STM Study Of The Supported Metals On The ZnO Surface Modulated By Small Gaseous Moleculars

ZnO-based catalysts have been used in a number of important reactions such as CO2/CO hydrogenation,water-gas shift(WGS),reversed WGS,CO oxidation,and methanol reforming etc.These catalytic reactions are closely related to global issues covering the economics,energy as well as the environment.In order to develop highly efficient new catalysts,it is of critical importance to understand the relationship between the structure and performance of the catalysts.Assited by the rapid developments in the spectroscopies,microscopies as well as the high-precision theoretical calculations,new insights have been achieved in revealing the structure and composition of catalysts,determining the surface species and reaction processes.Nonetheless,the multicomponent and complex reaction environment of practical catalysts still constitute the huge barrier for the researchers in their understandings of the real catalytic reaction mechanism,and thus lead to many disputations.Model catalysis,starting with simple components and well defined surface/interface,allows a deeper understanding of what is happening at the surface.In this thesis,we have applied low-temperature scanning tunneling microscopy(STM)and density functional theory(DFT)calculations to reveal the structure of water on the ZnO(10-10)surface.Further on,the regulation effect of H2O,CO2 and CO on the growth mode of Cu and Pd atoms on ZnO(10-10)surface was studied with STM in combination with photoemission spectroscopy(PES).The main achieved results are as follows:1.The atomic structure and growth dynamics of H2O on the ZnO(10-10)surface were systematically investigated using low-temperature STM.We clearly identified the undissociated and dissociated states of isolated H2O species at liquid nitrogen temperature.Upon annealing to room temperature(RT),water assembles into onedimensional chains orientating along the[0001]direction.High-resolution STM images successfully resolved the dissociated monomer and half-dissociated dimers inside the water chains,as well as their bonding positions at the bridge and top sites of surface Zn ions,respectively.Moreover,variable temperature STM experiments clearly observed the dissociated intermediate states during the water assembling,which thus suggests that water diffuses through a dissociation-assisted process as supported by the DFT calculations.Finally,the distribution of different H2O-related species on the ZnO(1010)was analyzed.Their population against the total surface coverage of water was discussed.The water dimers were found with formation priority and dominant on the surface within wide range of coverage.2.Based on the understandings of the detailed interaction of water with the ZnO surface,we further investigated its regulation effect over the growth modes of Cu and Pd as well as their mutual charge transfer between the ZnO substrate.When deposted at RT,the STM experiments clearly observed both metals tend to form threedimensional(3D)clusters on the clean ZnO surface and aggregate at the step edges along the[0001]direction.Pre-adsorbed water effectively disperses the deposited Cu and Pd into single atoms or sub-nanometer clusters,which can be stabilized up to 373 K.High resolution STM images unambiguously confirmed that the ultrasmall metal species bind directly to the surface water.Moreover,detailed experiments revealed that Cu may preferentially form dimers and their interaction with water induces partial desorption of the later.In contrast,Pd interacts directly with the surface water and aggregates into small two-dimensional(2D)islands at low coverage although 3D particles still develop naturally at increased coverage and are arranged into linear pattern under the guidance of the water chains.Besides the STM experiments,PES characterizations were also performed to reveal the chemical interactions between the evaporated metals and the ZnO surface.We found Cu is oxidized upon interacting with the clean ZnO(10-10)and gets further oxidized by the pre-adsorbed H2O species.In contrast,Pd remains almost neutral on both clean and water pre-covered ZnO surfaces.In addition,we found the incorporation with Cu induces the recombination of dissociated water back into the molecular state,while the interaction with Pd plays an opposite role and induces the transformation of molecular water into dissociated water.3.Considering the similar existing universality of CO2 in the ZnO catalyzed reaction systems,we also explored its effect over the evaporated Cu and Pd on the ZnO(10-10)surface.Interestingly we found the pre-adsorbed CO2 molecules can also assist the dispersion of metal atoms into single atoms or sub-nanometer clusters,and these ultrasmall metals were found preferentially anchoring by the side of the CO2 chains.Morever,annealing experiments found that at 473 K the CO2-stabilized Pd atoms tend to aggregate into larger clusters with a general height of～0.6 nm and can be stabilized until above 673 K without further sintering.In contrast,the CO2-stabilized Cu atoms readily aggregate into higher 3D clusters along with the increase of the temperature.The PES measurements showed that the pre-adsorbed CO2 has no direct impacts on the chemical statuses of the added metals,indicating that its modulation over the metals may be mostly a spatial confinement effect.4.We further investigated the growth mode of Pd on the ZnO(10-10)surface in the CO atmosphere.Interestingly,we found once the CO pressure gets higher than 5×10-8 mbar,the evaporated Pd starts to grow into 2D islands which mostly possess a bilayer structure.In addition,these 2D Pd islands can be stabilized to above 373 K.At higher temperatures,3D clusters can develop owing to the desorption of CO.As a comparison study,we examined the evaporation of Pd in an O2 atmosphere which also turned out a 2D growth mode since the PdOx species can be formed.In contrast,Pd deposition in CO atmosphere onto a rutile TiO2(110)surface still takes a 3D growth mode,possibly due to the relatively weaker interactions between Pd and TiO2.In this thesis the effects of H2O,CO2 and CO on the growth of Cu and Pd on ZnO(l010)surface were studied.