Short Bridge Site Research Articles

Pathways to dissociation of O 2 on Cu (110) surface: first principles simulations

First principles simulations based on density functional theory have been used to determine absorption energies and pathways to dissociation of oxygen at finite temperature on the Cu (110) surface. Our results, which are in accord with recent experimental studies, suggest that adsorption kinetics play an important role in determining the mechanism of dissociation. Energy minimisation studies confirm that the HL site within the fourfold hollow is energetically most favourable for molecular adsorption where the O 2 is peroxo like. Studies using the NEB method show that asymmetric rather than symmetric dissociation is preferred at this site and that the energy barrier for dissociation is virtually zero along the [001] direction and 120 meV for the [11̄0] direction. We have identified the [11̄0] orientation at this site as a possible precursor state but only at very low temperatures. Ab initio dynamic simulations were used to examine dissociation on a substrate initially at 300 K for collisions normal to the surface with a kinetic energy of either 50 or 500 meV. Particularly for the lower impact energy molecules carry out complex motions once at the surface and significant steering is observed. In some cases molecules are observed to navigate several atomic distances across the surface before arriving at a fourfold hollow and adopting a favourable orientation prior to dissociation. On the time scale of the simulations (up to 5 ps) some molecules were observed to become trapped on short-bridge sites with the [11̄0] orientation; in this case the electronic structure is superoxo like.

Chemisorption of iodine on Ag(110): a density-functional theory approach

The adsorption of iodine atom (I) on Ag(110) surface modeled by two-layer Ag m+ n ( m, n) clusters has been studied by means of B3-LYP density-functional method. The effects of basis set and cluster size were tested. It is found that the adsorption properties are less sensitive to geometry than to basis set. The adsorption energy of I is found to converge after the coordination number of the Ag atoms forming the adsorption sites reaches its maximum. This determination allows us to extrapolate the adsorption properties of the iodine/Ag(110) surface from those of iodine/silver clusters. As a result, Ag 16(8,8) and Ag 21(12,9) are chosen to model four possible adsorption sites (long and short bridges, hollow and top) of Ag(110). The results show that the short bridge site is the most preferred for iodine adsorption, followed by top, hollow and long bridge sites. The present results about the effect of the adsorption of I atom on Ag(110) surface also provide a sound theoretical basis for the understanding of the role of I in heterogeneous catalysis.

Mechanism of the hydrogenation of CO2 to methanol on a Cu(100) surface: dipped adcluster model study

The mechanism of the hydrogenation of CO2 to methanol on a Cu(100) surface was studied using the dipped adcluster model (DAM) combined with ab initio Hartree–Fock (HF) and second-order Møller–Plesset (MP2) calculations. The Langmuir–Hinshelwood (LH) mode, which corresponds to the reaction between coadsorbed species on the surface, was adopted. Our calculations show that hydrogen and formate are adsorbed at short-bridge sites. The coadsorption of hydrogen and CO2, in which CO2 is chemisorbed in the bent anionic state, is described well by the DAM. Five successive hydrogenations are involved in the hydrogenation of adsorbed CO2 to methanol: the intermediates are formate, dioxomethylene, formaldehyde and methoxy. The geometries of these intermediates and the transition states, as well as the energy diagrams in the reaction process, are presented. The rate-limiting step is the hydrogenation of adsorbed formate. Subsequent steps occur relatively readily, and lead to the formation of methanol. Clearly, any factor that could enhance the hydrogenation of formate on copper should lead to enhanced activity in methanol synthesis.

Orientational anisotropy in oxygen dissociation on Rh(110)

Scanning tunneling microscopy study of ${\mathrm{O}}_{2}$ dissociation on Rh(110) at 170 K shows a low-energy dissociation coordinate for a precursor with the O-O axis aligned in the [001] azimuth, indicating a strong influence of the surface anisotropy on the potential-energy surface. This results in pairs of oxygen atoms oriented along [001], which aggregate in chains along $[11\ifmmode\bar\else\textasciimacron\fi{}0].$ The atomic distance in the pairs, 3.3 \AA{}, is shorter than the [001] lattice constant, which led to the conclusion that the oxygen sits in adjacent asymmetric short bridge sites and that the creation of ``hot'' oxygen atoms is not favored.

