Partial Oxidation Of Methane To Synthesis Gas Over Co/Mg/HZSM-5 Catalysts

This dissertation focuses on the partial oxidation of methane into synthesis gas over Cobalt-based catalysts.A series of supports and promoters have been widely screened.Moreover,the effects of prepareation method and calcination temperature on the catalyst performance have also been investigated.In order to reveal the relationship between the catalytic performance and the chemical and physical properties,Co/HZSM-5 and Co/Mg/HZSM-5 catalysts have been characterized in detail by BET,XRD,H₂-TPR,XPS,and TEM techniques.The results of the study are summarized as follows:1) Among the supported Co catalysts,only Co/HZSM-5 exhibites good catalytic performance.After 12 hours on stream,however,the catalyst is deactivated rapidly. Magnesium promoter can effectively improve the stability of Co/HZSM-5 catalyst, but the stabilities of Mg-modified catalysts depend on the preparation procedure and the calcination temperature to a great extent.The Co/Mg/HZSM-5 catalyst,prepared by sequential impregnation and calcination at high temperature(600 and 800℃for Mg/HZSM-5 and catalyst respectively),exhibites the best stability.Under the reaction condition of T=700℃,CH₄/O₂/Ar=2/1/4 and GHSV=100000 mL·g-cat^-1·h^-1, during 100 h running,the conversion of CH₄(～80%) and the selectivity to CO(～95%) remain high and almost unchanged.2) The results of fresh catalyst characterization including XRD,H₂-TPR and XPS show that the Co-species in the Co/HZSM-5 is Co₃O₄,while the Co-species in the Co/Mg/HZSM-5 is Co₃O₄ and/or(Co,Mg)O solid solution.The XRD results of reduced Co/Mg/HZSM-5 catalyst indicate that the Mg modification can increase the Co metal dispersion and decrease the Co metal particle size.3) Based on the TEM results of reduced catalysts and H₂-TPR and XPS results of used catalysts,it is reasonable to conclude that the deactivation of Co/HZSM-5 is mainly due to both the sintering of Co metal and the transformation of Co metal into CoAl₂O₄ spinel.In the case of Co/Mg/HZSM-5,magnesium interacts with Co oxides to produce(Co,Mg)O solid solution,which increase the dispersion and resistance to sintering of Co metal.In addition,magnesium effectively suppresses the formation of CoAl₂O₄ phase under the reaction conditions by providing a protective layer.Higher dispersion,resistance to sintering and magnesium protective layer is responsible for the excellent catalytic stability of Co/Mg/HZSM-5 catalyst.