Saturation coverage of deuterium on Ni(110)

The absolute deuterium coverages associated with the (2 × 1) and (1 × 2) phases on Ni(110) at 175 K have been measured by nuclear reaction analysis to be 1.0 and 1.5 monolayers, respectively. Our measured values are consistent with the previous models but are substantially larger than those reported recently by Stensgård and Feidenhans'l.     

Absolute coverage measurements: CO and oxygen on the (110) plane of tungsten

An effusion source, calibrated with a vibrating quartz microbalance, has been used to determine absolute coverages of CO and of oxygen adsorbed on a tungsten (110) plane by an extension of the field emitter detector method: The amount of gas reflected from the substrate is measured as a function of the absolute amount impinged per unit area; maximum coverage in the chemisorbed layer can be obtained very directly from this information. Work function increments vs. absolute coverages were measured in the same apparatus by the vibrating condenser method. Results were as follows: For virgin CO, adsorbed at 100 K, the maximum coverage obtained was CO/W = 0.71 ; this leads to a maximum coverage for beta or beta-precursor CO of CO/W = 0.28. The maximum work function increment for virgin CO was 0.8 eV. For oxygen, adsorption at 100 or 300 K a decrease in sticking coefficient by several orders of magnitude occurs when the coverage is O/W = 0.5. Adsorption at 20 K leads to the molecular precursor noted by Leung and Gomer, which converts to atomic oxygen at <50 K. By adsorbing at 20 K the O atom coverage can be increased to O/W = 0.62. Work function versus coverage data for this system are also presented.     

低覆盖度CO分子在 Ni（110）面的吸附研究

利用密度泛函理论研究了低覆盖度下CO分子在Ni（110）表面的吸附结构和电子态。研究结果表明：在低覆盖度情况下, CO分子优先垂直吸附在短桥位,其次是顶位和长桥位。垂直短桥位吸附、顶位吸附相应的振动频率分别是1850.52 cm-1、1998.08cm-1。态密度的研究结果表明：CO分子和Ni原子在-10 eV -8 eV,-8 eV—-6 eV及1 eV -5 eV能量范围内发生了杂化作用。-10ev -8ev能量范围内的杂化主要来源于C、O原子的s轨道、pz轨道与Ni原子s、p、d轨道的杂化作用。-8ev—-6ev能量范围内的杂化作用主要来源于C、O原子的py、 px轨道与Ni原子d、s轨道的杂化作用。轨道间的杂化作用是吸附作用的主要来源。 我们计算的吸附位置与相应的振动频率与相关实验结果基本一致。     

Absolute coverage and isostemc heat of adsorption of deuterium on Pt(111) studied by nuclear microanalysis

The absolute coverage (θ) of deuterium adsorbed on Pt(111) in the ranges 180< T<440 K and 5 × 10^?6 < P < 5 × 10^?2 Pa D₂ has been determined by nuclear microanalysis using the D(³He, p)⁴He reaction. From these data, the isosteric heat of adsorption (E_a) has been determined to be 67 ± 7 kJ mol^?1 at θ ? 0.3. This heat of adsorption yields values of the pre-exponential for desorption (10^?5 to 10^?2 cm² atom^?1 s^?1) that lie much closer to the normal range for a second order process than those determined from previous isosteric heat measurements. The E_a versus θ relationship indicates that the adsorbed D atoms are mobile and that there is a repulsive interaction of 6–8 kJ mol^?1 at nearest neighbour distances. At 300 K the coverage decreases to ? 0.05 monolayer (? 8 × 10¹³ D atoms cm^?2) as P→ 0, apparently invalidating a recent model of site exchange in the adsorbed layer.     

Rovibrational state distribution of deuterium molecules resulting from D-atom interaction with Ni(110)

Using the REMPI technique we have studied the internal state distribution of deuterium molecules produced by the interaction of atomic deuterium with chemisorbed deuterium on Ni(110) at 180 K. We observed molecules in vibrational states up to v = 3 with a mean vibrational energy of 220 meV. The mean rotational energies of the molecules in the vibrational states v = 0 to 3 are 185 meV, 133 meV, 75 meV and 37 meV, respectively. The overall mean rotational energy amounts to 150 meV, again far in excess of the mean rotational energy of molecules accommodated to the surface. The data are consistent with direct interactions of the impinging particles with the adsorbed particles (Eley-Rideal reaction and collision induced desorption), for which it is assumed that a considerable amount of the potential and kinetic energy of the impinging atoms is channeled into translational and internal energy of the reaction products.     

