Study On Preparation And Performance Of Catalysts For Carbon Dioxide Reforming Of Methane

Nowadays,the conversion of natural gas into resources is much more useful,and has received widely attention.Methane is a major component of natural gas,so the increase utilization of methane is then important,while carbon dioxide reforming of methane can increase methane utilization efficiency to a greater extent.From an environmental point of view,the reforming reaction consumes two gases that are directly related to the greenhouse effect.On the other hand,from the industrial point of view,the reaction produces a low H₂/CO ratio of synthesis gas,which is more suitable for the synthesis of liquid fuel.One of the obstacles encountered in the application of this process is the rapid deactivation of catalyst,which is mainly due to coke accumulation and sintering of catalysts.Ni-based catalysts became the hot spot in the field because of its high activity,high selectivity and lower cost.But the major drawback of this reaction,however,is the deactivation of catalysts as a result of carbon deposition.In this paper,a series of Ni-based catalysts were prepared.The samples were characterized by XRD,SEM,N₂ adsorption-desorption isotherms,TG-DSC,H₂-TPR,XPS and Raman spectrum on the structures and properties.The catalysts were placed at 800 ?,n?CH₄?:n?CO₂?=1:1,V?CH₄?=35 mL/min,airspeed GHSV=21000 mL·gcat-1·h-1 to catalyze carbon dioxide reforming of methane.The catalytic activity was evaluated by a fixed bed reactor.The effects of the carrier,the active component and the auxiliary agent on the catalytic performance of the reforming reaction were investigated.?1?The magnesium aluminate spinel catalyst carriar were synthesized by hydrothermal method,using NaAlO₂ and Mg?OH?₂ as the raw materials.The effects of sintering temperatures and hydrothermal reaction time on the structures of magnesium aluminate spinel were investigated.The results showed that the good flake skeletal structure is formed by hydrothermal reaction temperature of 180? for 24 h and calcination temperature of 800? for 5 h.The maximum surface area is 82.98 m2·g-1.MgAl₂O₄ with high surface area provides more active sites and improves the dispersion of the active components on the surface of the carrier,which provides a great advantage for MgAl₂O₄ as a catalyst carrier.?2?The magnesium aluminate spinel catalyst carriar were synthesized by coprecipitation method,using NaAl O₂ and Mg?NO₃?₂ as the raw materials.Ni/MgO,Ni/Al2O3,Ni/MgAl₂O₄?coprecipitation?,Ni/MgAl₂O₄?hydrothermal?catalysts with Ni loading of 10 wt% were prepared by equal volume impregnation method,using Ni?NO₃?₂ as nickel sources.The effect of the carrier on the catalytic performance was studied.The results show that the four catalysts are mesoporous.Ni/MgAl₂O₄?hydrothermal?has a higher specific surface area than Ni/Mg O,Ni/MgAl₂O₄?coprecipitation?granular and Ni/Al2O3 columnar structures.This structure provides more active sites and increases the dispersion of the active ingredient.Catalytic activity and stability are higher than the other three catalysts.CH₄ and CO₂ conversion rates were 79.73% and 90.02%.The catalyst activity varied little during the reaction time of 6 h,and had good thermal stability.Therefore,MgAl₂O₄ prepared by hydrothermal method is the best carrier for this problem.?3?The catalyst with 4 wt%,7 wt%,10 wt%,13 wt% and 16 wt% Ni loadings was prepared by equal volume impregnation method,using Ni?NO₃?₂ as nickel sources.The effect of Ni loading on the catalytic performance was investigated.The results show that Ni/MgAl₂O₄ catalyst has the best catalytic activity and high stability in CH₄-CO₂ reforming reaction when the nickel loading is 10%.The result of carbon deposition analysis combined with the catalyst characterization shows that the less nickel content and the less active component to disperse on the entire carrier surface result in not fully contacting with the reaction gas.So the catalyst carbon deposition is less,and the transformation activity is low.However,when the nickel content is more than 10%,the NiO particles are easier to agglomerate and the carbon is more fastly deposited.Considering the catalyst activity,stability and the carbon deposition factors,the best nickel load is 10%,but still 16.3% of the carbon deposited.?4?The catalytic performance of 10% Ni/Mg Al₂O₄ catalyst was enhanced with Mn promoter.The addition amounts of Mn were 0.5 wt%,2.5 wt%,4.5 wt%.The catalyst was prepared by equal volume impregnation method,using Mn?NO₃?₂ as manganese sources.The results show that Mn is successfully supported on the surface of the catalyst.Mn reduces the grain size of the active component Ni and increases the dispersibility of Ni.With the increase of manganese content,the catalytic activity of reforming reaction increased firstly and then decreased.The optimum doping amount of Mn is 2.5%.CH₄ and CO₂ conversion rates were 83.32% and 92.79%.Based on the analysis of the properties of 10%Ni-2.5%Mn/MgAl₂O₄,it is found that the addition of Mn reduced the interaction between Ni and the carrier,which was beneficial to the reduction of the active component,thereby enhancing the activity.And Mn plays the role of electronic additives.The addition of Mn increases the electron density of Ni,which facilitates the activation of CO₂ and improves the catalytic performance.When the incorporation of manganese was 2.5%,the carbon deposition was 11.8%.It can be seen that the incorporation of Mn enhances the carbon deposition resistance of the reforming catalyst.