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.     

Alternate interpretation of final state relaxation for chemisorbed molecules

Ab initio Xα electronic spectra have been calculated for CO before and after chemisorption onto Ni(001) in the observed c(2 × 2) symmetry. The electronic spectra of a monolayer of C₂H₂ has also been calculated and referenced to E_F of Ni. The theoretical eigenvalue spectra of these two molecules agrees colosely with ionization potentials measured by UPS when these molecules are chemisorbed onto Ni. Charge density calculations for CO indicate that the molecular orbitais are delocalized by the COCO and CONi bonds. These results suggest that final state relaxation effects are small and possibly zero for the chemisorbed molecule and that the relaxation energy of chemisorbed molecules is approximately the correlation energy.     

Molecular cluster calculations for the interpretation of CO chemisorption on a Ni(100) surface

Self-consistent Hartree-Fock-Slater molecular cluster calculations for the chemisorption of carbon monoxide on a Ni(100) surface are presented. In earlier calculations of this type the CO molecule has been assumed to be chemisorbed in a hollow position of C_4v symmetry. A recent EELS experiment shows however that in the most stable configuration CO is linearly bonded to the Ni atoms, i.e. a top position of the CO-molecule. This experiment indicates also that there exists an additional bridge bonding of the CO molecule to the two nearest neighbour Ni atoms. The variation of the energy levels, binding energies and charge distribution with the height of the CO molecule above the nickel surface is calculated for the top position using the NiCO and Ni₅CO clusters and for the bridge bonding configuration using the Ni₂CO cluster. The CO 1π level is found to be split by about 0.8 eV in bridge bonding geometry. For both hollow and top positions the 1π and 5σ levels are separated by about 0.5 eV. The energy separation to the 4σ level is about 3 eV, which is in good agreement with experimental data. Theoretical ionization energies relative to the Fermi energy for top position geometry at a bond distance of 3.5 au between the carbon atom and nickel surface were found to be 25.7, 11.7, 8.7 and 8.2 eV for the 3σ, 4σ, 5σ and 1π levels which should be compared with the experimental data of 29.0, 10.8, 8.4 and 7.8 eV, respectively. The corresponding ionization energies for a bond angle of 99° in bridge bonding were 23.7, 12.1, 7.3, 7.0 and 7.9 eV. The two last values represent the 1π level which is split into two levels in this geometry. The variation of the C-O stretch vibrational frequencies with the height of the CO molecule above the surface for the top-position geometry is estimated from the 5σ and 2π gross orbital populations and from the CO σ and π overlap populations.     

Unified theory of photoelectron and vibrational spectra of chemisorbed molecules on metal surfaces

Photoelectron spectra of diatomic molecules such as CO and N₂ weakly chemisorbed on metal surfaces exhibit a multitude of satellite lines due to multi-electron excitations in which the photoinduced hole is screened by charge transfer from the metal into the low-lying unoccupied level. Recently Heskett et al. [Surface Sci. 139 (1984) 558] noticed that in the systems which display the satellite lines, the molecular vibrational frequencies decrease with increasing molecular coverage, in contrast to what is expected from dipole-dipole coupling theory. They then claimed that instead of the 2π¹-metal bonding, the 5σ orbital is responsible for the bond to the substrate. This leads to a shift of the C-O stretching frequency to lower energy as the molecular coverage increases, since the 5σ orbital has an antibonding character with respect to the stretch vibration. Other experimental phenomena which should be taken into account are the negative shift and substantial broadening of the vibrational spectra of these molecules upon chemisorption on metal surfaces. Experimental results seem to reveal that the magnitude of negative shift and width increases with the strength of chemisorption, thereby suggesting that an unique mechanism is responsible for these vibrational properties. We propose a unified theory which enables us to understand these apparently unrelated electronic and vibrational properties in terms of the occupancy of the low-lying 2π¹ level in the neutral ground state and in vibrational excited states of the chemisorbed molecules. Also discussed is a change of the vibrational frequencies with coverage within the framework of the chemical frequency shift caused by charge transfer between the 2π¹ level and the metal.     

