| The partial oxidation of methane to synthesis gas has been attracting lots of attentions both in academia and in industry. Among the catalysts reported, nickel- and cobalt-based catalysts are particularly promising due to their low prices. Compared to nickel, cobalt has higher melting, vaporizing points and less activity for syngas methanation, thus cobalt-based catalysts have been considered to be an interesting alternative to traditional nickel-based catalysts. However, the major drawbacks with cobalt catalysts are their lower activities and stabilities. The main purpose of this thesis is to gain the excellent Co-based catalyst with high activity and high stability.To improve the catalytic performance of supported cobalt catalysts for conversion of methane to syngas, a series of supports and promoters have been widely screened in this work. Moreover, the effects of catalyst compositions and reaction conditions on the catalytic performance have been investigated. In order to reveal the relationship of the catalytic performance with the chemical and physical properties of the catalysts, Co/Ca/Al2O3 catalysts have been characterized by BET, XRD, H2-TPR, XPS, Raman and in-situ techniques. The main results of the study are summarized as follows:1) Among the Co-based catalysts researched in this work, the 6Co/6Ca/Al2O3 catalyst has the best catalytic performance for the partial oxidation of methane to synthesis gas reaction.2) The stability test demonstrates that 6Co/6Ca/Al2O3 catalyst has favourable duration and resistance of carbon deposition. Under the reaction condition of T = 750℃, CH4/O2/Ar = 2/1/4 and GHSV = 12000 ml·g-1·h-1, the catalyst performs with the CH4 conversion of-88%, CO selectivity of-94% and undetectable carbon deposition during 120 h.3) The results of catalyst characterization including XRD,TEM,Raman indicate that Co3O4 is the main active species in the fresh Co/Al2O3 and Co/Ca/Al2O3 catalysts, and the calcium modification can effectively increase the dispersion of Co3O4 and decrease the Co metal particle size.4) H2-TPR characterization shows that the calcium modification can effectively suppress the metal-support interaction and decrease the reduction temperature of Co3O4 of the catalysts, which is responsible for their higher catalytic performance for partial oxidation of methane to syngas.5) XPS and in-situ characterization results indicate that the deactivation of 6Co/Al2O3 catalyst is due to the oxidation of Co0 and the formation of CoAl2O4 spinel phases under the reaction conditions. The metal oxidation and the phase transformation can be suppressed by the calcium modification. This could be relatedto the excellent catalytic stability of Co/Ca/Al2O3 catalysts.
|
PDF Full Text Request