Preparation And Microstructure Of Sepiolite Nanofibers Loaded Ni-NiAl₂O₄ Composites

Nickel-based catalysts have received widespread attention,because they can be used in many catalytic processes such as reforming,hydrogenation,oxidation and tandem.However,nickel-based catalysts have problems such as poor dispersion,low temperature stability and short service life.Sepiolite nanofibers have a special pore structure,which makes them excellent in adsorption,catalytic and thermal stability and can be used as an ideal nickel-based catalyst support.Using sepiolite nanofibers as a carrier can increase the surface area of the composite and solve the problem that the active ingredients of the catalyst are easily agglomerated.Spinel-type nickel aluminate(NiAl₂O₄)has excellent thermal stability and acid-base resistance and can be used as a transition layer between the support and active components in nickel-based catalysts.It can not only as the source of active components,but also improve the high temperature stability and the dispersibility,and increase the use time.In this work,Ni-NiAl₂O₄/sepiolite nanocomposites were prepared by sol-gel-reduction method and coprecipitation-reduction,respectively.The phase composition,crystal structure,and micromorphology of the composites were characterized by thermogravimetric-differential thermal analyzer(TG-DTA),X-ray diffraction(XRD),Fourier transform infrared spectrometer(FTIR),scanning electron microscope(SEM),transmission electron microscopy(TEM)and hydrogen temperature-programmed reduction(H₂-TPR).The results are shown below.The best conditions for preparing composites by sol-gel-reduction method were as follows:the citric acid as complexing agent with a content ratio to the metal salt is 1:1,the calcination temperature is 700°C,the additive is 10%ethylene glycol,the ratio of metal salt to sepiolite is 7.5:1,the reduction temperature is 550°C and the holding time is 6 h,respectively.NiAl₂O₄ particles are distributed uniformly on the surface of the sepiolite nanofibers with a particle size of about 25 nm.Ni particles attach to the surface of NiAl₂O₄with a particle size of less than 10 nm.During the gelation process,the polymer particles adsorbed on the surface of sepiolite grow up into chains and intertwine with each other to form a continuous network structure.The long chain is broken during the heat treatment,and the spinel-type nickel aluminate formed can grow on the surface of sepiolite.The best conditions for preparing composites via coprecipitation-reduction method were as follows:the calcination temperature is 700°C,the p H value is 9,the ratio of alcohol to water is 1:1,the ratio of metal salt to sepiolite is 7.5:1,the reduction temperature is 600°C and the holding time is 3 h,respectively.NiAl₂O₄ particles are evenly distributed on the surface of sepiolite and the particle size is between 10-20 nm.Ni particles are attached to the surface of NiAl₂O₄ with a particle size is less than 10 nm.Because sepiolite surface is electronegative,metal hydroxides are preferentially formed on the fiber surface during the coprecipitation process,and spinel-type nickel aluminate was produced after heat treatment.The coprecipitation-reduction method has the advantages of short synthesis cycle and easy control of preparation conditions compared with the sol-gel-reduction method.The particle size of the nickel aluminate in the prepared composites is smaller,and the distribution on the surface of sepiolite is more uniform.In this study,composites with sepiolite as the carrier and nickel aluminate as the transition layer were prepared,which provided a new idea for the preparation of new nickel-based catalysts,and laid a theory and experimental basis for the application of sepiolite with high added value.