The initial oxidation of Cr(100)

Electron energy loss spectroscopy (EELS), Auger spectroscopy, and low-energy electron diffraction (LEED) have been used to study oxygen chemisorption on and the initial oxidation of Cr(100). With O₂ exposures up to 5 L, CrO vibrational frequencies between 495 and 545 cm⁻¹ are observed. A CrO stretching frequency at 635 cm⁻¹, probably due to rhombohedral Cr₂O₃, is observed to emerge strongly by ≈ 60 L. Based on a sequence of O₂ exposures at 300 K and on a second sequence with 625 and 1175 K anneals, a model of the initial oxidation of Cr(100) is presented. Subsurface oxygen in interstitial sites with Cr atoms maintaining bulk positions is proposed to act as a nucleus for subsequent oxide growth. According to this model, oxide growth at 300 K occurs primarily through domain expansion, while frequent creation of new domains occurs at 625 K. At elevated temperatures, competition between domain growth and diffusion into the bulk is observed.     

LCGTO-Xα model cluster study for the chemisorption of CO on twofold sites of Ni surfaces

The cluster Ni₂CO is studied as a simplified model for the chemisorption of CO on twofold bridging sites of transition metal surfaces. Using the LCGTO-Xα method we have calculated the potential energy surface for the totally symmetric stretching motion keeping the NiNi distance fixed at the bulk value. The minimum energy is found at a NiC distance of 1.72 Å and a CO bond length of 1.19 Å. The vibrational frequency for the CO bond (1850 cm^?1) shows reasonable agreement with EELS data (1810, 1870 cm^?1), whereas the (Ni₂)C frequency of 495 cm^?1 is remarkably higher than the experimental values (380, 400 cm^?1) indicating an overestimation of the chemisorption bond strength in this simple cluster model. The bonding between CO and Ni is analyzed using orbital correlations, ionization energies and Mulliken population analysis. Important bonding contributions from π backdonation are identified while the a₁orbital manifold exhibits strong antibonding effects.     

A density functional theory study on the adsorption of CO and O2 on Cu-terminated Cu2O （111） surface

The adsorptions of CO and 02 molecules individually on the stoichiometric Cu-terminatcd Cu20 （111） surface are investigated by first-principles calculations on the basis of the density functional theory. The calculated results indicate that the CO molecule preferably coordinates to the Cu2 site through its C atom with an adsorption energy of-1.69 eV, whereas the 02 molecule is most stably adsorbed in a tilt type with one O atom coordinating to the Cu2 site and the other O atom coordinating to the Cul site, and has an adsorption energy of -1.97 eV. From the analysis of density of states, it is observed that Cu 3d transfers electrons to 2π orbital of the CO molecule and the highest occupied 5σ orbital of the CO molecule transfers electrons to the substrate. The sharp band of Cu 4s is delocalized when compared to that before the CO molecule adsorption, and overlaps substantially with bands of the adsorbed CO molecule. There is a broadening of the 2π orbital of the 02 molecule because of its overlapping with the Cu 3d orbital, indicating that strong 3d-2π interactions are involved in the chemisorption of the 02 molecule on the surface.     

Photoemission of adsorbed CO and H2O on Pt: Valence band studies

In this paper, using monochromatic He and Ne discharge lines, we report the ultraviolet photoemission results for CO and H₂O adsorbed on a Pt crystal. For chemisorbed CO, we use the energy dependence of the photoionization cross section to deduce that the 5σ?1π splitting is approximately 1 eV and that the 5σ is located at about ?8.1 eV while the 1π is at ?9.1 eV. The distribution of metal electrons changes in two ways upon CO chemisorption: a d character peak is found at ?4 to ?5 eV and emission at E_f is strongly depleted. For the weakly adsorbed water molecule, we find preferential attenuation of Pt states near E_f that has heretofore been observed for strongly chemisorbed systems.     

Theoretical studies of CO/Ni(100): Geometry,vibrational frequencies and ionization potentials for the on-top site

The chemisorption of CO on the (100) surface of Ni has been studied using an Ni₁₄ cluster and generalized valence bond (GVB) methods. CO is found to bond perpendicular to the Ni surface with optimized NiC and CO bond distances of 1.94 and 1.15 Å, respectively. The calculated NiCO bond strength is 29.7 kcal (experimental values 30–32 kcal). Vibrational frequencies are calculated to be 401 cm^?1 for NiC stretch, 327 cm^?1 for NiCO bend, and 2129 cm^?1 for CO stretch. This decrease of the CO frequency by 71 cm^?1 from the free molecule value is consistent with experiment based on self-consistent calculations of the positive ion states. We propose a new explanation for the loss of one PES peak upon chemisorption.     

