Oxygen adsorption on silver (110): Dispersion,bonding and precursor state

Angle-resolved photoemission, both in the laboratory and with synchrotron radiation, has been used to investigate three states of adsorbed oxygen on Ag(110): chemisorbed atomic oxygen, undissociated chemisorbed dioxygen and physisorbed molecular oxygen. For atomic oxygen, dispersion of adsorbate-induced levels was observed indicating strong oxygen-oxygen interaction in the [001] direction. Polarised light combined with selection rules was used to determine the symmetry of the adsorbate-derived bands. The adsorption geometry and symmetry of the oxygen-induced levels for the chemisorbed dioxygen species were also investigated with the selection rules. The molecule appears to lie parallel to the surface with the OO axis oriented in the [11̄0] direction. The adsorbate-induced feature of lowest ionisation potential at 1.1 eV relative to E_F is 1 π_g-derived. The very low frequency reported for the OO stretch and the analogy with coordination chemistry also suggest that the chemisorbed dioxygen species is lying down on the surface. At sufficiently low temperature, oxygen was found to physisorb on the bare metal also with its OO axis parallel to the surface. We identify physisorbed oxygen as an intrinsic precursor state for chemisorption.     

氧在Nb(110)表面吸附的第一性原理研究

通过第一性原理赝势平面波方法研究了氧在Nb(110)表面的吸附性质随覆盖度变化规律. O在Nb(110)表面最稳定吸附位是洞位,次稳定吸附位是长桥位. 在长桥位吸附时, O诱导Nb(110)表面功函数随覆盖度的增加而几乎线性增加;但当O在洞位吸附时, 与干净Nb表面相比, 覆盖度为0.75 ML和1.0 ML时功函数增加, 而覆盖度为0.25 ML和0.5 ML时功函数减小.通过对面平均电荷密度分布和偶极矩变化的讨论, 解释了由吸附导致功函数复杂变化的原因.通过对表面原子结构和态密度分析, 讨论了O在Nb表面吸附时引起表面原子结构变化以及O和Nb(110)表面原子的相互作用.     

Oxygen adsorption and oxide formation on Cr(100) and Cr(110) surfaces

The adsorption of oxygen on (100) and (110) clean chromium surfaces was studied by electron diffraction (LEED and RHEED) and scanning microscopy. The beginning of the oxidation process was investigated using LEED technique. The successive patterns, observed with increasing oxidizing atmosphere, allow us to propose an oxidation mechanism for these chromium surfaces. For both orientations, rhombohedral chromium oxide grows on the surface, as shown by RHEED diagrams. The behaviour of the two planes is markedly different: the oxide film is polycrystalline on the (100) surface, while the pseudo six-fold symmetry of the (110) plane permits a parallel oriented growth.     

Oxygen adsorption and the surface electronic structure of GaAs (110)

The surface electronic structure of cleaved GaAs (110) is found to be very sensitive to small amounts of adsorbed oxygen. For example, adsorbing oxygen on only a few percent of the surface Ga or As atomic sites can produce changes of a factor of two in the surface electronic structure. Thus, long range effects must be involved, and these are associated with rearrangement of the surface atoms.     

Potassium adsorption on Fe(110)

LEED, AES, UPS and XPS were used to study submonolayer coverages of potassium on Fe(110). At room temperature the maximum potassium coverage is characterized by a LEED superstructure. This LEED pattern is interpreted as being due to a hexagonal close-packed K layer on Fe(110), resulting in a maximum atom density of 5.3 × 10¹⁴ cm^?2, i.e.θ k = 0.31. The work function change and the shift of the K(2p) and K(3p) core levels with potassium coverage indicate a charge transfer from potassium to iron at low potassium coverages.     

Oxygen adsorption on Pt(110)-(1×2): new high-coverage structures

From an interplay between scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations, a comprehensive picture is obtained for oxygen adsorption on the Pt(110)-(1×2) surface, from single isolated oxygen atoms chemisorbed in FCC sites along the platinum ridges to the formation of a new high-coverage oxide-like structure with a local coverage of two oxygen atoms per platinum surface atom. We find that the repulsive O–O interactions for the O/Pt(110) system are compensated by an effective O–O attractive interaction originating from a strong coupling between oxygen adsorption and platinum lattice distortions.     

