Long-bridge Site Research Articles

A computational investigation of the adsorption of small copper clusters on the CeO2(110) surface.

We report a detailed density functional theory (DFT) study of the geometrical and electronic properties, and the growth mechanism of a Cun (n = 1-4) cluster on a stoichiometric, and especially on a defective CeO2(110) surface with one surface oxygen vacancy, without using pre-assumed gas-phase Cun cluster shapes. This gives new and valuable theoretical insight into experimental work regarding debatable active sites of promising CuOx/CeO2-nanorod catalysts in many reactions. We demonstrate that CeO2(110) is highly reducible upon Cun adsorption, with electron transfer from Cun clusters, and that a Cun cluster grows along the long bridge sites until Cu3, so that each Cu atom can interact strongly with surface oxygen ions at these sites, forming stable structures on both stoichiometric and defective CeO2(110) surface. Cu-Cu interactions are, however, limited, since Cu atoms are distant from each other, inhibiting the formation of Cu-Cu bonds. This monolayer then begins to grow into a bilayer as seen in the Cu3 to Cu4 transition, with long-bridge site Cu as anchoring sites. Our calculations on Cu4 adsorption reveal a Cu bilayer rich in Cu+ species at the Cu-O interface.

The Adsorption and Diffusion Manners of Hydrogen Atoms on Pt (100), Pt (110), and Pt (111) Surfaces

In this study, the interactions between H atoms and the (100), (110), and (111) surfaces of platinum have been investigated by using the London-Eyring-Polanyi-Sato (LEPS) potential function. The adsorption zones (sites) and LEPS energy values of these sites have been determined theoretically. In addition, the potential-energy surfaces for each Pt surface have been obtained in detail. Further, the adsorption sites on the surface, scattering from the surface, diffusion paths on the surface, and transition regions to the subsurface, have been determined and the differences have been examined in detail among the surfaces. From these results, it is found that an H atom has the lowest binding energy at the hollow sites on the Pt (100) and Pt (111) surfaces and that it has the lowest binding energy at the long-bridge sites on the Pt (110) surface. It has also been determined that the hollow sites on the three surfaces are the regions through which H atoms can penetrate into the subsurface. In addition, it has also been found that, for each of the three Pt surfaces, the diffusion of an H atom across the surface may follow a bridge-hollow-bridge pathway. These results are in agreement with previous experimental and theoretical results. Besides, the adsorption and diffusion manners of hydrogen atoms on each of the Pt surfaces have been analyzed deeply.

H2S adsorption and dissociation on Rh(110) surface: a first-principles study

First-principle study based on density functional theory are used to scrutinize the mechanism of H2S adsorption and dissociation over Rh(110) surface. For adsorption mechanisms, we probe the most favorite sites of H2S monomers over Rh(110) surface. It is determined that H2S vigorously adsorbed over high symmetry adsorption sites with preferred long-bridge (LB) site having adsorption energy − 1.00 eV, with no more than 0.50 eV, binding energy. Also we found that HS chemisorption is higher as compared to H2S on Rh(110) surface having − 3.76 eV adsorption energy, where atomic S and H binding at hollow and short-bridge site is more stronger. The energy barriers to split the bond of S–H in first and second H2S dehydrogenation are 0.18–0.36 and 0.30 eV. To further investigate, electronic density of state are employed to illustrate the interaction of adsorbed H2S with the surface of Rh(110), which is able to account for energy divergences of all species adsorbed on Rh(110) surface. Hence, our calculated results confirm that H2S dissociation over Rh(110) surface is exothermic as well as an easy process, however kinetically and thermodynamically the existence of atomic S avoid the breaking of H–S bond procedure.

Complex reactions on a convertible catalyst surface: A study of the S-O-Cu system

The interaction of clean and partially oxidized Cu(110) with sulphur was studied by scanning tunneling microscopy and density functional theory calculations in the low-coverage range. On the clean Cu surface individual S atoms adsorb in the troughs between the Cu atom rows. Hollow sites are preferred, but long-bridge sites are occasionally occupied as well. The majority of adsorbed S, however, seems to be involved in the formation of highly mobile CuxSy clusters of various sizes. The clusters preferentially attach to steps thus changing the step morphology completely. Some of the clusters form aggregates on the terraces. On the partially oxidized surface similar clusters form and cause long-range mass transport to steps. Additionally, nanowires form in [001] direction on and along the surface oxide stress domains. These nanowires have a complex composition, exhibit different corrugations and appear sometimes as three-dimensional needles. Occasionally they flip their direction by 90°, but doing so they partially decompose. Finally, annealing of the S-O-Cu surface leads to consumption of the surface oxide stripes indicating loss of oxygen presumably via SO2 formation. Simultaneously, linear sulphur chains suspended between the [001] –O-Cu-O- chains form in [11¯0] direction. The surprising multitude of processes and products even at low-pressure, low-temperature conditions in the comparatively simple S-O-Cu system highlights the difficulty of controlling reactivity and selectivity on such convertible catalyst surfaces.

