Chalcogen adsorption on Ni(100)

The successive stages of the oxidation of Ni(100) have been investigated by angle-resolved uv photoemission. The adsorption spectra are very similar to that measured previously for the same surface saturated with sulphur. Results are interpreted using recent theoretical calculations. It is found that this interpretation can be extended to the other chalcogens adsorbed on Ni(100). When increasing oxygen exposures, photoemission spectra have shown a continuity of the electronic character in the oxidation process.     

Hydrogen adsorption on Ni (100)

The adsorption of hydrogen on the (100) plane of nickel at room temperature has been investigated using the technique of flash desorption spectroscopy. It is shown that no variation in the adsorption enthalpy of 23.1 kcal/mole occurs during the chemical cleaning of the surface by repeated oxidation and reduction. The number of adsorption sites does however increase to 3.3×10¹⁴/cm² during this process. Determination of the partition functions of the adsorbed species and of the activated complex indicates that the hydrogen atoms are localised on specific adsorption sites but that greater liberty exists in the activated complex. Finally the experimental desorption spectra may be described using a model with a repulsive interaction of 400 cal/mole between nearest neighbours.     

Els study of alkali metal adsorption on Ni(100)

Electron energy-loss spectra on clean and alkali-covered Ni(100) surfaces have been measured. The changes in the characteristic Ni(100) loss spectra in the presence of Na, K and Cs are interpreted as a result of large charge transfer to Ni. Alkali metal losses due to core-level excitation and plasmon excitations are also discussed.     

原子分子在Ni(100)表面吸附的第一性原理研究

本文采用密度泛函理论,结合周期性平板模型,通过对原子H、N、O、S和C,分子CO、N2、NH3、NO,以及自由基CH3、CH、CH2、OH在Ni(100)表面吸附的研究,比较了它们的吸附能,稳定吸附位点,吸附结构及扩散能垒等信息. 这些吸附质与表面结合能力从小到大依次是N2相似文献   

原子分子在Ni(100)表面吸附的第一性原理研究

本文采用密度泛函理论,结合周期性平板模型,通过对原子H、N、O、S和C,分子CO、N2、NH3、NO,以及自由基CH3、CH、CH2、OH在Ni(100)表面吸附的研究,比较了它们的吸附能,稳定吸附位点,吸附结构及扩散能垒等信息.这些吸附质与表面结合能力从小到大依次是N2NH3COCH3NOHOHCH2CNSONCHC.在所有的原子中,O原子倾向于吸附在桥位,而其余的原子则倾向于吸附在空位.除N2之外的分子吸附物(CO、NO、NH3),最佳吸附位点均为四重空位,而N2的最稳定吸附位置为顶位.对于自由基吸附物(CH、CH2、CN、OH)而言,它们倾向于吸附在四重空位,而CH3则稳定吸附在桥位.     

CO adsorption on alkali-metal covered Ni(100)

The effect of preadsorbed alkali metal atoms Na, K and Cs on CO adsorption on Ni(100) has been studied using Auger spectroscopy and thermal desorption. It was found that the presence of alkali metals causes an appearance of several more tightly bound states in the CO thermal desorption spectra. The observed difference in carbon and oxygen Auger peak line shape on a bare and alkali modified Ni(100) is indicative that the presence of alkali adatoms induces CO decomposition on the Ni(100) surface. The fraction of dissociated CO increases with the amount of alkali adatoms present. At the same overlayer coverage the dissociation probability increases in the sequence Na, K, Cs. A comparison of the strength of the promoting effect on CO dissociation with the changes in the surface electron density in the presence of alkali adatoms has shown that at low overlayer coverages the electronic factor plays a major role in explaining the action of the surface modificators.     

Oxygen adsorption on an alkali metal-covered Ni(100) surface

The oxygen chemisorption on an alkali (Na, K, Cs) covered Ni(100) surface and its initial oxidation were studied by Auger and electron energy loss spectroscopy (ELS). It was found that in the presence of an alkali metal, the sticking coefficient S remains unity up to a given oxygen coverage of θ_O^cwhose value depends on the alkali overlayer concentration and the ionicity of the Ni-alkali metal bond. At a given oxygen coverage, the line shapes of Auger and loss spectra are almost the same for alkali-covered and clean Ni(100), which suggests that alkali metals cause no change in the character of the Ni-O bond. The effect of alkali metals is associated with increasing electron charge in the surface region, which facilitates oxygen chemisorption. The enhanced surface oxygen concentration in the presence of an alkali metal results in the formation of an oxide phase at lower oxygen exposures than is the case of clean Ni surfaces.     

