Preparation,Characterization And Property Of Ni-based Catalyst For Methanation Of Carbon Oxides

CO₂ emission to the atmosphere is one of main causes of global warming.CO₂methanation,which is an efficient method in CO₂ recycling,has both potential commercial application and environmental benefits.Meanwhile,our country lacks of oil and gas,but is rich in coal.This makes coal a dominant energy resource in China.Producing synthetic natural gas from coal through syngas methanation is a competitive way for its clean utilization.Therefore,it is significant to study CO₂ and CO methanation process.The key technology in methanation is preparation of highly efficient catalysts.Unfortunately,the traditional nickel catalyst is not an ideal candidate,which exhibits poor low-temperature activity,and is easy to sinter and induces carbon deposition at high temperature.This thesis mainly studies the preparation and performance of some novel Ni-based catalysts.NiSi-P catalyst with enhanced metal-support interaction was successfully synthesized by ammonia evaporation method.Compared with conventional NiSi-C catalyst,NiSi-P catalyst exhibits advantages such as low-temperature activity,stability,anti-sintering and anti-carbon deposition.NiSi-P catalyst has smaller nickel particle size,stronger metal-support interaction and promoted nickel dispersion,which contribute to anti-carbon deposition.Using urea as precipitant,NiAl catalysts（NMx）and NiMgAl catalysts（NMA-x）were prepared from HTLcs.There are two weight lose stages in the thermal decomposition process of NAx catalysts precursors.The layered structure collapses completely and complex metal oxide forms above 500oC.Compared with the traditional impregnated NA1-I catalyst,NA1 catalyst exhibits the excellent reactivity and stability.The factors that contribute to its excellence include high Ni dispersion,small nickel particles,strong metal-support interaction,and anti-coke formation ability.N₂ adsorption-desorption curve of NMA-x is a typical type IV isothermal one,implying that NMA-x belongs to mesoporous materials.Among NMA-x catalysts,NMA-60 has the largest basic sites and the highest CO₂ methanantion activity.Characterization results indicate that the addition of MgO weakens metal-support interaction,suppresses the generation of spinel NiAl₂O₄,and enhances Ni dispersion on catalysts.However,the further increase of MgO content shows limited influence on the Ni dispersion and the nickel specific surface area,so CO conversion of NMA-x（x=20,40,60）is nearly same.Ordered mesoporous NiAl catalysts（OM-NA-x）were prepared by solvent evaporation self-assembly method,using P123 as template agent.Compared with NA-13 which is free of the template agent,OM-NA-13 catalyst exhibits better reactivity in methanation.60-hour reaction test at 650oC showed that OM-NA-13catalyst had good resistance to sintering and carbon deposition,which was ascribed to its high nickel dispersion,high Ni active-specific surface area,and unique"confinement effect".NiCeSBA-15-P catalyst was prepared by the dielectric barrier discharge plasma method（DBD）.Experimental results show that adding CeO₂ and DBD treatment to the catalyst significantly improves its activity and stability in CO₂ and CO methanation.Factors that contribute to the improvement include smaller Ni particles anchored in pores of NiCeSBA-15-P catalyst,and higher Ni dispersion as well.In addition,Ni/CeAl-p catalyst with improved low-temperature CO₂ methanation activity was prepared by DBD.Starting temperature and maximum CO₂ conversion of Ni/CeAl-p catalyst are 195oC and 87.1%at 350oC,while for Ni/CeAl-c by calcination method those are 217oC and 80.9%at 375oC,respectively.So,DBD can replace calcination method to discompose the catalyst precursor to prepare the catalyst,with higher Ni dispersion and CO₂ adsorption activity.