The Study Of N-doped Carbon Material Supported Cobalt Based Catalyst For CH₄-CO₂ Reforming To Synthesis Gas

With the excessive use of fossil fuels and more prominent environmental issues,the share of natural gas which is used as fuel is increasing in the total energy consumption.The main component of natural gas is methane,converting methane into high-valued chemicals and liquid fuel is an efficient way to use fuel.Moreover,the development of industrial production also leads to an increase of carbon dioxide emission.Irrational use of methane and carbon dioxide is the main causes of greenhouse effect and global climate warming.The reduction of carbon dioxide emissions and high efficiency using of methane have become the focus all over the world.The dry reforming of methane can not only convert methane and carbon dioxide into syngas,the product gases could also be used directly as the raw gases of the Fischer-Tropsch synthesis to synthesize hydrocarbons.As for dry reforming reaction,the main problem is that catalyst has poor performance due to the cock deposition and active metal sintering.Therefore,the aim of this study is to develop a catalyst with high activity as well as high resistance to carbon deposition and sintering.This thesis mainly focuses on preparing a kind of Co-based catalyst,the catalyst performance has been conducted from some important factors such as the dispersion of active metals,the interaction between active metal and support as well as catalyst redox property.Firstly,the effect of different activated carbon support on catalytic performance of Co-based catalyst was investigated.Through comparison of different activated carbon,it was demonstrated that the support with larger surface areas and good porosity can improve the dispersibility of active metal and the interaction between active metal and support.The support with high quantity of basic sites can enhance the absorption property of reaction gases and reduce the activation energy.In order to improve the catalytic performance,modifying activated carbon by melamine for improving the dispersion of active metal and electron transfer properties.In addition,the effects of calcination temperature and the doping amount of nitrogen on catalytic performance also investigated during process of nitrogen doping modification and the optimizing modification condition was obtained.Compared with the unmodified catalyst,Co-based N-doped activated carbon supported catalyst exhibited excellent catalytic performance.The results demonstrated that the generation of pyridine on N-doped catalysts was basic group,which enhanced the adsorption capacity and activation of CO₂,the formation of pyrrole promoted electron transfer and enhanced the redox ability of the catalysts.The content of Co²⁺on N-doped catalysts increased,the existence of Co²⁺and Co³⁺promoted the redox cycle of catalyst itself and reduced the accumulation of carbon deposition.The suitable second metal was added in the Co-based N-doped activated carbon supported catalyst to adjust the interaction between Co and second metal.The effect of different second metal as well as cobalt and second metal ratio on catalytic performance was also studied.The experiment results indicated that the addition of second metal had significant influence on Co²⁺/Co³⁺ratio,the introduction of Fe or Zr made the most of Co²⁺was oxidized to Co³⁺,however,Co²⁺and Co³⁺co-existed on Ce promoted catalyst.The ratio of Co/Ce also had great influence on catalytic activity,when the Co-Ce ratio was 1:1,the catalyst surface has almost same content of Co²⁺and Co³⁺,the stronger interaction between two metal as well as high content of chemisorbed oxygen increased the stability of the catalyst.The different promoters were introduced to further improve the catalyst performance.The influence of different promoters and the sequence of dipping bimetal and promoter were also studied.The results demonstrated that the catalyst modified by promoter Ca and prepared by co-impregnation method exhibited higher catalytic,the addition of promoter improved the properties of catalyst and reflected a good synergy effect.The higher interaction between Co and Ce as well as the higher content of chemisorbed oxygen improve the anti-coking performance.The characterization of spent catalyst indicated active metal particles were well dispersed on support surface and the degree of graphitization for this catalyst was lowest.The catalysts prepared with sequential impregnation generated great amount of carbon nanotubes,this was the main reason why this catalyst had poor stability.The reaction mechanism and kinetics of dry reforming of methane over1Co-1Ce-1Ca/AC-N catalyst was studied.The action mechanism of methane and carbon dioxide on catalyst surface was discussed through FTIR and XPS analysis results.In addition,the kinetic experiment data were fitted by different models such as PL model,ERⅠmodel,ERⅡmodel and LH model.The highest determination coefficient in the LH model was obtained and the determination coefficients in other models were relatively low.The LH model can clearly demonstrate the changing regularity of reaction gas at different reaction temperature.The temperature dependence of these parameters was displayed as:k=2.11×exp（124,00/RT）,K_CH4=3.09×10^-6×exp（-117,000/RT）,K_CO2=0.014×exp（-47,000/RT）.