Study On The Zinc-based Catalysts For CO₂ Hydrogenation

Combining the "green hydrogen" produced by clean energy with CO2 will be one of the effective ways to achieve the carbon neutral goal,and this process will also play an important role in the energy and chemical industry in the future.Thermal catalytic hydrogenation of CO2 to methanol is currently the most promising technology for large-scale application.Zinc based catalysts,for high temperature CO2 hydrogenation to methanol,have been widely used to construct bifunctional catalysts for selective synthesis of various chemicals.It is very important to identify the surface structure of Zn-based catalysts under reaction conditions for understanding the structure-activity relationship and designing and synthesizing highperformance catalysts,although it remains a grand challenge.In this work,through in situ FTIR,XRD,XPS and other characterizations,the dynamic evolution of the surface and interface structure,active sites and intermediates on Zn-based oxides under in situ conditions were studied.We identified the active phase of CO2 hydrogenation to methanol on ZnAl2O4,and designed and synthesized highly active ZnO/ZrO2 catalysts to investigate the active sites for the formation of CO and methanol.Finally,the optimized catalyst is coupled with ZSM-5 to realize the direct conversion of CO2 to aromatics.The main contents and conclusions are summarized as follows:Herein,the surface reconstruction of ZnAl2O4 spinel during CO2 hydrogenation reaction was investigated using detailed spectroscopic characterization.The results indicate that the reaction-driven surface reconstruction leads to the formation of amorphous zinc oxide on the ZnAl2O4 surface.The reconstructed catalyst promoted the hydrogenation of carbonate species to formate species.Under the reaction conditions,H2 cleaves the Zn-O bond on the surface of ZnAl2O4 and promotes the generation of oxygen vacancies from ZnAl2O4,after which CO2 oxidizes the partially reduced spinel surface.The redox cycle breaks the spinel surface and produces amorphous zinc oxide with methanol generating activity.Kinetic analysis,STEM and in situ IR results indicate that the amorphous zinc oxide boosts the methanol formation.Based on the understanding of the active phase of CO2 hydrogenation on ZnAl2O4,the supported ZnO/ZrO2 catalyst was studied.The change of structure and surface properties of the catalyst and its effect on the catalytic performance were investigated.It is found that the methanol formation rate on ZnO/ZrO2 catalyst increases gradually under the reaction conditions,indicating that the structure of the supported zinc oxide has changed.The changes of the surface structure of the catalyst were traced by in situ IR.It was found that the zinc oxide grew up gradually during the reaction.By adjusting the dispersion of zinc oxide,the inference of zinc oxide aggregation was verified.With the aggregation of zinc oxide cluster,the activation ability of hydrogen increases,which accelerates the synthesis of methanol.the modification of the interface sites shows that the ZnO-ZrO2 interface is the main sites for the formation of CO,and the surface of the zinc oxide mainly contribute to the synthesis of methanol.Based on the study of the performance of ZnO/ZrO2 catalyst,the tandem catalysts were constructed to broaden the CO2 conversion path and realize the further upgrading of methanol products.The synthesis of aromatics from one-pass CO2 hydrogenation is still a great challenge owing to the high aromatization barrier.Here a tandem catalyst comprising of ZnO/ZrO2 and ZSM-5 zeolite was proposed for one-pass CO2 conversion to aromatics.By adjusting the pore structure and acidity of zeolite and optimizing the reaction conditions,the selectivity of aromatics in hydrocarbon products is close to 70%,while the selectivity of by-product methane is less than 1%.The present research has found that the production of methanol comes directly from the hydrogenation of CO2 rather than through CO intermediates.By exploring the changing trend of hydrocarbon products with contact time and reaction time,a reaction path from CO2 to aromatics on ZnO/ZrO2-ZSM-5 tandem catalyst was proposed.