Oxygen-exchange reaction between O2 and NO coadsorbed on a Pt(111) surface: reactivity of molecularly adsorbed oxygen

The oxygen-exchange reaction between N¹⁶O and ¹⁸O₂ coadsorbed on Pt(111) has been studied by temperature-programmed desorption (TPD). Reaction products of N¹⁸O and ¹⁸O¹⁶O are desorbed from Pt(111) initially saturated with ¹⁸O₂ at 94 K followed by exposure of N¹⁶O. Three distinct desorption peaks are observed in N¹⁸O TPD spectra at 145, 310, and 340 K, and two peaks in ¹⁸O¹⁶O at 155 K and between 600 and 1000 K. In contrast, the exchange reaction is greatly suppressed when oxygen molecules are replaced with oxygen adatoms at three-fold hollow sites of Pt(111). These results strongly suggest that adsorbed oxygen molecules are responsible for the exchange reaction. NO₂ or NO₃ is postulated as a reaction intermediate. However, since desorption signals corresponding to these species are not detected, the oxygen-exchanged products are not due to the cracking processes of the higher order nitrogen oxides in the mass spectrometer. Thus, the reaction proceeds via the intermediate that is dissociated during the elevation of surface temperature.     

The Tamm surface state on Cu(111) and Ag(111)

It is shown, that the well-known d-electron Tamm surface-state emission observed in photoelectron spectra from Cu(111) at the M̄ point in the surface Brillouin zone, is indeed due to such a surface state and not a bulk band transition as recently suggested L. Wallden, Solid State Commun. 59, 205 (1986). A similar surface state on Ag(111) is reported.     

A density functional theory study of CF3CH2I adsorption and reaction on Ag(111)

The adsorption and reaction behaviors of CF₃CH₂I on Ag(111) were systematically studied by density functional theory (DFT) calculations. Physical adsorption of CF₃CH₂I on Ag(111) occurs due to the weak interactions between surface Ag atoms and iodine atom of CF₃CH₂I; while strong chemisorption occurs for CF₃CH₂ fragment on Ag(111). Electronic analysis indicates that the singly occupied molecular orbital (SOMO) of CF₃CH₂ strongly interacts with the surface Ag atoms. It is very interesting to find that the most stable structures of CF₃CH₂ on Ag(111) locate at the top site, instead of the hollow sites. This might be attributed to the facts that CF₃CH₂ adsorbed at the top site will maximize the sp³-type hybridization, and the possible weak interaction between the fluorine lone pair electrons of p orbitals for CF₃CH₂ and surface Ag(111) occurs, which is supported by the charge density difference (CDD) analysis with a low isosurface value. We propose that the charge density difference (CDD) analysis with a high or low isosurface value can be widely applied to analyze the strong or weak electronic interactions upon adsorption. Transition state calculations suggested that the energy barrier of CF bond rupture for CF₃CH₂I on Ag(111) (1.44 eV) is much higher than that of CI bond breakage for CF₃CH₂I (0.43 eV); and the activation energy of the CF bond dissociation for CF₃CH₂(a) is 0.67 eV.     

M2Al2(M=Au,Ag,Cu)混合小团簇的密度泛函研究

采用基于密度泛函理论的B3LYP方法,利用小核实赝势LANL2DZ,优化了含重金属二元混合团簇M2Al2(M=Au,Ag,Cu)的稳定结构,并得出具有C2v(1A1)对称性的蝴蝶结构比平面构型更加稳定,其中团簇Au2Al2最稳定.计算了稳定结构的结合能、电离势、电子亲和能、最高占据轨道能级和最低空轨道能级及二者间的能隙,得出参杂团簇M2Al2比非参杂团簇M4(M=Au,Ag,Cu)更稳定的结论.     

Theoretical study of CO2 activation on Pt(111) induced by coadsorbed K atoms

The adsorption of CO₂ on the clean and potassium-precovered Pt(111) surface has been studied by means of the cluster model approach within the hybrid B3LYP density functional theory-based method. On the clean surface, CO₂ is undistorted and weakly bound. The equilibrium position of this physisorbed species appears at a rather large distance from the surface. However, when coadsorbed K atoms are included in the model, a chemisorbed, bent CO₂ species on top of a surface Pt atom is found. The presence of the coadsorbed K is found to be necessary to promote CO₂ chemisorption and activation, the key step in activating the CO₂ molecule being a direct interaction with the coadsorbate. In addition, the calculated vibrational frequencies for this chemisorbed species are in agreement with available experimental data.     

