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The adsorption of oxygen on a Ba-covered Ni(110) surface has been investigated mainly by Auger electron spectroscopy (AES) and work function (WF) measurements. Low energy AES lines indicate that O interacts with Ba and Ni as well. Both Ba-O and Ni-O interactions take place simultaneously on the surface, progressively leading to BaO and NiO formation. Ba enhances the oxidation of the substrate due to the higher sticking coefficient of 0 on the Ba/Ni surface. The oxygen interacts with the nickel substrate even at high adsorbate coverage, incorporating under the Ba layer. A part of the Ba adatoms remains non-oxidized even at a high O exposure. Journal of Physics-Condensed Matter

Sustainable energy sources are of great importance, and will become even more important in the future. Gasification of biomass is an important process for utilization of biomass, as a renewable energy carrier, to produce fuels and chemicals. Density functional theory (DFT) calculations were used to investigate i) the effect of co-adsorption of water and CO on the Ni(111) catalysed water splitting reaction, ii) water adsorption and dissociation on Ni(111), Ni(100) and Ni(110) surfaces, as well as iii) formyl oxidation and dissociation, iv) hydrocarbon combustion and synthesis, and v) the water gas shift (WGS) reaction on these surfaces. The results show that the structures of an adsorbed water molecule and its splitting transition state are significantly changed by co-adsorption of a CO molecule on the Ni(111) surface. This leads to less exothermic reaction energy and larger activation barrier in the presence of CO which means that far fewer water molecules will dissociate in the presence of CO. For the adsorption and dissociation of water on different Ni surfaces, the binding energies for H2O and OH decrease in the order Ni(110) > Ni(100) > Ni(111), and the binding energies for O and H atoms decrease in the order Ni(100) > Ni(111) > Ni(110). In total, the complete water dissociation reaction rate decreases in the order Ni(110) > Ni(100) > Ni(111). The reaction rates for both formyl dissociation to CH + O and to CO + H decrease in the order Ni(110) > Ni(111) > Ni(100). However, the dissociation to CO + H is kinetically favoured. The oxidation of formyl has the lowest activation energy on the Ni(111) surface. For combustion and synthesis of hydrocarbons, the Ni(110) surface shows a better catalytic activity for hydrocarbon combustion compared to the other surfaces. Calculations show that Ni is a better catalyst for the combustion reaction compared to the hydrocarbon synthesis, where the reaction rate constants are small. It was found that the WGS reaction occurs mainly via the direct pathway with the CO + O → CO2 reaction as the rate limiting step on all three surfaces. The activation barrier obtained for this rate limiting step decreases in the order Ni(110) > Ni(111) > Ni(100). Thus, the WGS reaction is fastest on the Ni(100) surface if O species are present on the surfaces. However, the barrier for desorption of water (as the source of the O species) is lower than its dissociation reaction on the Ni(111) and Ni(100) surfaces, but not on the Ni(110) surface. Therefore the direct pathway on the Ni(110) surface will dominate and will be the rate limiting step at low H2O(g) pressures. The calculations also reveal that the WGS reaction does not primarily occur via the formate pathway, since this species is a stable intermediate on all surfaces. All reactions studied in this work support the Brønsted-Evans-Polanyi (BEP) principles.

In this work, we study the coadsorption of barium (Ba) and hydrogen (H) on Ni(100) surfaces using mainly thermal desorption spectroscopy (TDS) in correlation with Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and work function (WF) measurements. Two different processes have been used for the coadsorption experiments: (1) Ba deposition on hydrogenated Ni(110) and (2) H adsorption on Ba-covered Ni(110). In both cases, H gives two different TD energy states, β1 and β2. The first state is the same with that of H adsorption on clean Ni(110), pointing to a direct H Ni bonding, while the second, which is attributed to a H Ba interaction, shifts to higher temperature as Ba-coverage increases up to ΘBa=0.9 ML. This shift indicates a gradually developing attractive H Ba interaction leading to BaH2 formation. The formation of BaH2 does not depend on the sequence of Ba and H deposition. For ΘBa?0.3 ML, H adsorption on Ba/Ni(110) does not induce any WF change, as H2 does on alkali-covered surfaces. This means that the Ba/Ni(110) surface might be a stable negative H ion source. Applied Surface Science

A comparative study of the structure of free-standing parallel bundles of single-wall carbon nanotubes (SWCNTs), multiwalled carbon nanotubes (MWCNTs), and highly oriented pyrolytic graphite (HOPG) was achieved by means of transmission electron microscopy and electron energy loss spectroscopy analyses. In particular, the carbon K (1s) extended fine structure of SWCNTs is found to be characterized by an apparent contraction of the nearest neighbors distance. This contraction is interpreted here to originate from an asymmetric pair distribution function, mostly due to the high out-of-plane vibrational motion of the C atoms, as for the case of chemisorbed atoms on clean surfaces. In contrast, the MWCNTs did not exhibit any signature of such an anharmonic effect because of their more rigid structure. This indicates that the SWCNTs pair potential is significantly broader and its effect is much weaker than that experienced by the same C-C pair embedded in a multiwall nanotube.

