Design Of Highly Efficient Zn-based Catalyst And Its Performance Of Reverse Water Gas Shift Reaction

People’s use of fossil fuels will produce a large amount of CO₂,and these continuous emissions of CO₂ will cause the greenhouse effect and ecological crisis.Therefore,the catalytic conversion of CO₂ into valuable chemicals and fuels is an effective way to reduce dependence on fossil fuels and achieve carbon neutrality.Among them,the reverse water gas shift（RWGS）reaction is the main step in the CO₂ hydrogenation reaction,and the important platform molecule CO generated by it can be further value-added conversion reaction,which attracts widespread attention.However,based on the thermodynamic characteristics of RWGS reaction,high temperature conditions are conducive to the forward reaction,but the catalyst is prone to sintering.Therefore,how to design high-performance catalysts is a great challenge.Transition metal oxides with strong reducibility are widely used as catalysts for the conversion of CO2 to CO,methanol,low carbon olefins and aromatic hydrocarbons due to their good storage capacity of oxygen,which can deprive the lattice oxygen on the catalyst surface and generate surface oxygen vacancies to promote the adsorption of CO₂ in the H₂atmosphere.However,this reducible transition metal oxide catalyst has poor stability under high temperature reaction conditions,which greatly affects its industrial application.In particular,among reducible transition metal oxide catalysts,Zn-based oxide catalysts are widely used in RWGS reaction,methanol synthesis and fischer-Tropsch synthesis due to their easy construction with heteroatoms to form mixed metal oxides with high stability and activity.In this thesis,a high performance CO₂ hydrogenation catalyst ZnCrOx catalyst was constructed by adding Cr promoter based on Zn base oxide.On this basis,combined with the reaction performance evaluation results and characterization methods such as XRD,XPS,HR-TEM,H₂-TPR and In Situ DRIFTS,the effects of Cu promoters on the morphology,structure,active center and reaction mechanism of ZnCrOx catalyst were analyzed.The main research contents and conclusions are as follows:（1）ZnCrOx catalysts with different Zn/Cr molar ratios were prepared by co-precipitation method,and the molar ratios were 1:0,1:1,1:2,1:4 and 0:1,respectively.The effects of temperature,space velocity and pretreatment conditions on RWGS reaction performance were investigated.The XRD results show that when the Zn/Cr mole ratio is 1:2,the crystal composition is ZnCr₂O₄ spinel phase,and the catalyst phase is stable.XPS characterization analysis shows that the oxide catalyst with spinel structure has more oxygen vacancies,that is,it can provide more active sites for CO₂ adsorption,so ZnCrOx catalyst shows good stability and activity.In addition,the RWGS reaction rate increases further with the increase of temperature,and the CO₂ conversion rate increases with the increase of pressure.Moreover,when the space velocity of reaction gas is 30 m L/min,the contact time between reaction gas and catalyst is the best,and then the reaction can reach the best activity.（2）ZnCrOx catalysts doped with different metal promoters such as Ce,Fe,K and Cu were prepared by co-precipitation method（The molar ratio of Zn/Cr was 1:2）.The effects of different promoters on the activity,selectivity and stability of the catalysts were investigated.Compared with other metal promoters,Cu/Zn₁Cr₂ catalyst achieves the best activity and CO selectivity is up to 97%when Cu mass fraction is 3%.The results of XRD and HR-TEM showed that the phase structure of the catalyst remained stable after 48h reaction.On the other hand,combined with the characterization results of H₂-TPR,XPS and In Situ DRIFTS,it is proved that compared with Zn₁Cr₂ catalyst without metal promoters,the Cu-Zn interface is formed between Cu and Zn O with 3%mass fraction.On the other hand,combined with the characterization results of H₂-TPR,XPS and In Situ DRIFTS,the analysis shows that compared with Zn₁Cr₂catalyst without metal additives,the addition of 3%mass fraction of Cu and Zn O forms a rich Cu-Zn interface.In addition,the Cu-Zn interface promoted the reaction of CO₂ with surface hydroxyl species to generate more intermediate species such as formates and carboxyl groups,and the final product CO was further generated by these intermediate species.At the same time,the redox reaction rate calculation further proves that the addition of metal Cu promoter contributes to the formation of new active sites on the Cu-Zn interface,and the co-existence of associative and redox paths significantly improves the RWGS reaction activity of the catalyst.