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1.
S. Funk 《Applied Surface Science》2007,253(17):7108-7114
We attempt to correlate qualitatively the surface structure with the chemical activity for a metal surface, Cr(1 1 0), and one of its surface oxides, Cr2O3(0 0 0 1)/Cr(1 1 0). The kinetics and dynamics of CO2 adsorption have been studied by low energy electron diffraction (LEED), Aug er electron spectroscopy (AES), and thermal desorption spectroscopy (TDS), as well as adsorption probability measurements conducted for impact energies of Ei = 0.1-1.1 eV and adsorption temperatures of Ts = 92-135 K. The Cr(1 1 0) surface is characterized by a square shaped LEED pattern, contamination free Cr AES, and a single dominant TDS peak (binding energy Ed = 33.3 kJ/mol, first order pre-exponential 1 × 1013 s−1). The oxide exhibits a hexagonal shaped LEED pattern, Cr AES with an additional O-line, and two TDS peaks (Ed = 39.5 and 30.5 kJ/mol). The initial adsorption probability, S0, is independent of Ts for both systems and decreases exponentially from 0.69 to 0.22 for Cr(1 1 0) with increasing Ei, with S0 smaller by ∼0.15 for the surface oxide. The coverage dependence of the adsorption probability, S(Θ), at low Ei is approx. independent of coverage (Kisliuk-shape) and increases initially at large Ei with coverage (adsorbate-assisted adsorption). CO2 physisorbs on both systems and the adsorption is non-activated and precursor mediated. Monte Carlo simulations (MCS) have been used to parameterize the beam scattering data. The coverage dependence of Ed has been obtained by means of a Redhead analysis of the TDS curves.  相似文献   

2.
X-ray photoelectron spectroscopy was applied to study the hydroxylation of α-Al2O3 (0 0 0 1) surfaces and the stability of surface OH groups. The evolution of interfacial chemistry of the α-Al2O3 (0 0 0 1) surfaces and metal/α-Al2O3 (0 0 0 1) interfaces are well illustrated via modifications of the surface O1s spectra. Clean hydroxylated surfaces are obtained through water- and oxygen plasma treatment at room temperature. The surface OH groups of the hydroxylated surface are very sensitive to electron beam illumination, Ar+ sputtering, UHV heating, and adsorption of reactive metals. The transformation of a hydroxylated surface to an Al-terminated surface occurs by high temperature annealing or Al deposition.  相似文献   

3.
We have studied adsorption of CO on Fe3O4(1 1 1) films grown on a Pt(1 1 1) substrate by temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS) and high resolution electron energy loss spectroscopy (HREELS). Three adsorption states are observed, from which CO desorbs at ∼110, 180, and 230 K. CO adsorbed in these states exhibits stretching frequencies at ∼2115-2140, 2080 and 2207 cm−1, respectively. The adsorption results are discussed in terms of different structural models previously reported. We suggest that the Fe3O4(1 1 1) surface is terminated by 1/2 ML of iron, with an outermost 1/4 ML consisting of octahedral Fe2+ cations situated above an 1/4 ML of tetrahedral Fe3+ ions, in agreement with previous theoretical calculations. The most strongly bound CO is assigned to adsorption to Fe3+ cations present on the step edges.  相似文献   

4.
Adsorption of alkali atoms on the (1 1 1) and (1 0 0) noble metal surfaces has been shown recently to induce long-lived resonances located inside the surface projected band gap. However, the width of these resonances, as it appears in two-photon photo-emission experiments, is much larger than the inverse of their lifetime. We report on a theoretical study of some broadening mechanisms of these resonance lines in the Na/Cu(1 1 1) and Cs/Cu(1 1 1) systems at low coverage, including the homogeneous natural line broadening and the inhomogeneous statistical broadenings due to the distribution of adsorption heights associated to the quantal vibration of the alkali adsorbate and to the lateral disorder of alkali adsorption on the surface. The inhomogeneous mechanisms are shown to induce a very large broadening of the resonance line, in quantitative agreement with experimental results. The most important broadening effect appears to be the effect of the distribution of alkali adsorption heights.  相似文献   

