Vibrational excitations of p(2×2) oxygen and c(2×2) hydrogen on Pd(100)

The p(2×2) oxygen and c(2×2) hydrogen structures on Pd(100) have been investigated by angle-resolved high-resolution electron energy loss spectroscopy. Dipole excited vibrational modes are observed at 44 and 64 meV for the oxygen and hydrogen structures respectively and are interpreted to correspond to atomic adsorption in the hollow site.     

Vibrational excitations and structure of oxygen on Fe(110)

Vibrational spectra of oxygen adsorbed on a clean Fe(110) surface at 300 K have been measured by high resolution electron energy loss spectroscopy (EELS). In the exposure range up to 6 L a single loss around 500 cm^-1 is observed which is interpreted to be due to the stretching vibration of atomic oxygen adsorbed at a 2-fold long-bridge site. Above exposures of 6 L a second loss around 400 cm^-1 appears which is attributed to the formation of a disordered oxide layer. Subsequent heating of the sample leads to the observation of a (5×12) LEED pattern which is explained by a mixed oxygen-iron surface structure which is nearly identical to the (111) face of bulk FeO. A weak loss around 910 cm^-1 appears after oxygen exposures at elevated sample temperatures. This loss is attributed to the formation of bulk oxide.     

Vibrational states of the hydrogen isotopes on Pd(111)

The ground and excited vibrational states for the three hydrogen isotopes on the Pd(111) surface have been calculated. Notable features of these states are the high degree of anharmonicity, which is most prominently seen in the weak isotopic dependence of the parallel vibrational transition, and the narrow bandwidths of these states, which imply that atomic hydrogen is localized on a particular surface site on time scales of 100 ps or more. Experiments to resolve ambiguities concerning the present system are suggested.     

Vibrational motion of hydrogen atoms chemisorbed on Ni(100)

The vibrational motion of hydrogen and deuterium atoms adsorbed, at different coverages, on the Ni(100) surface has been investigated using high-resolution electron-energy-loss spectroscopy. In specular scattering the vibrational spectra show only one prominent fundamental excitation, which can be reconciled with the perpendicular motion of a localized adatom in a fairly harmonic potential well. Substantial coverage induced vibrational energy shifts are observed. These are caused by static and dynamic adsorbate-adsorbate interaction effects, as revealed by spectra for a substitutionally disordered p(1 × 1)(H + D) structure and a model calculation based upon the effective medium theory.     

Adsorption of hydrogen and deuterium on potassium-promoted Pd(100) surfaces

The adsorption of H₂ and D₂ has been studied on clean and K-promoted Pd(100) surfaces using thermal desorption, work function changes, ultraviolet photoelectron and Auger spectroscopy. The potassium adlayer significantly lowers the sticking coefficient (from 0.6 to 0.06 at θ_k = 0.2), and the uptake of hydrogen, but increases the desorption energy for H₂ desorption. Calculation showed that each potassium adatom blocks approximately 4–5 adsorption sites for H₂ adsorption. Atomization of hydrogen led to an increase of hydrogen uptake. The adsorption of potassium on the H-covered surface caused a significant decrease in the amount of hydrogen adsorbed on the surface (as indicated by less desorbing hydrogen below 500 K) and promoted the dissolution of H atoms into the bulk of Pd. The dissolved hydrogen was released only above 600–650 K. In the interpetation of the results the extended charge transfer from K-dosed Pd to the adsorbed H atoms and the direct interaction between adsorbed H and K adatoms are taken into account.     

Magnetic properties of (Co Pd ) superstructures on Pd(100) and Pd(111)

The magnetic properties of (Co_nPd_m)_r superstructures on Pd(100) and Pd(111) are evaluated using the fully-relativistic spin-polarized screened Korringa-Kohn-Rostoker method. It is found that only in the case of a Pd(111) substrate such superstructures exhibit perpendicular magnetism, while on a Pd(100) substrate the magnetization is oriented in-plane. Also investigated is the effect of interdiffusion in repeated superstructures. By using the inhomogeneous coherent potential approximation (CPA) for layered systems the effect of ordering into (repeated) superstructures can be described in an ab-initio-like manner. It is found that already small amounts of interdiffusion can be decisive for the actual value of the magnetic anisotropy energy. Received 3 November 1999 and Received in final form 18 January 2000     

Uptake,transport, and release of hydrogen from Pd(100)

Understanding the interactions of hydrogen atoms with the surface and the subsurface regions of Pd is critical to the development of advanced energy technologies for hydrogen storage, hydrogen separations, and catalytic conversion processes. While many of the physical and chemical characteristics of the H₂–Pd system are known, the kinetics and thermodynamics of H atom absorption into the bulk, transport from the bulk back to the surface, and desorption from the surface remain unclear. In this work, the kinetics of D₂ release from Pd following exposure to D₂ over a range of pressures and temperatures were measured using temperature programmed desorption. To accurately simulate the kinetics of D₂ release, the continuum-based model of Mavrikakis, et al. (J. Chem. Phys 105, 8398, 1996) was extended to include activation barriers for desorption and transport that depend on D atom concentration in the surface, subsurface and bulk regions of the Pd. The use of concentration dependent barriers improves the ability of the model to predict the hydrogen uptake and release kinetics observed across temperatures ranging from 100 to 600 K.     

