Study On The Performance Of Cobalt-based/Nitrogen Doped Activated Catbon Catalyst For Combined Steam And Dry Reforming Of Methane

The efficient use of natural gas resources(CH₄)and the reduction of CO₂emissions are in line with the needs of the“National Medium and Long-Term Carbon Emission Reduction Path and Energy Structure Optimization Strategy”.CH₄ and CO₂ are the main carbon resources,but they are also the main greenhouse gases.Their unreasonable use and large amounts of emissions are one of the important causes of global warming or extreme weather.The synthesis of syngas from combined steam and carbon dioxide reforming of methane(CSCRM)can not only make methane and carbon dioxide greenhouse gases become treasure,but also flexibly adjust the ratio of hydrogen(H₂)to carbon monoxide(CO)to provide good raw materials for subsequent Fischer-Tropsch,which is conducive to alleviating the energy crisis.However,since the reforming reaction is a strongly endothermic reaction,the reaction requires a higher temperature.In the process of high temperature reaction,the catalyst will have problems such as sintering,carbon deposition and deactivation.Therefore,the development of catalysts with low temperature,high activity,high stability and resistance to carbon deposition is the key to the industrial application of the combined reforming reaction.Thus,based on the previous research of the research group,a series of non-noble metal/nitrogen-doped carbon material reforming catalysts were prepared by using nitrogen-doped activated carbon as a carrier through metal loading.Mainly studyed the influence of different preparation methods,different active components,metal impregnation sequence and different Co:Ni ratio on the catalytic performance of the prepared catalyst;and the relationship between the pore structure,surface properties,support metal interaction and redox performance of the prepared catalyst and the catalytic performance of the prepared catalyst was investigated using modern characterization techniques such as BET,XPS,TEM,H₂-TPR,etc.The main contents and results of this paper are as follows:1.Four Co-based/nitrogen-doped carbon materials catalysts were prepared by impregnation method,precipitation method,sol-gel method and direct mixing method.The study found that the catalyst prepared by the impregnation method showed the best catalytic activity and stability among the four types of catalysts prepared.Under the reaction conditions of 650?,the conversion rates of CH₄and CO₂ reached 71.6%and 64.4%,respectively,and the H₂/CO ratio was 1.5;after 12 hours of continuous reaction,the conversion rate of CH₄ and CO₂decreased by about 2%,and the catalyst showed good performance.BET characterization results show that the catalysts prepared by the four different preparation methods are all type IV isotherms of a typical H3(a)type hysteresis ring,which has the same structure as the original support.At the same time,it is found by comparison that the catalyst prepared by the impregnation method has a relatively high specific surface area and a large pore size.H₂-TPR characterization results show that the catalyst support prepared by the impregnation method also has strong interaction with metals.SEM characterization results show that the catalyst active metal prepared by the impregnation method also has good dispersibility on the support.Larger surface area and high metal dispersion improve oxygen storage capacity and oxygen mobility,thereby improving catalyst activity.The stronger interaction between the metal and the carrier improves the stability of the catalyst,so that the catalyst prepared by the impregnation method exhibits the best catalytic performance.The BET and SEM of the catalysts prepared by the sol-gel method and the precipitation method can clearly find that the pores are obviously narrowed and partially clogged,and the metal active component has a larger particle size,so the catalyst shows poor catalytic performance.2.(Ni,Co and Fe)M-Ce/AC-N catalysts with different active components were prepared by the impregnation method and used in the methane-carbon dioxide-steam combined reforming reaction.By comparison,it is found that the performance of Co-based catalysts is better than that of Ni-based and Fe-based catalysts.The Co²⁺species present in the Co-Ce/AC-N catalyst changed the chemical morphology and redox performance of the Co-Ce composite oxide,it enhanced the oxygen content and the chemically adsorbed oxygen mobility,thus it improved the catalyst of activity.The effects of the water content of the reaction raw materials and the CH₄/CO₂ ratio on the catalytic performance were studied.It was found that when the water content is 0.010 m L/min and CH₄/CO₂=2/1,the conversion rate of CH₄ and CO₂ is higher,and H₂/CO is about 1.5.TEM and TG characterization results show that there is obvious carbon deposition after the dry reforming reaction,but no carbon deposition after the combined reforming reaction.It shows that during the combined reforming process,water as an oxidant can promote the gasification of carbon and inhibit the production of carbon deposits.3.Based on the previous research,the effect of different impregnation sequences of Co and Ce on the performance of CSCRM was further investigated.The results show that the Co-Ce/AC-N catalyst prepared by the co-impregnation method exhibits a strong surface interaction between Co and Ce.Moreover,the presence of Ce in the catalyst improves the dispersibility and reducibility of Co,so that the catalyst shows good catalytic activity and stability.The catalyst prepared by the step impregnation method occupies part of the active site on the carrier due to the Ce impregnated first.When Co is impregnated afterwards,there is competitive adsorption in the catalyst,which reduces the active sites and reduces Co-Ce interaction.Thus,the catalyst shows lower catalytic activity and stability.The order of catalytic performance of the catalysts prepared by different impregnation sequences is:Co-Ce/AC-N>Co/AC-N>Co/Ce/AC-N>Ce/Co/AC-N>Ce/AC-N.4.Using the impregnation method,by adjusting the ratio of the active components Ni and Co,a Ni-Co dual active component combined reforming catalyst was successfully prepared.It is also used in the combined steam and carbon dioxide reforming methane reaction(CSCRM).The results show that as the Ni content increases,the catalytic activity of the prepared Co-Ni bimetallic active component catalyst increases first and then decreases.When the Ni loading is 7 wt.%,The catalyst activity is the best.The addition of Ni improves the dispersion of the active component Co on the nitrogen-doped carbon material,and significantly improves the catalytic activity of the catalyst.At the same time,the formation of Co-Ni alloy improves the catalyst surface chemical properties,improves the adsorption and dissociation of the reaction gas CH₄ and CO₂,and enhances the carbon deposition resistance of the prepared catalyst.The characterization results indicate that the addition of too much Ni reduces the stability of the catalyst precursor.In turn,the force between the metal and the carrier is weakened and the dispersion of the catalyst Co is reduced.After the reaction,the active metal sinters and generates more carbon deposits,resulting in a reduction in the catalytic activity of the catalyst.