The adsorption and reaction of 2-butanol on clean and oxygen-covered Pd(100)

The surface chemistry of 2-butanol is explored on clean Pd(100), c(2 × 2)-O/Pd(100) and p(2 × 2)-O/Pd(100) surfaces by means of temperature-programmed desorption, reflection–absorption infrared and X-ray photoelectron spectroscopies. 2-Butanol adsorbs molecularly on clean and oxygen-covered Pd(100) below ～ 190 K, but then appears to react to form 2-butoxide species at ～ 200 K. Both 2-butanone and 2-butanol desorb from the clean surface at ～ 226 K, by β-hydride elimination from the 2-butoxide species and rehydrogenation of the 2-butoxide, respectively. In contrast, almost exclusively 2-butanone is formed on oxygen-covered surfaces. Butanone desorbs at ～ 195 K and ～ 260 K from c(2 × 2)-O/Pd(100) with the 195 K peak being the most intense. However, on p(2 × 2)-O/Pd(100), 2-butanone desorbs at ～ 195 K and ～ 295 K, and the latter peak is the most intense. The ～ 195 K, 2-butanone state is proposed to occur due to abstraction by adsorbed atomic oxygen and the change in relative intensity of these features is ascribed to the lower ability of surface hydroxyl groups to facilitate β-hydride elimination on oxygen-covered surfaces. Further heating results in the formation of hydrogen and carbon monoxide and leaves a small amount of carbon deposited on the surface.     

The adsorption and reaction of vinyl acetate on Au/Pd(100) alloy surfaces

The surface chemistry of vinyl acetate monomer (VAM) is studied on Au/Pd(100) alloys as a function of alloy composition using temperature-programmed desorption and reflection–adsorption infrared spectroscopy. VAM adsorbs weakly on isolated palladium sites on the alloy with a heat of adsorption of ~ 55 kJ/mol, with the plane of the VAM adsorbed close to parallel to the surface. The majority of the VAM adsorbed on isolated sites desorbs molecularly with only a small portion decomposing. At lower gold coverages (below ~ 0.5 ML of gold), where palladium–palladium bridge sites are present, VAM binds to the surface in a distorted geometry via a rehybridized vinyl group. A larger proportion of this VAM decomposes and this reaction is initiated by CO bond scission in the VAM to form adsorbed acetate and vinyl species. The implication of this surface chemistry for VAM synthesis on Au/Pd(100) alloys is discussed.     

The adsorption of acetic acid on Au/Pd(1 1 1) alloy surfaces

The adsorption of acetic acid is studied as a function of gold content by temperature-programmed desorption and reflection-absorption infrared spectroscopy on Au/Pd(1 1 1) alloys formed by depositing 5 ML of gold onto a Pd(1 1 1) surface and heating to various temperatures. For mole fractions of gold greater than ∼0.5, acetic acid adsorbs molecularly and desorbs intact with an activation energy of ∼52 kJ/mol. This acetic acid is present as catemers, where the nature of the catemer is found to depend on gold concentration. When the relative gold concentration is less than ∼0.33, adsorption of acetic acid at 80 K and heating to ∼207 K forms η¹-acetate species on the surface. On further heating, these can either thermally decompose to eventually evolve hydrogen, water and oxides of carbon, or form η²-acetate species, where the coverage of reactively formed η²-acetate species increases with decreasing gold concentration in the near surface region.     

Oxygen adsorption on clean Mo (100) surfaces

Oxygen adsorption on clean Mo (100) surfaces has been studied by LEED, AES, work function changes and energy loss spectroscopy. At room temperature, the oxygen uptake as determined by AES is linear up to one third of the saturation value. Data obtained with CO adsorption have been used to determine the oxygen coverage. With increasing oxygen exposure LEED shows three stages: a c (2 × 2) phase growing simultaneously with a (6 × 2) structure, a stage with (110) microfacets covered by two-dimensional structures and finally a p (3×1) structure together with a p (1×1) structure, probably due to an oxide phase. Even in the low temperature range (370–500 K) remarkable effects are observed: adsorption at 370 K produces a disordered c (4×4) structure which is followed by a (√5 × √5)?R 26° 33 structure. The same occurs when the inital c (2 × 2) structure formed at 295 K is heated above 370 K. Measurements of the work function indicate a minimum at the end of the c (2×2) structure, then a rapid increase and at saturation a value of about 1.5 V above that of the clean surface. Energy loss spectroscopy measurements point to an increase of the surface plasmon energy during the faceting stage. New transitions are observed which are due to new electronic levels induced by the adsorption. They are comparable with photoemission results on W and Mo.     