Scattering and recoiling imaging spectrometry (SARIS) study of chlorine chemisorption on Ni(1 1 0)

The clean and chlorine chemisorbed Ni(1 1 0) surface has been investigated by scattering and recoiling imaging spectrometry (SARIS). Rare gas ion beams at 4 keV were scattered at different incident ( α) and exit ( β) angles along different azimuthal directions of the surface. The LEED pattern changed from a sharp (1 × 1) to a weak (3 × 1) after exposure to Cl 2. The He + images from the Cl/Ni surface exhibited very little differences compared to those of the clean surface. This is due to the high penetration depth of the He + ions, which results in low surface sensitivity. The images obtained from the Cl/Ni surface with the heavier ions exhibited obvious changes due to their low penetration depths, which facilitates high surface sensitivity. Scattering and Recoiling Imaging Code (SARIC) simulations were carried out in order to interpret the perturbations induced by the chlorine adatoms. Different chemisorption sites for chlorine were tested. The results of the experiments and simulations agree that Cl atoms are chemisorbed in the short-bridge sites above the [1 1 0] rows.

Adsorption of the formate species on copper surfaces: a DFT study

The density functional theory and the cluster model approach have been applied to study the interaction of the formate species with the copper (100), (110) and (111) surfaces. The short-bridge, long-bridge and cross-bridge sites of the copper surface have been modelled by Cu 7 and Cu 8 metal clusters after checking the validity of the results against those obtained with much larger clusters. The results show that the formate species is stabilized strongly on the short-bridge site of the surfaces considered and this is in agreement with available experimental data. For adsorption on the short-bridge site, and for the three surfaces considered, the Cu surfO is close to 2.02 Å, the CO bond length is 1.26 Å and the OCO angle is 128°. On this adsorption site, formate is bridge-bonded with the two oxygens almost in a top position on two copper atoms. For the long-bridge site, the oxygen atoms of the adsorbate are not located above the two copper atoms. The two OC bonds are equivalent when formate is adsorbed on these two sites. On the cross-bridge site, the formate species is bonded to the surface in a monodentate conformation. The two OC bonds are different with two clearly different bond lengths. The difference is larger for adsorption on the Cu (100) surface. In this case, the bond lengths are typical of a bond order of one and two. The bonding to the surface is essentially ionic, and the total charge in the adsorbate is 0.75 ±0.05 e, approximately.

NO monomer and (NO)x polymeric chain chemisorption on Pt{110}: Structure and energetics

The chemisorption of NO on Pt{110}-(1×1) and -(1×2) has been studied using density functional theory slab calculations with the generalized gradient corrections. On both surface phases the monomeric species is the most stable and the short-bridge sites are energetically the most favorable adsorption sites. Monomeric NO is adsorbed upright with its molecular axis normal, bonded to the surface through the N atom. On the (1×2) surface at high coverage a polymeric (NO)x chain structure is identified; this may well correspond to the structure experimentally observed at high coverage on the (1×2) surface formed after heating a multilayer to temperatures between 80 K and 200 K, characterized by an NO IR band at 1760 cm−1.

Reconstruction of small Si cluster after ethylene adsorption: A full-potential linear-muffin-tin-orbital molecular-dynamics study

Using full-potential linear-muffin-tin-orbital method, we have performed molecular-dynamics simulations for the ethylene adsorption on the Si5–7-cluster surfaces. The calculations show that the most favored adsorption site is the short bridge site for Si5 cluster, with the adsorption energy 1.78 eV. The adsorption structure of ethylene molecule is similar to that of the dimer-maintained structure for C2H4+Si(100)-(2×1). It indicates that ethylene is di-σ bonded to the Si5 cluster. At the same time, the Si5 cluster reconstructs after ethylene adsorption. The three-center bond among side atoms breaks, and new bonds form. For Si6 cluster, the most favored adsorption site is the atop site on the side atom. After adsorption, Si6 cluster reconstructs from tetragonal bipyramid to edge-capped trigonal bipyramid. The short bridge is the only available site to adsorb ethylene for Si7 cluster.

