| China are rich in coal bed methane reserves,there are some problems in underground coal mining.Large amounts of methane are diluted by moist,ventilated air and eventually released into the atmosphere.Methane emissions are a large and important source of powerful greenhouse gases and its global warming potential is 20 times that of carbon dioxide,which is a major source of greenhouse gases.In addition,the methane concentration is less than 1%,far below the flammability limit.The use of conventional combustion process to consume methane is blocked.Because methane molecules are difficult to be activated,its oxidation is challenging.Therefore,catalytic combustion is a potential solution for treating low-concentration methane.Pd-based catalysts have proven to be the most effective catalyst for low concentration methane catalytic oxidation.The catalytic activity of methane depends on many factors,including the size and dispersion of noble metal particles,the nature of the supports and its interaction with the noble metal.Compared with other supports,zeolite have striking attention due to their regular pores,adjustable pH and high thermal and chemical stability.In addition,MFI-type zeolite was the most widely used molecular zeolite.Based on this,the noble metal Pd was selected as the active center,and MFI zeolite was used as the support to conduct in-depth research on low concentration methane.In this thesis,Pd/S-1-in catalysts were prepared in situ by hydrothermal synthesis for the catalytic combustion of low concentration methane.The temperatures for methane ignition(T10%),the half-conversion temperature(T50%)and total conversion temperature(T90%)over Pd/S-1-in?in situ?catalyst are261?,329?and 382?,respectively,which are 45?,49?and 68?lower than the corresponding temperatures of Pd/S-1-im catalyst?incipient impregnation method?catalyst.The H2-TPR analysis showed the Pd/S-1-in catalyst has a high hydrogen reduction temperature.The XPS analysis showed that Pd clusters were encapsulated within the porous rather than on the surface of the crystal by in-situ method.The HRTEM studies demonstrated that the average particle size of Pd prepared by in situ method?1.7 nm?is less than prepared by impregnation method?2.8 nm?.Therefore,Pd/S-1-in catalyst has good methane catalytic activity.In addition,the stability of the catalyst is further improved by introducing a CeO2 promoter into the Pd/S-1-in catalyst,which due to the addition of CeO2 improves the stability of PdO in high temperature regions.The conversion rate remained around 90%when the reaction is carried out at 450?for 50 h at the gas hourly space velocities?GHSV?of 112500 mL×g-1×h-1.In this study,synthesis of Na-ZSM-5 with different Si/Al ratio?Si/Al=20,40 and 80?by simple hydrothermal method study on catalytic combustion performance of low concentration methane after loading Pd.The catalytic results indicate that the temperatures for methane ignition(T10%),the half-conversion temperature(T50%)and the total conversion temperature(T90%)over of the Pd/Na-ZSM-5-40 catalyst are 248?,311?and 373?,which are lower than the corresponding temperatures of Pd/Na-ZSM-5-40 and Pd/Na-ZSM-5-40 catalysts,respectively.From the NH3-TPD,the Bronstein acid sites of the synthesized Na-ZSM-5-40 support are the lowest,which is beneficial to methane catalytic reaction.The H2-TPR analysis showed the Pd/Na-ZSM-5-40 catalyst has a high hydrogen reduction temperature.The XPS analysis showed that the Pd/Na-ZSM-5-40 catalyst have the highest PdO and Oads content.In addition,the low-concentration methane catalytic combustion performance of Pd/Na-ZSM-5-40catalyst under different gas hourly space velocities?GHSV?was also studied.The results show that the methane conversion rate decreases with the increase of GHSV.In order to evaluate the high-temperature stability of the catalyst,we tested the stability of the catalyst at different GHSV at 450?running stable for 50 h,the conversion rate of methane hardly decreases within running stable for 50h. |
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