Synthesis Of Novel Ni-based Catalysts And Their Catalytic Performance

Recently,carbon dioxide reforming of methane（DRM）,which can consume the two primary greenhouse gases（CH₄ and CO₂）into low H₂/CO molar ratio syngas,has received considerable attention.Furthermore,its high endothermicity makes the DRM into a promising method for converting abundant renewable energies,especially solar energy,into chemical energy.Noble metals catalysts,such as Ru,Pt and Pd,possess high catalytic activity and excellent coke resistance for DRM reaction,but the high price and limited availability restrict their large-scale commercial application.Therefore,low-cost nickel-based catalysts have become the major research objects.Herein,a seies of mesoporous crystalline NiAl₂O₄/γ-Al₂O₃ and La,Ce,Ca,Mg modifed NiAl₂O₄/γ-Al₂O₃ composites were prepared through the one-pot partial hydrolysis of metal nitrate salts.The obtained materials were systematically characterized by various measurements,and used for the DRM reaction.The main research contents and results are summarized as follows:（1）A series of NiAl₂O₄/γ-Al₂O₃ composites with various Ni contents have been successfully prepared via the one-pot method in the absence of surfactants and used for carbon dioxide reforming of methane.The characterization results demonstrated that the NiAl₂O₄/γ-Al₂O₃ materials possessed wormhole-like mesoporous structures with high specific surface areas,large pore volumes and narrow pore size distributions;and the Ni²⁺ ions were completely reacted with alumina to single-phase NiAl₂O₄/γ-Al₂O₃ solid solution in the matrices using N₂ sorption,XRD,TEM,and XPS.The NiAl₂O₄/γ-Al₂O₃ materials exhibited excellent catalytic properties and superior long-term stability for carbon dioxide reforming of methane.The effects of Ni content on the intrinsic activities and the amounts of coke disposition of the NiAl₂O₄/γ-Al₂O₃ catalysts were discussed in detail for the carbon dioxide reforming of methane.The results revealed that the Ni particle sizes did not affect the intrinsic activity of metallic Ni,but smaller Ni particles could reduce the rate of coke deposition.（2）La-modified NiAl₂O₄/γ-Al₂O₃-La composites with homogeneous mesopores in the γ-Al₂O₃ frameworks were successfully obtained and applied for dry reforming of methane to syngas.The evaluation results showed that the reduced catalysts exhibited excellent catalytic properties as well as long-term stability for DRM reaction.Addition of La showed little influence on the catalyst structure and the mean sizes of metal Ni particles,but could enhance the medium-strength basicity and the accumulation of Ni²⁺ on the catalyst surface,resulting in the enhancement of intrinsic activity,the reduction of apparent activation energy,and the suppression of carbon deposition for DRM reaction.The catalyst containing 3 wt% La possessed the best excellent catalytic performance.The characterization of spent catalysts also demonstrated that La could effectively prevent the phase transformation of γ-alumina in the DRM process.（3）Mesoporous NiAl₂O₄/MO_x（M=La,Ce,Ca,Mg）—γ-Al₂O₃ nanocomposites were successfully prepared.Promotion effect of modifiers on the physicochemical properties and catalytic performance of the catalysts was systematacially investigated,and in comparision with NiAl₂O₄/La₂O₃/γ-Al₂O₃—imp prepared via traditional impregnation method.Characterization and evaluation results indicated that the modified Ni/MO_x（M=La,Ce,Ca,Mg）—γ-Al₂O₃ catalysts showed higher activities and better coking-resistance than Ni/γ-Al₂O₃,and Ni/La₂O₃—γ-Al₂O₃ was found to be the most effective one.All the Ni particles derived from the reduction of NiAl₂O₄ precursor presented similar average sizes,and highly dispersed throughout mesoporous γ-Al₂O₃ frameworks.However,more medium-strength basic sites on the catalyst surface were obtained by adding promoters,which could facilitate the adsorption/activation of CO₂ and the gasification of amorphous carbon,improving the catalytic properties and accelerating the coke elimination rate.Additionally,the incorporation of promoters also prevented the phase transformation of γ-alumina.（4）The removal of CO in hydrogen-rich reformate gases through selective CO methanation has stimulated particular attention with the development of proton exchange membrane fuel cell（PEMFC）.On the basis of the previous research achievements by our group,we expanded the one-pot partial hydrolysis method to prepare mesoporous Ru-Ni/γ-Al₂O₃ and Zr-modified Ru-Ni/γ-Al₂O₃ materials after 450 ℃ calcination,and employed them in selective CO methanation.The effects of contents of Ru,Ni and Zr on the catalysts structure,Ni particle sizes,metal-support interaction,and activity and selectivity of CO methanation were investigated in detail.Ru-Ni/γ-Al₂O₃ catalysts not only can reduce the CO concentration lower than 10 ppm,but also have a wide working temperature window up to 109 ℃.The addition of moderate Ru and Zr can effectively decrease the Ni particle sizes and improve the catalytic performance.The interaction between NiO species and support gradually become weaker with the Ni contents,simultaneously,the Ni crystallite sizes increased.The optimization results showed that 3Ru-25Ni/15ZrO₂—γ-Al₂O₃ catalyst present the best catalytic performance.The catalysts have excellent stability during 300 h on stream with no detectable change in CO and CH₄ concentrations under the standard reaction conditions with 15 vol% H₂ O,showing a huge commercial application value.