5.
6.
We have investigated the interaction of molecular oxygen with the Ag(1 0 0) surface in a temperature range from 130 K to 470 K and an oxygen partial pressure ranging up to 10 mbar by scanning tunneling microscopy, low electron energy diffraction, Auger electron spectroscopy and ab initio density functional calculations. We find that at 130 K, following oxygen exposures of 6000 Langmuirs O2, the individual oxygen atoms are randomly distributed on the surface. When the sample is exposed to 10 mbar O2 at room temperature, small, p(2 × 2) reconstructed patches are formed on the surface. After oxidation at ≈470 K and 10 mbar O2 pressure the surface undergoes a c(4 × 6) reconstruction coexisting with a (6 × 6) superstructure. By ab initio thermodynamic calculations it is shown that the c(4 × 6) reconstruction is an oxygen adsorption induced superstructure which is thermodynamically stable for an intermediate range of oxygen chemical potential.  相似文献   

7.
Magnetotransport properties of magnetite thin films deposited on gallium arsenide and sapphire substrates at growth temperatures between 473 and 673 K are presented. The films were grown by UV pulsed laser ablation in reactive atmospheres of O2 and Ar, at working pressure of 8 × 10−2 Pa. Film stoichiometry was determined in the range from Fe2.95O4 to Fe2.97O4. Randomly oriented polycrystalline thin films were grown on GaAs(1 0 0) while for the Al2O3(0 0 0 1) substrates the films developed a (1 1 1) preferred orientation. Interfacial Fe3+ diffusion was found for both substrates affecting the magnetic behaviour. The temperature dependence of the resistance and magnetoresistance of the films were measured for fields up to 6 T. Negative magnetoresistance values of ∼5% at room temperature and ∼10% at 90 K were obtained for the as-deposited magnetite films either on GaAs(1 0 0) or Al2O3(0 0 0 1).  相似文献   

8.
Low temperature scanning tunneling microscopy (LT-STM) and scanning tunneling spectroscopy (STS) have been used to investigate adsorbed copper phthalocyanine (C32H16N8Cu) molecules on an ordered ultrathin Al2O3 film on the Ni3Al(1 1 1) surface as a function of coverage and annealing temperature. For sub-monolayer coverage and a deposition temperature of 140 K two different planar molecular adsorption configurations rotated by 30° with respect to each other were observed with submolecular resolution in the STM images. The template effect of the underlying oxide film on the CuPc orientation, however, is only weak and negligible at higher coverages. For θCuPc ≈ 1 ML, before completion of the first layer, the growth of a second layer was already observed. The measured spacing of 3.5 Å between first and second layer corresponds to the distance between the layers in the α-modification of crystalline CuPc. The molecules deposited at 140 K are thermally stable upon prolonged annealing to temperatures up to 250 K. By the use of STS the lowest unoccupied molecular orbital (LUMO) of the adsorbed copper phthalocyanine molecules has been identified at an energy of 1.2 eV above EF. The lateral distribution of the electronic states of the CuPc has been analyzed and mapped by STS.  相似文献   

9.
We have studied the adsorption of Pb on the Rh(1 0 0) and (1 1 0) surfaces by photoemission and low energy electron diffraction (LEED), and tested the chemical properties by adsorption of CO. Pb forms two distinct c(2 × 2) phases on Rh(1 0 0), according to the temperature of the substrate. The phase formed below about 570-620 K, denoted α-c(2 × 2), reduces the coverage of adsorbed CO but does not affect the valence band spectrum of the molecule. The phase formed above this temperature, denoted β-c(2 × 2), also reduces the coverage of adsorbed CO but the valence band spectrum of the adsorbed CO is strongly affected. The two phases are also characterised by a slightly different binding energy of the Pb 5d5/2 level, 17.54 eV for the α phase and 17.70 for the β phase. The Pb/Rh(1 1 0) surface shows two ordered Pb induced phases, c(2 × 2) and p(3 × 1). CO adsorbs on the first with reduced heat of adsorption and with a valence band spectrum that is strongly altered with respect to CO adsorbed on clean Rh(1 1 0), but does not adsorb on the p(3 × 1) structure at 300 K. We compare the present results with previous results from related systems.  相似文献   