NO dissociation on Cu(111) and Cu2O(111) surfaces: a density functional theory based study

NO dissociation on Cu(111) and Cu(2)O(111) surfaces is investigated using spin-polarized density functional theory. This is to verify the possibility of using Cu-based catalyst for NO dissociation which is the rate limiting step for the NO(x) reduction process. The dissociation of molecularly adsorbed NO on the surface is activated for both cases. However, from the reaction path of the NO-Cu(2)O(111) system, the calculated transition state lies below the reference energy which indicates the possibility of dissociation. For the NO-Cu(111) system, the reaction path shows that NO desorption is more likely to occur. The geometric and electronic structure of the Cu(2)O(111) surface indicates that the surface Cu atoms stabilize themselves with reference to the O atom in the subsurface. The interaction results in modification of the electronic structure of the surface Cu atoms of Cu(2)O(111) which greatly affects the adsorption and dissociation of NO. This phenomenon further explains the obtained differences in the dissociation pathways of NO on the surfaces.     

Electronic structure of the Cu(111) surface

Self-consistent electronic structure calculations are reported on bulk Cu, and 3- and 5-layer Cu films. These yield a size insensitive work function, φ = 5.0±.1 eV, and a surface energy of 0.75 eV, in agreement with experiment. Good size convergence of the film potential permits the construction of a self-consistent potential for an 11-layer Cu(111) film, whose spectral properties we studied. A prominent p-like surface band was found within 0.1 eV of experiment, serving as a check on the surface potential.     

A DFT study of H adsorption on Pt(111) and Pt-Ru(111) surfaces

In this work a comparative analysis between different Pt-Ru(111) surface models and pure Pt(111) surface is presented. Some aspects of the electronic structure of the surfaces and hydrogen adsorption are analysed based on density functional theory calculations. The hydrogen adsorption energy is significantly reduced when Ru is present on the surface. The substitution of Pt atoms by Ru atoms reinforce the Pt-H bond while the metal-metal bond is strongly modified, making the system less stable.     

Cu(100)表面c(2×2)-N原子结构与吸附行为研究

用密度泛函理论的总能计算研究了金属铜(100)面的表面原子结构以及氮原子的c(2×2)吸附状态.研究结果表明：在Cu(100) c(2×2)-N表面系统中，氮原子处于四度配位的空洞(FFH)位置，距离最表面铜原子层的垂直距离为0.20?，最短的Cu—N键长度为1.83?.结构优化的计算否定了被吸附物导致的表面再构模型，即c(2×2)元胞的两个铜原子在垂直于表面方向发生相对位移，一个铜原子运动到氮原子之上的模型.该吸附表面的功函数约为4.65eV, 氮原子的平均吸附能为4.92 eV(以孤立氮原子为能量参考点).计算结果还说明，Cu—N杂化形成的表面局域态的位置在费米面以下约1.0 eV附近出现，氮原子和第一层以及第二层铜原子均有不同程度的杂化作用.该结果为最近有关该表面的STM图像的争论提供了判据性的第一性原理计算结果. 关键词： Cu(100) c(2×2)-N 表面吸附态 密度泛函总能计算     

Density functional study of the adsorption of K on the Cu(111) surface

The adsorption of potassium on the Cu(111) surface in a (2×2) pattern has been simulated with all-electron full-potential density functional calculations. The top site is found to be the preferred adsorption site, with the other highly symmetric adsorption sites being nearly degenerate. The bond length from potassium to the nearest copper atom is computed to be 2.83 ?. Population analysis and density of states indicate that there is no evidence for covalent bonding so that the binding mechanism appears to be a metallic bond. Received 11 April 2001     

First-principles calculation of ZnS monolayer on Cu(111) surface

First-principles calculation is carried out on the interface of the ZnS(001) monolayerand Cu(111) surface. It is found that the ZnS monolayer significantly reconstructs aftergeometry optimization. The out-of-plane S atom has a positive displacement in thez directionwhile other atoms (Zn and S) have small displacements on the ZnS monolayer. The interfacestacking sequence has an influence on the flatness of the ZnS monolayer and the bindingenergy of the interface. There are two approaches for the ZnS monolayer to reach thelowest energy state which take place on the two kinds of S atoms in the ZnS monolayer andresult in the bulging feature. The van der Waals (vdW) interaction exists between ZnSmonolayer and Cu surface.     

