Studies On The Structure Control Of Cu₂O Nanocrystal Catalysts And The Corresponding Propylene Epoxidation Performance

Catalysis,as an essential part of modern industry,paves a highway for the development of our society in energy,materials and environment protection,which significantly reduces energy consumption,waste emission and increases atom economy.However,due to the high complexity of structure and composition,it is of great difficulty to study the structure-property relationship of industrialized catalysts for the rational development of excellent catalysts.Conventionally,the surface scientists utilized the single crystals to investigate the dependence of property on the structure under high vacuum,which,however,is too far away from the practical conditions.As an alternative,the catalytic system based on well-defined nanocrystals at ambient or high-pressure conditions could better mimic the practical industrial applications and thus provides a more realistic platform for the study of relationship between the structure and property.On the other hand,as an important organic compound,the production of propylene oxide(PO)was about ten million tons every year in the world,but its industrial production methods are still suffering from severe drawbacks such as high cost,too many by-products and serious pollution,etc.By contrast,direct epoxidation of propylene with O2 attracts more and more attention owing to its advantages in environment production and atomic economy.However,the competing process of ?-H abstraction of propylene leads to many reaction paths and it is being a large challenge to obtain 60%selectivity of PO or higher,even in the case of excellent Cu₂O-based catalysts.Therefore,it is of significant value to deeply understand the structure-property relationship of catalysts and precisely control their structure for high catalytic selectivity of PO.In this dissertation,Cu₂O nanocrystals were set as model catalysts for direct epoxidation of propylene with O2,whose structure-property relationship were systematically studied by precisely controlling their structure.Firstly,the synergistic effect of the loading Cl and crystal facets of Cu₂O nanocrystals on the selectivity of PO was studied.Secondly,by synthesizing Cl doped Cu₂O nanocrystals with certain facets,the role of doping structures in stabilizing Cl and affecting the performance in propylene epoxidation were studied.In addition,the synthetic methodology of hollow nanocrystals for Cu₂O nanocrystal and noble metals were explored.The corresponding contents and progresses are listed as following:1.The synergistic effect of the loading Cl and Cu₂O nanocrystals enclosed by different facets in direct epoxidation of propylene with O2 was studied.Cu₂O cubic and rhombic dodecahedral nanocrystals were fabricated,then desired amount of NH4Cl were loaded on them.The results showed that the loading amount of Cl had a volcanic type curve relationship with the selectivity of PO.Cu₂O cubic nanocrystals with 100%NH4Clloading(mono atomic layer loading)displayed highest selectivity(57%)toward PO and Cu₂O rhomic dodecahedral nanocrystals with 100%NH4Cl loading showed 65%selectivity of PO,which demonstrated the synergistic effect of Cl loading and specific facet of Cu₂O could further increase the catalytic selectivity.However,serious Cl loss occurred during the catalytic process,leading to the compromise of PO selectivity.2.Cl doped Cu₂O rhombic dodecahedral(Cl-RD-Cu₂O)nanocrystals were successfully synthesized and its structure-property relationship in direct epoxidation of propylene with O2 was studied.Initially,Cl-RD-Cu₂O and Cl doped Cu₂O octahedral(Cl-OCT-Cu₂O)nanocrystals were synthesized via wet-chemical method.In comparison,Cu₂O rhombic dodecahedral nanocrystals(RD-Cu₂O)and Cu₂O octahedral nanocrystals(OCT-Cu₂O)with NH4Cl loading were also prepared to deeply study the superior effect of the doped Cl and exposed facets of Cu₂O in propylene epoxidation.Through the XPS,EDS and ion etching technology analysis of Cl-RD-Cu₂O,Cl was found to be uniformly distributed in the lattice of Cu₂O and its weight percentage is 2 wt.%.Interestingly,the catalytic performance of different catalysts in propylene epoxidation was in the order of Cl-RD-Cu2o>Cl-OTC-Cu₂O>Cl loaded Cu₂O nanocrystals.Among them,Cl-RD-Cu₂O displayed the best performance,showing 100%selectivity of PO at 125? and 12 h-1 turnover frequency(TOF)at 200?.Moreover,its Cl loss during the catalytic process was very small and could be negligible.This result illustrates that Cu₂O nanocrystals doped with Cl is stable and favorable for propylene epoxidation.Additionally,Cl doping could work with specific facets of Cu₂O to further increase catalytic selectivity via synergistic effect.Density function theory(DFT)calculations indicated that Cu₂O(1 10)doped with Cl was favorable for the activation of O2 and the formation of middle transition state during the propylene epoxidation.3.Hollow Cu₂O nanocrystals were fabricated to reveal the influence of hollow structure in direct epoxidation of propylene with O2.The hollow Cu₂O nanocubes were fabricated from the transformation of sacrificial templates via one-pot surfactant free wet-chemical method.In addition,the growth mechanism of hollow Cu₂O nanocubes and their catalytic performance for propylene epoxidation were further investigated.During the study of the growth mechanism,the formation of hollow Cu₂O nanocubes involved a series of transformations,from Cu2(OH)3C1 to CuCl and final Cu₂O.The presense of Cl and pH value played essential roles in the formation of hollow structure.When used as catalysts,the hollow Cu₂O nanocubes showed enhanced catalytic performance than that of solid Cu₂O nanocubes in propylene epoxidation.4.Taking Pt as an example,ultrathin Pt-based icosahedral nanocages with a single hole opening was designed and fabricated via wet chemical method.The synthesis started from the selective deposition of Ag domain on a Pd icosahedra to from Pd-Ag dimer,which will then evolve into Pt-Ag2O dimer through the oxidation of Ag.Afterwards,Pt shells were selectively deposited on the Pd seeds to form Pd@Pt-Ag2O dimer.Finally,the Ag2O domain and Pd core were removed by wet chemical etching for the production of Pt-based icosahedral nanocages with a single hole opening.Owing to the complete exposure of the inside part and thus convenient mass transfer as well as the stable structure,nanocage with a single hole opening could further increase the catalytic activity and durability relative to the conventional nanocages.