Construction Of Ni-Based Catalysts And Its Catalytic Performance For CO₂ Hydrogenation

The massive consumption of fossil fuels has increased the concentration of CO₂in the atmosphere year by year,leading to aggravation of the greenhouse effect and causing many environmental problems.But from the perspective of resource utilization,CO₂is also a"carbon source"compound with a wide range of sources and low prices.CO₂hydrogenation conversion is an effective CO₂utilization method.Among them,the hydrogenation reduction of CO₂to CH₄or CO is the two most basic conversion forms.In the catalyst system,the two often coexist and compete with each other,which results in the decrease of the specific selective products in the target reaction.How to selectively control the hydrogenation products has been a hot and difficult topic in this field.In this paper,by constructing a Ni-based alloy catalyst and doping anions(S^2-,Cl^-),the traditional Ni-based CO₂methanation catalyst is transformed into a high-efficiency catalyst for CO₂hydrogenation to CO.The catalysts were characterized by XRD,N₂adsorption-desorption Test,SEM,TEM,XPS,CO₂-TPD,H₂-TPR and other characterization methods.（1）Using nickel-based layered metal hydroxides（Ni-LDHs）as precursors,after calcination,reduction and in-situ carbon doping,Ni-based intermetallic compounds（Ni-IMCs）with different Ni/In/Al ratios were obtained.The effect of Al³⁺doping on the catalyst structure and CO₂hydrogenation performance is investigated.The research results show that the best Ni/In ratio is 3:1.At T=500℃,the CO₂conversion rate of the Ni₃In catalyst is 42.7%,and the CO selectivity is 100%.On this basis,based on the replaceability of trivalent ions in LDH,Ni In Al LDHs were prepared by replacing part of In in Ni₃In-LDH with Al.Experiments show that the addition of Al can promote the complete transformation of the Ni₃In and Ni₄In phases into the highly dispersed In Ni₃C_0.5active phase,increase the concentration of oxygen vacancies on the catalyst surface,increase the CO₂adsorption capacity,and maintain the original two-dimensional structure of the catalyst.At T=450℃,the CO₂conversion rate of the Ni₃In C_xcatalyst is 34%,and the CO selectivity is 100%;while the Ni₃In_0.7Al_0.3Cx catalyst with Al added has the CO₂conversion rate increased to 41.2%,but the CO selectivity decreases slightly（96.2%）,this is due to the relatively increased metal Ni loading on Al₂O₃ to generate Ni/Al₂O₃ phase,which further promotes deep hydrogenation of part of CO₂to generate CH₄.（2）A series of Ni/ZrO₂ catalysts were synthesized sequentially by urea precipitation method and impregnation method.Verification of the regulation of anions(S^2-,Cl^-)on the selectivity of CO₂hydrogenation.The research results show that the Ni/ZrO₂(S^2--Cl^-)catalyst has 100%CO selectivity throughout the temperature range,and the CO₂conversion rate of the Ni/ZrO₂-W catalyst obtained after washing is higher than that of the Ni/ZrO₂(S^2--Cl^-)catalyst has been significantly improved,but the catalytic product is mainly CH₄(S_CH4is91.65%)and a small amount of CO(S_COis 8.34%)is generated,which is a typical methanation catalyst.Subsequently,by adding S^2-and Cl^-to the Ni/ZrO₂-W catalyst,the CO selectivity was successfully increased from 8.3%to 94.2%,confirming that the anions(S^2-,Cl^-)play a key role in the regulation of CH₄/CO selectivity.The related characterization results show that:Compared with the Ni/ZrO₂(S^2--Cl^-)catalyst,the Ni/ZrO₂-W catalyst is more basic and has more surface oxygen vacancies,so it has stronger CO₂adsorption and activation capabilities,which shows relatively high CO₂hydrogenation activity.The trace amounts of S^2-and Cl^-in the Ni/ZrO₂(S^2--Cl^-)catalyst have a significant co-promoting effect on the improvement of CO selectivity.