The interaction of oxygen with evaporated Ni films on an Fe-doped SrTiO3 (100) substrate was investigated by means of LEED, AES and work function measurements (WF) at room temperature. The adsorption of oxygen takes place on the nickel overlayer firstly by chemisorption on nickel step sites, accompanied by a reduction of the WF, and secondly on terrace sites, followed by a WF increase. After the chemisorption phase, the oxidation of the nickel overlayer starts with NiO island formation followed by bulk NiO development, which is marked by a second WF reduction. The adsorption phase's of oxygen correspond closely to those of oxygen on single crystals of nickel. This indicates that the-character of the Ni predeposited layers on strontium titanate seems to be metallic. Journal of Physics-Condensed Matter

(T)his study involves the adsorption of Cs on Si(111)-(7 x 7) surfaces at room and elevated temperatures, with increasing coverage of Cs to saturation, in ultrahigh vacuum (UHV) conditions. The techniques of low energy electron diffraction (LEED), Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS) and work function (WF) measurements were utilized. A WF change at the minimum, found to be Deltaphichi= -3.2 eV at an approximate 0.23 ML Cs coverage, corresponds to half the saturation coverage. We propose that, at that coverage, the adatom dangling bonds of the Si(111)-(7 x 7) substrate are completely filled. At saturation coverage (0.47 ML), Cs forms a single saturation layer in a near-metallic surface state, where the atomic radius of the Cs adatoms is likely to be 2.2 x 10(-8) cm. The surface exhibits a high degree of disorder with 0.47 ML of adsorbed Cs, and it is likely that the adsorbate remains in cluster-like domains within the center of the disordered 7 x 7 unit cells, instead of evenly covering the Si surface. Saturation coverage of Cs on Si(111)-(7 x 7) surfaces exhibits a WF value 0.5 eV less than that of pure metallic Cs. Cesium adatoms form a stronger bond to the Si(111)(7 x 7) surface than to Ni(100), with a calculated energy of 1.64 eV/atom. The induced surface disorder and strong binding energy are evidence of the strong Cs-Si interaction, which most likely prevents the formation of a purely metallic Cs overlayer. Surface Review and Letters

It is demonstrated how the crystal field splitting (CFS) fine structure can be used to characterize a coadsor-bate system. We have applied K 2p x-ray absorption spectroscopy (XAS) to the c(2x2) CO/K/Ni(100) system. The CFS fine structure is shown to be sensitive to the the local atomic environment, the level of interaction, and the chemical state of the alkali atoms. From angle dependent XAS measurements, combined with x-ray photoelectron spectroscopy, a significant K-CO electrostatic adsorbate-adsorbate interaction is found, whereas the K-Ni interaction is substantially weaker. The present results provide evidence for a coad-sorbed overlayer best described in terms of the properties associated with an ionic (two-dimensional) crystal.

Ziel dieser Arbeit war der Aufbau eines Systems zur Summenfrequenzerzeugungs-(SFG-) spektroskopie bei Gasdrücken vom UHV bis zu einigen 100 mbar und die Messung der CO-Adsorption an unterschiedlichen Modellkatalysatoren. Die SFG-Spektroskopie vereinigt die Oberflächenselektivität nichtlinear-optischer Methoden mit der Adsorbatspezifik der Schwingungsspektroskopie und ist für Messungen bei hohen Gasdrücken geeignet. Die SFG-Messungen der CO-Adsorption an einem Ni(100)-Einkristall unter UHV-Bedingungen stehen in Übereinstimmung bekannten IRAS-Messungen. Auf einem Pt(111)-Einkristall wurden SFG-Untersuchungen in einem CO-Druckbereich vom UHV bis 200 mbar durchgeführt. Die bei einer Temperatur von 300 K und CO-Drücken oberhalb von 10 mbar gemessenen SFG- Spektren ergaben eine deutliche Abweichung von bisher veröffentlichten SFG- Messungen. Bei einer Probentemperatur von 160 K konnte der Einbau des CO in eine Eisschicht beobachtet werden. Am Modellkatalysatorsystem Pd/Al2O3 /NiAl(110) wurden die ersten SFG-Messungen an Nanopartikeln durchgeführt. Es wurde die CO-Adsorption auf Partikeln unterschiedlicher Größe in einem CO- Druckbereich von 10-7 mbar bis 200 mbar und bei unterschiedlichen Temperaturen untersucht. Die in den Spektren sichtbaren Signalmaxima konnten dem an die Partikel bridge- bzw. ontop-gebundenen CO zugeordnet werden. Bei hohen CO Drücken war ein von UHV-Messungen abweichendes Adsorptionsverhalten der Partikel zu beobachten., The goal of this thesis was the set-up of an apparatus for the performance of Sum-Frequency-Generation-(SFG-) Spectroscopy at gas pressures from UHV to several hundred mbar and the measurement of the adsorption of CO on several model catalysts. SFG spectroscopy combines the surface sensitivity of nonlinear-optical methods with the adsorbate specificity of vibration spectroscopy and can be applied also at high gas pressures. The SFG measurements of the CO adsorption on Ni(100) are in good agreement with known IRAS measurements. The CO adsorption on Pt(111) was studied in a pressure range from UHV to 200 mbar. The spectra obtained at a temperature of 300 K and a CO pressure above ten mbar were in disagreement with previously published SFG measurements. At a temperature of 160 K, the integration of the CO into an ice layer could be observed. The SFG spectra obtained from the model catalyst Pd/Al2O3/NiAl(110) were the first published measurements of that kind on nanoparticles. The CO adsorption on particles with different sizes was studied. The spectra obtained show two features which can be assigned to the bridge- and ontop-bound CO respectively. At higher CO pressures, an adsorption behaviour different from the UHV measurements was detected.

