Oxygen Adsorption Sites Research Articles

A ARUPS INVESTIGATION OF TWO SURFACE SUPERSTRU-CTURES (4×1)-O AND (2×2)-S ON VANADIUM(001) SURFACES

Two surface superstructures (4×1)-O and (2×2)-S, induced respectively by oxygen and sulphur segregation on the V(001) surface, are studied in detail by ARVPS. The energy levels and symmetry of the adsorbate surface states derived from S- and O-ordered adsorbate over-layers are determined. Experimental evidence shows that the adsorption sites of oxygen on V(001) surface are four-fold hollow sites. All the experimental results consist with theoretical calculations, and are similar to that for oxygen on Ni(001) and Cr(001) surfaces.

The adsorption site of oxygen on Si(100) determined by sexafs

We present a SEXAFS (surface extended X-ray absorption fine structure) investigation of oxygen adsorbed on Si(100) at the oxygen K absorption edge (EK=535 eV), in the photon energy range 500–800 eV. The oxygen uptake on Si(100) at room temperature reaches a saturation at about one monolayer coverage, where the (2×1) reconstruction of the clean surface is lost. Our results demonstrate the following: the oxygen atoms occupy two types of bridge positions, namely between Si atoms belonging to the first layer and between Si atoms belonging to different topmost layers. The bridge site is characterized by a bond distance of 1.65 A and a Si−O−Si bond angle of ∼120°. No evidence of other types, of adsorption geometries is found nor the presence of adsorbed molecular oxygen is observed.

The adsorption site of oxygen on Si(100) determined by SEXAFS

The adsorption site of oxygen on Si(100) determined by SEXAFS

Interaction of atomic oxygen with copper clusters

The chemisorption of atomic oxygen on Cu(001) is treated using a many-electron embedding theory in which the copper lattice is modeled as a 25-atom cluster. Of the three high symmetry sites investigated—fourfold, bridge, and atop-atom—the fourfold site is found to be most stable, with a dissociation energy of ∼6.1 eV (r⊥=0.7 Å), while the atop-atom configuration corresponds to the least stable site for atomic oxygen adsorption. The atop-atom site yields a radical-like state, ‘‘O−’’, whereas the open sites involve an oxide-like state, ‘O2−’, for oxygen. The principal valence ionization from oxygen is found to occur ∼8–10 eV below the vacuum level. The results of calculations on smaller clusters, CunO, n=2–5, are found to be in qualitative agreement with the results for Cu25O.

Dissociative adsorption of CO and O 2, on Cr(100), Cr(110), and Cr(111) in the temperature range 300–1175 K

Electron energy loss spectroscopy (EELS), low energy electron diffraction (LEED), and Auger spectroscopy have been used to characterize the adsorption of CO and O 2 on Cr(100), Cr(110), and Cr(111) at 300 K and above. No evidence of molecular CO or O 2 was observed on these surfaces at any coverage. For each surface, the vibrational spectra for dissociated CO and oxygen are very similar. Broader EEL spectra are observed on Cr(111) than on Cr(110) or Cr(100), indicative of multiple sites on the very open (111) surface. For O 2 exposures on Cr(110) and at low coverages on Cr(111), similar Cr-O stretching frequencies at ≈ 560–580 cm −1 and at ≈ 420–440 cm −1 are suggestive of similar adsorption sites for oxygen on the two surfaces. These sites are proposed to be the long bridge and three-fold sites on (110) planes of the two surfaces. A four-fold hollow site is consistent with the Cr-O stretching frequency at 520 cm −1 for low coverages of oxygen on Cr(100). Adsorption sites for carbon are the same. Frequency shifts with coverage and temperature are reported for both chemisorption systems. A (6 × 1) LEED pattern is observed after an 825 K anneal for a 0.75 L exposure of CO Cr(110) and a model consistent with the LEED pattern and sites based on EELS data is presented.

Edge plane segregation in cobalt-doped MoS 2 crystals

Single crystals of cobalt-doped MoS 2 have been examined by Auger spectroscopy. Segregation of cobalt to growth steps (edge planes) of the faceted crystals has been detected. Edge planes are preferred sites for oxygen adsorption. Some evidence has been obtained that oxygen adsorption enhances cobalt segregation. These findings are consistent with the pseudo-intercalation model of segregation in MoS 2 crystals.

