A Study On Preparation And Performance Of Highly Dispersed Nickel-based Catalysts

Catalysts are the foundation of the modern chemical industry,and widely used in various fields of the national economy.The research and development of high-performance catalysts is still the core of current chemical production.In comparison with precious metal catalysts,nickel-based catalysts has many outstanding advantages,such as wide source of raw materials,low price and satisfactory performances.As of now,nickel-based catalysts have been widely applied in various industrial productions such as hydrogenation,dry reforming of methane(DRM),methanation and dehydrogenation.At present,the research and development of nickel-based catalysts mainly focus on how to improve their catalytic activity and stability,especially the durable capabilities of resisting sintering and carbon deposition in hightemperature reactions.Compared with other catalyst preparations,the sol-gel method has unique advantages in the preparation of highly dispersed supported catalysts.Using a simple and controllable sol-gel process,the nickel precursors can be effectively dispersed and encapsulated in the gel network,which promote the formation of small metal nanometers.The interaction between metal species and supports could be adjusted by the modified sol-gel method to tune catalytic active sites for improving the high-temperature catalytic activity and stability of the catalyst.In this work,one novel modified sol-gel method was proposed as a construction strategy of catalyst materials.Tetraethyl silicate was used as the silicon source of the catalyst network carrier(Si O2),and the natural polysaccharide polymers were introduced as green chelating agents to uniformly disperse the nickel precursors in the homogeneous 3D structural hybrid matrix.During the heat treatment process,the nickel precursors were strongly embedded and restricted in the gel network,which could effectively prevent the thermal migration and agglomeration of Ni species.This study reported a green and facile synthesis of highly dispersed supported metal catalysts using a modified sol-gel method.Furthermore,the influencing factors including the additive amounts,metal loadings,and metal types on the physicochemical properties of supported catalysts,and the catalytic performance and the related mechanism were primarily studied.Subsequently,the catalyst preparation strategy was successfully applied in the preparation process of the bimetallic Ni-Ce/Si O2 catalyst,and the preparation procedure of the method was optimized,which significantly improved the hightemperature thermal stability.The main relevant conclusions were as follows:1.SP was selected from a variety of natural polysaccharide polymers,and the effects of SP addition amount,metal loading,and metal type on the physical and chemical properties of the supported catalyst were systematically investigated.The characterization test results suggested that the additive SP significantly inhibited the thermal migration and aggregation of metal species during the heat treatment process,which led to the formation of ultra-small nanoparticles(<4 nm)and the high dispersion of the supported metal.In comparison with the10%Ni/Si O2 catalyst without SP addition,10%Ni/Si O2 0.3SP catalyst(the optimal weight ratio of nickel salt to SP was 1:0.3): the particle size of Ni O was reduced from 16.9 nm to 3.8 nm;and the particle size of Ni was reduced from 14.5 nm is reduced to 3.2 nm;and Ni dispersion was increased from 2.1% to 33.3%.In the DRM reaction(700°C),the CH4 and CO2 conversions of the catalyst were increased by about 13% and 14%,and significantly improved resistance to sintering and carbon deposition.In consequence,the facile synthesis method in this work could easily be applied to design other Si O2-supported metal catalysts such as Me/Si O2(Me = Co,Fe,Ce,Cu).2.In order to further improve the ability of 10%Ni/Si O2 0.3SP to resist sintering and carbon deposition,the second component of Ce(1%～10%)was added during the preparation of the catalyst.Through catalyst characterization and DRM test(700℃,30h),we found that adding Ce(1%～10%)could further promote the dispersion of Ni,and the dispersion of Ni increased with the content of Ce.In addition,Ce O2 could further inhibit the sintering and carbon deposition of Ni metal during the long-time reaction.The catalyst showed the best performance and stability under 5% Ce loading.3.Using citric acid(CA)instead of SP as a promoter,the highly dispersed 20% Ni/Si O2 catalyst was prepared by sol-gel.During the preparation process,the effect of different heattreatments on the hydrogenation performance was studied,and the relative preparation parameters were further optimized.The experimental results suggested that: the precursor was treated in hydrogen atmosphere(H2),nitrogen and hydrogen in sequence(N2-H2),nitrogen,air,and H2 in stages(N2-air-H2)prepared by the catalyst for naphthalene hydrogenation performance: H2>N2-H2>N2-air-H2,indicated that different heat treatment methods of the catalyst affect the performance of the catalyst.When the 20%Ni/Si O2 catalyst was heat-treated in a hydrogen atmosphere at 400°C(Ni/CA = 1),its hydrogenation activity was comparable to that of Ru/C at low temperature(55°C).