Study On The Modification Of Ni/CeO₂ Catalyst And The Performance For Dry Reforming Of Methane

Global climate change induced by greenhouse gases has received increasing attention in recent years.Methane and carbon dioxide are the two main greenhouse gases in the atmosphere.Dry reforming of methane(DRM)is one of the most effective ways to utilize CO2 and CH4,which can simultaneously convert two main greenhouse gases into syngas(H2+CO).Syngas is a critical intermediate in the chemical industry,Which with high added value and can be used as the feedstock for the production of Methanol,carbonyl and fischer-tropsch synthesis.DRM has significant environmental and economic benefits and has broad development prospects.Ni-based catalyst is widely used in DRM reactions because of high activity and low cost.However,due to the serious coking and sintering problems the elevated temperature,it has not been applied in industry at present.Therefore,it is necessary to find a new catalyst with strong resistance to sintering and coking to further optimize the catalytic performance of methane dry reforming,which is still a major research challenge in the field of heterogeneous catalysis.One of the commonly used methods is to use alkaline carriers or carriers containing a large number of oxygen ion vacancies,such as mixed metal oxides containing CaO,La2O3,Al2O3,SiO2,MgO,Y2O3,ZrO2 or CeO2.Among them,oxide CeO2 is widely used.Due to the constant valence conversionbetween Ce4+and Ce3+,the storage and oxygen release capacity of the catalyst will be improved to ensure the transfer of oxygen in the system,and will effectively inhibit the generation of carbon deposition in the reaction.Based on this,this paper focuses on the modification of Ni/CeO2 to improve the stability of DRM reaction.The main research are as follows:1.The effects of different promoters boron(B),gallium(Ga)and indium(In)on the performance of Ni/CeO2 catalysts show different results.Only Ga doped Ni/CeO2 catalyst improves the stability of the activity.The activity of B doped catalyst is decreases rapidly.while the activity of the In doped catalyst is very low at the initial,and then increases slowly.Strong Lewis acid sites are generated after B doping,which promotes the complete dissociation of CH4 and generates more carbon depositions,leading to a rapid decline of activity.The In doping due to its own and the electronic effect of Ni,inhibit the dissociation of CH4.The In-doped catalyst show a very low initial activity,but the activity gradually increases,which because under high temperature and reducing atmosphere the indium oxide reduction as In after sublimation.According to the the characterization results of XRD,H2-TPR,XPS,and CO2-TPD,it is found that the activity and stability of Ga doped catalyst are directly related to basicity,while the activity of B,In-doped catalyst decreases and increases,the chemical properties of dopant itself play a significant role.The exploration of these three promoters and the experimental results provide reference for the next step of experimental design and exploration direction.2.Based on the content of the above part of the experiment,the relationship between basicity maintaining good stability of the whole reaction was further studied.Firstly,we doped different proportion of Ga in the Ni/CeO2 catalyst to adjust the basicity of the catalysts,found that have more weak-medium basiv sites will show better activity.Then through comparing the hydrothermal method and the sol-gel method to synthetize the Ni/CeO2 catalysts,it was found that the Ni/CeO2-SG catalyst prepared by sol-gel method with more basic sites showed good stability,which was highly consistent with the conclusion of Ga doping.Finally,by doping different proportions of B promoter in the Ni/CeO2-SG catalyst and conducting activity evaluation,it is found that B with lower doping amount shows the best activity and stability.Combined with the CO2-TPD results,it was proved that the basic sites play an important role in the DRM reaction,which can promote the adsorption and activation of CO2,eliminate carbon deposition,maintain the stability of the reaction.Combined with in situ DRIFTS and pulse experimental characterization results at the same time,confirmed the existence of intermediate carbonate species,that can promote the elimination of carbon deposition,and explain the reason of catalyst with high stability.