Preparation Of Novel And Efficient Nickel Catalysts Based On Perovskite Materials For CO₂ Reforming Of CH₄

In past decades,Greenhouse effect and energy shortage have become pressing issues.CO2 reforming of methane can convert two potent greenhouse gases(CH4/CO2)into valuable syngas(H2/CO)for clean energy production.Ni-based catalysts have been widely studied in CO2 reforming of methane because of moderate cost and good availability.But their reforming activity and stability still cannot meet the requirement of industrial applications.Therefore,it is urgent to explore new strategies to improve the reforming performance of Ni-based catalyst.Perovskite exhibits excellent catalytic performance in particular reaction systems due to intriguing physicochemical properties,such as high thermal stability,good oxygen mobility and unique redox behavior.Therefore,in this work,perovskite was used as support to develop efficient Ni catalysts for CO2 reforming of methane and the structure-activity relationship of the catalysts was explored.Firstly,the effect of preparation methodologies for LaAlO3 perovskite support was investigated.Subsequently,the effects of alkali earth metal(Mg and Ca)substitution on catalytic performance of Ni/perovskite catalysts were studied.Finally,on the basis of Ca-substituted catalyst system,the effect of Ni loading was examined.The catalysts were systematically characterized by N2 sorption,X-ray diffraction(XRD),transmission electron microscopy(TEM),energy-dispersive X-ray spectrometry(EDX)elemental mapping,thermogravimetry-differential scanning calorimetry(TG-DSC),H2 temperature-programmed reduction(H2-TPR),CO2 temperature-programmed desorption(CO2-TPD)and X-ray photoelectron spectroscopy(XPS).The main conclusions are as follows:1.The catalytic activity followed the sequence of Ni/LaAlO3(HT)>Ni/LaAlO3(CP)>Ni/LaAlO3(SG)>>LaNiO3 in CO2 reforming of methane in terms of different preparation methods.XRD patterns showed that the LaAlO3 perovskite support with a high crystallinity was obtained by co-precipitation and sol-gel method.N2 sorption results showed that Ni/LaAlO3(HT)catalyst had the largest specific surface area.H2-TPR profiles documented that the reduction temperature of Ni/LaAlO3(HT)catalyst was relatively lower.All of the above results preliminarily explained the high activity observed on the Ni/LaAlO3(HT)catalyst.The initial reforming activity was enhanced after partial substitution of La by Mg.The Ni/La0.8Mg0.2AlO2.9 catalyst exhibited the highest initial activity.According to N2 sorption,XRD and TEM results,Mg partial substitution benefits the dispersion of Ni nanoparticles,which may be the reason for the enhanced initial activity of the Ni/La0.8Mg0.2AlO2.9 catalyst.CO2-TPD profiles and O 1s XPS spectra respectively demonstrated that Ni/La0.8Mg0.2AlO2.9 catalyst has more basic sites and more oxygen vacancies,both of which could promote carbon gasification and thus inhibit carbon deposition.H2-TPR profiles documented that the catalyst with Mg partial substitution possesses stronger metal-support interactions,which leads to that the metal sintering is alleviated during reaction.The above characterization results explain the enhanced activity and the improved stability observed on the Ni/La0.8Mg0.2AlO2.9 catalyst.2.The initial reforming activity was enhanced after partial substitution of La by Ca.The Ni/La0.9Ca0.1AlO2.95 catalyst exhibited the highest initial activity,but it is not very stable.In order to improve the stability of the catalyst,the strategy of reducing the Ni content to adjust metal particle size was adopted.The results showed that 2.5 wt%Ni/La0.9Ca0.1AlO2.95 catalyst showed the best stability.XRD results show that the perovskite structure is still maintained and the Ni(2.5)and Ni(10)catalysts possessed carbon characteristic peak.N2 sorption indicated that 2.5 wt%Ni/La0.9Ca0.1AlO2.95 catalyst possesses the largest specific surface area,which would benefit the dispersion of Ni nanoparticles.The H2-TPR profiles documented that the metal-support interactions are stronger as the Ni loading decreased,which improves the anti-sintering ability of the catalysts.CO2-TPD profiles demonstrated that the catalyst with lower Ni content has more basic sites,which would facilitate carbon gasification and thus suppress carbon deposition.The result may rationalize the the good anti-carbon deposition performance.The thesis highlights the significance of surface properties in perovskite-supported metal catalysts and optimizes the amount of metal loading,which provide new insights for the preparation of highly active and stable(low carbon deposit)reforming catalysts with the lowest content.In addition,it provides experimental foundation and theoretical guidance for reducing greenhouse gas emission and producing clean energy.