10.
C. Gatel  E. Snoeck 《Surface science》2007,601(4):1031-1039
We have studied the epitaxial growth of Au and Pt layers on Fe3O4(1 1 1) as a function of the deposition temperature and thickness. The layers were deposited by UHV sputtering and the structural properties were investigated by reflection high energy electron diffraction (RHEED), X-ray experiments and transmission electron microscopy (TEM). The epitaxial growth of both metals was obtained whatever the deposition conditions but the wetting is however different for the two metals. Comparison between the coverage ratios of Au and Pt is correlated with their surface and interfaces energies. The optimum conditions to achieve a 2D flat epitaxial metallic layer are determined.  相似文献   

11.
The previously developed kinetic Monte Carlo model of molecular oxygen adsorption on fcc (1 0 0) metal surfaces has been extended to fcc (1 1 1) surfaces. The model treats uniformly all elementary steps of the process—O2 adsorption, dissociation, recombination, desorption, and atomic oxygen hopping—at various coverages and temperatures. The model employs the unity bond index—quadratic exponential potential (UBI-QEP) formalism to calculate coverage-dependent energetics (atomic and molecular binding energies and activation barriers of elementary steps) and a Metropolis-type algorithm including the Arrhenius-type reaction rates to calculate coverage- and temperature-dependent features, particularly the adsorbate distribution over the surface. Optimal values of non-energetic model parameters (the spatial constraint, a travel distance of “hot” atoms, attempt frequencies of elementary steps) have been chosen. Proper modifications of the fcc (1 0 0) model have been made to reflect structural differences in the fcc (1 1 1) surface, in particular the presence of two different hollow sites (fcc and hcp). Detailed simulations were performed for molecular oxygen adsorption on Ni(1 1 1). We found that at very low coverages, only O2 adsorption and dissociation were effective, while O2 desorption and O2 and O diffusion practically did not occur. At a certain O + O2 coverage, the O2 dissociation becomes the fastest process with a rate one-two orders of magnitude higher than adsorption. Dissociation continuously slows down due to an increase in the activation energy of dissociation and due to the exhaustion of free sites. The binding energies of both molecular and atomic oxygen decrease with coverage, and this leads to greater mobility of atomic oxygen and more pronounced desorption of molecular oxygen. Saturation is observed when the number of adsorbed molecules becomes approximately equal to the number of desorbed molecules. Simulated coverage dependences of the sticking probability and of the atomic binding energy are in reasonable agreement with experimental data. From comparison with the results of the previous work, it appears that the binding energy profiles for Ni(1 1 1) and Ni(1 0 0) have similar shapes, although at any coverage the absolute values of the oxygen binding energy are higher for the (1 0 0) surface. For metals other than Ni, particularly Pt, the model projections were found to be too parameter-dependent and therefore less certain. In such cases further model developments are needed, and we briefly comment on this situation.  相似文献   

12.
Oxidation of Cu3Au(1 1 0) using a hyperthermal O2 molecular beam (HOMB) was investigated by X-ray photoemission spectroscopy in conjunction with a synchrotron light source. From the incident energy dependence of the O-uptake curve, the precursor-mediated dissociative adsorption occurs, where the trapped O2 molecule can migrate and dissociate at the lower activation-barrier sites, dominantly at thermal O2 exposures. Dissociative adsorption of O2 on Cu3Au(1 1 0) is as effective at the thermal O2 exposure as on Cu(1 1 0). On the other hand, at the incident energies of HOMB where the direct dissociative adsorption is dominant, it was determined that the dissociative adsorption of O2 implies a higher activation barrier and therefore less reactivity due to the Au alloying in comparison with the HOMB oxidation of Cu(1 1 0). The dissociative adsorption progresses with the Cu segregation on Cu3Au(1 1 0) similarly as on Cu3Au(1 0 0). The growth of Cu2O for 2 eV HOMB suggests that the diffusion of Cu atoms also contribute to the oxidation process through the open face, which makes the difference from Cu3Au(1 0 0).  相似文献   

