Synthesis of Novel Ni-based Catalysts and Their Catalytic Properties for Methane Reforming
Author:Chen Chen Ju
Supervisor:wang xue guang
As the two main greenhouse gases,the excess emission of methane and C02 would cause greenhouse effect.Therefore,eliminating and utilizing the greenhouse gases has been more and more interested.Catalytic converting these gases into high-value syngas which was composed of H2 and CO is a viable option.However,the dry reforming of methane(DRM)is usually carried out at high temperatures because of the endothermic nature.Thus,it is challengeable that development of innovative coke-and sinter-resistant catalysts at high temperatures for DRM.Ni-based catalysts were often preferred due to its comparable activity with noble catalysts as well as the wide availability.In this context,a series of mesoporous Ni-La-Si mixed oxides were first synthesized via the one-pot hydrolysis and condensation of TEOS and metal nitrates in a water-polyols solution of nitric acid and applied for methane reforming.The obtained Ni-La-Si materials were characterized in detail and the catalytic properties of Ni/La-Si catalysts were studied in detail.The maiiuresearch contents and results are summarized as follows:(1)Mesoporous silica-supported Ni-La oxides with homogeneously-dispersed NiO nanoparticles were first synthesized via one-pot sol-gel route of tetraethoxysilane,metal nitrates,and poly(ethylene glycol).Upon reduction with H2,the NiO nanoparticles were reduced to in situ generate metallic Ni particles distributed in the matrices.The obtained Ni/xLa-Si-P catalysts exhibited high activity and excellent stability for DRM with an almost 100%H2 selectivity or a H2/CO molar ratio of unity.The presence of La significantly inhibited the reverse water gas shift reaction,resulting in a H2/CO ratio close to unity.(2)Ni-La-Si mixed oxides with different La contents were first prepared via one-pot polyethylene glycol-assisted hydrolysis and condensation of tetraethoxysilane and inorganic salts in a water-ethylene glycol solution of nitric acid and used for carbon dioxide reforming of methane.The obtained Ni-La-Si materials had mesoporous structures,narrow pore size distributions and large specific surface areas.The effects of La loadings on material structure,surface characteristics,interaction between Ni species and the supports,and dispersion of Ni species were investigated in detail.Homogeneously-dispersed NiO species were reduced in situ with H2 to form ultrafine metallic Ni nanocrystallites of 5-7 nm throughout the mesoporous silica,and the obtained Ni/La-Si catalysts exhibited high catalytic activity and excellent resistance to coke depositon.Addition of La in the Ni-La-Si catalysts not only improved the dispersion of Ni crystallites,the reforming rate of Ni active sites and catalytic stability,and more importantly,but also strongly inhibited the reverse water gas shift reaction,resulting in almost 100%H2 selectivity or a synthesis gas with H2/CO ratio of 1:1.(3)nRhNi-La-Si(n=0,0.5,1.0,1.5,2.0)mixed oxides with different weight percentage of Rh were synthesized with the one-pot hydrolysis and condensation of TEOS and metal nitrates in a water-ethylene glycol solution of nitric acid and applied to the DRM.Upon reduction,metallic NiRh alloy particles were obtained and homogeneously dispersed in the SiO2 matrix.The addition of Rh not only enhanced the dispersion of Ni species,but also improved the catalytic activity and stability of nRhNi/La-Si catalysts.In addition,the formation of NiRh alloy significantly enhanced the coking resistance of the nRhNi/La-Si catalysts.The 1.5RhNi/La-Si catalyst exhibited the most excellent catalytic activity,stability and coking resistance after the 50-h DRM.(4)The highly dispersed mNi/xLa-Si catalysts with varied weight percentages of Ni and La were synthesized via one-pot sol-gel process and vm subsequently applied to combined carbon dioxide and steam reforming of methane(CSDRM)for syngas production.The addition of La improved the catalytic activity and stability as well as the coke resistance of the mNi/xLa-Si catalysts.The effects of preparation routes,Ni contents and CO2/steam(C/S)ratios on the performances of the Ni/La-Si catalysts were studied in detail for the CSDRM.The 17.5Ni/3.0La-Si catalyst synthesized with the assistance of poly(ethylene glycol)and ethylene glycol exhibited the most excellent catalytic activity,stability and coke resistance.In addition,the H2/CO ratios in the product gas could be tuned by changing the C/S ratios in the feed.When the C/S ratio was 0.5,the H2/CO ratio of about 2 was achieved for the 17.5Ni/3.0La-Si catalyst.