A quadratic configuration interaction study of N2O and N2O·−

The geometries and vibrational frequencies of both N₂O and N₂O^·– were calculated at the QCISD and QCISD(T) levels of theory using aug-cc-pVDZ and aug-cc-pVTZ basis sets. The electron affinity of N₂O was determined to be ?0.15 eV. This work corroborates an earlier G2 study and suggests that the currently accepted value for the electron affinity, 0.22eV, is in error. This study represents the best calculation to date for the geometry and vibrational frequencies of N₂O^·?     

Oxygen chemisorption on Cr(110): II. Evidence for molecular O2(ads)

Oxygen chemisorption and dissociation on Cr(110) at 120 K have been studied using high resolution electron energy loss spectroscopy (HREELS), electron stimulated desorption ion angular distribution (ESDIAD), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Dissociative adsorption dominates although vibrational and stimulated desorption data provide evidence for a coexisting minority molecular binding state. An O₂(ads) vibrational frequency of 1020 cm⁻¹ and a six beam ESDIAD pattern are suggestive of super-oxo O₂(ads) bonding at six local sites each with the O-O molecular axis tilted away from the surface normal. These results are compared with data for chemisorbed oxygen on other transition metal surfaces.     

The adsorption of N2: Chemisorbed on Ni(110) and physisorbed on Pd(111)

The bonding of molecular N₂ has been investigated with angle resolved photoelectron spectroscopy and inelastic electron scattering. The spectra obtained from N₂ chemisorbed onto a Ni(110) surface are compared to CO chemisorbed onto Ni(110) and to N₂ physisorbed onto Pd(111). The N₂ molecular axis was found to be normal to the crystal surface for the chemisorbed state on Ni(110) and random for the physisorbed state on Pd(111). The NN and NiN₂ stretching frequecies indicate that the N₂ molecule is terminally bonded to a single Ni atom on Ni(110). The binding energies of the two outer σ states and one π state of chemisorbed N₂ were measured, indicating that the bonding of N₂ to a metal surface is different than CO. Both σ states drop in energy compared to the π level due to the fact that both of them are involved in the N₂ substrate bond. The symmetry of the gas phase N₂ molecule is reduced upon adsorption. The consequences of this are seen in the dipole active NN vibrational mode, the large intensity of the Ni-N₂ vibrational mode and the coupling of the adsorbate 4σ(2σ_u) level to the final state resonance which is forbidden by symmetry in the gas phase. Many electron excitation satellite lines are observed in the valence spectra of both the chemisorbed and physisorbed N₂. The physisorbed satellite lines are nearly identical to those seen in gas phase N₂, while the chemisorbed N₂ spectra has new satellite structure, due to the interaction with the substrate.     

Electron energy loss spectroscopy of ethylene on Ag(110) precovered with oxygen: energy dependence of the cross section

The intensities of the CH out-of-plane bending (v₇) and CH stretch (v₁) vibrations of ethylene adsorbed on Ag(l 10) precovered with oxygen have been measured in EELS as a function of beam energy from 2 to 20 eV. The energy dependence of the v₇ vibration is satisfactorily described by the dipole approximation. For the v₁ vibration, which is purely dipole forbidden, an entirely different energy dependence is found, with a maximum intensity at a beam energy of 2 eV. Comparison of the EEL intensities observed experimentally with those predicted from the dipole moments of the free molecule confirms the conclusion that the molecular plane of chemisorbed ethylene is parallel to the surface.     

Adsorption and dissociation of O2 on CuCl(1 1 1) surface: A density functional theory study

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.     

Untersuchung über den einfluss von adsorbierten gasen auf die elektronen-energieverlust-spektren einer Ni(110)-oberfläche

Electron energy loss spectra on a (110) nickel surface exhibit characteristic changes upon adsorption of H₂, CO and O₂. The clean surface shows only the surface and bulk plasmon losses at 8 eV and 18 eV respectively. Adsorption of CO produces two new loss peaks at 13.5 eV and 5.5 eV. Loss peaks due to hydrogen adsorption at 15 eV and 7.5 eV show a strong correlation with the well known adsorption characteristics of this system. The oxygen induced losses are different for chemisorbed O on Ni and NiO. In any case the chemisorption-induced losses are well established for primary energies below 120eV. In the loss spectra with higher excitation energies only a drastic decrease of the surface plasmon loss peak-height is visible. If the new losses can be attributed to one-electron excitations from molecular orbital levels due to the chemisorption bond, with assumptions of the final state of the excited electron a determination of the postition of these levels can be made. In case of CO and H₂ reasonable results are evaluated.     