13.
The adsorption and dissociation of O2 on CuCl(1 1 1) surface have been systematically studied by the density functional theory (DFT) slab calculations. Different kinds of possible modes of atomic O and molecular O2 adsorbed on CuCl(1 1 1) surface and possible dissociation pathways are identified, and the optimized geometry, adsorption energy, vibrational frequency and Mulliken charge are obtained. The calculated results show that the favorable adsorption occurs at hollow site for O atom, and molecular O2 lying flatly on the surface with one O atom binding with top Cu atom is the most stable adsorption configuration. The O-O stretching vibrational frequencies are significantly red-shifted, and the charges transferred from CuCl to oxygen. Upon O2 adsorption, the oxygen species adsorbed on CuCl(1 1 1) surface mainly shows the characteristic of the superoxo (O2), which primarily contributes to improving the catalytic activity of CuCl, meanwhile, a small quantity of O2 dissociation into atomic O also occur, which need to overcome very large activation barrier. Our results can provide some microscopic information for the catalytic mechanism of DMC synthesis over CuCl catalyst from oxidative carbonylation of methanol.  相似文献   

14.
Jooho Kim  Bruce E. Koel 《Surface science》2006,600(19):4622-4632
Nanosized gold particles supported on reducible metal oxides have been reported to show high catalytic activity toward CO oxidation at low temperature. This has generated great scientific and technological interest, and there have been many proposals to explain this unusual activity. One intriguing explanation that can be tested is that of Nørskov and coworkers [Catal. Lett. 64 (2000) 101] who suggested that the “unusually large catalytic activity of highly-dispersed Au particles may in part be due to high step densities on the small particles and/or strain effects due to the mismatch at the Au-support interface”. In particular, their calculations indicated that the Au(2 1 1) stepped surface would be much more reactive towards O2 dissociative adsorption and CO adsorption than the Au(1 1 1) surface. We have now studied the adsorption of O2 and O3 (ozone) on an Au(2 1 1) stepped surface. We find that molecular oxygen (O2) was not activated to dissociate and produce oxygen adatoms on the stepped Au(2 1 1) surface even under high-pressure (700 Torr) conditions with the sample at 300-450 K. Step sites do bind oxygen adatoms more tightly than do terrace sites, and this was probed by using temperature programmed desorption (TPD) of O2 following ozone (O3) exposures to produce oxygen adatoms up to a saturation coverage of θO = 0.90 ML. In the low-coverage regime (θO ? 0.15 ML), the O2 TPD peak at 540 K, which does not shift with coverage, is attributed to oxygen adatoms that are bound at the steps on the Au(2 1 1) surface. At higher coverages, an additional lower temperature desorption peak that shifts from 515 to 530 K at saturation coverage is attributed to oxygen adsorbed on the (1 1 1) terrace sites of the Au(2 1 1) surface. Although the desorption kinetics are likely to be quite complex, a simple Redhead analysis gives an estimate of the desorption activation energy, Ed, for the step-adsorbed oxygen of 34 kcal/mol and that for oxygen at the terraces near saturation coverage of 33 kcal/mol, values that are similar to others reported on Au surfaces. Low Energy Electron Diffraction (LEED) indicates an oxygen-induced step doubling on the Au(2 1 1) surface at low-coverages (θO = 0.08-0.17 ML) and extensive disruption of the 2D ordering at the surface for saturation coverages of oxygen (θO ? 0.9 ML). Overall, our results indicate that unstrained step sites on Au(2 1 1) surfaces of dispersed Au nanoparticles do not account for the novel reactivity of supported Au catalysts for CO oxidation.  相似文献   