Adsorption and disproportionation reaction of OH on Ag surfaces: dipped adcluster model study

The adsorption and surface disproportionation reactions of OH on various silver surfaces were studied by the dipped adcluster model (DAM) combined with ab initio HF and MP2 methods. Our studies show that OH binds strongly to Ag surfaces; the adsorption energies were calculated to be 118.3kcalmol−1 at the short bridge site on Ag(110), 108.6kcalmol−1 at the fourfold hollow site on Ag(100), and 97.3kcalmol−1 at the threefold hollow site on Ag(111). The H–O axis is preferentially perpendicular to the surface for OH on Ag(100) and Ag(111), but tilts about 50° along the [001] azimuthal orientation on Ag(110). The Ag4d orbital plays an important role in adsorbing OH, whose molecular orbital analysis is described. Coadsorption of OH at the nearest fourfold sites on Ag(100) is stable with the H–O axis perpendicular to the surface; an inclined OH structure is proposed for the coadsorption of OH at the bridge sites and it is shown to be very reactive regarding the surface disproportionation reaction of OH. The structures and energy surfaces for the disproportionation reaction of OH to form H2O on Ag(100) are presented. The present study provides clear information regarding the adsorption, coadsorption and disproportionation of OH on Ag surfaces.

A quantum chemical investigation of imide adsorption at model Cu(110) surfaces

The adsorption of imide (NH) on small clusters of copper atoms representing a Cu(110) surface has been investigated using density functional methods. The calculations show that the imide adsorbs in the short bridge site with a Cu–N distance of 1.856 A and an N–H bond length of 1.031 A. The formation of imide '‘chains’' in the 〈110〉 direction gives additional stabilisation where the imide spacing within the chains is 5.1 A (i.e. two copper lattice spacings). An isolated imide prefers a linear geometry with the N–H bond orientated perpendicular to the surface plane but in the chains further stabilisation is obtained if the imides are tilted alternately in the 〈100〉 direction. Despite the relatively small cluster sizes used, the results show very good agreement with experimental results and help elucidate some experimental observations.

DFT–LDA study of NO adsorption on Rh(110) surface

We examine the interaction between NO and the Rh(110) surface using ab initio DFT–LDA pseudo-potential plane-wave total energy calculations. Four different adsorption sites for perpendicular NO are considered. The short-bridge site with linear NO is found to be the optimal adsorption configuration. It is also possible for NO to bond parallel to the surface, and this may be the precursor to NO dissociation.

Anomalous hydrogen adsorption sites found for the c(2×2)-3H phases formed on the Re(101̄0) and Ru(101̄0) surfaces

Hydrogen adsorption on the (101̄0) surfaces of Ru and Re leads to the formation of c(2×2)-3H phases. As determined by quantitative low-energy electron diffraction (LEED) and density functional theory calculations, hydrogen atoms, as expected, occupy threefold coordinated hcp sites along the densely packed rows and the unexpected short-bridge sites along the ridges in both c(2×2) phases. The Ru and Re substrates reconstruct only weakly and in a very similar fashion under hydrogen chemisorption. Most notably, there is a buckling in the third substrate layer of about 0.06 Å. Probably (though not outside the limits of error), there are also slightly lateral displacements (0.02 Å) of top-layer substrate atoms which are bridge-coordinated to hydrogen. The metal–hydrogen bond lengths determined for both surfaces correspond to hydrogen radii in the expected range of 0.4–0.7 Å.