The effect of oxygen coverage on surface relaxation of Ni(110) measured by medium energy ion scattering

Changes have been observed in the upper layer spacing of a clean and an oxygen covered Ni(110) single crystal by employing medium energy ion scattering, combined with channeling and blocking. We find a contraction of 4% for the clean surface and a minor expansion of 1% for a surface with $\text{1}\text{3}$ monolayer of adsorbed oxygen.     

Structure of graphene on the Ni(110) surface

The structure of graphene on Ni(110) was studied using scanning tunneling microscopy (STM) and low-energy electron diffraction spectroscopy. STM images show a Moire structure, depending on the orientation of the domains making up the graphene layer. A simple model has been proposed which permits prediction of the Moire structure and interpretation of STM images based on calculation of the distances between the nearest neighbor carbon and nickel atoms. Our theoretical calculation suggests that the final orientation of graphene domains forming in the course of synthesis is defined by the angle of rotation of small clusters in the initial stages of growth.     

Electronic structure of graphitic carbon on Ni(110)

Photoemission experiments show that graphitic overlayers obtained by cracking ethylene on Ni(110) have a significantly different structure from graphite carbon on Ni(111) or Ni(100). Analyses of our data suggest that a complete overlayer of graphite in register with the substrate cannot be formed because of the rough structure of the Ni(110) face. Nevertheless a graphitic-like structure with much of the p_z orbitals saturated can grow along the channels of the (110) surface. These findings are consistent with a previous model deduced by surface extended energy loss experiments performed on the same system.     

Hydrogen desorption from Ni sites on (110) CuNi surfaces

Hydrogen adsorption on Ni-rich (110) CuNi alloy surfaces has been studied by means of thermal desorption spectroscopy. After adsorption near room temperature the hydrogen desorption spectra exhibit a coverage dependence similar to that known from pure (110)Ni. Besides a slightly composition dependent desorption energy the alloy surfaces behave like a (110)Ni surface diluted by practically inert Cu. These results are compared to those reported by Yu Ling and Spicer.     

Surface coverage measurements by sims for CO adsorption on a number of metals and for CO and H2S CO-adsorption on Ni(110), (100) and (111)

A detailed study of CO adsorption on Ni(100) utilizing static SIMS and a comparison of the data with surface coverage data from the literature shows that there is a linear relationship between CO coverage and the peak intensity ratios (MCO⁺/M⁺ and M₂CO⁺/M⁺₂) of the CO-containing secondary ions, in the region of coverage below which the adlayer becomes compressed. Adsorption isobares were obtained using the intensity ratios and from these, adsorption isosteres were derjved to give heats of adsorption as a function of coverage. These data are in very close agreement with the literature. Confirmatory data were obtained for this relationship for CO adsorption on polycrystalline Ni, Pd, Pt and Cu and Cu(100). The application of this technique of surface coverage measurements to a study of the extent to which H₂S coadsorption reduces the coverage of adsorbed CO on Ni(110), (100) and (111) shows that these faces are poisoned in the order (100) > (111) > (110). Surface coverage measurements on the non-closepacked (110) face are affected by the apparent insensitivity of SIMS to adsorbates located in the “channels”.     

Angle-resolved resonant photoemission of Ni(100) and Ni(110) single crystals

We have studied the valence band photoemission spectra of Ni(100) and Ni(110) single crystals near the excitation threshold for 3p core electrons. The resonant behavior of the 6 eV satellite does not depend on both the surface orientation and the polarization of the electric vector of an incident light for excitation. These results indicate that the 6 eV satellite should be under little influence of spatial symmetry of the valence band. In the angle-resolved photoemission spectra of Ni(100), we have observed another broad feature near the 6 eV satellite. It shows the large energy dispersion and is interpreted as due to the interband transition. In Ni(110), we have observed the weak valence band satellites at binding energies of about 9.3 eV and 13.4 eV. They do not show well-defined resonance around the 3p threshold.     

