Oxygen on Fe(100) and Fe(110)

Investigations of the electronic and magnetic properties of oxygen adsorbed on magnetized iron films have been carried out by means of angle and spin resolving photoelectron spectroscopy. Iron, epitaxially grown on W(100) and W(110) crystals, served as the ferromagnetic substrate. Exchange splittings of the O 2p_x derived level have been detected, demonstrating a magnetic coupling between the chemisorbate and the iron layer. Variations of the exchange splitting have occurred as a function of the oxygen coverage, photon energy, and emission angle. High oxygen exposures have lead to a FeO overlayer at the surface.     

HCN Adsorption on the Fe(100), Fe(111) and Fe(110) Surfaces: a Density Functional Theory Study

Twenty kinds of adsorptions of HCN on the Fe(100), Fe(111) and Fe(110) surfaces at the 1/4 monolayer coverage are found using the density functional theory. For Fe(100), the adsorption energy of the most stable configuration where the HCN locates at the fourfold site with the C-N bonded to four Fe atoms is 1.928 eV. The most favored adsorption structure for HCN on Fe(111) is f-η3(N)-h-η3(C), in which the C-N bond is almost parallel to the surface, and the adsorption energy is 1.347 eV. On Fe(110), the adsorption energy in the most stable configuration in which HCN locates at the two long-bridge sites is 1.777 eV. The adsorption energy of the parallel orientation for HCN is larger than that of the perpendicular configuration. The binding mechanism of HCN on the Fe(100), Fe(111) and Fe(110) surfaces is also analyzed by Mulliken charge population and the density of states in HCN. The result indicates that the configurations in which the adsorbed HCN becomes the non-linear are beneficial to the formation of the addition reaction for hydrogen. The nature that the introduction of Fe into the catalyst could increase the catalytic activity of the bimetallic catalyst in the addition reaction of hydrogen for nitriles is revealed.     

Competitive segregation of Si and P on Fe96.5Si3.5 (100) and (110)

Annealing an Fe(96.5)Si(3.5) (100)/(110) bicrystal, containing 90 ppm P, leads immediately to a strong segregation of silicon. The Si atoms, however, desegregate subsequently and are displaced by P, whose segregation enthalpy is larger than that of silicon. The corresponding surface structures formed on both faces have been studied using complementary methods: Scanning tunneling microscopy (STM) to obtain atomically resolved geometrical information and Auger electron spectroscopy (AES) for the determination of the surface composition. Si substitutes surface Fe atoms on both faces and forms ordered surface alloys, whereas P occupies hollow sites on the surface. Si and P form c(2 x 2) superstructures on the (100) surface, whereby each segregated phosphorus atom blocks in the average one silicon segregation site. The (110) surface, on the other hand, is characterized by a c(1 x 3) Si superstructure. Due to the anisotropy of this surface the P/Si exchange proceeds by the formation of silicon coverage decreasing domain boundaries within the silicon structure, which are simultaneously occupied by P atoms. Furthermore the comparison of the AES and STM derived phosphorus coverages indicates a P multilayer segregation on the (110) surface.     

氧原子在Al(100)、(110)、(111)面上的吸附与振动

利用原子与表面簇合物相互作用的5参数Morse势方法(简称5-MP)系统研究了O-Al低指数面表面吸附体系, 获得了吸附几何、结合能、振动频率等吸附态的临界点特性. 通过把这些临界点特性作为研究吸附体系的探针, 系统分析了吸附体系的性质. 理论计算结果表明, 在Al(111)面上, 氧原子在表面三重位处存在吸附态, 其垂直表面的振动频率为621(619) cm-1, 平行表面的振动频率为880(887) cm-1. 在子表面八面体处也存在吸附态, 其垂直振动频率为464 cm-1, 平行振动频率为437 cm-1; 在Al(100)面上, 氧原子在表面四重洞位、子表面四面体处存在吸附态;在Al(110)面上,氧原子优先吸附在表面膺势三重位而非长桥位,同时子表面八面体处也存在吸附态.     