Density functional study of O2 molecule and O atom adsorption on α-U(0 0 1) surface

First-principle calculations have been performed to study both O2 molecule and O atom adsorption on α-U(001) surface and subsurface. The results show that the long-bridge site is the most stable adsorption site for O2U(001) and OU(001) surface adsorption. The tetrahedral (Tet) 1 site is the most stable adsorption site for OU(001) subsurface adsorption. The partial electronic density of states (PDOSs) of O2U(001) and OU(001) shows the hybridization of U and O atoms orbitals causes the electrons of 5f/6d of U more non-local and leads to the formation of OU bond. The OU bond is partly ionic and partly covalent. The analysis of the diffusion path of O atom on U(001) shows that O atom can easily transfer from hollow (hol) 1 site to hollow 2 site with no energy barrier. O atom can hardly transfer from surface to the subsurface.

Surface atomic relaxation and magnetism on hydrogen-adsorbed Fe(110) surfaces from first principles

We have computed adsorption energies, vibrational frequencies, surface relaxation and buckling for hydrogen adsorbed on a body-centred-cubic Fe(110) surface as a function of the degree of H coverage. This adsorption system is important in a variety of technological processes such as the hydrogen embrittlement in ferritic steels, which motivated this work, and the Haber–Bosch process. We employed spin-polarised density functional theory to optimise geometries of a six-layer Fe slab, followed by frozen mode finite displacement phonon calculations to compute Fe–H vibrational frequencies. We have found that the quasi-threefold (3f) site is the most stable adsorption site, with adsorption energies of ∼3.0eV/H for all coverages studied. The long-bridge (lb) site, which is close in energy to the 3f site, is actually a transition state leading to the stable 3f site. The calculated harmonic vibrational frequencies collectively span from 730 to 1220cm−1, for a range of coverages. The increased first-to-second layer spacing in the presence of adsorbed hydrogen, and the pronounced buckling observed in the Fe surface layer, may facilitate the diffusion of hydrogen atoms into the bulk, and therefore impact the early stages of hydrogen embrittlement in steels.

Structure of Low and High Coverage Phases of Bromine on Pd(110)

Halogens can be used to steer the catalytic selectivity of transition metal surfaces. In the present work the structures of bromine on Pd(110) in the low-coverage c(2 × 4) and the high-coverage c(4 × 2) adlayers are investigated by density functional theory calculations and quantitative low-energy electron diffraction. Excellent agreement is achieved between theory and experiment. Both find the high-coordinated hollow site to be favored at low coverage, while in the high coverage phase also adsorption close to long-bridge sites occurs due to the Br–Br repulsion. This results in an almost uniform distribution of adatoms in the troughs of the Pd(110) surface as in the majority of halogen fcc(110) transition metal adsorption systems, but not on Pt(110). The different coordination of the hollow and quasi long-bridge adsorption sites is reflected by different distances to the top-layer Pd atoms, 271 pm for the high-coordinated hollow site and 248 pm for the low-coordinated quasi long-bridge site. With increasing bromine coverage the contraction of the first interlayer distance d12 of clean Pd(110) is reduced and inverted to an expansion of ∼3% at high bromine coverage. As shown by DFT, this change in substrate relaxation depends markedly on the adsorption site. A population analysis reveals a non-negligible ionic contribution for the Br–Pd bonding. Finally, we discuss the, at first sight, surprising DFT prediction of pure long-bridge adsorption at intermediate coverages.

Density functional study of H2O molecule adsorption on α-U(001) surface.

Periodic density functional theory (DFT) calculations were performed to investigate the adsorption of H2O on U(001) surface. The metallic nature of uranium atom and different adsorption sites of U(001) surface play key roles in the H2O molecular dissociate reaction. The long-bridge site is the most favorable site of H2O-U(001) adsorption configuration. The triangle-center site of the H atom is the most favorable site of HOH-U(001) adsorption configuration. The interaction between H2O and U surface is more evident on the first layer than that on any other two sub-layers. The dissociation energy of one hydrogen atom from H2O is -1.994 to -2.215eV on U(001) surface, while the dissociating energy decreases to -3.351 to -3.394eV with two hydrogen atoms dissociating from H2O. These phenomena also indicate that the Oads can promote the dehydrogenation of H2O. A significant charge transfer from the first layer of the uranium surface to the H and O atoms is also found to occur, making the bonding partly ionic.

