| 1. |
- Dannetun, Helen, 1957-, et al.
(författare)
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Dehydrogenation of acetylene and ethylene studied on clean and oxygen covered palladium surfaces
- 1986
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 173, s. 148-
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Tidskriftsartikel (refereegranskat)abstract
- The interaction of acetylene and ethylene with a clean and oxygen covered Pd surface has been studied at a temperature of 473 K. The measurements were performed on a hydrogen sensitive Pd-MOS structure making it possible to obtain direct information on the dissociation of both hydrogen and oxygen containing species on a palladium surface. Desorption studies were also performed as well as ultraviolet photoelectron spectroscopy and work function measurements. The studies show that both acetylene and ethylene adsorb dissociatively at this temperature leaving mainly carbon on the surface. When an oxygen covered Pd surface is exposed to C2H2 or C2H4 carbon dioxide and water will be formed and desorb until the surface is oxygen free. In the case of acetylene the presence of preadsorbed oxygen does not block or prevent the C2H2 dissociation on the surface. For C2H4, a large preadsorbed oxygen coverage ( 0.45) will have an impeding effect on the dissociation. The CO2 desorption is oxygen coverage dependent contrary to the H2O desorption. This is due to the fact that hydrogen has a large lateral mobility on the surface while carbon has not. Both the CO2 and H2O reactions are, however, due to the same type of mechanisms.
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| 2. |
- Dannetun, Helen, 1957-, et al.
(författare)
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Reactions between hydrocarbon and an oxygen covered palladium surface
- 1988
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 193, s. 109-
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Tidskriftsartikel (refereegranskat)abstract
- Isothermic desorption studies where an oxygen covered polycrystalline Pd surface (T = 325–575 K) is exposed to small unsaturated hydrocarbons have been performed. The studied gases were acetylene, propene, propadiene, propyne, butene, butadiene, butyne, and also for comparison hydrogen. Electron energy loss spectroscopy (EELS) was available for adsorbate studies and a hydrogen sensitive Pd-MOS structure was used as a sample. The temperature limit for total dissociation is observed to decrease slightly for an oxygen covered surface compared to the clean surface. On an oxygen covered surface the dissociation is complete above 400 K and the reaction products are H2O, CO2, and adsorbed C. Varying the adsorption temperature, the carbon dioxide production is not detectable at room temperature, where all carbon is adsorbed and only water is formed, but increases to reach a constant level above 475 K. For temperatures above 400 K the licking coefficient is close to unity and there is no blocking due to preadsorbed oxygen (as there is for H2). Both the water and carbon dioxide formations are of Langmuir-Hinshelwood type but while the H2O desorption rate is found to depend only on impinging hydrocarbon flux the CO2 desorption rate depends also on the oxygen coverage.
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| 3. |
- Dannetun, Helen, 1957-, et al.
(författare)
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The H2-O2 reaction on palladium studied over a large pressure range : Independence of the microscopic sticking coefficients on surface condition
- 1985
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 152/153, s. 559-
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the H2---O2 reaction on Pd over a large pressure range by letting the reaction take place on the Pd gate of a Pd-MOS structure. By measuring shifts in the capacitance versus voltage (C(V)) curve of the structure, the variation of the “free” hydrogen atom concentration can be monitored during the reaction. By combining these results with surface potential measurements and with photoelectron spectroscopy, we find a critical ratio of 0.4 where the surface turns from being oxygen dominated to being hydrogen dominated. The critical ratio is independent of the absolute pressures and of surface condition and shows no significant temperature dependence in the temperature range 50–200°C. This implies that hydrogen and oxygen have similar sticking coefficients at both clean and technical Pd surfaces. The conclusion is drawn that hydrogen (and oxygen) dissociation on Pd is a property of the Pd atom or cluster of Pd atoms and is not critically dependent on any long-range order parameter such as the surface electronic density of states.
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| 4. |
- Lindgren, S.Å., et al.
