Study On Properties Of The Interface And Surface Of The Oxygenated Metal Compound Semiconductor Catalyst

Catalytic technology has played a great role in solving the increasingly serious environmental pollution and energy crisis,however there are still lots of problems under solved.How to efficiently improve the catalytic activity,increase its service life,and dramatically enhance the catalytic conversion efficiency,without causing other pollution and the waste of resources,is the primary problem deserving our in-depth consideration and solution.Among many novel and traditional catalysts,oxygenated metal compounds have been favorited by people.The oxygenated metal compounds are consisted in metal elements and oxygen,which possess abundant of electrical,optical and magnetic properties.They are abundant in reserves on Earth,easy to be exploited,low cost,excellent thermal stability,and have the good corrosion resistance.With the development of nano-technology,the oxygenated metal compounds can also be prepared in the low-dimensional form,which can further improve their catalytic performance and broadens their application.Of course,in practical application,metallic oxygen compounds still have some unsatisfactory aspects.Therefore,in order to let them play a better role,we must improve and design these compounds in the targeted wayThis thesis contains three parts:(1)using the first-principles calculation,we have studied the segregation properties of Pt on the Ce O₂?3(111)grain boundary(GB),and the interaction between Pt and oxygen vacancy at the GB.(2)Using the same method,we have also studied the electronic structures for the heterostructures composed of Bi OX(X=Cl and I)and Bi₂Mo O₆,and revealed their photocatalytic mechanism under visible light.(3)we have prepared highly ordered titanium dioxide(Ti O₂)nanotube arrays(TNTAs)using a secondary anodic oxidation method,and also examined the photocatalytic properties of the samples annealed in different conditions on the degradation of the methyl orange dye.The main researches are presented as follows:(1)In this study,we have systematically investigated the Pt segregation and Pt-oxygen vacancies interaction at the?3(111)GB in Ce O₂.The Pt atom has a stronger tendency to segregate to the?3(111)GB than to the(111)and(110)free surfaces,but the tendency is weaker than to(112)and(100).Lattice distortion plays a dominant role in Pt segregation.At the Pt-segregated-GB(Pt@GB),oxygen vacancies prefer to form spontaneously near Pt in the GB region.However,at the pristine GB,oxygen vacancies can only form under O-poor conditions.Thus,Pt segregation to the GB promotes the formation of oxygen vacancies,and their strong interactions enhance the interfacial cohesion.We propose that GBs fabricated closed to the surfaces of nanocrystalline ceria can trap Pt from inside the grains or other types of surface,resulting in the suppression of the accumulation of Pt on the surface under redox reactions,especially under O-poor conditions.(2)First-principles calculations are used to explore the interfacial sturctures and photocatalytic mechanisms of the Bi OI/Bi₂Mo O₆ and Bi OCl/Bi₂Mo O₆heterostructures.We find that both the Bi OI/Bi₂Mo O₆ and Bi OCl/Bi₂Mo O₆composites have small lattice mismatch and good stability.The DOS analysis show that the Bi OI/Bi₂Mo O₆ belongs to type-II heterostructure,while the Bi OCl/Bi₂Mo O₆composite is a type-I heterostructure.The analyses of the electron difference density and Mulliken charge suggest that charge transferring and redistribution occurs at the interfaces.Moreover,a built-in electric field directed from Bi₂Mo O₆ to Bi OI or Bi OCl can be formed at the interfaces of the heterostructures.The band alignment and the built-in electric field in the Bi OI/Bi₂Mo O₆ and Bi OCl/Bi₂Mo O₆heterostructures can promote the separation of the photo-generated carriers,resulting in their improved photocatalytic activities compared with the single component.Our work may provide useful information for understanding the photocatalytic mechanism of the Bi OI/Bi₂Mo O₆ and Bi OCl/Bi₂Mo O₆heterostructures and developing more efficient heterostructure photocatalysts.(3)In this part,we have studied the effect of the annealing environment on the photocatalytic activity of Ti O₂ nanotube arrays.A series of TNTAs samples with regular arrangement were prepared by the secondary anodic oxidation method.Subsequently,these samples were annealed in air and argon(Ar)for 2 hours and 4 hours,respectively.After that,the samples were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),UV-visible spectroscopy(UV-VIS),X-ray photoelectron spectroscopy(XPS)and fluorescence spectrophotometer.To understand the photocatalytic properties of the TNTAs,four annealed samples(25 mg)were mixed into the50 ml methyl orange solution with a concentration of 20%,and irradiated by a xenon lamp(sunlight)with the power of 500W.After sunlight irradiation for 3 hours(180 min),the sample annealed in air for 4 hours could decompose methyl orange solution completely.However,the sample annealed in Ar can adsorb a large amount of methyl orange molecules under dark reaction conditions,reducing the concentration of the mixed solution by 50%.However,under the irradiation of sunlight,the degradation effect of the sample on the dye is very poor,compared with that of the sample annealed in air.Finally,on the basis of the characterized results,we explained the above phenomena.