Controllable Construction And Catalytic Mechanism Of Cu-based Catalysts For Water Gas Shift Reaction

With the rapid development of social economy,the environment and energy crisis are increasingly rigorous.So,it meets the strategic demand for sustainable development to develop hydrogen technology vigorously.Steam reforming of Hydrocarbons and oxygenated hydrocarbons is the main technology in the industry for hydrogen production.However,the by-product CO is inevitably produced,which is toxic to the Pt electrode of fuel cells.Water-gas shift reaction is one of the effective ways to solve the problem.Yet the composition of the catalysts for the water gas shift catalytic reaction is complex,and there are ambiguities in the reaction mechanism.In this paper,efficient Cu based catalysts were constructed on Si O₂and Al₂O₃,respectively.And the structure-activity relationship and reaction mechanism of the catalysts were studied in depth.The selected topic of thesis has great social significance and potential economic value for the rational design and development of high-efficiency water-gas shift catalysts.Firstly,a series of Cu/Si O₂-AE catalysts with different Cu content were prepared by the steaming ammonia method without any additives.As indicated by in-situ XRD results,the bulk metallic copper nanoparticles with a size of～4 nm exist on Cu/Si O₂-AE catalyst after hydrogen reduction at 350°C,and no Cu₂O is found.However,as indicated by in-situ XAFS results,there are also a small amount of Cu⁺active sites in the catalyst after reduction in addition to a large number of Cu⁰ active sites,but no Cu-O coordination is found in the radial structure function curve,which means the scarcity of Cu₂O species.Subsequent results of in situ XPS experiment show that there is a large amount of Cu⁺active sites on the surface of the reduced catalyst,accounting for 58%of the total Cu atoms on the surface.These findings show that the copper species in the reduced 25%Cu/Si O₂-AE catalyst are mainly in the form of bulk metallic copper nanoparticles and a small amount of Cu⁺enriched on the catalyst surface.The results of activity evaluation show that the 25%Cu/Si O₂-AE catalyst exhibits the best WGS activity at low temperature,and its reaction rate value reaches 9.36×10^-5 mol _CO g _Cu^-1 s^-1at 200°C,which is～9 times higer than that of the traditional Cu/Zn O/Al₂O₃ catalyst.Simultaneously,it exhibits excellent stability for WGSR.After 100 h of stability test and 9 cycles of start-stop experiments,the CO conversion of the catalyst remains～90%,which implies promising potential for vehicle fuel cells.In order to make clear the water gas shift reaction mechanism of Cu/Si O₂-AE catalyst,a series of in-situ structure characterizations of the 25%Cu/Si O₂-AE catalyst was carried out in reaction atmosphere.As indicated by in-situ XRD results,the bulk metal copper structure of the catalyst can be transformed into Cu₂O after 4 h treatment with water vapor.When switching into CO atmosphere,the bulk metal copper structure is restored.While the water gas shift reaction atmosphere is introduced,the bulk metal copper structure is unchanged.Subsequent results of in situ XAFS and in situ HRTEM experiment also confirm the dominant existence in the form of bulk metal Cu on the25%Cu/Si O₂-AE catalyst during the reaction.Therefore,the near atmospheric pressure XPS（APXPS）of the synchrotron radiation source is conducted,and the results show that Cu₂O with abundant oxygen vacancy exists on the surface of the catalyst.At the same time,through the surface reactions of CO-TPD,CO-TPR and mass spectrometry online detection,it is found that CO₂ can be generated on the reduced catalyst at room temperature,indicating that there are active oxygen species on the surface of the reduced catalyst.Moreover,the on-line detection of the surface reaction by mass spectrometry（TPSR-MS）also find that more water can be dissociated on the surface of CO-pretreated catalyst to generate larger amounts of hydrogen,indicating that the vacancy is of great importance for H₂O dissociation.These findings indicate that the structure of active site on the Cu/Si O₂ catalyst in the water gas shift reaction atmosphere is that Cu₂O with aboundant oxygen vacancies bonds on the surface of bulk metal Cu and the reaction pathway in WGS over this site follows the redox mechanism.On this basis,density functional theory calculations are carried out,which proves that the formation energy of oxygen vacancies on the surface of the Cu-Cu₂O structure is lower.So,it is easier to form abundant oxygen vacancies.In addition,by constructing a Cu₂O structure model with aboundant oxygen vacancies attached to the surface of metal Cu for theoretical calculation,it is found that the redox pathway proceeds at the Cu₂O-Cu₄-cluster structure model with lowest energy,which formed by partial oxygen deficiency on the surface of Cu₂O.Therefore,an efficient“reaction pathway on vacancy-rich Cu_xO site”is proposed.In addition,Cu/Al₂O₃ catalyst was prepared by sol-gel method.As indicated by EXAFS results,there is nearly total Cu-O coordination of copper oxide without Cu-Cu coordination in the sample,suggesting that the Cu species on the catalyst are in the form of single atoms(Cu₁²⁺).And the RSF curves of reduced samples exhibit Cu-O coordination for Cu₂O and Cu-Cu coordination for metallic Cu,respectively.No Cu–Cu shell for Cu₂O is detected in all samples.Thus,the EXAFS study confirms the coexistence of metallic Cu nanoparticles(Cu _NP⁰)and Cu⁺single atoms（Cu₁⁺）.Further,the reduced sample is tested by AC-HAADF-STEM,and the result shows that the coexistence of Cu single atoms and Cu nanoparticles,and especially,the latter is tightly surrounded by the formers.In addition,the catalyst exhibited excellent low-temperature performance during the water gas shift reaction,and the reaction rate value of the 12%Cu₁⁺+Cu _NP⁰/Al₂O₃catalyst reaches 8.1×10^-5 mol _CO g _Cu^-1 s^-1 at 160°C,which is about5 times higher than that of the conventional Cu/Zn O/Al₂O₃ catalyst.Meanwhile,the 12%Cu₁⁺+Cu _NP⁰/Al₂O₃catalyst maintains a relatively stable catalytic performance during the test up to 100 h and 8 start-stop cycle experiments.Due to the formation of more Cu₁⁺,its adsorption capacity is～2 orders of magnitude higher than that of the traditional Cu/Zn O/Al₂O₃ catalyst.Therefore,it greatly increases the CO concentration on catalyst surface,which will shift the eqilibrium to the right.Thus,the catalytic activity of the WGS is improved.