Several Catalytic Processes On Precious Metal Surface: A First Principles Study

Chemistry on solid surface is focused on the physical and chemical property of surface and interface, the series of resulting phenomena, and the applications, which is related to physics, chemistry, biology, and material subjects. It is central to many areas of practical interest such as heterogeneous catalysis, tribology, electrochemistry, and materials processing. For heterogeneous catalysts, their surface catalysis is a typical example. The reaction kinetics on their surfaces is mainly determined by the surface composition, structure, size or support. At the microscopic level, however, little is known about the microscopic origin of these effects. This may be induced by the vast complexity of typical heterogeneous catalysts and several experimental difficulties to research the detailed reaction steps. With the development of many surface-sensitive analytical techniques in the past decades, great advances have been possible in our understanding of such surface chemistry at the molecular level. Earlier studies with model systems, single crystals in particular, have provided rich information about the adsorption and reaction kinetics of simple inorganic molecules. Now, researchers have paid more attention to organic molecules, even macro-molecules. For precious metal catalysts Pt and Ag, they play an important effect in chemical industry and fuel cell technology, where the main reactions are the decomposition and oxidation of the organic molecules. However, the reactive sites and catalysis mechanism are still unknown or controversial, which is mainly due to the difficulties of detecting the transition state, even intermediate, and the detailed reaction process. This limits the application of the catalysts in industry. Thus, the research of these catalytic reactions is important to enhance the understanding their catalytic mechanisms. Furthermore, it is also helpful to improve their catalytic ability and develop the new catalysts.Surface adsorption is also an important domain in surface science. It is not only the important prophase research of surface catalysis but also the central topics of tribology, electrochemistry, Self-assembled, store hydrogen and sensor. GaAs is a promising semiconductor; however, it is limited in applications due to the poor thermal oxidation property and electronic passivation characteristics. Self-assembled monolayers (ASM) on the semiconductor can protect it effectively, especially the thiolate-ASM. Unfortunately, it remains controversial about the GaAs-ASM bonding, which may be caused by its complex surface structure. We study the adsorption of sulfide on the surface of GaAs to elucidate the property of the formed bonding, which is helpful to enhance the knowledge of the ASM structure.Molecular junctions are central to the nano-electronics, in which the structure between the single molecule and electrode is also the important topic in surface and interface science. The electronic properties of single molecules have been studied by spectroscopy for almost a century, since photons can blithely interrogate molecules in the gas phase, in the solid state, or in solution without the need of electrical leads. From the molecular diode behavior proposed in 1974, different experimental techniques were developed. Not only the possible molecules and nano materials for molecular junctions are detected, but also their structure property and conductance are got. Using computer simulation, the bonding between the single molecule and metal electrode were researched recently, which the corresponding geometry and electronic structures were got. However, it is known that the geometry structures of molecular junctions are inconstantly, as well as the conductance. But the effect of this geometry instability to the electronic structure is still unknown. Considered about the key effect of the electronic structure to the conductance, the relationship between the geometry and electronic structure should be researched, which is very helpful for the understanding of the molecular junction principle and the development of the new junctions.Computer simulations can simulate not only experiments with shorter time scales but also experiments that are difficult to realize. In addition, they can provide detailed information that is difficult to extract from experimental data. In this contribution, first-principles density functional theory (DFT) calculations are performed to determine the catalytic reaction on Pt and Ag surface, the adsorption property of GaAs surface, and the relationship between the geometry and electronic structures for Au-C6H4O2-Au. The detailed contents are listed as follows:1. The electronic structure characteristics of P(4×4) or Ag12O6/Ag(111) overlayer structure and its catalysis for ethylene epoxidation are determined using DFT. The adsorption characteristic of ethylene is obtained according to the partial density of state (PDOS) of Ag12O6/Ag(111) overlayer. Moreover, the relative energetic diagrams for the ethylene oxidation process at different adsorption sites are determined using linear synchronous transit/quadratic synchronous transit (LST/QST) tools in DMOL3 code, which show reaction mechanisms and associated barriers for the complete catalytic cycle of conversion of ethylene to ethylene epoxide.2. Non-CO involved oxidation of methanol (NCOIOM) on the Pt(111) surface has been investigated using density functional theory. The relative energetic diagrams for the NCOIOM are established, in which the reaction mechanisms for a catalytic cycle including the associated barriers, the reactive energies, the intermediates, and the transient states are shown. Results indicate that the reaction proceeds in the kinetically favored pathways of HCOH→HC(OH)2→HCOOH→HCOO– [–COOH]→CO2 and CHO→HCOOH→ HCOO– [–COOH]→CO2 where OH plays a key role in the entire process of reaction. The vibrational frequencies of the intermediate states derived from the calculations are in agreement with experimental measurement.3. The dehydrogenation of benzene on the Pt(111) surface is studied by Ab initio density functional theory. The minimum energy pathways for benzene dehydrogenation are found with the nudge elastic band method including several factors of the associated barriers, the reactive energies, the intermediates and the transient states. The results show that there are two possible parallel minimum energy pathways on Pt(111) surface. Moreover, the tilting angle of the H atom in benzene can be taken as an index for the actual barrier of dehydrogenation. In addition, the properties of dehydrogenation radicals on Pt(111) surface are explored through their adsorption energy, adsorption geometry and electronic structure on the surface. The vibrational frequencies of the dehydrogenation radicals derived from the calculations are in agreement with literature data.4. The adsorption of the methylthiolate (MT) on the As-rich GaAs (001) surface has been studied using DFT calculations with a three-dimensional periodic boundary condition. A complete characterization of their structures, binding energies, and type of the bonding are obtained. It is found that the most reactive binding sites are related to empty Ga dangling bonds located at threefold-coordinated second-layer Ga atoms with the binding energy value of 2.8 eV, which results in the formation of stronger covalent S-Ga bonds. On the other side, the binding energy of 2.1 eV is obtained for the atop As dimer. The characteristic of the thiolate binding also accounts for an anisotropic thiolate migration to stabilize the binding of MT. In addition, the analysis for this shortest chain binding is helpful to realize the electrical passivation and chemical protection of GaAs surfaces.5. For a model system consisting of a benzenedithio (BDT) molecule sandwiched between two Au plates, the electronic properties as a function of different BDT geometry is investigated using density functional theory. The distorted BDT structures are got through stretching the electrode distance. The corresponding electronic properties, including the spatial distribution of the frontier orbits, the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital levels and density of states at the Fermi energy are determined. It reveals that the molecular distortion essentially determines electronic structures. The result should be beneficial to understand the stress-dependent or structure-dependent transport mechanism of electrons of the BDT junction.