Oxygen Overlayer Research Articles

Density-Functional Study of Chemisorption of Oxygen on Al (111)

ABSTRACTWe present ab initio studies of oxygen adsorption on Al (111) using a density-functional local-orbital method. Our results indicate that, for very low oxygen exposures, the oxygen atoms are incorporated in the first Al (111) layer. For exposures corresponding to approximately a monolayer of oxygen, an ordered oxygen overlayer with the O atoms sitting on the 3-fold sites was seen. These results are consistent with EXAFS experiments. We have also studied clean Al surfaces and have analyzed their surface relaxations.

Electronic structure of the (1 x 1) oxygen overlayer on TiC(111): Angle-resolved photoemission study.

Angle-resolved photoemission spectroscopy with synchrotron radiation has been used to study the electronic state of oxygen adsorbed on a TiC(111) surface at room temperature. Oxygen is adsorbed dissociatively forming a (1×1) overlayer on the TiC(111) surface at saturation coverage. The band structure of the (1×1)-O overlayer is measured in the two symmetry directions; (Γ M) and (Γ K)of the TiC(111) surface

Optical anisotropy of Ge(001)2 × 1

We have measured the change in the optical reflection anisotropy of a clean Ge(001) surface upon exposure to molecular oxygen up to saturation coverage. Both phase and amplitude changes have been recorded with a normal-incidence ellipsometer. They have been found to be related by a Kramers-Kronig transformation. The change in the complex reflection ratio could be interpreted as an anisotropy of the clean Ge(001)2 × 1 surface dielectric function, using a three-layer McIntyre-Aspnes approach and neglecting the oxygen overlayer. The surface dielectric function anisotropy can be described fairly well by optical selection rules, based on symmetry arguments. This model was applied to the possible optical transitions at this surface between filled dimers, dangling bonds and back-bonds and the empty dangling bonds and dimers.

The geometric and electronic structure of oxygen on a Re(10[formula omitted]0) surface investigated by LEED and ARUPS

The geometric and electronic structure of oxygen dissociatively adsorbed on a Re(1010) surface has been studied by means of LEED and ARUPS. Five ordered oxygen LEED phases could be observed, namely, a (2×3)-O structure at θO = 16, a c(2×4)−20 structure at θO = 14, a (1×5)−20 structure at θO = 25, a (1×4)−20 structure at θO = 12, and a (1×3)−20 structure at θO = 23. The two dimensional band structures of the (1×5)−20 and the (1×3)−20 phase were mapped at BESSY. Both ordered oxygen overlayers showed quite similar band structures with a pronounced dispersion along Σ ([1210]-direction) but a lack of any dispersion along Δ ([0001]-direction). The oxygen-induced bands could be assigned to certain symmetric and antisymmetric states of two chemisorbed oxygen atoms per unit cell by considering dipole selection rules in combination with final state symmetries. The uniform behaviour of both band structures can be interpreted by a similar local arrangement of the oxygen atoms in the unit cells of both structures.

Unoccupied orbitals of C and O on Fe(100)

Inverse photoemission experiments from c(2 × 2) carbon and 1 × 1 oxygen overlayers on Fe(100) reveal adsorbate states at 2.5 and 3.9 eV for C and at 1.7 eV for O. They are assigned to antibonding combinations of C 2p and O 2p states with Fe 3d states. For oxygen on Fe(100) the adsorbate state resonates with the minority spin Fe 3d band, but not with the majority band, in agreement with a recent first principles calculation. Together with previous photoemission results we obtain the energy levels for a complete surface reaction, i.e., dissociation/recombination of CO on Fe(100).