Structural, electronic and vibrational properties of indium oxide clusters

Geometric, electronic and vibrational properties of the most stable and energetically favourable configurations of indium oxide clusters In_mO_n (1≤m, n≤4) are investigated using density functional theory. The lowest energy geometries prefer the planar arrangement of the constituent atoms with a trend to maximize the number of ionic In-O bonds. Due to the charge transfer from In to O atoms, the electrostatic repulsion occurs between the atoms with the same kind of charge. The minimization of electrostatic repulsion and the maximization of In-O bond number compete between each other and determine the location of the isometric total energy. The most stable linear In-O-In-O structure of In₂O₂ cluster is attributed to the reduced electrostatic repulsive energy at the expense of In-O bond number, while the lowest energy rhombus-like structure of In₂O₃ cluster reflects the maximized number of In-O bonds. Furthermore, the vibrational frequencies of the lowest energy clusters are calculated and compared with the available experimental results. The energy gap and the charge density distribution for clusters with varying oxygen/indium ratio are also discussed.     

Energy threshold of Cs-induced chemisorption of oxygen on a GaAs(Cs,O) surface

The probability of Cs-induced chemisorption of oxygen on a p-GaAs(Cs, O) surface is experimentally shown to be close to unity only when the work function does not exceed ～3.1 ± 0.1 eV. The measured adsorption energy threshold likely corresponds to the energy of the unoccupied level of the antibonding 2π* orbital of the O₂ molecule in the preadsorption state on the semiconductor surface.     

The chemisorption of nitrogen on the (001) surface of ruthenium

High resolution electron energy loss spectroscopy (EELS), thermal desorption mass spectrometry (TDMS) and low energy electron diffraction (LEED) have been used to investigate the molecular chemisorption of N₂ on Ru(001) at 75 K and 95 K. Adsorption at 95 K produces a single chemisorbed state, and, at saturation, a (√3x√3) R30° LEED pattern is observed. Adsorption at 75 K produces an additional chemisorbed state of lower binding energy, and the probability of adsorption increases by a factor of two from its zero coverage value when the second chemisorbed state begins to populate. EEL spectra recorded for all coverages at 75 K show only two dipolar modes — ν(RuN₂) at 280–300 cm^?1 and ν(NN) at 2200–2250 cm^?1 — indicating adsorption at on-top sites with the axis of the molecular standing perpendicular to the surface. The intensities of these loss features increase and ν(NN) decreases with increasing surface coverage of both chemisorbed states.     

Structural,electronic and elastic properties of potassium hexatitanate crystal from first-principles calculations

The structural, electronic and elastic properties of potassium hexatitanate (K₂Ti₆O₁₃) whisker were investigated using first-principles calculations. The calculated cell parameters of K₂Ti₆O₁₃ including lattice constants and atomic positions are in good agreement with the experimental data. The obtained formation enthalpy (−61.1535 eV/atom) and cohesive energy (−137.4502 eV/atom) are both negative, showing its high structural stability. Further analysis of the electronic structures shows that the potassium hexatitanate is a wide-band semiconductor. Within K₂Ti₆O₁₃ crystal, the TiO bonding interactions are stronger than that of KO, while no apparent KTi bonding interactions can be observed. The structural stability of K₂Ti₆O₁₃ was closely associated with the covalent bond interactions between Ti (d) and O (p) orbits. Further calculations on elastic properties show that K₂Ti₆O₁₃ is a high stiffness and brittle material with small anisotropy in shear and compression.     

Theoretical study of the chemisorption of H2 on boron cluster surfaces

“Ab initio” RHF calculations are used to investigate the chemisorption of a H₂ molecule on boron cluster surfaces. Potential energy surfaces and electron charge difference density plots are given. The results obtained indicate that the H₂ molecule in certain cases is dissociated on the surface, and that the hydrogen atoms are individually bound to different boron atoms. It is also found that the chemisorbed hydrogen atoms can move almost freely in certain directions parallel to the boron surface.