Adsorption And Reaction Of Molecules On CoO(001)and Co(0001)Surfaces Studied By UHV-FTIRS

Environmental pollution and energy shortages have been bothering and hindering the development of today's society.The treatment of pollution and the development and utilization of renewable clean energy sources are two important ways to solve the problem at present.The use of catalysts for the decomposition of water to hydrogen,solar energy conversion,degradation of pollutants and reduction of carbon dioxide will play a vital role in the future.And studying the basic process of catalytic reaction at the atomic and molecular scale is of great significance for designing a reasonable catalyst and improving the high efficiency and selectivity of the catalyst.In heterogeneous catalysis,the surface of the catalyst is the place to react,and adsorption is the first step of the reaction.Therefore,studying the adsorption behavior of molecules on the catalyst surface and the interaction between molecules is very important for the in-depth understanding of the heterogeneous catalysis process.Cobalt monoxide(CoO)and Cobalt metal(Co)are two important catalytic materials in the development and utilization of new energy and pollution control.The former is a rock salt structure,while the latter is a hexagonal close-packed structure.These two materials with different structures could be regarded as model catalysts for the study of catalytic mechanisms.With the development of various surface analysis methods,plenty of progress has been made in the research of the surface of these two materials.However,there are still some fundamental problems that need to be resolved at the atomic and molecular scale.Therefore,it is necessary to study the molecular behavior on the surface from different aspects by using different experimental methods.Infrared Reflection Absorption Spectroscopy(IRRAS)can discriminate different species of absorbed molecules and is very sensitive to the surrounding environment on the surface.Thus,it is an effective technique for studying the adsorption sites,adsorption configurations,reaction paths and interactions of molecules.However,IRRAS researches on the single crystal metal oxides(insulators or semiconductors)surface are insufficient due to the weak infrared reflection and absorption signal of molecular vibration on these material surface.In this thesis,the novel state-of-the-art Ultrahigh vacuum Fourier Transform Infrared Spectrometer(UHV-FTIRS)is mainly used in our study.This system is optimized for the optical path with high sensitivity and long-term stability to realize the in suit measurement of molecular behaviors on the surface of the metal and single crystal metal oxides.On the basis of this system,we have systematically studied the adsorption sites,adsorption configurations,intermolecular interactions and molecules-surfaces interactions of typical molecules such as CO,CO2,O2 0n CoO(001)and Co(0001)surfaces.The main results are as follows:1.CO molecules were used to investigate the properties of the CoO(001)surface.It is found that CO adsorbed on the CoO(001)surface with C atom bonding to fivefold coordinated Co2+ions at low temperature with approximately vertical behavior.The vibration frequency of CO is redshift relative to the gas phase value at low coverage because the 2?*antibonding orbital of CO accepts 3d electrons from Co2+,which weakens the bond strength of C-O.As the coverage increases,the vibration frequency of CO continues to redshift.According to the DFT simulation,it is found that the continuous redshift is caused by the gradual increase of the bond length of adsorbed CO as the coverage increases.2.The controllable manipulation of surface oxygen vacancies(Ov)was studied by LEED,AFM and IRRAS.Two different superstructures((?))R18.4° and((?))R45° were observed after changing the conditions of Ar+sputtering and annealing.Both superstructures are related to the distribution of Vo on the surface confirmed by using CO as the probe molecule.Surface O atoms are removed randomly by Ar+sputtering,while the distribution of Ov will become orderly after annealing.Furthermore,numerous steps distribute along<110>and<110>directions and resulting rectangular pits were observed by atomic force microscopy(AFM)on the surface,and there are a large number of threefold coordinated Co2+ at the step edges.The presence of these structures was also supported by the IRRAS.3.We studied the adsorption of NO on the CoO(001)surface.It is found that NO adsorbed on the surface obliquely with N atoms bonding to Co5c2+ ions.However,the vibration frequency of NO has a larger redshift relative to the gas phase value(90 cm-1),resulting from the inclined adsorption configuration and its 2?*antibonding orbitals are easier to accept electrons from the 3d orbitals of Co2+.As the coverage of NO increases,the repulsive force between adsorbed molecules weakens the electron transfer effect and makes the obliquely adsorbed NO gradually turn into vertical adsorption,resulting in the blue shift of NO vibration frequency.When NO was adsorbed on the CO pre-adsorbed CoO(001)surface,the frequency was red-shifted by 30 cm-1 again,attributing to more electrons transfer from the 5? orbit of CO to the substrate and then to 2?*antibonding orbitals of NO.4.We also studied the adsorption and reaction of CO and CO2 on the single crystal Co(0001)surface.We found that CO can be adsorbed on the top,bridge and hollow sites on the flat surface under UHV conditions.There are two adsorption configurations at the step defects,that is,CO is adsorbed on the top position of the step edge at low coverage,while adsorbed at the bridge position of the step edge at high coverage.At low temperature,the CO2 will directly dissociate into CO and O at the hollow site on the terrace and step site on the clean surface;when the temperature is raised,the physisorbed CO2 on top sites of the flat terrace will diffuse onto the active sites and then dissociate.Meanwhile,the CO produced at step sites will transfer to the terrace.On the effect of CO2 dissociation at room temperature,the pre-adsorbed H atoms play both positive and negative roles by means of two different mechanisms:when the H atoms are adsorbed on the clean Co(0001)surface,they can accelerate CO2 dissociation via transferring electrons to the 2?u antibonding orbital of the CO2;on the other hand,when the H atoms form the hydroxyl groups(-OH)with the O from dissociated CO2,they occupied the active hollow sites and consequently reduces the CO2 dissociation rate.