Oxygen adsorption on stepped Pd(100) surfaces

We use scanning tunneling microscopy (STM) and high-resolution core-level spectroscopy (XPS) measurements to study the initial oxidation of vicinal Pd(100) surfaces exhibiting close-packed (111) steps. The XPS data analysis is supported by detailed surface-core level shift calculations based on density-functional theory. Both STM images and the XPS spectra are found to be perfectly consistent with a characteristic zigzag O decoration of the Pd steps predicted by a preceding cluster-expansion based theoretical study [Y. Zhang and K. Reuter, Chem. Phys. Lett. 465, 303 (2008)]. Continued oxygen uptake leads to the additional stabilization of a p(2 × 2)-O overlayer on the Pd(100) terraces, and ultimately to step bunching with the resulting large Pd(100) terraces covered by the PdO(101) surface oxide.     

The adsorption of ethylene on Au/Pd(1 1 1) alloy surfaces

The adsorption of ethylene on gold-palladium alloys formed on a Pd(1 1 1) surface is investigated using a combination of temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). Various alloy compositions are obtained by depositing four monolayers of gold on a clean Pd(1 1 1) surface and annealing to various temperatures. For gold coverages greater than ∼0.7, ethylene adsorbs primarily on gold sites, desorbing with an activation energy of less than 55 kJ/mol. At gold coverages between ∼0.5 and ∼0.7, ethylene is detected on palladium sites in a π-bonded configuration (with a σ-π parameter of ∼0.1) desorbing with an activation energy of between ∼57 and 62 kJ/mol. Further reducing the gold coverage leads to an almost linear increase in the desorption activation energy of ethylene with increasing palladium content until it eventually reaches a value of ∼76 kJ/mol found for ethylene on clean Pd(1 1 1). A corresponding increase in the σ-π parameter is also found as the gold coverage decreases reaching a value of ∼0.8, assigned to di-σ-bonded ethylene as found on clean Pd(1 1 1).     

Adsorption of CH3Cl on clean and Cl-dosed Pd(100) surfaces

The adsorption of methyl chloride on a Pd(100) surface has been investigated by ultraviolet photoelectron spectroscopy (UPS), electron energy loss spectroscopy (in the electronic range, EELS), temperature-programmed desorption (TPD) and work function change. CH₃Cl adsorbs with high sticking probability at 80–100 K. UPS and TDS spectra suggest that the adsorption of CH₃Cl is molecular at 100 K, with a little distortion of the corresponding gas-phase molecular electronic structure. No dissociation of CH₃Cl was observed even up to 550 K. By means of TPD, we distinguished two adsorption states with desorption energies of 46.9 and 33.4 kJ/mol. The formation of a condensed layer at 105–110 K was also observed. Adsorption of CH₃Cl caused a significant work function decrease, Δ = −0.91 eV, indicating a dipole with positive end pointed away from the surface. The effects of electronegative additives, preadsorbed Cl and O were also examined. Preadsorbed Cl caused a slight destabilization of adsorbed CH₃Cl at lower concentration, prevented the adsorption of CH₃Cl at higher concentration and facilitated the formation of a condensed layer. No such effect was experienced in the presence of preadsorbed O.     

Photoemission studies of clean and oxygen-covered Pt 6(111) × (100)

Employing the enhanced sensitivity obtained by using synchrotron radiation near the Cooper minimum for the 5d valence electrons, we have located the oxygen 2p and 2s levels for oxygen chemisorbed on a Pt 6(111) × (100) crystal. We find the oxygen 2p level located ?6 eV with a FWHM of 3 eV and the 2s at ?21.6 eV. A factor of four difference in saturation coverage is measured between temperatures of 300 and 120 K, but the position and width of the 2p level is independent of temperature. We observe also the 1b₁ orbital of weakly adsorbed H₂O molecules, which has pure O 2p parentage; from the intensity of this orbital, we are able to suggest why it is difficult to observe the oxygen 2p signal at low photon energies. In addition, we note a strong preferential attenuation in the Pt states near E_f for the adsorbed H₂O in spite of the weak nature of the bond.     

