| The increasing demand of biodiesel,which has generated the excessive surplus of glycerol as byproduct,intensified the great interest in exploring the efficient utilization of glycerol.Due to the highly functionalized character,glycerol could be utilized as a platform chemical for the production of high-added-value products via catalytic oxidation.Normally,glycerol selective oxidation is confronted with the challenges referring to the transformation pathway(primary versus secondary hydroxyl group)as well as the oxidation depth(aldehyde/ketones or acids).Therefore,in order to realize the high yield of target product,rational design and construction of advantageous active sites that could match the continuous reaction path of glycerol oxidation is of great significance.Supported noble-metal catalysts are well-known as the most promising materials for glycerol oxidation.Among various researches,the influence of active metal on the catalytic activity and product distribution,especially on the selectivity towards primary or secondary hydroxyl groups has been more widely reported.It has been revealed that Pt-based catalysts are considered to possess unique advantage for the preferential oxidation of primary hydroxyl groups.However,there is still a large promotion space in terms of activation capacity of C-OH bond over supported Pt catalysts.Generally,introducing a second metal component to form alloy is considered to be a vital method to modify the performance of single metal catalysts.Zinc,as a highly abundant,non-toxic and cheap metal element,has been drawing more attention on promoting the catalytic performance of Pt catalysts in many reactions.In spired by the promotional effect of PtZn alloy on activating C-H bond in the reaction of dehydrogenation,alloying Pt with Zn could also be a promising way to enhance the conversion efficiency of glycerol as the cleavage of the C-H bond is the rate-determining step(RDS)in the activation of C-OH.However,the common reducing agents cannot reduce Zn2+ Cefficiently and the preparation of PtZn alloy is challenging due to the relatively negative redox potential of Zn(Zn2+/Zn,-0.76 V versus standard hydrogen electrode).In addition,the Metal@MOx interface structure can change the adsorption mode of the C=O bond due to the geometric effect,thereby affecting the catalytic reaction rate.Therefore,it is feasible to improve the oxidation selectivity of glycerol by constructing a specific interface structure.Therefore,this thesis aims to enhance the performance of the selective oxidation reaction of glycerol.The structure-adjustable anionic layered compound LDHs was used as the catalyst precursor,and the lattice confinement and topological transformation characteristics are used to construct a highly dispersed homogeneous alloy.In the process of building a highly dispersed uniform alloy,a low-coordination oxide interface coating layer was induced.At the same time,a variety of characterization methods were used to explore the essential relationship between the alloy and the low-coordination interface structure in the selective oxidation of glycerol and the catalytic performance,and the catalytic mechanism was clarified.(1)In view of the difficulty of preparing PtZn alloy due to the large redox potential difference between Pt and Zn,Zn2+was introduced into the laminate according to the principle of LDHs laminate construction to obtain the ZnTi-LDHs precursor.Pt was loaded on the surface by impregnation method,and the lattice positioning effect of LDHs in the topological transformation process under reducing atmosphere was used to construct a highly dispersed uniform PtZn alloy structure.Further combining with the geometric structure characteristics of LDHs precursors,it is easy to form oxide network trap confined metal nanoparticles during the reduction process.By precisely controlling the topological transformation conditions,a low-coordination oxide interface coating structure was induced on the surface of the nanoparticles.At the same time,the evolution mechanism of the constructed alloy@low coordination interface structure was revealed.The TEM and XANES characterization results showed that the nanoparticles in the catalysts at different reduction temperatures are highly dispersed.With the increase of the reduction temperature,the continuous Pt sites were separated by Zn atoms.When the temperature rised to 700℃,alloying occured between Pt and Zn to form an alloy structure;when the temperature rised to 800 oC,a lowcoordination oxide interface coating structure was induced on the surface of the PtZn alloy nanoparticles.After gaining an understanding of the geometric structure changes during the formation of the catalyst,the electronic structure of the catalyst was analyzed using XPS and XAFS characterization.The results show that as the reduction temperature increased,the oxygen vacancy content in the reducible oxide support increased,the electronic interaction between the metal and the carrier enhanced with the electrons transfermation from Zn to Pt,and the electron cloud density of Pt increased.When the temperature reached 800℃,the coordination unsaturation degree of Zn was the highest,and the lowcoordination interface coating induced by the high-temperature reducing atmosphere further promoted the transfer of electrons to Pt,and the content of electron-rich Pt0 species is the largest.In summary,the construction of this alloy and low-coordination interface structure can isolate continuous Pt sites and change the geometric configuration of the catalyst.At the same time,it could facilitate the transformation of the electron between the metal and the support,changing the electronic structure of the active component.(2)Based on the LDHs precursor method,a supported catalyst with high dispersion alloy and low coordination coating interface structure is used to develop the role of alloy and interface structure in the glycerol oxidation reaction mechanism research.In this study,PtCl62-/ZnTi-LDHs was used as the precursor to obtain PtZn@ZnTiOx/ZnTiO3 catalyst with PtZn alloy structure and low coordination oxide interface layer as the main sample.And PtCl62-/ZnTiO3 was used as the precursor to obtain the PtZn/ZnTiO3 catalyst with alloy structure prepared by the atmosphere induction method and the single metal Pt/ZnTiO3 catalyst were used as comparative samples.Through the study of the catalytic performance of glycerol oxidation and its structure-activity relationship,it is found that the formation of the PtZn alloy structure enables the transfer of electrons from Zn to Pt,increasing the electron cloud density of Pt,to promote the activation of O2,and forms O-and O2-active oxygen species.At the same time,due to the topological transformation of the LDHs precursor to form the oxide-coated interface structure,the strong interaction of metal and support could further enhance the electron-rich degree of Pt.And a large amount of oxygen vacancies are generated at the interface of the lowcoordination oxide and the surface of supports,which enhance the ability of the catalyst to activate O2 to a certain extent,could significantly improve the activity of the catalyst.In addition,for the active components in the catalyst,the changes in the electronic environment and the steric hindrance of the low-coordination interface structure have a synergistic effect,effectively changing the adsorption form of glyceraldehyde molecules on the catalyst surface.As for the glyceraldehyde molecules,C=O bond is adsorbed on the surface of PtZn@ZnTiOx/ZnTiO3 catalyst supports or the low coordination interface formed with active components in the form of bidentate which is prone to desorption and dissociation.Its oxidation and desorption rate is extremely fast,which is a catalyst for glyceric acid,which is a key factor for the catalyst to increase the selectivity of glycerol to glyceric acid. |