Theoretical study of NH3 adsorption on Fe(110) and Fe(111) surfaces

With the aim of understanding the nature of the interactions between organic molecules and metal surfaces, the adsorption of NH3 onto model Fe(110) and Fe(111) surfaces has been studied with use of the molecular orbital and density functional theories. B3LYP calculations have revealed that the on-top site is most suitable for adsorption of NH3 both on Fe(110) and on Fe(111). Mulliken population analysis in terms of the MO's of the two fragment systems suggested that electron delocalization from NH3 to the Fe surface should play a key role in the adsorption. Then, our transformation scheme of fragment orbitals has demonstrated that the electron delocalization is represented well only by a pair of interaction orbitals. The NH3 molecule provides the occupied interaction orbital bearing a close resemblance to the highest occupied (HO) MO, whereas the Fe surface prepares the paired unoccupied orbital that is localized at the adsorption site and overlaps in-phase with the orbital of NH3. Not only the lowest unoccupied (LU) MO but also other unoccupied MO's have been shown to participate significantly in the interaction. The reason the on-top site is the most preferable position for NH3 attack has been elucidated by investigating the interaction orbitals.     

Density functional theory study of hydrogen adsorption on Fe5C2(001), Fe5C2(110), and Fe5C2(100)

Density functional theory calculations have been carried out for hydrogen adsorption on the (001), (110), and (100) surfaces of Fe5C2. At 1/3 and 2/3 monolyer (ML) on (001), the most stable hydrocarbon species is CsH, while CsH and CsH3 can coexist at 1 ML. On (110), only dissociated hydrogen is found at 2/5 ML, while CsH is the most stable hydrogen carbon species at 4/5 ML, and CsH and CH3 coexist at 6/5 ML. On (001) and (110) surfaces, CsH2 is less stable and can dissociate into CsH or convert into CsH3, respectively. These results are in agreement with the experimental observations. On the metallic Fe5C2(100) surface which lacks surface carbon atoms on the surface monolayer, dissociated hydrogen is found at 1/2 ML, while both dissociated hydrogen and activated H2 are found at 1 ML.     

Adsorption of oxygen on potassium-covered Pt(111) and Fe(110)

Photoemission measurements (UPS, XPS) show that potassium oxide is not formed when adsorbing oxygen on potassium-covered Pt (111) and Fe(110). Two oxygen states are observed. One is chemisorbed oxygen on the surface. The other can be interpreted as being due to either incorporated atoms or oxide.     

Structures and energies of coadsorbed CO and H2 on Fe5C2(001), Fe5C2(110), and Fe5C2(100)

Density functional theory calculations have been carried out on the CO/H2 coadsorption on the (001), (110), and (100) surfaces of Fe5C2 for the understanding of the Fischer-Tropsch synthesis (FTS) mechanism. The stable surface species changes with the variation of the H2 and CO coverage. Along with dissociated hydrogen and adsorbed CO in 2-, 3-, and 4-fold configurations, methylidyne (C(s)H) (C(s), surface carbon), ketenylidene (C(s)CO), ketenyl (C(s)HCO), ketene (C(s)H2CO), and carbon suboxide (C(s)C2O2) are computed as thermodynamically stable surface species on Fe5C2(001) and Fe5C2(110) containing both surface iron and carbon atoms. These surface carbon species can be considered as the preliminary stages for FTS. On Fe5C2(100) with only iron atoms on the surface layer, the stable surface species is dissociated hydrogen and CO with top and 2-fold configurations. The bonding nature of these adsorbed carbon species has been analyzed.     

A comparative DFT study of Fe3+ and Fe2+ ions adsorption on (100) and (110) surfaces of pyrite: An electrochemical point of view