A new approach to the mechanism of heterogeneously catalysed reactions: the oxydehydrogenation of ammonia at a Cu(111) surface

The oxydehydrogenation of ammonia at a Cu(111) surface is a highly efficient process at 295 K, with the selectivity sensitive to the dioxygen-ammonia ratio. However, there is no evidence from either XPS or HREELS for surface oxygen being present during the reaction and, in effect, catalysis occurs at a clean Cu(111) surface. The rate of NHx(a) formation is indistinguishable from the rate of the dissociative chemisorption of oxygen at close to zero coverage suggesting that the reactive oxygen species are the hot transients O-(s). The chemisorbed oxygen overlayer, the O2-(a)-like species are, by comparison, unreactive. The reaction is, therefore, not characteristic of either Eley-Rideal or Langmuir-Hinshelwood mechanisms but involves the interaction of rapidly diffusing ammonia molecules and hot transient O-(s)-like species. Models for this type of reaction have been discussed previously, while very recent studies by scanning tunnelling microscopy have provided further evidence for such oxygen transients.

Induced ordering of ethylidyne on the Pd(111) surface by the preadsorption of oxygen: a LEED and HREELS study

Overlayers of oxygen coadsorbed with acetylene and ethylene on a Pd(111) surface were studied by low-energy electron diffraction (LEED) and high-resolution electron-energy-loss spectroscopy (HREELS) in the temperature range 150–320 K. Low temperature adsorption of both C 2H 2 and C 2H 4 resulted in non-dissociatively chemisorbed molecules, and the preadsorption of oxygen did not change the LEED or HREELS data. At room temperature both C 2H 2 and C 2H 4 formed ethylidyne (C 2H 3), which poorly ordered in a (√3 × √3)R30° structure. We found that preadsorption of oxygen induced better ordering for C 2H 3 formed from acetylene exposure, but not for C 2H 3 derived from ethylene adsorption. We propose that preadsorbed oxygen helped the ordering of the C 2H 3 overlayer by: (a) efficiently removing surface hydrogen and (b) attractively interacting with C 2H 3 to give coadsorbate induced ordering.

Adsorbed states of acetone and their reactions on rhodium(111) and rhodium(111)-(2.times.2)oxygen surfaces

The binding and decomposition of acetone on clean and modified Rh(111) surfaces were studied by using TPD (temperature-programmed desorption) and HREELS (high-resolution electron energy loss spectroscopy). On the clean surface the {eta}{sup 2}(C,O) configuration was the dominant form of acetone. This species was characterized by a {nu}(CO) frequency of 1,380 cm{sup {minus}1}. This value was 430 cm{sup {minus}1} below the {nu}(CO) frequency of liquid acetone, indicating a substantial reduction in bond order arising from the binding of acetone to the metal surface via the {pi} and {pi}* orbitals of the carbonyl. The strength of this interaction was also indicated by the reactivity of the {eta}{sup 2}(C,O)-acetone intermediate. All acetone molecules adsorbed in the first monolayer on the clean surface decomposed to CO, H{sub 2}, and surface carbon. This decomposition exhibited a primary kinetic isotope effect upon deuterium substitution in both TPD and HREELS experiments. Thus C-H rather than C-C scission was the rate-determining step in acetone decomposition on Rh(111). Three different methods of estimating the acetone decomposition kinetics from the hydrogen TPD were compared. Accurate modeling of the kinetics required inclusion of the effect of hydrogen atom recombination kinetics on the rate of hydrogen evolution from the surface. Modification ofmore » the surface by addition of a (2 {times} 2) overlayer of oxygen resulted in a change in the acetone binding configuration from {eta}{sup 2}(C,O) to {eta}{sup 1}(O). This shift was the result of electronic modification of the surface by the electronegative oxygen atoms.« less

Coadsorption‐induced effects at surfaces: Thermal desorption spectroscopy and x‐ray photoelectron spectroscopy study of oxygen compression by Au on Ru(001)