(författare)
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Sodium induced structure in UPS spectra of Cu(111)/Na
- 1985
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Ingår i: Surface Science. - 0039-6028 .- 1879-2758. ; 155:1, s. 165-172
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Tidskriftsartikel (refereegranskat)abstract
- UPS Spectra of sodium monolayers evaporated onto Cu(111) are presented. We interpret sodium induced structure below the Cu d-band as due to photoemitted 3d electrons which have experienced an energy loss by exciting a sodium monolayer plasmon. An experimental difficulty is that even small amounts (<1/10 monolayers) of water contamination produces structure in the same energy range. This emission is due to the OH 1π orbital. Hydroxide groups may also origin from the alkali source if this is operated at too elevated temperatures.
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| 5. |
- Paul, Jan, et al.
(författare)
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Hydrogen adsorption on alkali modified aluminum
- 1988
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Ingår i: Surface Science. - 0039-6028 .- 1879-2758. ; 194:3, s. 419-437
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Tidskriftsartikel (refereegranskat)abstract
- Coadsorption experiments of alkali metals and hydrogen on Al(100) are reported. The probability for H-H (D-D) dissociation and subsequent atomic adsorption is less than 1:104 at all alkali coverages. As a consequence, each overlayer had to be prepared by exposing the sodium or potassium modified surface to a beam of hydrogen (deuterium) atoms. We observed an attractive alkali-hydrogen interaction at all coverages. This interaction is best described as the formation of an alkali hydride in the presence of excess alkali atoms. The formation of the hydride shifts the recombination and desorption temperature of hydrogen adatoms on Al(100) from around 350 to around 500 K. While no isotope effect was detected on the clean aluminum surface (diffusion limited recombination), a significant 10-15 K shift was observed for both alkali hydrides (bond breaking). Furthermore, both alkali hydrides observed intense vibrational losses, thus revealing the ionic character of the metal-hydrogen bond. Electron energy loss spectra of an annealed monolayer of sodium hydride (deuteride) showed metal-hydrogen stretching bands at 1850 (1350) cm-1 and 1715 (1250) cm-1, a deformation band at 800 (600) cm-1, and a metal-metal band at 200 (200) cm-1. Corresponding peak positions for potassium hydride (deutride) were 1650 (1200) cm-1, 1500 (1100) cm-1, and 775 (585) cm-1. The metal-metal vibration could not be separated from the elastic peak because of the larger mass of potassium compared to sodium.
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| 6. |
- Paul, Jan, et al.
(författare)
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TDS and EELS observations for CO, O2, and CH3OH bound to Ru(0001)/Cu
- 1986
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 172:1, s. 151-173
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Tidskriftsartikel (refereegranskat)abstract
- The electronic properties of monolayers of copper atoms adsorbed onto a Ru(0001) single crystal surface have been studied with thermal desorption spectroscopy (TDS) and high resolution electron energy loss spectroscopy (EELS) utilizing carbon monoxide (CO), dioxygen (O2), methanol (CH3OH), and to some extent water (H2O) as chemical probes. Whereas a three-monolayer-thick film exhibits most properties of a Cu(111) crystal distinct deviations are found at lower Cu coverages. TDS as well as EELS show a weakened Ru---CO bond and a strengthened Cu---CO bond as a result of metal-metal interaction. The stronger Cu---CO bond is accompanied by a higher probability for O2 dissociation. The mobilities of copper and oxygen atoms are such that annealing to 650 K produces an overlayer structure which is independent of adsorption sequence: Cu/O2 or O2/Cu, but where Ru---O as well as Cu---O vibrations can be identified. Methanol adsorbs reversibly on a monolayer of copper atoms. Metal bound methoxy species are formed in the presence of oxygen atoms. The decomposition paths of such methoxy intermediates alter towards more formaldehyde (CH2O) relative to CO with increasing copper and methoxy coverages.
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| 7. |
- Paul, Jan, et al.