Oxygen adsorption on the LaB6(100), (110) and (111) surfaces

Oxygen adsorption on the LaB₆(100), (110) and (111) clean surfaces has been studied by means of UPS, XPS and LEED. The results on oxygen adsorption will be discussed on the basis of the structurs and the electronic states on the LaB₆(100), (110) and (111) clean surfaces. The surface states on LaB₆(110) disappear at the oxygen exposure of 0.4 L where a c(2 × 2) LEED pattern disappears and a (1 × 1) LEED pattern appears. The work function on LaB₆(110) is increased to ～3.8 eV by an oxygen exposure of ～2 L. The surface states on LaB₆(111) disappear at an oxygen exposure of ～2 L where the work function has a maximum value of ～4.4 eV. Oxygen is adsorbed on the surface boron atoms of LaB₆(111) until an exposure of ～2 L. Above this exposure, oxygen is adsorbed on another site to lower the work function from ～4.4 to ～3.8 eV until an oxygen exposure of ～100L. The initial sticking coefficient on LaB₆(110) has the highest value of ～1 among the (100), (110) and (111) surfaces. The (100) surface is most stable to oxygen among these surfaces. It is suggested that the dangling bonds of boron atoms play an important role in oxygen adsorption on the LaB₆ surfaces.     

Co adsorption on potassium promoted Fe(110)

CO adsorption on potassium covered Fe(110) has been studied using UPS, XPS, AES and flash desorption. It was found that CO adsorbs molecularly at room temperature with a larger binding energy than on clean Fe(110). The CO saturation coverage increases and the sticking coefficient decreases with increasing potassium coverage. On heating, the probability of adsorbed CO dissociating increases with the amount of potassium present. The UPS spectra show that the CO 4σ peak is shifted by 0.8 eV to higher binding energies on Fe(110) + K and that at 21.2 eV the peak due to the 1π + 5σ orbitals is split into a double peak. The catalytic relevance of the measurements is discussed with reference to the Fischer-Tropsch synthesis.     

The adsorption of CO on Cu(110)

The adsorption of CO on Cu(110) has been studied by LEED, surface potentials and infrared spectroscopy. With increasing surface coverage the s.p. passes through a maximum value of 0.29 V and than falls to 0.17 V at saturation. The heat of adsorption is nearly constant (~55 kJ mol^?1) up to the maximum s.p. but then falls rapidly. A ( 2× 1) structure is formed near the s.p. maximum, followed by a structure which is compressed in the [11?0] direction and poorly ordered in the [001] direction but tending towards c(1.3 × 2). At low coverage two infrared bands appear at 2088 and 2104 cm^?1; their relative intensity is similar at 77, 195 and 295 K. As the coverage increases, the bands shift in frequency and merge into a single band at 2094 cm^?1. The origin of the two bands is discussed in relation to the overlayer structure. Strong interaction between CO molecules is shown by the spectra of mixtures of ¹³CO and ¹²CO.     

Oxygen adsorption on the tungsten (110) plane. Electron impact and thermal desorption at high temperatures

When a layer of oxygen on the (110) plane of tungsten at coverages O/W≦0.5 is heated from 100 K, O⁺ evolution under electron impact becomes almost negligible at 600 K. On further heating, however, a slow, temperature-dependent evolution of O⁺ current is observed atT≳1500 K. For O/W>0.3 there is also desorption under massive bombardment. Once an equilibrium value of O⁺ current has been established, there is rapid adjustment to the appropriate equilibrium value when the temperature changes in the range 1500–1700 K. On cooling toT<1000 K, O⁺ decreases rapidly; on reheating toT>1500 K, O⁺ increases slowly again. Above 1700 K there is thermal desorption which is also reflected in the O⁺ signal. These facts indicate that there is a slow activated evolution of an electron sensitive state above 1500 K, from a reconstructed state formed by heating the low temperature layer toT≧1000 K. The latter state seems to be reformed on cooling below 1500 K.     