Coadsorption of gold on an oxygen saturated ruthenium surface causes dramatic changes in the energetics and kinetics of oxygen desorption and the selectivity of surface processes. The activation energy for desorption of oxygen (EA) is lowered, it is coverage independent, and the process is governed by first‐order kinetics. The lower EA prevents oxygen bulk diffusion, which otherwise competes with desorption from a pure oxygen layer, thus leading to a twofold increase in the amount of desorbing oxygen. A complete analysis of desorption data yields EA=81±4 kcal/mol, prefactor ν(1)=(4±2)×1014 s−1, and reaction order n=0.94. There is no direct interaction between Au and O, as inferred from gold desorption and x‐ray photoelectron spectroscopy data. We propose that changes in EA, ν, and n arise as three‐dimensional gold islands compress the oxygen overlayer causing a local increase in oxygen coverage. Desorption of oxygen occurs before the onset of Au desorption and as oxygen leaves the Ru surface, gold adatoms spread over Ru keeping the local oxygen coverage constant.

Coadsorption-induced effects at surfaces: compression of a chemisorbed oxygen layer by gold on ruthenium(001)

An experimental study of the Au/O/Ru(001) system has shown that coadsorption of gold causes dramatic changes in the parameters of oxygen desorption and, consequently, in the selectivity of surface processes. E{sub A}, the activation energy for desorption of oxygen, is lowered, it is coverage independent, and desorption of oxygen is governed by approximately first-order kinetics. Lower E{sub A} prevents oxygen bulk diffusion, which otherwise competes with desorption from a pure oxygen layer, thus leading to a 2-fold increase in the amount of desorbing oxygen. A complete analysis of desorption data yields E{sub A} = 339 {plus minus} 17 kj/mol, prefactor {nu} = (4 {plus minus} 2) {times} 10{sup 14}s{sup {minus}1}, and reaction order n = 0.94 {plus minus} 0.03. Observed effects are not due to chemical interactions between Au and O, as inferred from TDS, AES, and XPS data. The authors propose that changes in E{sub A}, {nu}, and n arise as gold 3-D islands compress oxygen 2-D islands, keeping the local oxygen coverage higher than in a saturated pure oxygen overlayer. As oxygen leaves the Ru surface (before any significant Au desorption), gold adatoms spread over the liberated Ru surface, keeping oxygen compressed and local oxygen coverage constant.

Zinc oxide and oxygen overlayers on Cu(110): a model for Cu [sbnd]Zn [sbnd]O catalysts

Submonolayers to multilayers of zinc and oxygen deposited in UHV on Cu(110) were characterized by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and temperature-programmed desorption (TPD) of CO, CO 2 and zinc. It is shown that initially, ZnO x grows two-dimensionally on Cu(110), but heating above 300 K leads irreversibly to three-dimensional island formation. These three-dimensional ZnO x islands were then used with various amounts of chemisorbed oxygen on the exposed Cu(110) as models for Cu Zn O methanol synthesis catalysts. Methanol produces formates on the surface, the concentration of which can be controlled by the amount of chemisorbed oxygen. The oxygen coverage found on Cu Zn O methanol synthesis catalysts after reaction corresponds to the oxygen coverage at which maximum formate formation occurs on ourZnO x/O/Cu(110) surfaces.

He-diffraction investigations of the (2 × 2)-phases of oxygen and hydrogen on Ni(111)

He-diffraction results are reported for the clean Ni(111)-surface as well as for the (2 × 2)-chemisorption phases of oxygen and hydrogen. Intensity analyses confirm the early LEED-results concerning both the oxygen overlayer with one adatom and the honeycomb arrangement of the hydrogen overlayer with two adatoms per (2 × 2)-unit cell. Charge density calculations to reproduce the measured amplitudes of the adatom corrugation hills yield bond lengths for both species to the Ni-substrate atoms in good agreement with LEED. For the (2 × 2)2H-case the best-fit corrugation suggests a slight outward relaxation of those Ni-atoms, which lie in the centers of the H-honeycomb arrangement and are thus not bound directly to the H-adatoms; the possibility of a H-induced substrate reconstruction was not considered in the previous LEED-analyses.