15.
Au/TiO2/Ru(0 0 0 1) model catalysts and their interaction with CO were investigated by scanning tunneling microscopy and different surface spectroscopies. Thin titanium oxide films were prepared by Ti deposition on Ru(0 0 0 1) in an O2 atmosphere and subsequent annealing in O2. By optimizing the conditions for deposition and post-treatment, smooth films were obtained either as fully oxidized TiO2 or as partly reduced TiOx, depending on the preparation conditions. CO adsorbed molecularly on both oxidized and reduced TiO2, with slightly stronger bonding on the reduced films. Model catalyst surfaces were prepared by depositing submonolayer quantities of Au on the films and characterized by X-ray photoelectron spectroscopy and scanning tunneling microscopy. From X-ray photoelectron spectroscopy, a weak interaction between the Au and the TiO2 substrate was found. At 100 K CO adsorption occurred on both the TiO2 film and on the Au nanoparticles. CO desorbed from the Au particles with activation energies between 53 and 65 kJ/mol, depending on the Au coverage. If the Au deposit was annealed to 770 K prior to CO exposure, the CO adsorption energy decreased significantly. STM measurements revealed that the Au particles grow upon annealing, but are not encapsulated by TiOx suboxides. The higher CO adsorption energy observed for smaller Au coverages and before annealing is attributed to a significantly stronger interaction of CO with mono- and bilayer Au islands, while for higher particles, the adsorption energy becomes more bulk-like. The implications of these effects on the known particle size effects in CO oxidation over supported Au/TiO2 catalysts are discussed.  相似文献   

16.
J. Wang 《Surface science》2006,600(21):4855-4859
Presented are thermal desorption spectroscopy (TDS) and adsorption probability measurements of iso-butane on the Zn-terminated surface of ZnO. The initial adsorption probability, S0, decreases linearly from 0.57 to 0.22 (±0.02) with impact energy, Ei = 0.74-1.92 eV, and is independent of adsorption temperature, Ts = 91-114 K (±5 K), indicating non-activated molecular adsorption. The coverage, Θ, dependent adsorption probabilities, S(Θ), show a cross-over from adsorbate-assisted adsorption (S increases with Θ) to Kisliuk-like dynamics at about the desorption temperature of iso-butane bi-layers (∼110 K). Thus, the adsorption dynamics are precursor-mediated. The enhanced (gas-surface) mass-match, caused by forming a second layer of the alkane, leads to adsorbate-assisted adsorption. A direct fitting procedure of the TDS data yields a pre-exponential factor of 2.5 × 1013/s and a coverage dependent heat of adsorption of Ed(Θ) = 39 − 6 ∗ Θ + 2.5 ∗ exp(−Θ/0.07) kJ/mol.  相似文献   

17.
The growth mode and electronic structure of Au nano-clusters grown on NiO and TiO2 were analyzed by reflection high-energy electron diffraction, a field-emission type scanning electron microscope, medium energy ion scattering and photoelectron spectroscopy. Au was deposited on clean NiO(0 0 1)-1 × 1 and TiO2(1 1 0)-1 × 1 surfaces at room temperature with a Knudsen cell at a rate of 0.25-0.35 ML/min (1 ML = 1.39 × 1015 atoms/cm2:Au(1 1 1)). Initially two-dimensional (2D) islands with thickness of one Au-atom layer grow epitaxially on NiO(0 0 1) and then neighboring 2D-islands link each other to form three-dimensional (3D)-islands with the c-axis oriented to the [1 1 1] direction. The critical size to form 3D-islands is estimated to be about 5 nm2. The shape of the 3D-islands is well approximated by a partial sphere with a diameter d and height h ranging from 2.0 to 11.8 nm and from 0.95 to 4.2 nm, respectively for Au coverage from 0.13 to 4.6 ML. The valence band spectra show that the Au/NiO and Au/TiO2 surfaces have metallic characters for Au coverage above 0.9 ML. We observed Au 4f spectra and found no binding energy shift for Au/NiO but significant higher binding energy shifts for Au/TiO2 due to an electron charge transfer from Au to TiO2. The work function of Au/NiO(0 0 1) gradually increases with increase in Au coverage from 4.4 eV (NiO(0 0 1)) to 5.36 eV (Au(1 1 1)). In contrast, a small Au deposition(0.15 to 1.5 ML) on TiO2(1 1 0) leads to reduction of the work function, which is correlated with an electron charge transfer from Au to TiO2 substrate.  相似文献   

