Study On Effect Of Support And Promoter On Ni-based Catalysts For Methanation

The conversion of coal to natural gas is an effective approach to achieve clean utilization of coal and ease the shortage in supply of natural gas.The developments of methanation reaction process and highly efficienct catalyst are greatly critical for this technology.Nickel-based catalysts are widely used in CO methanation reaction due to their high activity and low cost.However,the CO methanation reaction is a strongly exothermic reaction,which can cause deterioration of the catalyst,such as sintering and coking,further lead to deactivation.Herein,we focus on the Ni-based catalysts with good low-temperature activity and excellent high-temperature stability.Hence,oxides(CeO₂ and WO₃)and carbides(SiC)with specific physicochemical properties were chosen as supports to improve the catalytic performance of Ni catalysts by controlling the morphology of the support and adding promters and to clarify the influence of texture,thermal conductivity,electron transfer,hydrogen spillover and oxygen vacancy of supports on CO methanation performance.The main research contents and results are as follows:(1)Ordered mesoporous silicon carbide(OM-SiC)was prepared by the nano-casting with polycarbosilane as the raw material,and ordered mesoporous silica(KIT-6)as the hard template.Ni/OM-SiC,Ni/SBA-15 and Ni/SiC catalysts were synthesized by ultrasonic impregnation and the Ni/OM-SiC exhibited the best activity and stability for CO methanation.Fristly,the enchanced catalytic performance can be ascribed to the highly dispersed Ni paritcles and the increased number of active sites,which is promoted by large specific surface area of OM-SiC support.Secondly,the improved catalytic stability can be assigned to stronger interaction between Ni and support generated from the unique pore structure of OM-SiC that facilitates to anchoring the nickel particles,further hampering the aggregation and exfoliation of Ni particles.In addition,OM-SiC with high thermal conductivity can transfer the heat rapidly and decrease the temperature in the catalyst bed,which inhibits the sintering and coking of the catalyst effectively and eventually improves the stability;(2)WO₃ with surface area of 45 m²/g was prepared by the hard template method and was used as support to synthesize Ni/WO₃ catalysts for CO methanation.The 25Ni/WO₃ catalyst displayed a relatively high CO conversion,but a poor CH₄ selectivity(80%).After the addition of MgO,both CO conversion and CH₄ selectivity over x Mg25Ni/WO₃ were significantly enhanced.The CH₄selectivity reached up to 92%at 440°C and CO conversion was closed to the thermodynamic theoretical values at 600°C over 7Mg25Ni/WO₃.Frist,the enhanced hydrogen spillover of WO₃ by MgO promotes the adsorption of hydrogen at high temperature,further improves the conversion of CO at high temperature.The improved CH₄ selectivity can be attributed to the enhanced CO dissociation,which was related to the reduced Ni particle size,as well as the enhanced Ni electron cloud density.On the one hand,MgO acts as physical barrier to decrease Ni particles size.On the other hand,Mg O promotes the electron transfer from support to Ni particles,which is induces an enhancement of the metal–support interactions,which are benefical to decrease the Ni particle size.Meanwhile,the electron transfer performance of Mg O constitutes a crucial factor in enhancing the Ni electron cloud density.Furthermore,with benefit from the inhibition of agglomeration of the Ni particles by the Mg O promoter,a significantly better catalytic stability was also observed on 7Mg25Ni/WO₃ than with the 25Ni/WO₃ catalyst;(3)CeO₂ nanorods were prepared by the hydrothermal method,and the Ni/La_xCe_1-xO_2-x/2 catalyst was further synthesized by the ultrasonic impregnation method and applied in the CO methanation.The effect of La₂O₃ amounton the structure,morphology,surface characteristics,catalytic activity and stability of the catalyst were systematically investigated.The results show that La₂O₃ as an electronic donator can effectively transfer more electrons to nickel,increase the electron cloud density of nickel,accelerate the dissociation of CO,and improve the catalytic activity in the low temperature.In addition,the substitution of Ce by La atom promotes the generation of more oxygen vacancies,and increases the mobility of oxygen species,and then reacting with carbon deposits,which maintained the excellent activity after the 50 h long time test.