(författare)
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The interaction of CO and O2 with the (111) surface of Pt3Ti
- 1986
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 177:1, s. 121-138
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Tidskriftsartikel (refereegranskat)abstract
- The electronic properties of clean and partly oxidized Pt3Ti(111) surfaces have been studied utilizing carbon monoxide both as a probe and as a reducing agent. Vibrational frequencies and desorption profiles of chemisorbed CO as well as ion scattering and angular resolved X-ray photoelectron spectroscopy (XPS) suggest that the first atomic layer of annealed Pt3Ti(111) is quasi-pure platinum. Scarcely any (θ ≈ 0.01) dissociation of CO was observed. Minor shifts of vibrational frequencies and desorption temperatures compared to Pt(111) and a p(2 × 2) "reconstruction" of the clean surface reveal some influence of the bulk. Auger spectroscopy, XPS, and ion scattering all show an increased titanium signal as a result of oxidation. Surface bound atomic oxygen gives a vibrational band around 650 cm-1 which coincides with infrared absorption spectra of TiO2. Flashing with CO shifts the band to 500 cm-1. Correlated with this shift we observe (i) CO2 desorption at a temperature well above that observed for Pt(111)/O, (ii) an altered Ti XPS signal, and (iii) a reduced oxygen concentration. Subsequently adsorbed CO molecules vibrate at the same frequencies as on the bare surface, give the same c(4 × 2) LEED pattern, and desorb at the same temperatures but with reduced intensity, in all proving that the surface oxide only acts as a site-blocker with respect to the metal surface. Our current understanding of these observations is that oxygen creates "islands of TiO2", segregated to the surface but with no electronic influence on remaining areas of the platinum enriched metal surface. The hexacoordinated Ti4+ ions on the surface of these islands are reduced by CO to pentacoordinated Ti3+ species. The vibrational shift, 650 to 500 cm-1, can be understood by the dipole active bands of a triatomic O-Ti4+ -O vibrator compared to a diatomic Ti3+-O vibrator.
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| 8. |
- Paul, Jan
(författare)
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UPS spectra of H2O, CH3OH and C5H9OH adsorbed onto Cu(111)/Na and Na(cp)
- 1985
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 160:2, s. 599-617
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Tidskriftsartikel (refereegranskat)abstract
- The present communication presents ultraviolet photoemission spectra (UPS) of three different "alcohols"; water (H2O), methanol (CH3OH), and cyclopentanol (C5H9OH), chemisorbed onto a Cu(111) surface partially covered by sodium atoms as well as onto closely packed sodium films, a free electron adsorbent. Whereas all three alcohols ROH bind reversibly and associatively to Cu(111) they react with adsorbed sodium atoms to metal bound alcoxides RO. The chemisorption bond, characterized by the interaction between O 2pπ orbitals and metal atoms as an electron donor, the alcoxide being the acceptor, is similar for all groups R. The O 2pπ orbitals shift to higher UPS binding energies with increasing electron density, i.e. decreasing rs/ao of the sodium overlayer. Only for HONa, the sterically smallest group R, does the alcoxide growth continue in three dimensions. Although, possibly failing to reproduce the electron density profile of a free electron surface, Hartree-Fock-Slater cluster calculations of small models ROH and RONa3 enable correlations to be made between UPS intensity peaks and one electron orbitals.
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| 9. |
- Paul, Jan
(författare)
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Vibrational spectra of CO and CO2 adsorbed on potassium modified Fe(100)
- 1989
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 224:1-3, s. 348-358
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Tidskriftsartikel (refereegranskat)abstract
- High resolution electron energy loss (EEL) spectra for CO adsorbed on potassium modified Fe(100) are presented. Five CO vibrational bands are clearly distinguishable. Two bands at or above 2000 cm-1 correspond to the *1- and *2-states on the clean surface. Two more bands, residing between 1450 and 1850 cm-1, reveal adsorption at potassium modified sites. The response of these bands to changes in potassium coverage and annealing temperature is highly predictable and similar to the situation on Fe(111) [1]. This suggests a local K: CO coordination. A fifth band is observed around 950 cm-1 and interpreted as CO adsorbed at potassium modified *3-sites, redshifted from 1200 cm-1 at unmofidied sites. The present results are complementary to a previous study of the same adsorption system utilizing photoemission spectroscopy and thermal desorption spectroscopy [2]. Finally, we present novel vibrational and thermal desorption (TDS) data for Fe(100)/K exposed to CO2 and point at similarities between intermediate states following CO and CO2 adsorption on different potassium promoted iron surfaces.