Pyrite acts as a catalyst in the mineral processing, and the speed of ferric ion reduction and mineral decomposition increases with increasing cathodic points. In this study, the ferric ion interaction on the (100) and (110) surfaces of pyrite was studied using the density functional theory calculations. The analysis of stability, density of states, and electron density were performed to understand the interaction between the ferric ion and pyrite surfaces. The results showed that pyrite surface is chemically active and tends to absorb ferric ion between two surface sulfur atoms. The hyperconjugation between the 3d orbital of ferric ion and the 3p or 3d orbitals of surface atoms provides the conditions for the Fe³⁺ ion adsorption. The molecular orbital (MO) and electron density analyses indicate that the 3p orbitals of S atoms play a more important role in bonds formations relative to the 3d orbitals. The (110) surface is more active, and the adsorption energy is larger than that of surface (100), which is the result of decreased cation coordination and the presence of sulfur at the surface. Subsequently, the interaction of the Fe²⁺ ion, as product of Fe³⁺ ion reduction and its competitor for adsorption, on the surfaces was studied. The Fe^{2 +} ion adsorbs stronger at the surface of (110), and the adsorption energies at (100) and (110) surfaces were obtained as −24 and −47 kcal/mol, respectively. In general, the Fe³⁺ ion is a stronger oxidizing agent than Fe²⁺ on pyrite surfaces.     

One-dimensional corrugation of the h-BN monolayer on Fe(110)

We report on a new nanopatterned structure represented by a single atomic layer of hexagonal boron nitride (h-BN) forming long periodic waves on the Fe(110) surface. The growth process and the structure of this system are characterized by X-ray absorption (XAS), core-level photoemission spectroscopy (CL PES), low-energy electron microscopy (LEEM), microbeam low-energy electron diffraction (μLEED), and scanning tunneling microscopy (STM). The h-BN monolayer on Fe(110) is periodically corrugated in a wavy fashion with an astonishing degree of long-range order, periodicity of 2.6 nm, and the corrugation amplitude of ～0.8 ?. The wavy pattern results from a strong chemical bonding between h-BN and Fe in combination with a lattice mismatch in either [111] or [111] direction of the Fe(110) surface. Two primary orientations of h-BN on Fe(110) can be observed corresponding to the possible directions of lattice match between h-BN and Fe(110), with approximately equal area of the boron nitride domains of each orientation.     

Hydrogen diffusion on (100), (111), (110) and (211) gold faces

Calculations showed that hydrogen adsorption into subsurface sites is most likely to occur on Au (110) and (211) faces. The presence of low-coordinated Au atoms on significantly reduces the barrier of subsurface adsorption of H. The barriers of H surface diffusion increase in the following Au series: (110) < (111) < (100) < (211). An analysis of the dependence of the surface diffusion barriers on the electronic structure of gold atoms on the respective faces revealed a U-shaped dependence of the centers of the s- and d-bands. This dependence is the result of the filling of the s- and d-bands on different faces of the gold. The results obtained suggest that it is possible to use band centers to determine surface diffusion barriers.     

Oxygen vacancies on TiO2 (110) from first principles calculations

We have carried out a systematic study of oxygen vacancy formation on the TiO2 (110) surface by means of plane-wave pseudopotential density-functional theory calculations. We have used models with the mean number of vacancies per surface unit cell being theta=0.25 and theta=0.5. The study comprises several kind of vacancies within the outermost layers of the surface. The use of a suitable set of technical parameter is often essential in order to get accurate results. We find that the presence of bridging vacancies is energetically favored in accordance to experimental data, although the formation of sub-bridging vacancies might be possible at moderate temperatures. Surprisingly, the spin state of the vacancy has little influence on the results. Atomic displacements are also analyzed and found to be strongly dependent on the particular arrangement of vacancies.     

Dissociative and non-dissociative adsorption dynamics of N2 on Fe(110)

We study the adsorption dynamics of N(2) on the Fe(110) surface. Classical molecular dynamics calculations are performed on top of a six-dimensional potential energy surface calculated within density functional theory. Our results show that N(2) dissociation on this surface is a highly activated process that takes place along a very narrow reaction path with an energy barrier of around 1.1 eV, which explains the measured low reactivity of this system. By incorporating energy exchange with the lattice in the dynamics, we also study the non-dissociative molecular adsorption process. From the analysis of the potential energy surface, we observe the presence of two distinct N(2) adsorption wells. Our dynamics calculations show that the relative population of these adsorption sites varies with the incident energy of the molecule and the surface temperature. We find an activation energy of around 150 meV that prevents molecular adsorption under thermal and hypothermal N(2) gas exposure of the surface. This finding is also consistent with the available experimental information.     