The adsorption of hydrogen and the reaction of hydrogen with oxygen on Pt (100)

The adsorption of hydrogen on Pt (100) was investigated by utilizing LEED, Auger electron spectroscopy and flash desorption mass spectrometry. No new LEED structures were found during the adsorption of hydrogen. One desorption peak was detected by flash desorption with a desorption maximum at 160 °C. Quantitative evaluation of the flash desorption spectra yields a saturation coverage of 4.6 × 10¹⁴ atoms/cm² at room temperature with an initial sticking probability of 0.17. Second order desorption kinetics was observed and a desorption energy of 15–16 kcal/mole has been deduced. The shapes of the flash desorption spectra are discussed in terms of lateral interactions in the adsorbate and of the existence of two substates at the surface. The reaction between hydrogen and oxygen on Pt (100) has been investigated by monitoring the reaction product H₂O in a mass spectrometer. The temperature dependence of the reaction proved to be complex and different reaction mechanisms might be dominant at different temperatures. Oxygen excess in the gas phase inhibits the reaction by blocking reactive surface sites. At least two adsorption states of H₂O have to be considered on Pt (100). Desorption from the prevailing low energy state occurs below room temperature. Flash desorption spectra of strongly bound H₂O coadsorbed with hydrogen and oxygen have been obtained with desorption maxima at 190 °C and 340 °C.     

The hydrogenation of CN on Pd(111) and Pd(100)

Adsorbed CN may be produced on Pd(111) and Pd(100) surfaces at RT by dissociative adsorption of cyanogen. HREELS measurements show that adsorbed CN forms adsorbed HCN or DCN on these Pd surfaces by reaction with H adsorbed from the residual gas or by dosing with H₂ or D₂. The reaction temperature is slighly lower for Pd(100) than for Pd(111), and the range of temperatures over which the reaction takes place much narrower. The reaction occurs on a time scale easily monitored with HREELS.     

LEED intensities from clean and hydrogen covered Ni(100) and Pd(111) surfaces

Hydrogen adsorbs on Ni(100) and Pd(111) surfaces without the formation of additional diffraction spots in the LEED patterns. Measurements of LEED intensities revealed that adsorbed hydrogen layers cause considerable changes even in such cases where displacements of surface atoms (“reconstructive adsorption”) may be excluded. After hydrogen adsorption on Ni(100) the intensities of Bragg beams are uniformly lowered whereas the background intensity increases which is attributed to the formation of a disordered adsorbed layer. With Pd(111) adsorbed hydrogen causes a slight decrease of the background intensity and characteristic modifications of the intensity/voltage curve of the (0,0) beam, suggesting the formation of an ordered 1 × 1 structure. In the latter case energy shifts of the primary Bragg maxima were observed and are interpreted as being caused by an expansion of the layer spacing in the surface region by about 2% owing the partial dissolution of the hydrogen.     

A helium diffraction study of the p(2 × 2) phase of oxygen on Pd(100)

The p (2 × 2) ordered structure of oxygen on Pd(100) was investigated with He diffraction, Best-fit intensity calculations of the very rich diffraction spectra yield a corrugation amplitude of 0.68 Å for the oxygen adatom hills, and require the oxygens to be located in the fourfold coordinated hollows. Surface charge-density calculations were performed in analogy to the well-studied O/Ni(100) system by superposition of atomic charge densities, taking into account the oxygen adatoms in a singly negative state. The corrugation amplitude measured was reproduced with the oxygens ～ 0.9 Å above the topmost palladium layer.     

Pd layers on a W(100) surface

Pd films from zero to several monolayers in thickness on a W(100) surface are studied by AES, LEED, work function change (A0) measurements and TDS. Similar to other metals on W, the adsorption and annealing behaviour differs drastically from that on the W(110) surface but resembles the behaviour of other metals on the W(100) and Mo(100) surface.     

Adsorbate band formation: The chemisorption of CO on Pd (100)

In ordered overlayers of adsorbed gases on metal surfaces, high coverage situations can lead to overlap between orbitals on adjacent species and hence to adsorbate band formation. We conclusively demonstrate the existence of this effect in chemisorption by examining the dispersion of the 4α level in the u.v. photoelectron spectrum of the CO/Pd (100) system. The results agree well with a first principles extended tight binding calculation of the two-dimensional band structure.     

Cobalt oxide nanolayers on Pd(100): The thickness-dependent structural evolution

The growth of interface-stabilized cobalt oxide (CoO_x) nanolayers on Pd(100) has been investigated and their structures are reported as a function of coverage. Several different phases have been observed by LEED and STM experiments, and they have been characterized spectroscopically by photoemission and X-ray absorption. The data indicate that in the low coverage regime (up to Θ_Co ≈ 2–3 ML) rock-salt CoO type phases are formed (defective in the single layer regime, and stoichiometric in multilayers) with (100) or (111) termination. At higher coverage (Θ_Co ≈ 10–20 ML) spinel Co₃O₄(111) and CoO(100) layers have been detected, in ratios dependent on the preparation conditions. The observed structures are discussed in relation to similar structures reported recently for CoO_x films on Ir(100) [W. Meyer et al., J. Phys.: Condens. Matter 20 (2008) 265011].     

Vibrational spectra of coadsorbed CO and H on Ni(100) and Ni(111)

High resolution electron energy loss spectra are reported for coadsorbed hydrogen and carbon monoxide on Ni(100) and Ni(111). On neither surface was there any evidence for either C-H or O-H bonds. On Ni(111) one CO stretching frequency is observed and it does not change significantly in the presence of coadsorbed hydrogen. This is consistent with segregation of CO and H into islands. On Ni(100) the situation is much different; one frequency is observed in the absence of H(a) while three CO stretching frequencies are observed for the coadsorbed layers. These are attributed to on-top, two-fold bridged and four-fold binding of CO to the Ni(100) surface. These results demonstrate significant structure sensitivity for the organization of these coadsorbed species.