Construction Of High-Efficiency Nickel-Iron Catalyst System And Its Methane Dry Reforming Performance Tailoring

Large-scale exploitation and utilization of fossil resources lead to the global greenhouse effect.As the main greenhouse gases,how to rationally use CO₂ and CH₄on a large-scale has become an important issue.Methane-carbon dioxide reforming technology can simultaneously convert CH₄ and CO₂ into syngas,and then synthesize a series of chemical products,that is an effective way to realize large-scale utilization of carbon dioxide.At present,the main problem of large-scale industrial utilization of methane dry reforming technology is the efficiency and stability of catalyst.Nickel-based catalysts are inexpensive and have good catalytic activity,but have defects of sintering and carbon deposition.Therefore,it is essential to improve the sintering resistance and carbon deposition resistance of nickel-based catalysts.In this paper,Ni-xFe/mayenite catalysts were prepared by loading Ni-Fe bimetals on the mayenite support.The synergistic effect of Ni-Fe bimetals and the oxygen conduction effect of mayenite support were comprehensively utilized to construct an efficient and stable catalytic system for dry reforming of methane.This research focuses on the tailoring of catalytic activity and carbon deposition resistance of Ni-Fe/mayenite catalyst in methane dry reforming.Firstly,the effective preparation method of Ni-Fe/mayenite catalyst was studied.Three preparation methods including wet mixing-impregnation method,sol-gel-impregnation method and one-pot method were investigated to prepare Ni-Fe bimetallic catalysts.The performance evaluation was carried out in the new designed dry reforming reaction system.And the physicochemical texture and carbon deposition of the catalyst were analyzed by XRD,BET,TG-DSC and other characterization methods.The results showed that the calcination temperature has a great influence on the specific surface area of the catalyst prepared by wet mixing-impregnation method.The activity of the catalyst was relatively high at 900°C,and the carbon deposition was the smallest in the reaction process.The increase of calcination temperature led to the increase of Ni grain size,but it could improve the specific surface area of the catalyst.The activity of the catalyst prepared at 900°C（calcination temperature of the carrier）-900°C（calcination temperature of the catalyst）decreased slightly,but the carbon deposition was the least in the reaction process.When the catalyst was prepared by sol-gel-impregnation method,the addition amount of citric acid and the doping time of Fe element in the preparation process affected the performance of the catalyst.When citric acid was added more（1.2:1）,the obtained catalyst had smaller Ni grain size,better catalytic activity and carbon deposition resistance.In the preparation process,the catalytic activity of the catalyst added with Fe element in the support preparation and impregnation process was slightly lower,but the anti-carbon deposition ability was effectively improved.Considering three preparation methods,the catalyst prepared by wet mixing-impregnation method at 900℃had a small Ni grain size（13.0 nm）,good Ni dispersion（6.2389%）and large specific surface area(56.86 m²g^-1),showing excellent catalytic activity and anti-carbon deposition in the dry reforming reaction.In order to systematically regulate the catalytic activity and carbon deposition resistance of Ni-xFe/mayenite catalysts during dry reforming,the effects of Ni metal loading,Fe doping,carrier composition,reaction temperature and space velocity on the performance of Ni-xFe/mayenite catalysts were further investigated.At the same time,the physicochemical properties of the catalyst were analyzed by XRD,BET,XPS,TPR and other characterization methods.The results showed that the 7.5%Ni-0.1Fe/Ca12Al9.45 catalyst showed a high conversion rate close to the thermodynamic equilibrium（CO₂ was 89.70%,CH₄ was 86.49%）and H₂/CO was0.987 in the dry reforming experiment.The minimum amount of carbon deposition was 4.77%in 30 hours dry reforming process and the catalyst could run stably for at least 60 hours at 700°C.The increase of Ni loading in the catalyst increased the Ni active site and the grain size of Ni O,reduced the dispersion of Ni element on the surface,and made the reduction of the catalyst decrease.The increase of active Ni sites promoted the dehydrogenation of methane and increased the methane conversion rate.However,the carbon deposition precursor generated by methane dehydrogenation cannot be oxidized in time,resulting in serious carbon deposition of the catalyst.Ni-based catalysts were more prone to carbon deposition（filamentous carbon and graphite carbon）in the dry reforming process,resulting in reaction tube blockage and catalyst deactivation,especially when the metal loading concentration was high.The addition of Fe element could lead to the formation of Ni-Fe alloy,reduce the grain size of Ni,improve the dispersion of Ni metal,increase the concentration of active oxygen species on the catalyst surface,and promote the adsorption and activation of carbon dioxide,which was beneficial to the oxidation of carbon deposition precursor.The existence of free oxygen species in the carrier mayenite structure enhanced the concentration of oxygen species in the catalyst.Due to the interaction between metal and carrier,it was conducive to improving the oxygen transfer rate and continuous oxygen supply capacity in the Ni-xFe/mayenite system,thereby improving the anti-carbon deposition performance of the Ni-xFe/mayenite catalyst.At the same time,the experiment proved that the conversion rates of CH₄ and CO₂ gradually increased with the increase of dry reforming temperature,while the catalytic activity gradually decreased with the increase of space velocity.The methane cracking reaction was more serious at high space velocity and high reforming temperature,while the reverse water-gas reaction was inhibited.