and the mechanism of dry reforming reaction was further discussed. The pulse reaction and in situ DRIFT indicated that CO2 adsorbed on La2Ni04 and Ni/MgAl2O4 surface can react with the catalyst and forming an "oxygen pool", which provided the active oxygen species to react with CHX and release CO. The main species existed in the "oxygen pool" maybe a form of carbonate species as evidenced by in situ DRIFT spectra and isotopic studies. CO2 can form carbonate species directly by reacting with a support with weaker metal-oxygen bond (i.e. La2O3), or via formate species formed between CO2 and surface hydroxyl species (i.e. A12O3 and MgAl2O4). The formation of CHx(x=0³) species through CH^ dehydrogenation was supported by in-situ DRIFT spectra, which also provided evidence of the reactions between CHX and surface carbonate species over a CO2 pre-conditioned La2O3. According to these foundings, a detailed reaction scheme of dry reforming methane was suggested in this thesis.The morphological and chemical reactivity of surface carbon species formed during reforming reaction over the supported Ni catalysts were also investigated by temperature programmed surface reaction (i.e. TPO, TPH, and CO2-TPSR) and Raman spectroscopy. Three types of carbonaceous species, Co, Cp and C,, were found to exist on the Ni catalyst during dry reforming condition. Same carbonaceous species were obtained during CH( or CO decomposition reactions. All the coke deposits can be removed by O2 below 800°C, while only Ca can be removed by H2 under this temperature. CO2 can partially remove all of the three type cokes due to its effective activation over La2Ni04 and 5%Ni/MgAl2O4 catalysts. The Ca and Cp species is suggested to be responsible for CO formation since the increase of these species with time on stream and have no influence on the catalytic performance during dry reforming of methane. The Cy species are mainly graphite-like coke and attributed to causing catalyst deactivation.