Preparation,Characterization And Methanation Performance Of Carbon Dioxide Over Nickel Based Catalyst

Carbon dioxide（CO₂）is the main component of greenhouse gases.Excessive emissions of carbon dioxide（CO₂）will cause a series of irreversible climate changes and seriously affect sustainable development.Thermally catalyzed CO₂ methanation converts room temperature gas CO₂ into CH₄,which can be used as fuel or industrial raw material.It has both environmental and economic benefits and has attracted much attention.Nickel-based catalysts have been widely studied because of their excellent catalytic performance and low price,but there are some problems such as poisoning,sintering and carbon deposition.In this paper,nickel-based catalysts with different supports（TiO₂,MCM-41 andγ-Al₂O₃）,modified with La additives and derived with hydrotalcite structure were prepared.The effects of the support,the content of La additives and the precursors of hydrotalcite structure on the activity and structure of nickel-based catalysts were systematically studied.The results showed that:（1）Ni/TiO₂ catalysts prepared with different supports had the best activity for CO₂ methanation,and the order of reaction activity was as follows:Ni/TiO₂>Ni/MCM-41>Ni/γ-Al₂O₃;（2）Under the reaction conditions of 400 ℃ and0.1 MPa,the activity of CO₂ methanation of Ni/TiO₂ catalyst is 79%,and the selectivity of CH₄ is 92%.（3）It was found that when TiO₂ was used as the carrier,the interaction between Ni O and TiO₂ carrier was suitable,and the reduction temperature was 455℃,which could form highly dispersed active sites of Ni.The results show that:（1）the addition of La can promote the formation of small size and highly dispersed Ni nanoparticles;（2）La as an electron assistant can change the electron environment around nickel atoms,which is conducive to the activation of CO₂;（3）La weakly interacts with Ni,which weakens the interaction between Ni and the carrier matrix and reduces the difficulty of Ni O reduction;（4）Among the catalysts with different La content,the catalytic performance of the catalyst is the best when the La content is 2wt.%.Under the reaction conditions of 400℃,0.1 MPa and H₂:CO₂=4:1,the CO₂ methanation activity of 2La-10Ni/TiO₂ catalyst is 83%,and the CH₄ selectivity is 94%.The results of the nickel-based catalyst derived from the hydrotalcite structure precursor show that:（1）The reaction conditions of 300℃,0.1MPa,the reactivity and selectivity of the catalyst meet the following rules:La-Ni Al（R）catalyst>Mg-Ni Al（R）>Ni Al（R）,and the CO₂ methanation activity of La-Ni Al（R）catalyst is 86%,and the CH₄ selectivity of La-Ni Al（R）catalyst is 97%.（2）As the roasting temperature increases,the precursor of Ni Al（R）catalyst tends to be sintered and usually forms spinel（Ni Al₂O₄）and solid solution,which leads to a decrease in surface area,pore volume,difficulty of reduction and average pore size.As a result,the increase in reduction temperature leads to insufficient reduction of active components.The Ni Al₂O₄ spinel phase decreases the reaction performance of the catalyst to a certain extent.（3）the addition of Mg species can improve the structural stability of the catalyst and prevent the formation of spinel;However,the Mg-Ni Al-HT hydrotalcite structure formed is too stable,and the interaction force between metals is too strong,leading to too high reduction temperature,increasing reduction difficulty,and insufficient reduction,resulting in the coexistence of Ni and Ni O,thus affecting the catalytic performance.（4）The addition of La species is beneficial to improve the dispersion degree of the active component,while the high dispersion degree of Ni²⁺will promote the reduction,reduce the reduction temperature of the catalyst,and make the reduction of active component Ni more fully,and the catalytic effect is better.