We present a density functional theory study of the first step of CH4 adsorption on the Ni(111) surface, dissociation into adsorbed CH3 and H. The rupture of the C-H bond occurs preferentially on top of a Ni atom, with a dissociation barrier of about 100 kJ/mol (including zero point corrections). The transition state involves considerable internal excitation of the molecule. The active C-H bond is both stretched to 1.6 Angstrom and tilted relative to the methyl group. A normal mode analysis shows that the reaction coordinate is mainly a C-H stretch, while the orientation of the C-H bond relative to the surface is responsible for the highest real mode. Alloying the surface with gold also affects the reactivity of the Ni atoms on adjacent surface sites. The dissociation barrier is increased by 16 and 38 kJ/mol for a Ni atom with one or two gold neighbors, respectively. We attribute these changes to a shift in the local density of d states at the nickel atoms in the neighborhood of gold. (C) 1996 American Institute of Physics.

A model is presented for describing the time dependence of the photoelectron and/or Auger signals during the overlayer formation in the case of Volmer Weber growth mode, i.e. 3D island formation. The impingement among clusters has been taken into account in the framework of Avrami's statistical approach. A first system has been considered in which nucleation occurs at a given number of preexisting sites randomly distributed throughout the whole surface. The results obtained by numerical computations indicate that particular conditions can be indeed realized for which the PES signal is chiefly related to the kinetics of the surface fraction that is covered by islands. A more involved system has been also modeled where nucleation does not occur at preexisting sites but through the formation of stable dimers. Under this circumstance, Avrami's treatment of island impingement can be still retained although now a system of integral differential equations has to be solved to get the kinetics. Such a modelling should be suitable for describing the metallic film growth studied by PES.

The interaction of Cs and O2 on NiO(100) has been studied by AES, TDS and WF measurements at room temperature. A correlation of the experimental results shows that Cs is initially adsorbed as ionized adatoms which interact strongly with the oxygen of the outmost layer of NiO. The initial dipole moment of Cs on NiO is 8 D, while the sticking coefficient found to be approximately 0.7. With increasing coverage Cs remains disordered and forms islands on the surface. Oxygen adsorption on Cs-saturated NiO takes place on top of Cs. The deposited oxygen interacts with Cs and increases the binding energy of the latter. Surface Science

It is demonstrated how the crystal field splitting (CFS) fine structure can be used to characterize a coadsorbate system. We have applied K 2p x-ray absorption spectroscopy (XAS) to the c(2X2) CO/K/Ni(100) system. The CFS fine structure is shown to be sensitive to the the local atomic environment, the level of interaction, and the chemical state of the alkali atoms. From angle dependent XAS measurements, combined with x-ray photoelectron spectroscopy, a significant K-CO electrostatic adsorbate-adsorbate interaction is found, whereas the K-Ni interaction is substantially weaker. The present results provide evidence for a coadsorbed overlayer best described in terms of the properties associated with an ionic (two-dimensional) crystal.

First-principles density-functional calculations of hydrogen adsorption on the Ni (001) surface have been performed in order to get a better understanding of adsorption and diffusion of hydrogen on metal surfaces. We find good agreement with experiments for the adsorption energy, binding distance, and barrier height for diffusion at room temperature, A model potential is fitted to the first-principles data points using the simulated annealing technique and the hydrogen band structure is derived by solving the three-dimensional Schrodinger equation, We find vibrational excitation energies slightly too high, with about 10%, compared with experiments and very narrow hydrogen bands. The experimentally observed absence of a pronounced isotope effect for hydrogen diffusion al low temperatures is discussed in terms of tunneling in a static three-dimensional potential.

