The Surface Structure Of Single Crystal ZnO（000（?））polar Surface And The Reduction Reaction Mechanisms Of CO₂ On Ni（110）Surface Studied By UHV-FTIRS

Catalysis is the modification of the rate of a chemical reaction induced by the catalyst by changing the activation energy of chemical reactants.Catalysis is closely related to modern human life.For example,ammonia fertilizer that significantly increases grain production is synthesized induced by iron catalysts;Catalytic reaction is widely used in petrochemical industry,such as catalytic cracking reaction,dehydrogenation reaction,hydrogenation reaction,redox reaction,etc;Platinum catalysts are commonly used in the treatment of automotive exhaust emissions.Therefore,designing and preparing efficient catalysts is crucial for energy conservation,pollution control,and improving people’s lives.Heterogeneous catalysts have the advantages of low cost,good thermal stability,high efficiency,good selectivity and easy mass production,and 90%of the chemical products in modern chemical industry are catalyzed by solid heterogeneous catalysts.If we can master the catalytic mechanism of heterogeneous catalysis,it will be of great significance to design new and efficient catalysts.Heterogeneous catalysis occurs on the surface of catalysts,but the surface structure and reaction environment of industrial catalysts are extremely complex,which makes it difficult to study the catalytic mechanism.Model catalysis is the in-situ study of catalytic reaction processes at the atomic and molecular scales on the surface of a single crystal catalyst in an ultrahigh vacuum,thereby revealing the catalytic reaction mechanism.We used a unique combination system of ultrahigh vacuum Fourier transform infrared spectroscopy and molecular-beam epitaxy（UHV-FTIRS-MBE）owned by the research group to carry out model catalysis research.This system combines ultra-high vacuum and vacuum infrared spectroscopy,avoiding the interference of various pollutants in the air,greatly improvs the signal-to-noise ratio,and successfully overcome the problem of poor signal-to-noise ratio of infrared reflection absorption spectroscopy（IRRAS）on the surface of semiconductors or insulators.In this paper,we choose zinc oxide ZnO（0001）polar surface and nickel metal Ni（110）surface as model catalysts.ZnO is widely used in the field of heterogeneous catalysis,such as industrial methanol production,water vapor synthesis reaction,etc.The polar surface of ZnO（0001）plays an important role in these reactions,which can improve the selectivity of inidustrial methanol production reactions.Theoretically,the accumulation of electric dipole moment along the c-axis direction of the crystal will lead to the surface energy divergence of ZnO（0001）polar surface.There is significant controversy in the academic community regarding the atomic structure and polarity compensation mechanism of ZnO（0001）polar surfaces.This hinders the study of the polar surface catalytic mechanism of ZnO（0001）.Ni is a low-cost catalyst and a potential substitute for precious metal catalyst platinum（Pt）.Compared with Pt catalysts,Ni based catalysts face an unresolved problem of carbon deposition in their application,Ni can be used to catalyze dry reforming of methane（DRM）,carbon dioxide to methane,and other reactions.In the Ni catalyzed DRM reaction,the slow decomposition rate of CO₂ may be one of the main factors causing carbon deposition.If we can understand the activation and reaction mechanisms of CO₂ on the surface of Ni catalysts,and try to improve their decomposition rate,it will have an important guiding role in designing new anti-carbon deposition Ni based catalysts.On the other hand,CO₂ is the main greenhouse gas,catalyzing the reduction of CO₂ into useful chemical raw materials has not only developed new clean energy sources but also alleviated the greenhouse effect.In this paper,we used UHV-IRRAS as the main technical means,combined with low-energy electron diffraction（LEED）,molecular-beam epitaxy（MBE）,Auger Electron Spectroscopy（AES）and density functional theory（DFT）,to systematically study the structure of ZnO（0001）polar surface and its polarity compensation mechanism,the adsorption and reaction of water molecules on it;The effects of activation,decomposition,hydrogenation of CO₂ on Ni（110）surface and modification of Cu and Au atoms on the chemical decomposition of CO₂ on Ni（110）surface.The main research contents and results are as follows:1.We prepared five types of Zn0（0001）periodic surfaces,including（（?））,（1×1）,（1×2）,（（?））and（3×3）.Their surface atomic configurations were determined for the first time using CO₂ molecular probes and UHV-IRRAS combined with DFT simulation calculations,and their polarity compensation mechanism were elucidated.2.We used CO and CO₂ molecular probes and DFT calculations to study the decomposition mechanism of water molecules at the atomic steps and oxygen vacancies on the surface of ZnO（0001）.The decomposition energy barrier of water molecules at the step is 0.4 eV;At the oxygen vacancy,the decomposition barrier is 0.15 eV.The H atom undergoes diffusion motion on the surface of ZnO（0001）with the assistance of water molecules:the diffusion barrier is 0.3 eV with the assistance of water molecules,while is 1.67 eV without water.3.We used UHV-IRRAS and DFT calculations to study the activation,decomposition,and hydrogenation processes of CO₂ on Ni（110）surface.Two chemical adsorption configurations of CO₂ were found on the surface,including SB-CO₂ and HU-CO₂.Experiments showed that the ground state HU-CO₂ is a direct precursor involved in surface decomposition and hydrogenation reactions.During the reaction between HU-CO₂ and H,we discovered carboxyl（HOCO*）intermediates experimentally for the first time and demonstrated that the HOCO*pathway can reduce the energy barrier for CO₂ decomposition.4.By using MBE and UHV-IRRAS,we found that a small amount of Cu or Au atom modification can accelerate the decomposition of CO₂ on the Ni（110）surface,and increases the final amount of CO₂ decomposition.This may be because the modified atom trans fers some electrons to the host Ni,so that more electron transfer on the Ni（110）surface transfer to the 2πμ antibonding molecular orbital of CO₂,which makes the C-O bond of CO₂ easier to be disconnected.