Study On Modification Of Ni/Al₂O₃ Fiber Catalyst

To efficiently utilize fossil energy,the use of natural gas as a raw material instead of a fuel has been studied intensively.Methane,the main component of natural gas,is reformed into syngas（H₂+CO）,which can be converted into liquid fuels or other chemicals via a Fischer-Tropsch process.Ni catalysts have high catalytic activity for cracking hydrocarbons or activating reactants,and Al₂O₃ is a cost-effective catalyst support.Therefore,the Ni/Al₂O₃catalyst system has been widely used in methane reforming and other catalytic processes.In addition,Ni/Al₂O₃ catalysts form NiAl₂O₄ after calcination.The formation of NiAl₂O₄increases Ni dispersion and the catalyst/support interaction.However,the low reducibility of NiAl₂O₄ leads to low Ni utilization and the re-formation of inactive NiAl₂O₄ in the presence of oxidant（e.g.O₂,H₂O and CO₂）during methane reforming.Therefore,it is meaningful to increase the catalytic activity of Ni/Al₂O₃catalyst via adding promoters.Due to the reformation of inactive NiAl₂O₄,the activity of Ni/Al₂O₃ catalyst is reduced.In this paper,the Ni/Al₂O₃ catalyst is modified by adding promoters.The promoter mainly improves the catalytic activity of the Ni/Al₂O₃ catalyst by inhibiting the formation of NiAl₂O₄ during the calcination and reaction process,and promoting the reduction of NiAl₂O₄.First,CeO₂ effectively promotes NiAl₂O₄ reduction via forming CeAlO₃,which prevents NiAl₂O₄ re-formation.The pathway of CeAlO₃ formation via Ni Al₂O₄ reduction has been proposed and thermodynamically confirmed for the first time.The other factors affecting CeAlO₃ formation have been identified:the presence of Ni in the composite promotes CeAlO₃formation via solid reaction between CeO₂ and Ni Al₂O₄ during reduction;high reduction and reaction temperatures favor CeAlO₃ formation and stabilization.Second,MgO has demonstrated promotion effect through securing Al₂O₃ to form MgAl₂O₄ during catalyst calcination so as to diminish NiAl₂O₄ formation.However,the promotion effect is limited by the formation of NiO-MgO solid solution with low reducibility.CeO₂ promotes NiAl₂O₄ reduction via forming CeAlO₃ and therefore prevents NiAl₂O₄re-formation during the reaction.This study,for the first time,demonstrates the synergistic promotion effect of MgO and Ce O₂ on the Ni/Al₂O₃ catalysts for methane partial oxidation through optimizing MgO and CeO₂ contents in the MgO-CeO₂-Ni/Al₂O₃ catalyst system.The thermally stable structure of fibrous catalysts prepared by one-step electrospinning enables the investigation of the synergistic promotion.Third,This study utilized the thermally-stable structure of fibrous catalyst to investigate the effects of La₂O₃ promotion and calcination temperature on Ni/Al₂O₃ catalysts.Adding La₂O₃ gradually diminished NiAl₂O₄ formation through forming LaAlO₃ perovskite,which is stable during reactions so as to prevent the reformation of inactive NiAl₂O₄.The higher crystallinity of fibrous LaAlO₃ perovskite achieved at the higher calcination temperature demonstrated the higher catalytic activity during methane reforming,which is attributed to the improved Ni reducibility and strengthened catalyst/support interaction.Therefore,a thermally-stable fibrous Ni/LaAl O₃ catalyst has been developed for high temperature methane reforming.