Adsorption and temperature-dependent decomposition of SO 2 on Ni(110): an XPS and XAFS study

The adsorption and temperature-dependent decomposition of SO 2 on Ni(110) has been studied by core-level photoemission and X-ray absorption fine structure measurements. The analysis shows that on adsorption at 160 K SO 2 partly decomposes, leaving SO 3 and atomic S on the surface. Two different, nearly flat-lying SO 2 species are observed which correspond to long-bridge and short-bridge S sites. The S atoms of co-adsorbed atoms of SO 3 occupy short-bridge sites with the C 3 axis of SO 3 being oriented approximately perpendicular to the surface, whereas atomic S is located in two-fold hollow sites. The average S–Ni bond length of all S-containing species measures 2.30±0.04 Å. On heating this co-adsorption phase to room temperature, SO 2 completely disappears from the surface, the coverages of adsorbed SO 3 and S increase and small amounts of atomic O show up. On further heating, SO 3 decomposes completely, increasing the amounts of atomic S and O on the surface while SO 2 is observed in the gas phase. At still higher temperatures, only atomic S remains adsorbed in a quasi five-fold coordinated site with an average S–Ni bond length of 2.26±0.04 Å.

Long-range periodicity in c(8 × 2) benzoate/Cu(110): a combined STM, LEED and HREELS study

The adsorption of benzoic acid on Cu(110) between 300 and 350K results in benzoate species oriented perpendicular to the surface with a c(8 × 2) periodicity at saturation coverage. STM images of the ordered structure and non-dipolar scattering in HREELS demonstrate alignment in the [110] azimuth. We interpret the tunnelling mechanism in STM, negative ion resonance scattering in HREELS, and assign low-frequency vibrational modes utilising ab initio molecular orbital calculations. We propose a model with four molecules per c(8 × 2) unit cell (ϑ = 0.25ML) arranged in two out-of-phase, zig-zag rows along the [001] direction with the car☐ylates bound in short bridge sites alternately three and five lattice constants apart along the [110] direction. Adsorbate-induced dipole-active phonons in the frequency range 65–185 cm −1 in HREELS support the adsorption model.

A density functional study of CO 2 adsorption on the (100) face of Cu(9,4,1) cluster model

The adsorption modes of inert CO 2 on the (100) face of Cu(9,4,1) cluster model have been studies by the Slater DFT code of the Amsterdam density functional (ADF) program. The side-on adsorption mode with near linear CO 2 lying at the short bridge site has the highest binding energy of 26.31 kJ mol −1. The binding energies of CO 2 in similar geometry on the cross bridge, hollow, and on-top sites vary from 19.66 to 21.8 kJ mol −1. The investigation also revealed that a number of CO 2 bent modes with O-C-O angles equal to ≈ 150° can coordinate with the surface with binding energies ranging from 9.66 to 23.56 kJ mol −1. Further calculations indicated that to cause the CO 2 molecule to bend to ≈ 150°, there is negative 9.66 to 23.56 kJ mol −1. Further calculations indicated that to cause the CO 2 molecule to bend to ≈ 150°, there is negative charge transferred from the copper cluster to the CO 2 molecule.

The structure and phase stability of CO adsorbates on Rh(110)

The structure of CO adsorbates on the Rh(110) surface is studied at full coverage using first-principles techniques. The relative energies of different adsorbate geometries are determined by means of accurate structure optimizations. In agreement with experiments, we find that a p2mg(2 × 1)-2CO structure is the most stable. The CO molecules sit on the short-bridge site (carbon below) with the molecular axis slightly tilted off the surface normal, along the (001) direction. Configurations corresponding to different distributions of tilt angles are mapped onto an anisotropic two-dimensional Ising model whose parameters are extracted from our ab initio calculations. We find that an order-disorder phase-transition occurs at a temperature T c ≈ 300 K.