A procedure for clean deposition of100Pd on surfaces

For a program of surface and interface experiments with the PAC isotope¹⁰⁰Pd, a procedure has been developed to chemically separate¹⁰⁰Pd from irradiated rhodium and deposit it on surfaces by evaporation under UHV conditions. First results have been obtained for¹⁰⁰Pd on an Ni(111) surface.     

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.     

The adsorption of Xe and CO on Au(100)

The adsorption of Xe and CO on Au(100) has been studied by LEED, Auger electron spectroscopy, electron energy loss spectroscopy (EELS) and surface potential measurements. The physical adsorption of xenon showed successive stages preceding the completion of a monolayer. The heat of adsorption was 22 (±2) kJ mol^?1 and the maximum surface potential was 0.45 V. Carbon monoxide gave a surface potential of 0.85 V at the highest coverage reached. The heat of adsorption showed a continuous fall from an initial value of 58 (±3) kJ mol^?1 as the coverage increased. Ordered adsorption structures were not observed in LEED for either Xe or CO. The EEL spectrum of clean Au(100) agreed well with spectra of polycrystalline gold. New loss features observed with adsorbed Xe and CO are discussed.     

The superstructures of the clean Pt(100) and Ir(100) surfaces

The clean and reconstructed surfaces of Pt(100) and Ir(100) were investigated by low energy electron diffraction (LEED). It is shown that two superstructures can be observed in the case of platinum. The structure Pt(100)-hex, which is commonly called Pt(100)-(5 × 20), transforms to Pt(100)-hex-R0.7° above 1100 K. It is shown that this stable phase differs from the first one by a slight rotation of the hexagonal surface layer by 0.7°. For Ir(100) only the well known (1 × 5) superstructure is observed without any rotation of the outer layer. The rotation angle of 0.7° for platinum and the stability of the unrelated structure for iridium can be interpreted by simple calculations of the coordination of surface atoms with those of the second layer. The method assumes that the surface layer is of ideal hexagonal structure in the case of platinum and nearly hexagonal in the case of iridium. The results are in good agreement with the experiment.     

Reflectometric study of surface states and oxygen adsorption on clean Si(100) and (110) surfaces

External differential reflection measurements were carried out on clean Si(100) and (110) surfaces in the photon energy range of 1.0 to 3.0 eV at 300 and 80 K. The results for Si(100) at 300 K showed two peaks in the joint density of states curve, which sharpened at 80 K. One peak at 3.0 ± 0.2 eV can be attributed to optical transitions from a filled surface states band near the top of the valence band to empty bulk conduction band levels. The other peak at 1.60 ± 0.05 eV may be attributed to transitions to an empty surface states band in the energy gap. This result favours the asymmetric dimer model for the Si(100) surface. For the (110) surface at 300 K only one peak was found at 3.0 ± 0.2 eV. At 80 K the peak height diminished by a factor of two. Oxygen adsorption in the submonolayer region on the clean Si(100) surface appeared to proceed in a similar way as on the Si(111) 7 × 7 surface. For the Si(110) surface the kinetics of the adsorption process at 80 K deviated clearly. The binding state of oxygen on this surface at 80 K appeared to be different from that on the same surface at 300 K.     

Electron energy loss spectroscopy measurement of surface vibration dispersion on clean and oxygen-covered Ni(100)

Inelastic electron scattering has been carried out at 300 K on Ni(100) and at 150 K on p(2 × 2) and c(2 × 2) oxygen overlayers adsorbed on Ni(100). Impact energies ranged from 4 to 300 eV in order to measure the dispersion curves of surface vibrations throughout the two-dimensional Brillouin zone in the [110] direction. The Rayleigh mode has been observed in all cases. On O-coved surfaces a surface resonance and two vibrations of different polarizations associated with oxygen motion have been detected. The polarizations of the detected modes and the origin of the resonances, arising from the folding of the Brillouin zone due to the adsorbate, have been analysed with the help of symmetry considerations and the EELS selection rules. The O-coverage dependence of the Rayleigh mode frequency suggests a continuous outwards expansion of the first Ni plane starting from a contracted clean surface. The O-dispersion data are consistent with an O layer distant from the first Ni plane by ～ 0.9 Å in both investigated overlayers.     