Adsorption-desorption properties and surface structural chemistry of chlorine on Cu(111) and Ag(111)

The adsorption, desorption, and structural properties of chlorine adlayers on Cu(111) and Ag(111) have been studied by LEED, Auger, Δ?, and thermal desorption measurements. Ancillary experiments were also carried out on cuprous chloride for purposes of comparison with the Cu(111)-Cl data. Chlorine adsorption is rapid on both metals and follows precursor kinetics, the absolute initial sticking probabilities being ~1.0 (Cu) and ~0.5 (Ag). Δ? results suggest that significant depolarisation of the chemisorption bond occurs at high coverages, the maximum values being + 1.2 eV (Cu) and + 1.8 eV (Ag). On Cu(111), adsorption leads to the formation of a sequence of well-ordered phases; in order of increasing coverage, these are as follows: (√3 × √3)R30°, (12√3 × 12√3)R30°, (4√7 × 4√7)R19.2°, and (6√3 × 6√3)R30°. On Ag(111) (√3 × √3)R30°, and (10 × 10) structures are observed. All six structures are susceptible to a straightforward interpretation in terms of coincidence lattices resulting from the progressive uniform compression of a hexagonal layer of Cl atoms. This interpretation is consistent with all the experimental results, and gives values for the nearest-neighbour ClCl spacing on both Cu(111) and Ag(111) which are in good agreement with other work on other surfaces. Chlorine desorbs exclusively as atoms from both metals with first-order desorption kinetics, and apparent desorption energies of 236 (Cu) and 209 (Ag) kJ mol^?1. These values, which depend on an assumed pre-exponential factor of 10¹³ s^?1, are shown to be inconsistent with the thermochemical constraints on the system necessitated by the complete absence of Cl₂ desorption. Lower limits for the pre-exponential factors are then deduced, and the values are found to be consistent with the differences between the CuCl and AgCl systems.     

LEED and Auger investigations of Cu (111) surface

After ultrahigh vacuum bake-out, electropolished Cu (111) surfaces were shown by Auger analysis to be contaminated by C, N, O, S and Cl. Other than C and S, which were contained in the bulk, the impurities were introduced by surface preparation; but all were easily removed by light Ar ion bombardment. Heating to ≈ 750°C caused diffusion of C and S from the bulk to the extent that a clear diffraction pattern corresponding to a √7 × √7 structure was produced by S on the surface. At ≈ b 900°C evaporation of Cu occurred to an observable degree, and S and C could no longer be detected on the surface. Auger analysis of clean Cu surfaces showed many details of the LMM and MMM types of transitions. Kinetic energies of all observed Auger electrons were in excellent agreement with calculated values. Also, the ≈ 62 eV MMM peak was resolved into two components related to the small differences in the M₂ and M₃ energy levels. The LMM transitions were classified according to their intensities, which could be rationalized on the basis of Coster-Kronig transitions and transition probabilities, as L₃MM > L₂MM > L₁MM.     

Size-dependent surface states of strained cobalt nanoislands on Cu(111)

Low-temperature scanning tunneling spectroscopy over Co nanoislands on Cu(111) showed that the surface states of the islands vary with their size. Occupied states exhibit a sizable downward energy shift as the island size decreases. The position of the occupied states also significantly changes across the islands. Atomic-scale simulations and ab initio calculations demonstrate that the driving force for the observed shift is related to size-dependent mesoscopic relaxations in the nanoislands.     

A photoelectron spectroscopy study of ultra-thin epitaxial alumina layers grown on Cu(111) surface

Thin epitaxial alumina layers were grown on the Cu(111) surface using simultaneous aluminum deposition and oxygen exposure. Different substrate temperatures during the deposition resulted in layers with different thicknesses, growth rates, crystallinity and epitaxy. Low energy electron diffraction measurements confirmed the epitaxial growth for substrate temperatures above 870 K. The Al 2p doublet was studied by means of photoelectron spectroscopy in order to determine the alumina termination at the metal-oxide interface. A strong dependence on the preparation temperature was found and both aluminum and oxygen terminated interfaces were created.     

Bulk and surface properties of liquid X-Zr (X=Ag, Cu) compound forming alloys

The phase diagrams of the Ag-Zr and Cu-Zr systems exhibit the existence of different intermetallic compounds in the solid state, and since the structure of a liquid alloy is in some respects similar to that of a crystal, the compound formation phenomenon in these liquid alloy systems has been analysed through the study of surface properties (surface tension and surface composition), dynamic properties (chemical diffusion and viscosity) and microscopic functions (concentration fluctuations in the long-wavelength limit and chemical short-range order parameter) in the frame of the compound formation model (CFM). Moreover, the associative tendency between unlike constituent elements qualitatively expressed by the microscopic functions indicates the glass-forming ability of both systems at higher Zr-concentrations. These results are in agreement with reported experimental data and confirm the applicability of a statistical mechanical theory in conjunction with the CFM to describe the mixing behaviour of compound forming alloys.