18.
Jakub Drnec 《Surface science》2009,603(13):2005-2014
The adsorption of Cs on Pt(1 1 1) surfaces and its reactivity toward oxygen and iodine for coverages θCs?0.15 is reported. These surfaces show unusual “anomalous” behavior compared to higher coverage surfaces. Similar behavior of K on Pt(1 1 1) was previously suggested to involve incorporation of K into the Pt lattice. Despite the larger size of Cs, similar behavior is reported here. Anomalous adsorption is found for coverages lower than 0.15 ML, at which point there is a change in the slope of the work function. Thermal Desorption Spectroscopy (TDS) shows a high-temperature Cs peak at 1135 K, which involves desorption of Cs+ from the surface.The anomalous Cs surfaces and their coadsorption with oxygen and iodine are characterized by Auger Electron Spectroscopy (AES), TDS and Low Electron Energy Diffraction (LEED). Iodine adsorption to saturation on Pt(1 1 1)(anom)-Cs give rise to a sharp LEED pattern and a distinctive work function increase. Adsorbed iodine interacts strongly with the Cs and weakens the Cs-Pt bond, leading to desorption of CsxIy clusters at 560 K. Anomalous Cs increases the oxygen coverage over the coverage of 0.25 ML found on clean Pt. However, the Cs-Pt bond is not significantly affected by coadsorbed oxygen, and when oxygen is desorbed the anomalous cesium remains on the surface.  相似文献   

19.
We report the iron isotope effect on a transition temperature (Tc) in an optimally-doped (Ba,K)Fe2As2 (Tc = 38 K) and SmFeAsO1−y (Tc = 54 K) superconductors. In order to obtain the reliable isotope shift in Tc, twin samples with different iron isotope mass are synthesized in the same conditions (simultaneously) under high-pressure. We have found that (Ba,K)Fe2As2 shows an inverse iron isotope effect αFe = −0.18 ± 0.03 while SmFeAsO1−y shows a small iron isotope effect αFe = −0.02 ± 0.01, where the isotope exponent α is defined by Tc  Mα (M is the isotopic mass). The results show that αFe changes in the iron-based superconductors depending on the system. The distinct iron isotope effects imply the exotic coupling mechanism in the iron-based superconductors.  相似文献   

20.
Weixin Huang 《Surface science》2006,600(4):793-802
The interaction of atomic hydrogen with thin epitaxial FeO(1 1 1) and Fe3O4(1 1 1) films was studied by TDS, XPS and LEED. On the thin, one Fe-O bilayer thick FeO film, partial reduction occurs in two steps during exposure. It ends after removal of 1/4 monolayer (ML) of oxygen with a 2 × 2 pattern appearing in LEED. This FeO0.75 film is passive against further reduction. The first reduction step saturates after removal of ∼0.2 ML and shows autocatalytic kinetics with the oxygen vacancies formed during reduction causing acceleration. The second step is also autocatalytic and is related with reduction to the final composition and an improvement of the 2 × 2 order. A structure model explaining the two-step reduction is proposed. On the thick Fe3O4 film, irregular desorption bursts of H2O and H2 were observed during exposure. Their occurrence appears to depend on the film quality and thus on surface order. Because of the healing of reduction-induced oxygen vacancies by exchange of oxygen or iron with the bulk, a change of the surface composition was not visible. The existence of partially reduced oxide phases resistant even to atomic hydrogen is relevant to the mechanism of dehydrogenation reactions using iron oxides as catalysts.  相似文献   

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