Study Of Adsorption And Catalytic Reactions Of Methanol On Anatase TiO₂?001?Surface

Nowadyas,there are several serious globle challenges,like the decrese of traditional energy storage,the increase of greenhouse gases,and environmental pollution.In 1972,Japanese scientists,Fujishima and Honda,found that under the irradiation of ultraviolet light,TiO2 can split water into hydrogen and oxygen.Since then,TiO2 and TiO2-based materials have attract widely attention for its promising applications in photocatalytic water splitting to produce hydrogen,photocatalytic degradation of organic pollutants,conversion of carbon dioxide to valueadded organic materials.In its three polymorphs of TiO2,the anatase phase has long been considered to have highly catalytic activity.In particular,mainly from theoretical calculations,it was predicted that the anatase TiO2(001)surface is the most active surface among the various facets of the different TiO2 polymorphs.However,due to the difficulty in growth of high quality anatase TiO2 single crystals,the experimental study on the(photo)catalytic properties of anatase TiO2 is quite insufficient,and the the activity of the anatase TiO2(001)has not been well examined.In the work of our group,the structure of antase TiO2(001)-(1×4)and the defect sites have been studied using scanning tunneling microscopy.The active sites due to the reduction of the surface have been revealed.STM has the advantage of high resolution in characterizing the structure,defects and adsorption behavior of surfaces,but lacks the ability in chemical resolution,which makes it difficulty in resolve the actual products.The goal of my doctoral research program to understand the activity of the TiO2(001)-(1×4)surface.For this purpose,I established a system by combining the temperature-programmed desorption(TPD)technique with STM.Using this system,I have systematically studied the adsorption and reactions of methanol on the TiO2(001)-(1×4)surface by TPD and STM.The TiO2(001)-(1×4)thin films were by epitaxially grown by pulsed laser deposition(PLD)method.In this thesis,I will introduce my investigation on the adsorption behavior and catalytic reactions of methanol on anatase TiO2(001)-(1×4)surface.In chapter 1,I briefly introduce the TiO2 materials,including three polymorphs of TiO2:rutile,anatase and brookite,their crystal structure,properties,and related applications.The research progresses of anatase TiO2,especially the preparation and research of anatase TiO2(001)film and characterizations are introduced.In chapter 2,I introduce the experimental methods and the instruments that I used in my research,including the pulsed laser deposition(PLD)system for the preparation of anatase TiO2(001)thin films,and the system of the scanning tunneling microscope,and temperature-programmed desorption(TPD)technique.In this chapter.I also introduce the details about our home-made TPD-STM apparatus.In chapter 3,I mainly introduce the preparation of anatase TiO2(001)thin films,and the structural models of the surface,two types of defects on the surface,and the adsorption and decomposition of methanol on the surface.The freshly prepared anatase TiO2(001)thin film surface contains only dark spot defects,and methanol decomposition does not occur on as-grown surface.On the reduced surface after treatments by Ar+ ion sputtering,the reduced defect sites appeared as bright spots are found to be the active sites for methanol dissociation through breaking O-H and C-O bond successively,to produce CH3 radical.After annealing in oxygen,the reduced defects can be partially oxidized to another type of defects,which produce formaldehyde.In chapter 4,I focus on the investigation of the photocatalytic reactions of methanol on anatase Tio2(001)surface.Our results show that the photocatalytic reaction of methoxy can be decomposed into formaldehyde in the partially oxidized Ti pair defects in the reoxidized surfaces under irradiation by weak-intensity UV light.While,under irradiation by high-intensity UV light,we also find that the molecular methanol adsorbed at non-defect lattice sites can also be dissociated to formaldehyde,evidenced by the observations of the reactions on all of the as-grown,the reduced,and the reoxidezed surfaces.By measuring the products of the second type of photocatalytic reactions,we find that the first step of the dehydrogenation reaction is much faster than the second step.