The role of defects in the dissociative adsorption of CO on Ni(100)

We present a molecular beam study on the dissociative sticking coefficient for CO on a Ni(100) surface. In the range from 1.6 to 20 kcal/mol the observed dissociative sticking coefficient at a surface temperature of 500 K is 0.02 and independent of beam energy. Investigations on a sputter-damaged surface result in a sticking coefficient of 0.40, again independent of beam energy. The observed behavior can be explained by CO dissociation at defect sites.     

On the surface phonons due to oxygen adsorption on Ni (100)

Assuming that an oxygen adsorbed atom is linked to four Nickel atoms by central harmonic forces, the surface phonons energies are calculated in the case of P 2 × 2 and C 2 × 2 structures. The comparison to measured low-energy electron losses indicates that the Ni-O bond strength in the P 2 × 2 adsorbed structure is twice that of the C 2 × 2. This result gives some information concerning the potential shape near equilibrium position.     

Vibrational characterization of carbon monoxide adsorption on sulfur modified Ni(100) surfaces

Carbon monoxide adsorption has been studied on a series of presulfided Ni(100) surfaces using vibrational spectroscopy. The sulfided Ni(100) surfaces were characterized using Auger electron spectroscopy and low energy electron diffraction, binding states were isolated by heating CO-dosed surfaces to prescribed temperatures, corresponding to the desorption temperatures of the CO. Adsorption of CO on Ni(100) with a p(2 × 2) array of sulfur lead to CO stretching frequencies of 1740 and 1930 cm^?1 corresponding to desorption temperatures of 370 and 290 K, respectively. Adsorption of CO into the c(2 × 2)S structure resulted in a CO stretching frequency of 2115 cm^?1 and a desorption peak near 140 K. The binding sites on the p(2 × 2)S structure were interpreted as metal four-fold hollows and bridging sites. The high frequency state was interpreted as weak bonding into the four-fold hollow with back donation into the π¹ orbital on CO restricted by stearic hindrance due to adsorbed sulfur. Both the thermal desorption and vibrational results indicated that local CO-sulfur interactions are dominant on the presulfided Ni(100) surface in the coverage range studied.     

The local adsorption of pyridine on Si(100) a combined PES and XPD study

The chemical and geometrical properties of the system pyridine on Si(100) are investigated in a combined photoelectron spectroscopy (XPS) and photoelectron diffraction (XPD) study. Synchrotron radiation was applied to achieve high spectral resolution and a high surface sensitivity. Our studies were performed at saturation coverage of pyridine on silicon. The XPS and XPD results, including diffraction patterns for all spectral resolved components, clearly show that pyridine is reacting with silicon dimer atoms of the (2 × 1)-reconstructed surface. We propose a tetra-σ-bonded structure model and provide detailed structure parameters.     

The decomposition of methanol on Ni(100)

The decomposition of methanol and deuterated methanol on Ni(100) has been investigated using mainly IR ellipsometric spectroscopy but also thermal desorption and LEED measurements. The IR data support the existence of two intermediate states, here assigned to methoxy and formal moieties respectively. Starting from saturation conditions at ～170 K and warming in 10 K steps, the initial chemisorbed state, believed to be a methoxy species, begins to decompose to a carbonyl-containing species, believed to be a formal species, at ～230 K. At ～260 K, the second intermediate begins to transform to adsorbed carbon monoxide with concurrent desorption of hydrogen. Totally deuterated methanol behaves similarly but shows substantial isotope effects.     