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| 10. |
- Petersson, L.-G., et al.
(författare)
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The water-forming reaction on Pd
- 1985
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 161, s. 77-
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Tidskriftsartikel (refereegranskat)abstract
- The water-forming reaction on Pd has been studied on a Pd---SiO2---Si (Pd-MOS) structure in the temperature range 323–473 K. The reaction is found to be of the Langmuir-Hinshelwood type with the formation of OH beeing rate limiting. Since the Pd-MOS structure works as a sensitive hydrogen detector unique information on the behaviour of hydrogen during this catalytic reaction has been obtained. The reaction can be described in a model where the hydrogen atoms on the Pd surface have a large temperature activated lateral mobility and with no evidence of beeing in hot precursor states. At T = 473 K this means that for oxygen coverages 0.01 monolayers all hydrogen adsorbed will also react with oxygen. For smaller oxygen coverages unreacted hydrogen will not initially desorb towards the vacuum but towards the internal Pd surface of the Pd-MOS structure. Futhermore, hydrogen adsorption is blocked by adsorbed oxygen. The sticking coefficient for hydrogen on the bare Pd surface is, however, close to one and only weakly temperature dependent. An effect giving rise to a hysteresis in the work function versus oxygen coverage curve during oxygen adsorption - desorption is also discussed.
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| 11. |
- Petersson, L.-G., et al.
(författare)
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Water production on palladium in hydrogen-oxygen atmospheres
- 1985
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 163, s. 273-
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the water production on Pd in various oxygen-hydrogen atmospheres in combination with work function measurements, photoelectron spectroscopy and hydrogen sensitive measurements on a Pd---SiO2---Si structure. The existence of a critical oxygen to hydrogen pressure ratio is confirmed. Above the critical ratio, the water production rate decreases with increasing oxygen pressure and increases with increasing hydrogen pressure. Below the critical ratio, the water production rate is proportional to the oxygen pressure and independent of the hydrogen pressure. These features, together with the variations in oxygen and hydrogen coverage, are described by a simple Langmuir-Hinshelwood model, assuming that hydrogen has a large lateral mobility and that both hydrogen and oxygen adsorption-dissociation is blocked by adsorbed oxygen.
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| 12. |
- Schmidt-May, J., et al.
(författare)
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Resonant electron and ion emission and desorption mechanism in rare earth oxides
- 1985
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 163:2-3, s. 303-314
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Tidskriftsartikel (refereegranskat)abstract
- The resonant enhancement in photoelectron spectra at the 4d edges of rare earth atoms and metals is also found in yield spectra of desorbed ions from the surfaces of the oxides of Sm, Eu and Yb following the photon excitation. The analysis of the 4d → 4f resonance leads to a picture of an indirect mechanism of ion desorption which is mainly caused by the flux of energetic 4f photoelectrons from the bulk. In this case the dominant desorption through secondary processes limits the use of the photon-stimulated desorption (PSD) to determine to which type of atom the desorbing species was attached.
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| 13. |
- Wallenberg, LR, et al.
(författare)
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On the crystal structure of small gold crystals and large gold clusters
- 1985
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 156:1, s. 256-264
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Tidskriftsartikel (refereegranskat)abstract
- A large gold cluster with the formula Au55[(C6H5)3P]12Cl6, was imaged with a high-resolution transmission electron microscope (HRTEM) and a proposed model was confirmed. Growth of gold crystals of approximately 4.0 nm size could be followed, row by row of atoms, with the use of a low light level TV camera and on-line image processing.
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