Comparative study of the carbide-modified surfacesC/Mo(110)andC/Mo(100)using high-resolution x-ray photoelectron spectroscopy

The preparations of single (monolayer) and bulk carbides on the $\mathrm{Mo}(110)$ and $\mathrm{Mo}(100)$ single crystals are followed in situ at 1200 K using synchrotron-based high-resolution x-ray photoelectron spectroscopy of the $\mathrm{C}\phantom{\rule{0.28em}{0ex}}1s$ and core Mo $3{d}_{5/2}$ levels. By comparing the experimental results to first principles calculations using density functional theory, we suggest real-space surface structures for the carbide-modified surfaces. For a monolayer carbide on $\mathrm{Mo}(110)$, carbon dimers adsorb in the long-bridge site, most likely at a coverage of $3/8\phantom{\rule{0.16em}{0ex}}\mathrm{ML}$ carbon atoms per Mo surface atom. For the bulk carbide, we find a coverage of $\ensuremath{\sim}0.5\phantom{\rule{0.16em}{0ex}}\mathrm{ML}$ on the surface, and the calculations show that single carbon atoms are more stable than dimers on the surface. The monolayer carbide on $\mathrm{Mo}(100)$ exhibits a coverage of $\ensuremath{\sim}1\phantom{\rule{0.16em}{0ex}}\mathrm{ML}$ and agrees with previous studies, while the bulk carbide preparation probably leads to a faceting of the surface.

Stability and diffusion properties of Ti atom on α-uranium surfaces: A first-principles study

Titanium (Ti) film is experimentally used to provide good protection for uranium (U) surface from corrosion, but the performance of the protector is limited by the strength of the Ti/U interface. Therefore, it is important to further understand the interaction between Ti atoms and U surfaces. Here the first-principles density functional theory approaches with spin polarization and non-spin polarization have been employed to study the stability and diffusion properties of Ti atoms on α-uranium (001) and (100) surfaces. The energetic, geometric parameters and electronic structures have been analyzed. For Ti/U(001) surface, it is found that the most energetically favoured configuration is Ti atom occupying the hollow2 site. Ti atom tends to diffuse from hollow1 site to hollow2 site via the longbridge site. The U–Ti bond is dominated by the hybridization between Ti 3d and U 5f states, and the negative charge is transferred to the first U layer from the adsorbed Ti atom. For Ti/U(100) surface, the hollow site is the most preferred site for the adsorption of Ti atom. Ti atom will overcome a low barrier of ∼0.2eV when it diffuses from one hollow site to another via the longbridge site. The topmost two U layers get electrons from Ti atom and the deeper U layers.

Site-dependent electronic structures of a single molecule on a metal surface studied by scanning tunneling microscopy and spectroscopy

Single-molecule observation of the electronic structures of para-cyanobenzoate (pCB) adsorbed on Cu(110) has been performed using scanning tunneling microscopy (STM) and spectroscopy (STS). We found that pCB has two types of the adsorption site on Cu(110); i.e., two oxygen atoms of pCB are bridged between adjacent Cu atoms at the short- or long-bridge sites. STS and STS mapping revealed that the pCB adsorbed at the short-bridge site has a resonant peak at 2.0V above the Fermi level, which is assigned to the lowest unoccupied molecular orbital (LUMO) of pCB. However, the LUMO state is shifted toward lower voltage (1.2V) when the pCB molecule is adsorbed at the long-bridge site. The energy levels of the LUMO state, depending on the adsorption site of pCB, can thus be ascribed to the degree of the electronic interaction between pCB and the Cu substrate. The site transformation of pCB induced by the injection of tunneling electrons from the STM tip has also been presented.

Theoretical investigation of uranyl ion adsorption on hydroxylated γ-Al2O3 surfaces

We herein report density functional investigations of uranyl adsorption on γ-Al2O3 surfaces. By applying periodic slab models, we report the possible configurations of uranyl adsorbed on the (100) and (110) surfaces of γ-Al2O3 by calculations using density functional theory. The above mentioned two surfaces are expected to be highly reactive and adsorb metal ions preferentially. We explored bidentate inner-sphere adsorption complexes of uranyl at short-bridge sites, AlOO, and long-bridge sites, AlOAlO(H). The results indicate several favorable adsorption configurations which fit the available TRLFS and EXAFS data well. Comparison of key calculated structure parameters with available experimental data suggests an extension of the prevailing interpretation and implies that a set of uranyl complexes may coexist on the edge surfaces of γ-Al2O3.