ELS study on initial oxidation of Ni(100) surfaces

Electron energy loss spectra of clean and oxygen-covered Ni(100) surfaces were observed with concomitant measurements of LEED, work function change, and Auger peak height ratio O(KL_{2, 3}L_{2, 3})/Ni(L_{2, 3}VV). The observed electronic transitions are interpreted on the basis of primary election energy dependence, and of comparison with the loss spectrum for a UHV-cleaved NiO(100) surface and optical data of Ni. The observed loss peaks at 9.1, 14, and 19 eV in the clean surface spectrum are ascribed to the bulk plasmon of the 4s electrons, the surface plasmon, and the bulk plasmon of the coupled 3d + 4s electrons, respectively, and the weak but sharp peak at 33 eV is tentatively attributed to the localized many-body effect in the final state. Three oxygen-derived peaks at 6.0, 8.0, and 10.3 eV in the low oxygen exposure region (?4 L) are ascribed to the O 2p(e) → Ni 3d, O 2p(a₁) → Ni 3d, and O 2p → Ni 4s transitions, respectively. In the high oxygen exposure region (?50 L), the spectra become quite similar to that of the UHV-cleaved NiO(100) surface. The oxidation process consistent with LEED, Auger peak height ratio and work function change measurements is discussed.     

H2O adsorption on Ni(100): Evidence for oriented water dimers

H₂O adsorption on clean Ni(110) surfaces at T ≦ 150 K leads at coverages below $\text{θ ? 0.5}$ to the formation of chemisorbed water dimers, bound to the Ni substrate via both oxygen atoms. The linear hydrogen bond axis is oriented parallel to the [001] surface directions. With increasing H₂O coverage $\text{(θ ≧ 0.5)}$, the accumulation of further hydrogen bonded water molecules induces some modification of the dimer configuration, producing at $\text{θ ? 1}$ a two-dimensional hydrogen bonded network with a slightly distorted ice lattice structure and long range order.     

The energetics of mercury adsorption on Cu(100)

We have investigated the adsorption of mercury overlayers on Cu(100) by atom beam scattering, low energy electron diffraction and angle resolved photoemission. From our data we have calculated the isosteric heats in the adsorbed Hg layer on Cu(100) and compared these with results obtained for mercury on Fe(100), W(100) and Ni(100). We observe changes in the isosteric heat of adsorption that can be associated with the ordering of a c(2 × 2) Hg overlayer phase and the transition from a c(2 × 2) overlayer to a c(4 × 4) overlayer. The isosteric heat of adsorption is 0.50 ± 0.07 eV/atom (48 ± 7 kJ/mol) at zero coverage and reaches a maximum of 0.73 ± 0.04 eV/atom (70 ± 4 kJ/mol). From a combination of ABS and LEED, the structures of the two equilibrium ordered phases of Hg on Cu(100) have been identified, as well as the structures of several non-equilibrium phases.     

The decomposition of formic acid on Ni(100)

The decomposition of HCOOD was studied on Ni(100). Low temperature adsorption of HCOOD resulted in the desorption of D₂O, CO₂, CO, and H₂. The D₂O was evolved below room temperature. CO₂ and H₂ were evolved in coincident peaks at a temperature above that at which h₂ desorbed following H₂ adsorption and well above that for CO₂ desorption from CO₂ adsorption; CO desorbed primarily in a desorption limited step. The decomposition of formic acid on the clean surface was found to yield equal amounts of H₂, CO, and CO₂ within experimental error. The kinetics and mechanism of the decomposition of formic acid on Ni (110) and Ni(100) single crystal surfaces were compared. The reaction proceeded by the dehydration of formic acid to formic anhydride on both surfaces. The anhydride intermediate condensed into islands due to attractive dipole-dipole interactions. Within the islands the rate of the decomposition reaction to form CO₂ was given by:

$\text{Rate = 6 × 10}{}^{15}\text{exp{?[25,500 + ω(}\text{c}\text{c}{}_{\mathrm{sat}}\text{)]/RT} × c}$

, where c is the local surface concentration, c_sat is the saturation coverage for the particular crystal plane, and ω is the interaction potential. The interaction potential was determined to be 2.7 kcal/mole on Ni(110) and 1.4 kcal/mole on Ni(100); the difference observed was due to structural differences of the surfaces relating to the alignment of the dipole moments within the islands. These attractive interactions resulted in an autocatalytic reaction on Ni(110), whereas the interaction was not strong enough on Ni(100) to sustain the autocatalytic behavior. Formic acid decomposition oxidized the Ni(100) surface resulting in the formation of a stable surface oxide. The buildup of the oxide resulted in a change in the selectivity reducing the amount of CO formed. This trend indicated that on the oxide surface the decomposition proceeded via a formate intermediate as on Ni(110) O.