Adsorbate site determination with the scanning tunneling microscope: C2H4 on Cu{110}

Scanning tunneling microscopy at T=4 K has been used to determine directly the binding site of a molecule chemisorbed on a metal surface, namely, ethene on Cu〈110〉, by simultaneous imaging of the adsorbate and the underlying lattice. The molecule is found to bond in the short bridge site on the close-packed rows with its C-C axis oriented in the 〈110〉 direction. \textcopyright{} 1996 The American Physical Society.

Structural determination of the Pd{110}(2 x 1)p2mg-CO system by means of high-energy x-ray photoelectron diffraction.

The adsorption geometry of CO in the (2\ifmmode\times\else\texttimes\fi{}1)p2mg phase on Pd(110) has been determined by means of an application of high-energy x-ray photoelectron diffraction, which achieves adsorption site sensitivity in a forward scattering geometry. Information about the surface structure is extracted from the azimuthal angle dependence of the surface component of the Pd 3${\mathit{d}}_{5/2}$ photoemission peak. Surface sensitivity is obtained by working at grazing emission angles. The experimental data are compared with single scattering cluster simulations, giving clear evidence that the CO molecules are located in short-bridge sites. The estimated Pd-C distance is 1.86\ifmmode\pm\else\textpm\fi{}0.09 \AA{}. A CO tilt angle of 24\ifmmode\pm\else\textpm\fi{}3\ifmmode^\circ\else\textdegree\fi{} from the surface normal was determined by means of the C 1s polar scan along the plane normal to the [001] direction. \textcopyright{} 1996 The American Physical Society.

Substrate scattering effects in the near-edge X-ray absorption fine structure of Ni(110)-p2mg(2 × 1)-CO

The near-edge X-ray absorption fine structure (NEXAFS) of Ni(110)-p2mg(2 × 1)-CO with titled CO molecules in short-bridge adsorption sites has been studied at the oxygen K edge. An additional structure is observed between the π and σ resonances the energy position of which is different in the 〈100〉 and 〈110〉 azimuths. It is shown that these features cannot be explained by Rydberg transitions, multi-electron excitations or vibrational effects. The analysis of the corresponding surface extended X-ray absorption fine structure (SEXAFS) suggests an assignment to SEXAFS wiggles due to backscattering from substrate atoms.

Orientation and periodicity in the c(4 × 8) and p(2 × 1) structures of 3-thiophene carboxylic acid on Cu(110)

The chemisorption of 3-thiophene carboxylic acid on Cu(110) between 300 and 350 K has been investigated by high resolution electron energy loss spectroscopy (HREELS), scanning tunnelling microscopy (STM) and low energy electron diffraction (LEED). Ab initio molecular orbital calculations of the molecular ion aided in vibrational frequency assignments, interpretation of STM images and estimation of intra- and inter-molecular interactions influencing formation of the c(4 × 8) and p(2 × 1) structures. HREELS shows that at low coverage, the molecule lays flat with its π orbitals interacting with the surface. Increasing the coverage induces the molecules to reorient perpendicular to the surface and form a c(4 × 8) intermediate structure. Impact scattering in HREELS demonstrates that the molecules are preferentially aligned with the thiophene ring in the [110] azimuth. STM images suggest that the upright carboxylate species form rows of four adjacent molecules face-to-face along the [001] direction separated by four lattice constants in [110]. Subsequent rows are shifted by two lattice constants along [110], resulting in an overall c(4 × 8) periodicity and a coverage of 0.25 ML. With increasing coverage, the c(4 × 8) structure changes to a p(2 × 1) structure. A model with the carboxylates bound in short bridge sites two lattice constants apart along [110] with a local coverage of 0.5 ML is proposed. Steric repulsion in the p(2 × 1) structure results in rotation of the thiophene ring by an estimated 30° away from the [110] direction, consistent with impact scattering HREELS measurements. Calculated dipole-dipole repulsion between the carboxylate groups is large compared to any dipole-dipole attraction which could result from anti-parallel alignment of the static dipole moments of the thiophene rings.