Study On The Fabrication And Catalytic Performance Of Lanthanium-Promoted Copper-Based Catalysts For Hydrogenation Of Carbon Dioxide To Methanol

Due to the increasing demand for energy by human beings,resources such as coal and oil are increasingly depleted.At the same time,the large amount of carbon dioxide released during industrial consumption and the environmental changes have always threatened the sustainable development of mankind.Scientists have proposed that carbon dioxide as a carbon source for methanol synthesis is one of the ways to solve the above problems.The carbon dioxide hydrogenation route can realize the recycling of carbon resources.From this perspective,the carbon source in methanol synthesis can be obtained without the production of syngas from fossil energy,and breakthrough in the utilization technology of carbon dioxide can provide a basic carbon source for industrial production in the further.Therefore,the hydrogenation of carbon dioxide to methanol is of great significance and has attracted worldwide attention.Based on the research foundation of the predecessors,this work aims to optimize the activity of copper-based catalysts,methanol selectivity and catalyst stability.A series of design experiments and advanced characterization techniques are used to develop higher catalytic efficiency and more stable copper-based catalyst.Copper-based catalysts are widely studied in hydrogenation of carbon dioxide to methanol.The catalytic performance is significantly affected by the support and co-catalyst,especially the Cu-MOx interface plays an important role in catalytic performance.The interfacial effects of Cu-ZrOx or Cu-ZnOX been extensively studied,but the report on the construction and research of Cu-LaOx interface is lacking in depth.In this paper,we fabricated four Cu-based catalysts,copper microspheres(Cu MPs)coated with lanthanium oxide(Cu@LaOx),active copper species dispersed on lanthanium oxycarbonate(Cu/La2O2CO3-R),SBA-15 mesoporous confined Cu/La/O species(Cu1Lax/SBA-15),and in situ exsolution copper species on lanthanium modified copper phyllosilicate nanotubes(CuLaPSNTs)respectively,to further clarify the influence of Cu-LaOx interface.The main research contents are as follows.1.Firstly,we prepared Cu@LaOx catalyst by depositing lanthanium oxide species on the surface of Cu MPs to construct Cu-LaOx interface.We investigated the catalytic activity of Cu MPs catalyst in the hydrogenation of CO2 to methanol before and after deposition.It was found that the activity of Cu MPs catalyst was poor at 240? and 3.0 MPa,the TOF was only 0.24 h-1,and the methanol yield was 1.29 mg/gcat·h.However,Cu@LaOx catalyst showed high efficiency of methanol synthesis,the TOF was 1.29 h-1,which increased by more than 5 times,and the methanol production rate increased more than 13 times,reaching 64.7 mg/gcat·h.2.Although the Cu MPs coated with lanthanium oxide showed significantly improved methanol yield,the CO2 conversion is unsatisfactory,because only the active copper species at the surface/interface are utilized in Cu@LaOx catalyst,resulting in the active sites for hydrogenation are very limited.In this regard,several copper-based catalysts supported on lanthanium oxycarbonate were prepared by changing the synthesis method.The catalysts were studied by various characterization methods using XRD,TEM,N2-adsorption,N2O chemical adsorption,CO2 chemical adsorption,XPS,CO2-TPD and TPSR experiment.The catalytic performance of each catalyst was also investigated.The results showed that copper supported on rod-shaped lanthanium oxycarbonate(La2O2CO3-R)have the highest copper dispersion(DCu)of 21.2%and copper specific surface area(SACu)of 68.8 m2/g.The Cu/La2O2CO3-R catalyst has higher TOF(25.1 h-1),and the methanol selectivity up to 92.5%.Different from the traditional addition of lanthanium promoter,the newly formed Cu?+ species and the adsorption/activation ability of basic oxide on Cu/La2O2CO3-R interface are two key factors related to catalytic performance.In situ DRIFTS studies have shown that synergistic basic sites in the copper-lanthanium oxide interface region can selectively stabilize methanol synthesis intermediates,which is an intrinsic factor in Cu/La2O2CO3-R catalyst.Since the water vapor generated by the CO2 hydrogenation reaction can convert La2O3CO3 into armorphous hydroxycarbonate phase,the structure was gradually dissolved,resulting in the deactivation of the Cu/La2O2CO3-R catalyst after 24 hours.3.The decrease of active sites and the separation of Cu-LaOx interface are the main reasons for the deactivation of Cu/La2O2CO3-R catalyst.Thus,we prepared a series of Cu catalysts supported on lanthanium-modified SBA-15 by step-by-step impregnation.A large number of highly dispersed Cu-LaOx interfaces were generated in the mesoporous channels of the SBA-15 support.Studies have shown that lanthanium species can not only significantly enhance the CO2 adsorption capacity of the catalyst,but also promote the dispersion of active copper nanoparticles(Cu NPs).The optimized Cu1La0.2/SBA-15 catalyst has a nanoparticle size of about 3.5 nm,and the catalytic performance showed a CO2 conversion of about 6%and a methanol selectivity of about 80%in 100 hours.4.Although the Cu NPs and Cu-LaOx interfaces are dispersed in the mesopores of SBA-15 by impregnation method,the supported metal content is limited,and the CO2 conversion needs to be improved.Here,we have fabricated five silica oxide supported catalysts(CuLa/SiO2)by impregnation,deposition-precipitation,ammonia-evaporation and hydrothermal synthesis methods.Specifically,CuLaPSNTs catalyst exhibited excellent catalytic performance and superior stability for the selective hydrogenation of CO2 to methanol,giving methanol selectivity at around 80%and STY up to 428.2 mg/gcat·h during 200 hours.This catalyst has the specialty of Cu-LaOx interfaces coupled with Cu NP which are socketed onto the parent copper phyllosilicate nanotubes by in situ exsolution under a reducing atmosphere,which can maintain good stability and methanol selectivity.5.The mechanism of lanthanium-promoted copper-based catalysts for the hydrogenation of carbon dioxde was explored.The measured activation energy Ea of Cu MPs and Cu@LaOx catalysts were 96.6 kJ/mol and 62.5 kJ/mol,respectively,which means that the apparent activation energy of the CO2 hydrogenation reaction can be greatly reduced after constructing the Cu-LaOx interface,thus the methanol intermediates can be more easily obtained on the catalyst surface.At the Cu-LaOx interface,the adsorption and dissociation of hydrogen occur on the copper sites,while the adsorption of carbon dioxide takes place over the lanthanium sites.The atomic-hydrogen transfer to lanthanium sites via spillover and hydrogenates the adsorbed CO2?-species.The isotope HGD exchange experiments showed that the methanol synthesis reaction on Cu1La0.2/SBA-15 catalyst has stronger thermodynamic isotope effect than reverse water-gas shift reaction.In situ DRIFTS studies indicated that*HCOO and*H3CO species are key intermediates in CO2 activation and methanol synthesis,and the formate route dominates CO2 hydrogenation to methanol at the Cu-LaOx interface.