Decomposition of methanol on oxygen-modified Fe(100) surfaces: II. Preadsorbed oxygen as poison, selectivity modifier and promoter

Decomposition of methanol (CH 3OH) on the Fe(100) surface modified by low temperature adsorption of oxygen has been studied, using high resolution electron energy loss spectroscopy (HREELS) and temperature programmed reaction spectroscopy (TPRS). Fe(100) surfaces studied were modified by adsorption of O 2 at 113 K, and methanol decomposition as a function of oxygen coverage was monitored. The effect of pre-heating the oxygen overlayers on the methanol decomposition was also examined. Decomposition of methanol on these O-modified surfaces passes through a methoxy (-OCH 3) intermediate. The thermal stability of methoxy increases in the presence of pre-adsorbed oxygen. At low coverage, atomic oxygen occupies four-fold hollow sites. In this case, the effect of oxygen on the methanol decomposition is similar to that observed previously on the annealed O-modified surfaces. At higher oxygen coverage, a more weakly bound non-hollow site oxygen also exists on the surface, which reacts with hydroxyl (-OH) hydrogen of the CH 3OH, promoting the formation of methoxy. At high oxygen coverage (close to saturation coverage at 113 K), decomposition of methanol results in the formation of formaldehyde (H 2CO), without production of carbon monoxide (CO). This is very different from the decomposition of methanol on the clean Fe(100) surface, where decomposition leads to the formation of CO without H 2CO. The effect of oxygen modification is discussed in terms of changing relative probabilities of competing reaction pathways.

Growth of Al oxide layers on GaAs (100) by reaction with condensed molecular oxygen

An Al oxide-GaAs (100) interface fabricated by the reactive deposition of Al into a molecular oxygen overlayer on a gallium terminated GaAs (100) surface at T=49 K is studied by synchrotron radiation photoemission. Al forms a stable oxide layer by reaction with O2 until all the oxygen is consumed. Limited oxidation of surface As atoms (≊20%) is observed during the initial deposition of Al, but further Al deposition reduces the AsO bond. The well-known exchange reaction between Al and Ga when Al is directly deposited on GaAs (100) is not observed.

Strong anisotropy in the Rayleigh phonon dispersion relation on Pt(111) induced by a p(2

Etude par diffusion inelastique de l'helium. La forte anisotropie s'explique par une absorption d'oxygene aux sites hexagonaux compacts surface Pt(111)

Structural steps to oxidation of Ni(100)

In this paper, we emphasize the temperature- and exposure-dependent development of low-energy electron diffraction patterns,measured quantitatively during oxidation of Ni(100) at 80 to 400 K. We find a strong temperature dependence in the development of LEED patterns associated with NiO. NiO(111) is favored by adsorption temperatures below 300 K, whereas a (7×7)-like structure is favored by adsorption temperatures of 300 to 400 K. Room temperature is a ‘‘crossover’’ point between these two forms of the oxide. The final oxide depth is independent of adsorption temperature and, therefore, of epitaxial orientation, between 80 and 400 K. When the sample is heated in vacuum after adsorption, massive rearrangements take place above 500 K. Some of the nickel reverts to metallic nickel covered by a c(2×2) oxygen overlayer, and some forms NiO crystallites which are probably deeper than the initial oxide skin. Effectively, the parent oxide disproportionates into a less-oxygen-rich phase and a more-oxygen-rich phase. This is again independent of the orientation of the initial oxide epitaxy.

Electronic structure calculations on Cu(110) and Ni (110) films with p(2 × 1) oxygen overlayers

