Modification Of Nickel-Based Catalysts Derived From Layered Double Hydroxides For CO₂ Reforming Of Methane

More attention has been paid to the comprehensive utilization of CH₄,main component of natural gas,and CO₂,main greenhouse gas for syngas production.Catalyst plays an important role in the industrial application.Compared with precious metals,the Ni-based catalyst has a comparable activity,extensive source and low price,seeming to be a good alternative catalyst.However,easy sintering and carbon deposit of Ni-based catalysts restrict the utilization in the process of CO₂ reforming of CH₄.Ni catalysts from [Ni,Mg,Al]-LDH can effectively reduce particle size of the active metal,improve surface acidity of the catalyst,and enhance interaction effect between active component and carrier.Porous carbon materials,with high specific surface area,lower price,and widely source,show specific ability for resistance of high temperature,sulfur poisoning,acid and alkali corrosion.Thus,the activity,stability and anti-coke ability of catalyst are improved by combination of Ni-based and carbon catalysts.Based on the advantages of Ni-based catalysts and carbon materials,[Ni,Mg,Al]-LDH as Ni precursor was used to prepare carbon-Ni/MgO-Al₂O₃ composite catalysts by incipient-wetness impregnation method.With furfural as carbon source,carbon content and pretreatment temperature of hydrotalcites were investigated.TG,H₂-TPR and XRD analysis showed that the introduction of carbon adjusted the interaction effects between active component Ni and carrier oxide,and suppressed the sintering and aggregation of active metal Ni particle.Under the preparation conditions of 20% carbon content and hydrotalcite calcination temperature of 200 oC,composite carbon-Ni/MgO-Al₂O₃ catalyst showed higher activity in the CRM process.The effect of carbon sources on the structure,composition and catalytic performance of composite catalysts were studied.When furfural was used as carbon source,the Ni crystallite size of FC-Ni/LDO remained stable before and after reaction and showed higher activity,stability and anti-coke ability.Based on the characteristics of adjustable cations and exchangeable braced anions in hydrotalcites structure,[Ni,Mg,Al]-LDH with nitrate were modified.Oxalic acid intercalated hydrotalcites were synthesized by anion exchange method,and the concentration was investigated.LDH-C₂O₄-10% sample showed hydrotalcite layered structure,and NO3-in the parent hydrotalcite was almost completely replaced,and its layer spacing was greater than LDH-NO3.Ni/LDO-C₂O₄-10% catalyst had a higher catalytic performance for activity,stability and resistance to carbon deposit.PLDH-10%,obtained by the direct thermolysis of oxalic acid intercalated hydrotalcites under vacuum,showed higher activity in the CRM reaction,and suggested that the active component Ni was almost completely reduced.In addition,NaOH solution was used to modify the parent hydrotalcites,and the effect of corrosion NaOH solution concentration on the structure of LDH was examined.It was found that,with the increase of concentration of alkali solution,the molar ratio of M²⁺/M3+ of samples gradually increased,indicating the dealumination of the raw hydrotalcites.Moreover,tiny particles appeared on the surface.The NaOH treatment resulted in the increasement of surface area and the centred pores shifted to wider diameters.During a long time test process,Ni/LDO-0.1 catalyst showed a better activity and stability.The growth of Ni crystal grain size become slower after reaction.With increasing CO₂ flow,the abilities of resistance sintering and carbon deposit were enhanced in a certain degree.