洁净和氧吸附的Cu(100)表面重构的理论研究

The interaction of atomic oxygen with the clean Cu(100) surface has been studied by means of cluster and periodic slab models density functional theory in the present paper. The Cu(4,9,4) cluster and a three-layer slab with c(2×2) structure are used to model the perfect Cu(100) surface. Three possible adsorption sites,top, bridge and hollow site, were considered in the calculations. The predicted results show that the hollow site is the prefer site for atomic oxygen adsorbed on Cu(100) surface energetically. This is in good agreement with the experiment. The calculated binding energies are respective 2.014, 3.154 and 3.942 eV for top, bridge and hollow sites at mPW1PW91/LanL2dz level for the cluster model. The geometry of Cu(100) surface has also been optimized theoretically with various density functional methods and the results show that the prediction from the B3PW91/LanL2dz and mPW1PW91/LanL2dz reproduce the experimental observation.The frontier molecular orbitals and partial density of states analysis show that the electron transfer from the d orbital of substrate to the p orbital of the surface oxygen atom.     

Tilted CO on metal surfaces; CO/Pt(110), CO/Ni(110) and CO/Cu(100)

Tilting of CO at coverages greater than half a monolayer is considered as a mechanism for reducing the CO-CO repulsion. We find qualitative agreement with the experiment for CO/Pt(110), and predict a slightly smaller tilt for CO/Ni(110). For CO/Cu(100), we find that a bend of about 10° greatly reduces the repulsion.     

Orbital mechanism of upright CO activation on Fe(100)

The knowledge of bond activation forms a cornerstone for modern chemistry, wherein symmetry rules of electronic activation lie in the heart of bond activation. However, the question as to how a chemical bond is activated remains elusive. By taking CO activated on Fe(100), herein, we have resolved the long-standing fundamental question; we have found that excitations in the adsorbate feature the bond activation. We essentially have discovered contrasting electronic processes in respective σ and π electron systems of the adsorbed CO molecule. The σ electron system is involved in reversible hidden excitations/deexcitations between two occupied σ orbitals, whereas the π electron system is subject to irreversible π to π* excitations dispersed along the d-band region, which is coupled to the rotational 2π electron couplings depending on the strength of molecule-metal interactions. The σ excitations pertain to the Pauli repulsion mediated quantum nature with energy and entropy marked by the two energy levels, whereas the π to π* excitations fall into a new category of electronic excitations contributing to energy and entropy exchanges in a wide and continuous d-band region. The findings that the internal states of the adsorbate are excited and that fundamental connections between the frontier orbitals and low-lying orbitals are established as the molecule comes to the surface may open up new channels to realize more efficient bond activation and renew our thinking on probing the quantum mechanical nature of bond activation at surfaces with further possible impact on manipulation of orbital activation in femtochemistry and attochemistry.     

HCOO在Cu(110)、Ag(110)和Au(110)表面的吸附

采用密度泛函理论(DFT)以及广义梯度近似方法(GGA)计算了甲酸根(HCOO)在Cu(110)、Ag(110)和Au(110)表面的吸附. 计算结果表明, 短桥位是最稳定的吸附位置, 计算的几何参数与以前的实验和计算结果吻合. 吸附热顺序为Cu(110)(-116 kJ·mol-1)>Ag(110)(-57 kJ·mol-1)>Au(110)(-27 kJ·mol-1), 与实验上甲酸根的分解温度相一致. 电子态密度分析表明, 吸附热顺序可以用吸附分子与金属d-带之间的Pauli 排斥来关联, 即排斥作用越大, 吸附越弱. 另外还从计算的吸附热数据以及实验上HCOO的分解温度估算了反应CO2+1/2H2→HCOO的活化能, 其大小顺序为Au(110)>Ag(110)>Cu(110).     