X-ray photoelectron diffraction study of oxygen adsorption on the stepped copper surfaces (410) and (211)

We report the first structural investigation of adsorption on stepped metal surfaces using core-level azimuthal X-ray photoelectron diffraction (XPD). The specific systems studied are oxygen adsorption on Cu(410) and Cu(211). Both surfaces show enhanced oxygen adsorption rates compared to the simple low-index surfaces of the terraces, suggesting adsorption sites on the steps. Kinematical LEED analyses verify the step structure of both clean Cu(410) and Cu(211), and also indicate a reconstruction of Cu(211) for exposures ≳ 3 L into a surface with diatomic step heights. At higher exposures of ≳ 75 L, Cu(211) is further observed to facet. Cu(410) is by contrast very stable in structure up to 1200 L exposure. Detailed azimuthal XPD data were obtained for O 1s emission from both surfaces at a low and a high exposure to oxygen. Pronounced diffraction features are seen; these are observed to be sensitive to both the polar angle of emission and the degree of exposure. By comparing the experimental data with single-scattering cluster calculations for different possible adsorption geometries, the most likely adsorption sites for atomic oxygen were deduced. Oxygen is found to lie predominantly on the steps for low exposures of both surfaces: for 0.75 L exposure on Cu(410) it occupies a two-fold site at the top of the step and for 5 L exposure on reconstructed Cu(211), a four-fold site at the bottom of the step. Vertical positions are determined with an estimated accuracy of ± 0.2 Å. At higher exposures ≳ 40 L for the Cu(410) surface, both step and (100) terrace sites are found to be occupied in a modified c(2 × 2) structure. O 1s polar-scan data for Cu(410) also verify that the initial adsorption site is on the step. Such angle-resolved XPS measurements thus have considerable promise for stepped-surface structural studies.

Adsorption Sites of Oxygen on Cu(001) and Ni(001) Determined from the Shape of the Low-Energy Electron-Diffraction Spots

The low-energy electron-diffraction patterns of the oxygen structures on the (001) pole of spherical Cu and Ni surfaces show characteristic differences: For Cu the (\textonehalf{} \textonehalf{}) and the (10) spots are elongated along $〈11〉$, while only the half-order spots are broadened for Ni. These unusual spot profiles are caused by antiphase scattering from neighboring domains, with the oxygen in the bridge position for Cu and in the fourfold hollow site for Ni.

Effects of oxygen adsorption on the leed fine structure features of Cu(001), Cu(110), Cu(111) and Ni(001)

The effect of adsorbed oxygen on the LEED fine structure features of Cu(001), Cu(110) and Cu(111) is investigated. The adsorption process is followed by AES and CPD measurements. Changes in the relative intensity of the fine structure features are related to changes in the surface order and smoothness. It is empirically established that a rough surface produces relatively weak fine structure features compared with a similar smooth surface. This is attributed to a reduction in reflectivity of the surface potential barrier. It is also shown empirically that increasing disorder on the crystal surface leads to a preferential damping of the fine structure close to the emergence. This effect is seen in the change of the relative intensity of the fine structure features during the adsorption of oxygen. Observations of these features are used to test several models proposed for the oxygen adsorption sites on these surfaces.

Adsorption of O2 and N2O on carbon-covered Ru(001) surfaces

The dissociative chemisorption of N2O and O2 on carbon-covered Ru(001) surfaces has been studied by Auger electron spectroscopy. Small amounts of carbon sharply lower the dissociative probability for N2O as compared to O2. Carbon blocks sites for atomic oxygen adsorption and one monolayer of carbon removes the dissociation activity for O2 and N2O. The results are interpreted in terms of a significant effect of carbon on the N2O molecular precursor state and in terms of more restrictive surface site requirements for N2O as compared to O2.

OXYGEN ADSORPTION SITES OF Fe2O3–Sb2O4 CATALYST DETERMINED BY SECONDARY ION MASS SPECTROMETRY-18O TRACER METHOD

Abstract In an attempt to determine oxygen adsorption sites of the Fe2O3–Sb2O4 catalysts, SIMS measurements of the catalysts were performed after 18O2 preadsorption (SIMS-18O tracer method). It was clearly shown that the adsorbed oxygen was accommodated by the surface Sb ions.

