Design Of High-performance Gold-copper Catalysts For Carbon Dioxide Hydrogenation To Methanol

Converting carbon dioxide（CO₂）with“green”H₂ from renewable energy into methanol by thermocatalysis can not only alleviate CO₂ emission but also produce value-added chemical.The industrialization of this technology is mainly impeded by the development of feasible catalysts.By virtue of high methanol synthesis rate,mild reaction conditions and acceptable fabrication cost,copper（Cu）-based catalysts are most extensively studied.However,current Cu-based catalysts still cannot deliver sufficient catalytic performance for industrial application.Therefore,the enhancement of methanol synthesis activity on Cu-based catalysts has always been the key point for researchers.In the present dissertation,the construction of AuCu bimetallic catalysts could significantly boost methanol synthesis activity.Subsequently,AuCu bimetallic catalysts were further optimized with the aid of MOFs materials,mainly including two routes.On one hand,MOFs materials was employed as precursors to prepare high-performance bimetallic oxide catalysts.On the other hand,MOFs materials was used as confinement supports to synthesize bimetallic confinement catalysts.In addition,multiple characterization methods were utilized to investigate surface-interface structure of catalysts and reaction mechanisms,which would thus reveal the relationship between catalyst structure and catalytic activity in methanol synthesis.The detailed studies are as follows:1.Adding trace amount of Au（0.4 wt%）into Cu/ZnO catalysts by the co-precipitation method would significantly enhance methanol synthesis performance.Among Au_xCu/ZnO bimetallic catalysts with various Au/Cu molar ratio（x=0.001,0.005,0.02 and 0.05）,the Au_0.005Cu/ZnO catalyst exhibited the highest methanol production rate.The space time yield of methanol(STY_{Me OH})on the Au_0.005Cu/ZnO bimetallic catalyst reached 314.4g _{Me OH} kg_cat^-1 h^-1 at 250℃ under 3.0 MPa,which was 1.75 times as high as that on the Cu/ZnO monometallic catalyst(179.7 g _{Me OH} kg_cat^-1 h^-1).Besides,the Au_0.005Cu/ZnO catalyst showed good stability for 30 h of reaction at 250℃.The formation of Au-Cu alloy was identified on the Au_xCu/ZnO bimetallic catalysts.Experimental characterizations and density functional theory（DFT）calculations indicated that oxygen vacancies-modified metal-oxide interfaces served as intrinsic active sties for both un-promoted and Au-promoted Cu/ZnO catalysts.The promotion effect of Au in bimetallic catalysts was correlated with the increase of active sites,facilitation of CO₂ activation and modification of intermediate adsorption.2.Three AuCu/ZnO bimetallic catalysts,AuCu/ZnO-BTC,AuCu/ZnO-BDC and AuCu/ZnO-MOF-74,were derived from various MOF precursors using facile hydrothermal methods.Among the resultant catalysts,the AuCu/ZnO-BTC catalyst exhibited the highest methanol production rate,whose STY_{Me OH} reached 359.0 g _{Me OH} kg_cat^-1 h^-1 at 250°C under 3.0 MPa.Structural characterization reveals that in comparison with the AuCu/ZnO-BDC and AuCu/ZnO-MOF-74 catalysts,the AuCu/ZnO-BTC catalyst possessed higher specific surface area(S_BET),smaller metal particle size,higher concentration of oxygen vacancies,stronger metal-support interaction（MSI）and more abundant medium basic sites.In situ DRIFTS result demonstrates that formate（*HCOO）species were readily generated and bridged-methoxy（*b-OCH₃）species were selectively formed on the AuCu/ZnO-BTC catalyst,which may be related with its abundant oxygen vacancies and responsible for its superior methanol synthesis activity.3.The AuCu/UiO-66-EDTA bimetallic catalyst with confinement structure was facilely synthesized by a method of using EDTA-modified UiO-66 to encapsulate metal nanoparticles for CO₂ hydrogenation to methanol.The STY_{Me OH} on the AuCu/UiO-66-EDTA(3.34 g _{Me OH} g_metal^-1 h^-1)was 3.21 times that of AuCu/Zr O₂ catalyst(1.04 g _{Me OH} g_metal^-1 h^-1)and 1.58 times as high as that of AuCu/UiO-66 catalyst(2.11 g _{Me OH} g_metal^-1 h^-1)at 250°C,respectively.Structural characterization indicates that the formation of confinement structure on the AuCu/UiO-66-EDTA catalyst could increase S_BET,decrease metal nanoparticles size,induce oxygen vacancies,enhance MSI and enlarge the amount of medium basic sites.In situ DRIFTS result suggests that faster conversion of reaction intermediates,more facile formation of*HCOO species and selective generation of*b-OCH₃ species were noted on the AuCu/UiO-66-EDTA catalyst.The discrepancy in the evolution of reaction intermediates may be associated with the formation of rich oxygen vacancies-modified metal-oxide node interfaces on the AuCu/UiO-66-EDTA catalyst,which rationalized its boosted catalytic performance.