Studies On Molecular Adsorption And Reaction On Ni(110)Oxidized Surfaces Using UHV-FTIRS

Metal Nickel and its oxides are long-standing classic catalyst systems in industry.They play important roles in Fischer-Tropsch reaction(FTS),hydrogen evolution and air purification,and thus have always been hot topics in scientific researches.Learning both the reactants' and the products' life processes at molecular level will definitly help us understand the catalytic reaction mechanism and get higher catalytic efficiency.The sites where heterogeneous catalytic reaction happen are on the catalyst surfaces,and thus the studies on the interactions between the catalyst surface and the adsorbed molecules,and between the adsorbed molecules on sufaces are vital for understanding the heterogeneous catalysis mechanism.Ni single crystals,(2×1)Ni-O/Ni chains and NiO/Ni films are three kinds of typical Ni-based catalysts with ordered structures,and could be regarded as model catylysts for studying reaction processes on Ni-based catalysts.By using the surface scientific techniques,abundant knowledge of molecular adsorption and reaction processes on Ni single crystals and NiO/Ni films have been got in recent years,though there still exist lots of controversies at molecular level.However,studies of molecular reactions on(2×1)Ni-O chain structures are still very scarce.To clarify the controversies and fill the reseach gaps,studies on these reactions from new sights with techniques that are sensitive to chemical environments of adsorbates are necessary.Infrared Reflection Absorption Spectroscopy(IRRAS)is very sensitive to the chemical adsorption environments of the molecules,and thus is ideal for studying the adsorption sites,adsorption configurations,reaction pathways and the intermolecular and the molecule-surface interactions.Thanks to the high reflectivity of metal surfaces,even very weak singnal on metal surfaces could be detected by IRRAS,and thus the IRRAS has been widely used in studying metal-based surface chemistry and catalytic mechanisms.In the work of this thesis,by optimizing path of the infrared light,we designed and built a state-of-the-art Ultrahigh Vacuum-Vacuum Fourier Transform Infrared Spectrometer(UHV-FTIRS)combined system.The high sensitivity and long-term stability of this system permit us to in situ measure molecular reaction processes on metal and oxide surfaces in ultrahigh vacuum.With this system,we have systematically studied the adsorption sites,adsorption configurations,the intermolecular and the molecule-surface interactions of small molecules like CO,NO and O2 on Ni(110)single crystal surfaces,(2×1)Ni-O/Ni(110)surfaces and NiO/Ni(110)surfaces.The specific results are following:1.CO adsorption and configuration evolutions with temperatures on(2xl)Ni-O chain structures on Ni(110)surfaces were studied by using UHV-FTIRS,low energy electron diffraction(LEED),thermal desorption spectroscopy(TDS)and density functional theory(DFT)calculations.Two kinds of adsorption configurations were identified for the first time.In the top configuration at low temperature(90 K),the carbon atom of one CO binds to one Ni atom in the Ni-O chains with a tilt angle of about 53°.In the bridge configuration near room temperature(280 K),the carbon atom of one CO binds to two Ni atoms from the neighboring Ni-O chians.By annealing the sample prepared at 90 K to 280 K,the top adsorption configuration gradually evolves into the ordered bridge adsorption configuration via a disordered state.In the disordered state,the top and bridge configurations are distributed in disorder,induce strong transverse distortion of the Ni-O chians along[110]direction,and thus lead to the significant CO frequency shifts.The distortion disappears after the complete desorption of CO above 300 K.2.DFT calculations were used to study exchange interactions between(2×1)Ni-O chains assembled on Ni(110)surfaces.The antiferromagnetic spin order had been determined by the DFT calculations.Scanning tunneling microscope simulation clearly distinguished two types of Ni with opposite spin-orientations,which means the information carried by such system is possible to be read out.Furthermore,we have introduced the CO molecules,which zipper the neighboring Ni-O chains and greatly enhance the inter-chain spin coupling.3.CO and O2 co-adsorption behaviours were studied on non-defective NiO/Ni(110)surfaces.It is found that O2 adsorption on NiO/Ni(110)surfaces depend closely on the CO pre-coverage.When CO pre-coverage is zero or very low,no O2 could adsorb on NiO/Ni(110)surfaces.Only when CO pre-coverage exceeds a certain value,could O2 molecules adsorb on Ni2+ sites next to the CO host Ni2+.This is related to the charge transfer from CO to the host Ni2+.Because of the limitation of electron transfer,only two co-adsorption configurations,CO-O2-CO and CO-O2-CO-O2-CO,can be formed.That is,two neighboring CO provide the transferred charge for one O2 adsorption or three CO for two O2.The IR absorption bands of CO show different frequency shifts depending on the number of O2 molecules adsorbed by Ni2+.The pure CO configuration and the CO-O2 co-adsorption configuration could be controlled reversibly by O2 adsorption or desorption.4.Combined with UHV-FTIRS and DFT calculations,we systematically studied the NO adsorption and reaction processes on Ni(110)single crystal surfaces.At initial adsorption,the earlier NO dissociate and leave N and O atoms on the surface,and the later NO molecularly adsorb on the short bridge(SB)and top(T)sites of the surface.With NO coverage increasing,the SB and T absorption bands blue shift heavily because of the intermolecular dipole interactions.DFT calculations indicate that the dissociation products N and O atoms occupy the long bridge(LB)sites and the 3-fold hollow(3H)sites on the surface respectively.Irradiation of UV light,N and O atoms on Ni(110)surfaces will recombinate to NO molecules.5.We studied NO configuration evolutions with NO coverages on ordered(2×1)Ni-O/Ni(110)surfaces combined with UHV-FTIRS and DFT calculations,and found a new method to produce the surface end-O.At low coverage,NO tiltedly adsorb on Ni sites in the(2×1)Ni-O chains.With NO coverage increasing,NO adsorb on the intrachain Ni sites both along and perpendicular to the Ni-O chains,which introduce big structure distorsions of the Ni-O chains.When NO coverage increases to more than 1/3 monolayer(ML),the increased intrachain stress will break the bonds between the host Ni atoms of NO and the neighbouring two O atoms,which introduces more end-O on the surface.The highly ordered surface with rich end-O can be obtained by heating the NO covered(2×1)Ni-O/Ni(110)surface to about 250 K.