Structural Regulation Of Ni - Based Catalyst And Behavior Of Methane Pressure Reforming Reaction

In recent years, the syngas from reforming of methane has received much attention due to the increasingly growing global environmental issue and the complex situation of the worldwide petroleum market. Generally, there are three basic reactions for reforming natural gas to syngas, i.e., the steam reforming of methane (SRM), carbon dioxide reforming of methane (CDR), and catalytic partial oxidation of methane (POM). At present, only the SRM with a high energy demand and a high H2/CO ratio of greater than3has been industrialized, and the more preferred CDR and POM are still at the beginning stage of the large-scale application. From an economical viewpoint, the mixed reforming of methane in the presence of CO2, H2O, and O2(tri-reforming) at elevated pressures is more acceptable because the syngas with a desired H2/CO can be achieved via simply adjusting the molar ratios of CO2, H2O, and O2. More importantly, when the molar ratio of the oxidants is suitable, autothermal reforming may be accomplished via the synergetic combination of the endothermic steam and CO2reforming reactions and the exothermic partial oxidation of methane. However, the severe coking and sintering of Ni are still a great challenge for industrializing the tri-reforming of methane. Therefore, the development of a high-performance catalyst, especially the commercially desirable Ni-based catalyst, is important in both theoretical studies and practical applications.In this dissertation, based on the results of our previous study and the literature reports, Ni-based catalysts were designed and prepared by different methods, in which the structural properties, the particle sizes and distribution of Ni, and the interactions between Ni and supports are effectively adjusted. Firstly, the ceria was introduced into the mixed oxide of NiO, MgO, and Al2O3(NMA) via the co-precipitation (NMACe) and the incipient impregnation (Ce/NMA) method, respectively. As a result, the redox properties of the catalysts could be adjusted. Secondly, to adjust the structural properties of Ni-SiO2and SiO2, the particle sizes and distribution of Ni, and the interactions between the Ni and SiO2, Ni-SiO2and Ni/SiO2were prepared by the complex-decomposition method by using the carboxylic acids, amino acids, and diethanol amine as a complexing agent, respectively. The catalysts were evaluated using CDR as a probe reaction, and the CDR results are quantitatively correlated with the structure of Ni-SiO2and Ni/SiO2, particle sizes and distribution of Ni, and the interactions between Ni and supports. The mechanistic relationship between the mentioned factors and CDR performance of the catalyst, especially the key factor on the deactivation, was determined. As a result, a highly active and stable catalyst for tri-reforming of methane was obtained. The experiment and main conclusions are summarized as follows:(1) The NMA was prepared from the hydrotalcite-like precursor, and the ceria was introduced into NMA with different content via the co-precipitation (NMACe) and the incipient impregnation (Ce/NMA) method, respectively. The NMACe and Ce/NMA were comparatively investigated to reveal the impact of ceria on NMA for carbon dioxide reforming of methane (CDR) under the conditions of P=5.0atm, T=750℃, CH4/CO2=1.0, and GHSV=53200mL·g-1·h-1. In comparison to NMA, the BET surface area, total pore volume, and average pore diameter were decreased. However, with increasing Ce/Al ratio, the change of textural properties of NMACe and Ce/NMA showed different trends. Although the (111) planes of cubic CeO2were preferentially exposed over all of the catalysts, the amount of mobile of oxygen from the redox properties of Ce3+(?) Ce4+was dependent on CeO2content and its introduction method. Moreover, the addition of CeO2into NMA leaded to an increased degree of reduction of Ni, the extent of which was dependent on its content and introduction method. The crystal size of Ni over Ce/NMA was slightly decreased with increasing Ce/Al ratio while a reverse changing pattern was obtained over NMACe. Because of the inhibited coke deposition, the alleviated graphitization of the deposited coke, and the delayed sintering of Ni, Ce/NMA with a Ce/Al ratio of0.5showed highly active and stable performance for the CDR.(2) The SiO2and Ni-SiO2were synthesized via the complex-decomposition method by using nickel nitrate and tetraethoxysilane as precursors of NiO and SiO2, respectively. Methane acid, acetic acid, oxalic acid, tartaric acid and citric acid are used as the complexing agent and fuel. For comparison, the Ni-supported SiO2from different sources was prepared by the incipient impregnation method. The characterization results of XRD, TEM, and N2adsorption-desorption at low temperature indicate that the textural properties of Ni-SiO2and Ni/SiO2were strongly dependent on the preparation method and the complexing agent. As revealed from H2-TPR and XPS patterns, Ni-SiO2and Ni/SiO2exhibited significantly varied interactions between Ni and SiO2, the extent of which was determined by the preparation method and the complexing agent used. The Ni-SiO2and Ni/SiO2were comparatively evaluated for CDR under the conditions of CH4/CO2=1.0, T=750℃, GHSV=53200mL·g-1·h-1, and P=1.0-10.0atm. Results indicate that the Ni-SiO2showed much higher activity and stability than Ni/SiO2-Ni-SiO2prepared with citric acid showed a high-performance under the conditions of P=1.0-10.0atm. By correlating the results of XRD, TEM, XPS and the pressurized CDR performance over the regenerated catalysts, coking and sintering of Ni leaded to catalysts deactivation was revealed. The excellent resistance to the sintering of Ni and the coking over Ni-SiO2originated from the strong interactions between Ni and SiO2, the small Ni particles and their much narrower distributions, and the uniformly distributed Ni in SiO2were responsible for its high activity and stability toward CDR.Based on the catalytic performance of Ni-SiO2prepared with carboxylic acid as complexing agent, different kinds of complexing agent were further expanded, a series of amino acids, i.e., glycine, alanine, serine, threonine, valine, proline, and lysine, and diethanol amine were used as complexing agents and fuel, respectively. For comparison, the urea and ammonium hydroxide are also used as the complexing agents and/or fuels. Ni-SiO2is synthesized via the complex-decomposition method. The Ni-SiO2were comparatively evaluated for carbon dioxide reforming of methane (CDR) under the conditions of CH4/CO2=1.0, T=750℃, GHSV=53200mL·g-1·h-1, and P=1.0atm. Results indicate that the Ni-SiO2prepared with glycine, alanine, serine, threonine, valine, and proline showed much higher activity and stability for CDR. Taking Ni-SiO2prepared with glycine as examples, it was also performed under the elevated pressures of5.0and10.0atm, the impact of the reaction pressures on the CDR performance of the catalysts was confirmed. The main factors determining the activity and stability of the catalysts were clearly revealed by the results of regenerated catalyst.A new method for synthesis of catalyst with highly active and stable performance for pressurized CDR was established. Importantly, the method provides a new strategy for the preparation of catalyst for tri-reforming of methane.(3) The Ni-SiO2was synthesized via the complex-decomposition method by using citric acid as the complexing agent and fuel, and it was evaluated for tri-reforming of methane. The effect of reaction temperature, pressure, raw materials ratio, and gas hourly space velocity on catalytic performance for tri-reforming of methane was systematic investigated. Under the condition of CO2/H2O=1.0, a desired H2/CO of2.0was successfully synthesized by adjusting the ratio of CO2, H2O, and O2. In compared to CDR, higher activity and stability can be obtained for the tri-reforming of methane under the same operation condition. Under the conditions of T=750℃, CH4/CO2/H2O/O2=1.0/0.3/0.3/0.2, GHSV=47880mL·g-1h-1, and P=10.0atm, the catalytic performance was kept constant for50h, indicating its high stability. Thus, it is very promising for preparing a highly efficient catalyst toward tri-reforming of methane.