Chemisorption of oxygen on the silver (110) surface

In the present work the interaction between oxygen and the silver (110) surface is investigated, mainly using LEED-Auger techniques and thermal desorption spectra. The formation and stability of adsorption layers is studied after exposures at pressures from 10^?3 to 1 torr. At maximum coverage, a (2 × 1) superstructure is formed which is stable up to the desorption temperature. At lower coverage, (3 × 1) and (4 × 1) superstructures are also observed. On the basis of the experimental evidence, tentative models for these structures are presented and discussed.     

Hydrogen adsorption on Pd/TiFe (110) surface

Adsorption of hydrogen on the TiFe (110) surface covered by palladium monolayer was investigated using the full potential linearized augmented plane wave method within the local density approximation. Influence of palladium coating to adsorption properties of the TiFe (110) surface as well as difference it from Pd/TiFe (100) are discussed.     

The adsorption entropy of H on W(110)

Recently, Nahm and Gomer have measured the increase in entropy associated with the adsorption of hydrogen on the W(110) surface. We discuss the implications of this data, and address it within the framework of a model introduced recently [Phys. Rev. Lett. 68 (1992) 2846; Surf. Sci. 287/288 (19930 837] to describe hydrogen dynamics on this surface.     

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.     

Monte Carlo simulations of hydrogen adsorption on the W(110) and Mo(110) surfaces

Kinetics of low-temperature hydrogen and deuterium adsorption on W(110) and Mo(110) surfaces have been studied by the real-time Monte Carlo simulations. Recently reported qualitative dependence of the adsorption characteristics on variation of the H₂ flux is described in terms of the dynamical equilibrium between incident and desorption fluxes and improved conditions for accommodation for the hydrogen molecules at high incident fluxes. The role of the intrinsic precursor state in hydrogen dissociative adsorption is analyzed.Received: 16 February 2004, Published online: 28 May 2004PACS: 82.65. + r Surface and interface chemistry; heterogeneous catalysis at surfaces - 02.50.Ng Distribution theory and Monte Carlo studies - 82.20.Wt Computational modeling; simulation     

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.     

Oxygen adsorption on a Pd(111) surface

The interaction of oxygen with Pd(111) has been studied by means of AES, ELS, thermal desorption (TD), electron stimulated desorption (ESD) and work function measurements. It was found that a very small part ( ～ 2–3%) of the available adsorption sites are contributing to the O⁺ electron stimulated yield, the population of the latter being accompanied by enormously large work function changes (up to ～ 0.9 eV). A mechanism of adsorption and depopulation of these sites involving oxygen bulk and surface diffusion has been proposed.     

The oxidation of methanol on a silver (110) catalyst

The oxidation of methanol was studied on a Ag(110) single-crystal by temperature programmed reaction spectroscopy. The Ag(110) surface was preoxidized with oxygen-18, and deuterated methanol, CH₃OD, was used to distinguish the hydroxyl hydrogen from the methyl hydrogens. Very little methanol chemisorbed on the oxygen-free Ag(110) surface, and the ability of the silver surface to dissociatively chemisorb methanol was greatly enhanced by surface oxygen. CH₃OD was selectively oxidized upon adsorption at 180 K to adsorbed CH₃O and D₂¹⁸O, and at high coverages the D₂¹⁸O was displaced from the Ag(110) surface. The methoxide species was the most abundant surface intermediate and decomposed via reaction channels at 250, 300 and 340 K to H₂CO and hydrogen. Adsorbed H₂CO also reacted with adsorbed CH₃O to form H₂COOCH₃which subsequently yielded HCOOCH₃ and hydrogen. The first-order rate constant for the dehydrogenation of D₂COOCH₃ to DCOOCH₃ and deuterium was found to be (2.4 ± 2.0) × 10¹¹ exp(?14.0 ± 0.5 $\text{kcal}\text{mole}$ · RT)sec^?1. This reaction is analogous to alkoxide transfer from metal alkoxides to aldehydes in the liquid phase. Excess surface oxygen atoms on the silver substrate resulted in the further oxidation of adsorbed H₂CO to carbon dioxide and water. The oxidation of methanol on Ag(110) is compared to the previous study on Cu(110).