A combined LEIS (low energy ion scattering)—LEED study of the V 6O 13(001) surface

The results of a combined LEIS-LEED study of the V 6O 13(001) surface as a function of oxygen coverage will be presented. The experiments have been performed on V 6O 13 layers, grown topotactically on V 2O 5(001) single crystal surfaces. The growth is initiated by low energy ion impact in UHV (O 2 partial pressure ⪡ 10 -8 Pa) at a temperature of 750 K, and proceeds spontaneously further into the bulk up to thicknesses allowing to identify the bulk V 6O 13 structure by means of X-ray diffraction. It is seen that under the given conditions of pressure and temperature, the outermost “vanadyl-oxygen” layer is populated by oxygen diffusing from the V 2O 5−V 6O 13 transition region towards the surface and tends to depletion at low oxygen supply rates (thick V 6O 13 layers). It has furthermore been shown that the outermost vanadium atoms can be considered to be adsorption sites for atomic oxygen in an on-top position.

A transient study of the co-adsorption and reaction of CO and oxygen on Ir(110)

The heterogeneously catalyzed reaction of CO and O 2 to form CO 2 over Ir(110) has been studied through measurements of the transient kinetics of the various elementary reactions that may limit the steady state rate. Rate expressions for these elementary reactions — the desorption of CO, the oxidation of CO via the Langmuir-Hinshelwood mechanism, the adsorption of CO and the adsorption of oxygen — were developed using thermal desorption mass spectrometry. Several phenomena were observed: (1) the activation energies for CO desorption and CO oxidation depend markedly upon the composition of the adlayer; (2) diffusion in the adlayer may limit the rates of CO desorption and CO oxidation; (3) the formation of a surface oxide modifies these four rate processes; and (4) chemisorbed CO blocks sites for oxygen adsorption, but chemisorbed oxygen does not block sites for CO adsorption.

ＣｄＳ粉末の粉砕に伴う光電導性の変化 ＣｄＳ粉末の表面状態の変化

The effect of ball milling on the surface condition of CdS powders was investigated. The sample powders were precipitated from aqueous solution of cadmium sulfate. The mean particle sizes of the precipitates were about 10 and 2μm. The sample powders were ground in an alumina ball mill for different periods from 30min to 240hr. In the early stage of the grinding process, the particle size of the specimen having the initial particle size of 10μm decreased rapidly to about 2μm and it did not decrease any more even if grinding time was prolonged. On the other hand, the particle size of the specimen having the initial particle size of 2μm did not decrease any more. However, the degree of structural imperfections increased with increasing grinding time for both specimens.Using an interdigital electrode, the decrease in photocurrent associated with an adsorption of oxygen has been measured. The magnitude of the decrease in the photocurrent of ground CdS powders became larger with increasing grinding time. This suggests that the structural imperfections, generated by ball milling, act as adsorption sites for oxygen molecules.Corona-charging potentials of ground CdS powders have been measured as a function of relative humidity. The initial surface potentials decreased with increasing grinding time over a whole range of relative humidity for all specimens. This is attributable to a decrease in apparent resistivity of the powder layer, which is associated with an increase in adsorbed moisture. The adsorption of ambient moisture is enhanced by the ball milling.

Chemisorption on some alkaline earth oxides. Part 1.—Surface centres and fast irreversible oxygen adsorption on irradiated MgO, CaO and SrO

A fast irreversible adsorption of oxygen at 20°C on γ- or neutron-irradiated MgO or CaO is correlated with observable surface paramagnetic centres (S centres) and colour centres. The S centre in MgO with gav= 2.0007 is believed to be an intrinsic defect consisting of an electron trapped at a surface anion vacancy. An additional oxygen-sensitive signal SH is related to the presence of some hydrogen contamination on the surface. The S centre in CaO, with g∥= 1.9992 and g⊥= 1.9969 after γ-irradiation to low doses, is similar in behaviour to that on MgO; however, at high γ-doses or after neutron irradiation a complex signal is produced. A surface centre with gav= 1.9798 and of low intensity has been detected on γ- and neutron-irradiated SrO, but this accounts for only a small fraction of the total oxygen adsorption on SrO. The formation of anion vacancies at the surfaces of unirradiated and irradiated oxides is discussed. These defects play a dual role in the irreversible adsorption of oxygen by providing a trap for an electron at the surface and a site capable of stabilizing the adsorbed oxygen species; other sites for oxygen adsorption are increasingly important in CaO and SrO and account for adsorption on the unirradiated oxides.