We describe the use of stimulated Raman pumping in a molecular beam to perform quantum state resolved gas-surface reactivity measurements for molecules prepared in totally symmetric vibrational states. Vibrational states of homonuclear diatomics as well as totally symmetric vibrations of polyatomic molecules cannot be prepared by direct infrared excitation but are accessible through stimulated Raman pumping by two laser fields when the difference between the incident laser frequencies matches the vibration. We generate a suitable resonant pair of high-energy pump and Stokes laser beams in an injection seeded Raman amplifier filled with the sample gas and equipped with internal gas recirculation. The ability to partially saturate the Raman pumping process in the molecular beam is used to quantify the fraction of vibrationally excited molecules in the irradiated volume, which is needed for quantitative reactivity measurements. We illustrate the method with state resolved reactivity measurements for CH4, prepared in its symmetric C-H stretch vibration on a Ni(100) single crystal surface.

In this thesis, steric effects in the dissociative chemisorption of quantum state-prepared methane on single crystal surfaces of nickel (Ni(100) and Ni(110)) are detected and quantified for the first time. Exploiting a new, continuous-wave, high-power, single-mode infrared optical parametric oscillator, I produced an intense, quantum state-prepared molecular beam by rapid adiabatic passage. During the infrared excitation of the antisymmetric (ν3) stretch of CH4 or the C-H (ν1) stretch of CD3H by linearly polarized radiation, the angular momentum and vibrational transition dipole moment of methane is aligned in the laboratory frame. The excited, aligned molecular beam is used to probe the stereodynamics of the chemisorption reaction of vibrationally excited methane. For the reaction on Ni(100), an increase in state-prepared methane reactivity of nearly 60% is observed when the laser polarization direction is changed from normal to parallel to the surface. The dependence of the alignment effect on the rotational branch used for excitation (P-, Q-, or R-branch) indicates that alignment of the vibrational dipole moment rather than the angular momentum is responsible for the alignment dependent reactivity. Dephasing of the initially prepared alignment due to hyperfine coupling is observed to be on the timescale of 5-15 microseconds which does not preclude the study of alignment dependent reactivity in our experimental setup. Reactivity decreased monotonically from parallel to perpendicular alignment for both methane isotopologues studied. The alignment effect is shown to be independent of incident velocity for CH4(ν3) and decreases with increasing velocity of CD3H(ν1). For the Ni(110) surface, which consists of parallel rows of closely spaced Ni atoms separated by one-layer deep troughs, I probed if the reactivity depends on the methane alignment relative to the direction of the surface rows. The CH4(ν3) reactivity increases a factor of two when the laser polarization direction is changed from normal to parallel to the surface. Alignment of the vibration perpendicular to the surface rows produced a ∼10% higher reactivity than alignment parallel to the surface rows. Steric effects in a chemical reaction reveal detailed information about the reactive potential energy surface, which makes experimental studies of stereochemistry a powerful probe of microscopic chemical dynamics. The results in this thesis demonstrate and quantify specific steric requirements for this benchmark gas-surface reaction and will serve as a stringent test of multi-dimensional dynamics calculations.

Ni-based catalysts are prone to deactivation (poisoning) of their active surface sites by sulphur and carbon species contained in the gas fed to the reactor. This study focuses on Ni/Al2O3-based catalyst samples which had allegedly been deactivated by sulphur poisoning. The samples had been collected from a 10 kW methanation reactor fed with producer gas from the industrial biomass gasifier in Gussing (Austria). The samples allowed intensive investigation using several analytical tools to identify the chemical nature (inorganic, organic) of the S-poisoning species. Temperature-programmed oxidation (TPO) allowed quantification of the sulphur content, but not the identification of the S species responsible. S 2p X-ray photoelectron spectroscopy (XPS) pointed at the presence of sulphide and sulphate, but the data were too noisy to reach more specific conclusions. Ni K-edge X-ray absorption spectroscopy (XAS) in the fine structure (EXAFS) region suggested the presence of elemental or thiophenic sulphur, but the contribution was masked heavily by other backscattering paths. Only S Kedge analysis in the near edge (XANES) region showed unambiguously that the catalyst could not have been deactivated by inorganic H2S only. This conclusion is supported by S K-edge XANES results with model catalysts which had either been poisoned by H2S or thiophene (C4H4S), representing a cyclic, aromatic S compound. Short-term H2S poisoning in the absence of air led to a white-line position characteristic for sulphide (2470 eV), whereas with thiophene the white-line position started at 3 eV higher energy. The XANES signatures changed with the catalyst samples after contacting air, but remained unique for each of the two S-poison types studied here. (C) 2009 Elsevier B.V. All rights reserved.