A Density Functional Study of Ozone and Oxygen Surface Structures on Ni(110)

The dissociative and molecular forms of O₃ adsorbed on nickel were studied by the density functional method (Becke3LYP exchange correlation functional). The energy profile of the ozone decomposition reaction (O₃)_gas (O₃)_ads (O₂)_ads+1/2O₂ on the Ni₁₀ surface cluster was calculated. As opposed to the weakly bound molecular form of oxygen on transition metals, ozone forms a tightly bound surface compound on the surface of nickel. It is attempted to evaluate the heat of oxygen adsorption on the defective surface of Ni using the model of the surface cluster Ni₁₃ with a vacancy . The positions of the surface nickel atoms before and after their interaction with the adsorbate were calculated. It is shown that substantial relaxation of the surface nickel atoms takes place in the course of adsorption. The possibility of stabilization of the molecular ozone-like form of oxygen on the surface of transition metals in conditions of high oxygen coating extent is discussed.     

甲醇在清洁和氧预吸咐Pd（100）面上的吸咐与分解

用高分辨电子能量损失谱（ＨＲＥＥＬＳ），热脱咐谱（ＴＤＳ）在１４０Ｋ—７００Ｋ温度范围内研究了清洁和氧改性Ｐｄ（１００）面上ＣＨ_３ＯＨ的吸咐与分解机理。ＨＲＥＥＬＳ结果表明：在１４０Ｋ吸咐甲醇能形成分子吸咐层；当加热甲醇分子层时，甲醇在１８５Ｋ先分解为ＣＨ_３Ｏ_（ａｄ）和Ｈ_（ａｄ），在２００Ｋ以上温度ＣＨ_３Ｏ_（ａｄ）逐步分解为ＣＯ_（ａｄ）和Ｈ_（ａｄ）；预吸咐氧后表面有甲酸物种生成。ＴＤＳ研究表明；除有少量甲醇在１８５Ｋ脱咐外，甲醇分解的主要脱咐产物为Ｈ_２（３２０Ｋ）和ＣＯ（４４０Ｋ）；次要脱附产物为甲烷（２００Ｋ—２１０Ｋ）。综合ＨＲＥＥＬＳ和ＴＤＳ研究指出，在清洁表面甲醇主要通过Ｏ—Ｈ键断裂，经甲氧基中间物种分解为ＣＯ和Ｈ_２，还有部分甲醇通过Ｃ—Ｏ键断裂分解为甲烷，预吸附氧后甲醇的分解除了存在以上两种方式外，氧的存在一方面能够转移ＣＨ_３Ｏ_（ａｄ）中的氢原子在表面形成一定量的甲酸中间物种，另一方面能够稍许提高少量ＣＨ_３Ｏ_（ａｄ）的热稳定性。     

Structure and reactivity of intermediates. N-overlayer on Pd(100), Rh(100) and Pt-Rh(110) and its reaction with H2

Rh(100), Pt(100), and Pt-Rh(100) surfaces are inert for the dissociative adsorption of N₂, but they are active for the catalytic reaction of NO with H₂ During the reaction on Rh(100) and Pt-Rh(100) surfaces, N atoms are accumulated by making a c(2x2)-N overlayer, but no accumulation of N atoms occurs on Pt(100) surface. The fact that N atoms on the Pt-Rh(100) surface gives the c(2x2) structure indicates that the N atoms have equal affinity to Pt and Rh on the alloy surface. When the c(2x2)-N surface was exposed to H₂ of 10^-7 to 10^-8 Torr, a prominent loss peak being assignable to NH_x appeared at 3200 – 3240 cm^-1 at around 400 K. The in-situ HREELS study proved that NH are prominent species which are formed during the hydrogenation of the c(2x2)-N, that is, a quasi-equilibrium of N + 1/2 H₂ - NH is established. When a clean Pt-Rh(100) (Pt/Rh = 1/3) alloy surface is exposed to NO at about 440 K, the LEED pattern changes sequentially as (1x1) → c(2x2) → c(2x2) + p(3x1) → p(3x1), where the c(2x2) pattern appears instantaneously on the alloy surface of any Pt/Rh ratio but the p(3x1) pattern accompanies a certain characteristic interval times being responsible to the segregation of Rh. The p(3x1) surface reflects the formation of an intermediate of Rh-O complex overlayer and it reacts rapidly with H₂.