The adsorption and reaction of CO and O2 on PdAu alloy wires

Temperature programmed desorption (TPD) of CO and O₂ on PdAu alloy wires has been studied. The heat of adsorption, sticking coefficient and maximum coverage of CO were recorded for Pd, 83 Pd 17 Au, 60 Pd 40 Au. For Pd and Pd-rich alloys the heat of adsorption remained fairly constant but the maximum coverage fell markedly from 0.42 for Pd to less than 0.05 for bulk palladium atom fraction X_B^pd ? 0.83. The heat of adsorption, sticking coefficient and maximum coverage of O₂ were investigated for pure Pd. A very limited adsorption was recorded on 83 Pd 17 Au and none on the more Au-rich alloys. The adsorption data are used to discuss the CO + O₂ reaction. Activation energy and frequency factor are estimated on Pd, for the TPD conditions used here. Earlier rate constants (0.2 Torr, 150°C) for CO + O₂ on PdAu as a function of Au content correlates with the maximum coverage of chemisorbed CO, which in turn is correlated with the probability of finding a Pd_9±1 ensemble in the surface. Modern results on the d-band structure of the PdAu alloys suggest that the Pd₉ ensemble, i.e. a surface Pd atom without an Au atom in its coordination shell, would tend to optimise both the donor and acceptor actions of the Pd atoms involved in chemisorbing CO.     

The surface chemistry of H2O on clean and oxygen-covered Cu(110)

Adsorption of water at 100 K. on clean and oxygen-covered Cu(110) has been studied using UPS, TDS, Δφ and LEED measurements. The results indicate that two-dimensional hydrogenbonded islands are formed on the clean surface. The long-range order in these islands is in registry with the substrate lattice and gives rise to a c(2×2) LEED pattern. Upon the formation of multilayer ice, the ordering disappears. The presence of oxygen on the surface disrupts the hydrogen bonding, and composite oxygen-water layers are formed. A model of the arrangement of oxygen atoms and water molecules is presented, based upon the LEED observations for these layers and an estimate of the relative oxygen and water coverages. The intensity variation of a thermal desorption peak at 290 K, attributed to adsorbed OH species, with oxygen coverage is in accordance with this model. For low oxygen coverages, the TDS and Δφ results indicate that small oxygen-water clusters with an enhanced ratio of water molecules per adsorbed oxygen atom are present.     

The adsorption of chlorine on clean germanium surfaces

The adsorption of chlorine on clean germanium surfaces was studied in an all-glass system by crushing a single crystal disc of germanium in the presence of the gas and following the pressure changes. Surface areas of the powders were measured by the B.E.T. method using krypton at liquid nitrogen temperature. The experimental techniques evolved for working with chlorine are described. Chlorine was found to adsorb on germanium to monolayer coverage at room temperature and at −78°C. A slow desorption of a fraction of the adsorbed chlorine followed the rapid adsorption. The nature of the adsorption bond and a possible explanation for the slow desorption are discussed.     

Selective adsorption of 4He on clean LiF (001) surfaces

The selective adsorption of ⁴He on in-situ cleaved LiF surfaces has been studied under improved resolution. The main results are as follows: (1) There are four bound states in the surface potential well, at energies of ?5.8, ?2.2, ?0.6 and ?0.1 meV. The lowest three levels were reported previously. (2) Most of the structure previously designated as “fine structure” is due either to transitions to these four levels via various small reciprocal lattice vectors or to the opening of diffraction channels. (3) The transitions involving the [01] and [01?] reciprocal lattice vectors (i.e., the ones nearly perpendicular to the incident wave vector) are strong; as much as 85% of the specular intensity may be removed. Transitions via the other small reciprocal lattice vectors are much weaker. (4) The widths of the lines are consistent with the velocity distribution, which has a half-width of about 2%. (5) The observed energies agree fairly well with those calculated by Tsuchida for a zeta-function potential, but are not consistent with a Morse potential.     

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.