We report scalar-relativistic electronic structure calculations on ultra-thin Cu(110) films and ferromagnetic Ni(110) films with p(2 × 1)-ordered oxygen chemisorbed on either side of the respective film. The latter consists of three atomic layers of Cu or Ni metal and displays a missing row reconstruction as suggested by various experiments. The calculations are carried out within a relativistic version of the density functional theory. The exchange-correlation potential has been approximated by the familiar local expressions of Gunnarsson and Lundqvist, and of v. Barth and Hedin. In solving the scalar-relativistic Kohn-Sham equations we have used the full potential linearized augmented plane wave (FLAPW) method put forward by Freeman and associates. The results on O-Cu(110) show only limited agreement with a Cu-O-Cu chain model that has been discussed in the literature and successfully been correlated with angular resolved photoemission data. In the case of O-Ni(110) we find a reduced ferromagnetic order of the top-most Ni layer, but there is a sizeable remnant moment of 0.42 bohr magnetons per atom concomitant with a finite magnetic moment of 0.17 bohr magnetons of the oxygen atoms. The latter fact gives rise to a visible exchange splitting of the excited bands. This is in keeping with recent inverse photoemission experiments.

Introductory lecture. Structural investigations of adsorbed layers using synchrotron radiation

A brief review is given of experiments aimed at elucidating the structure of adsorbed layers on single-crystal metal surfaces using synchrotron radiation. The techniques based on the phenomena of photoabsorption or photoemission, which require no long-range order in the adlayer and may prove to be of wider applicability than electron or X-ray diffraction, particularly for absorbed molecules. To illustrate the potential of the various methods the adsorption systems Cu{110}(2 × 1)/O and Cu{110}/H.CO2 are considered in more detail. In the case of the ordered oxygen overlayer, recent low-energy electron diffraction structural analyses are available for comparison. The application of a range of techniques is of particular advantage in this system because of the relative complexity of the structure: the adsorbate induces a so-called missing-row reconstruction. The energetics of such surface restructuring processes are also briefly discussed. In the case of the surface formate species the molecular orientation and adsorption site have been determined by X-ray absorption spectroscopy and photoelectron diffraction, respectively. This adsorption system shows no long-range order, a feature common to most adsorbed molecular fragments formed via simple heterogeneous reactions. Finally, some consideration is given to the requirement of high spectral brilliance for surface structural experiments, particularly in view of the construction of third-generation synchrotron radiation sources.

Missing-row reconstruction of Ag(110) induced by a 2 x 1) oxygen overlayer.

We present here a detailed study of the surface dynamics of the p(2\ifmmode\times\else\texttimes\fi{}1)O-Ag(110) system. The phonon dispersion curves along the 〈001〉 direction were measured using the He-beam-scattering technique. The lattice-dynamical calculations with nearest-neighbor interactions and central forces were performed assuming (a) unreconstructed, (b) missing-row-type reconstructed, and (c) saw-tooth-type reconstructed arrangement of the surface Ag atoms. A comparison of the observed frequencies of the modes along both 〈11\ifmmode\bar\else\textasciimacron\fi{}0〉 and 〈001〉 directions to the calculated values gives strong evidence that the Ag(110) surface undergoes a missing-row-type reconstruction.

The role of coverage in determining adsorbate stability: phenol reactivity on rhodium(111)

The reaction of phenol on Rb(111) has been studied by use of temperature programmed reaction and X-ray photoelectron spectroscopies under ultrahigh vacuum. Phenol undergoes competing molecular desorption and O-H bond cleavage to form adsorbed phenoxy below 300 K. Phenoxy quantitatively reacts to form carbon monoxide (400-500 K) and stoichiometric amounts of surface carbon and dihydrogen. O-H bond cleavage of phenol occurs at temperature as low as 120 K, but no C-H bond cleavage occurs until above 350 K. The decomposition kinetics of the adsorbed phenoxy are strongly dependent on its coverage. At low coverage phenoxy decomposes below 400 K to form adsorbed CO which desorbs near 500 K, while at saturation coverage, phenoxy decomposes about 450 K to CO, a large fraction of which is evolved directly into the gas phase at 495 K. Comparison of the reactivity of phenoxy on Rh(111) with previous studies of Mo(110) suggests that the strength of the metal-oxygen bond results in different selectivities on the two surfaces. On Mo(110), all C-O bonds are broken by 450 K leaving an oxygen overlayer on the surface whereas no C-O bond breaking is induced by the Rh(111) surface.