Angular-resolved photoemission from NH3 on Ni(110)

NH₃ adsorbed on Ni(110) at a temperature of 100 K has been studied by angular-resolved ultraviolet photoelectron spectroscopy (ARUPS) using synchrotron radiation in the energy range 10 ? ? ω ? 35 eV. From ARUPS it is concluded that NH₃ bonds via its nitrogen lone-pair orbital with its molecular axis normal to the surface. For both the 3a₁ and 1e level dispersion has been found indicating lateral interactions within a compact NH₃ layer. Increasing the NH₃ coverage beyond the chemisorbed monolayer produces a physisorbed monolayer followed by a multilayer deposition of NH₃ phase on top which has been identified by photoemission and thermal desorption. Desorption and partial decomposition result from annealing. At 300 K NH₃ or NH is left at the surface which desorbs almost completely by annealing to 400 K.     

Lattice gas model for O on Ni(110)

We consider the unusual behavior of the chemisorption system O on Ni(110). At a coverage of about$\text{1}\text{3}$, as the temperature is increased from room temperature to about 300°C, a wellordered (3 × 1) structure changes reversibly into a poorly organized (2 × 1) structure. This has been observed for a Ni crystal saturated with oxygen in the interior. We propose a two-dimensional lattice gas model with anisotropic and competing interactions between the adsorbed atoms to explain this behavior. Monte Carlo calculations on a 30 × 30 lattice show a transition of this type for coverages near$\text{1}\text{3}$.     

The autocatalytic decomposition of acetic acid on Ni(110)

The flash decomposition of CH₃COOH was studied on a clean nickel (110) surface following adsorption at 30° C in order to access the applicability of chemical reaction rate theory to a homologous series of reactants on a well-defined surface. As was observed for formic acid, acetic acid adsorbed at 30° C to yield gaseous H₂O and to form islands of adsorbed anhydride intermediates; the decomposition proceeded by a two-dimensional auto-catalytic mechanism to form H₂, CO₂, Co and surface carbon. The decomposition of the anhydride was rate determining for the formation of CO₂ and H₂. The rate of decomposition was well described by the equation governing the formic acid decomposition on the same surface. The activation energy for this first order decomposition was determined to be 28.2 $\text{kcal}\text{gmol}$ and the pre-exponential factor, v, was found to be 6.4 × 10¹⁴ s^?1 with a fraction of initiation sites of 0.004. These values were nearly the same as those observed for the decomposition of HCOOH, suggesting identical intramolecular mechanisms for the unimolecular decomposition of the adsorbed intermediates. The relative values of v for the decomposition of HCOOH, DCOOH and CH₃COOH indicated that the motion of the H, D or CH₃ group was involved in the rate-limiting step.     

First-principles calculations of ammonia decomposition on Ni(110) surface

First-principles calculations based on density functional theory (DFT) have been performed to study the adsorption and decomposition of NH₃ on Ni(110). The adsorption sites, the adsorption energies, the transition states and the activation energies of the stepwise dehydrogenation of NH₃ and the associative desorption of N are determined, and the zero point energy correction is included, which makes it possible to compute the rate constants of the elementary steps in NH₃ decomposition. Combined DFT calculations and kinetic analysis at 350 K indicate that the associative desorption of N has a reaction rate lower than NH_x dehydrogenation and is therefore the rate determining step. The distinctly different rate constants over Ni(110), Ni(111) and Ni(211) imply that ammonia decomposition over Ni-based catalyst is a structure-sensitive reaction.     

The vibrational modes of hydrogen adsorbed on Ni(110)

By studying the vibrational modes of H on Ni(110) as a function of coverage and temperature, a local picture of H site occupation is obtained in the lattice gas regime and on the (1 × 2) reconstructed surface at low temperature and for the irreversibly disordered surface formed by thermal conversion. Threefold sites are deduced from our data in both the lattice gas and the (1 × 2) reconstructed low temperature phases, with an additional loss in the latter phase ascribed to a second layer site. After thermal conversion, the threefold sites are depleted with a ? 0.5 monolayer (ML) transfer of H to second layer sites which appear to stabilize the surface and with ~ 0.5 ML H desorbing. Readsorption on the disordered surface indicates that a small amount of empty threefold Ni sites are still present after conversion. Various other models and site assignments are also discussed for comparison to the results of this study.