Oxygen adsorption on Nb(110) surface by first-principles calculation

The adsorption of atomic oxygen on the Nb (110) surface is systematically investigated through the first-principles method for oxygen coverage ranging from 0.25 to 1 monolayer (ML)．It is found that the hollow site is the most energetically favorable for the whole coverage range considered and that the long-bridge site takes the second place．The work function increases almost linearly with the increase of oxygen coverage for the long-bridge site adsorption, whereas for the hollow site adsorption the work function decreases when the coverage is 0.25 ML or 0.5 ML and increases when the coverage is 0.75 ML or 1 ML．Using the planar averaged charge density and the dipole moment change we can explain the complicated change of work function induced by atomic oxygen adsorption．In addition, the interaction between O and Nb is analyzed by the surface atomic structure and electronic density of states．

Unexpected Deformations Induced by Surface Interaction and Chiral Self‐Assembly of CoII‐Tetraphenylporphyrin (Co‐TPP) Adsorbed on Cu(110): A Combined STM and Periodic DFT Study

In a combined scanning tunnelling microscopy (STM) and periodic density functional theory (DFT) study, we present the first comprehensive picture of the energy costs and gains that drive the adsorption and chiral self-assembly of highly distorted Co(II)-tetraphenylporphyrin (Co-TPP) conformers on the Cu(110) surface. Periodic, semi-local DFT calculations reveal a strong energetic preference for Co-TPP molecules to adsorb at the short-bridge site when organised within a domain. At this adsorption site, a substantial chemical interaction between the molecular core and the surface causes the porphyrin macrocycle to accommodate close to the surface and in a flat geometry, which induces considerable tilting distortions in the phenyl groups. Experimental STM images can be explained in terms of these conformational changes and adsorption-induced electronic effects. For the ordered structure we unambiguously show that the substantial energy gain from the molecule–surface interaction recuperates the high cost of the induced molecular and surface deformations as compared with gas phase molecules. Conversely, singly adsorbed molecules prefer a long-bridge adsorption site and adopt a non-planar, saddle-shape conformation. By using a van der Waals density functional correction scheme, we found that the intermolecular π–π interactions make the distorted conformer more stable than the saddle conformer within the organic assembly. These interactions drive supramolecular assembly and also generate chiral expression in the system, pinning individual molecules in a propeller-like conformation and directing their assembly along non-symmetric directions that lead to the coexistence of mirror-image chiral domains. Our observations reveal that a strong macrocycle–surface interaction can trigger and stabilise highly unexpected deformations of the molecular structure and thus substantially extend the range of chemistries possible within these systems.

Density Functional Model Study of Uranyl Adsorption on the Solvated (001) Surface of Kaolinite

We studied the adsorption of uranyl on bare and solvated models of the octahedral (001) surface of kaolinite using first-principles density functional calculations. Inner-sphere bidentate complexes adsorbed at partially deprotonated short-bridge sites AlOO(H) and long-bridge sites AlO−AlO(H) were modeled as the most probable adsorption complexes. The uranyl complex at the doubly deprotonated AlO−AlO long-bridge site exhibits a third contact to the surface, not present in the complex at the corresponding short-bridge site. Adsorption at short-bridge sites is energetically favored compared to complexes at long-bridge sites. We were unable to determine stable adsorption complexes of uranyl at singly deprotonated AlO−AlOH long-bridge sites. Surface solvation, approximated via an adsorbed monolayer of water molecules, hardly affects the adsorption complexes of uranyl. Contacts U−Oeq in the equatorial plane shorten by 2 pm, U−Al distances elongate by up to 4 pm. In contrast to the bare surface, adsorption complexes at long-bridge and short-bridge sites of the solvated surface exhibit similar stabilities.

Single-Molecule Vibrational Spectroscopy and Inelastic-Tunneling-Electron-Induced Diffusion of Formate Adsorbed on Ni(110)

We have investigated the inelastic-tunneling-electron-induced vibrational excitation of single formate (HCOO− and DCOO−) molecules adsorbed on Ni(110) using a low-temperature scanning tunneling microscope (STM). Formate molecules adsorbed on the long-bridge site can be moved laterally along [11̅0] by the injection of tunneling electrons from the STM tip. Using an isotope-labeled molecule, the diffusion probability and distance were found to be enhanced significantly when the applied voltage reaches the energies of specific vibrational modes, that is, C−H bending and C−H stretching modes. Inelastic electron tunneling spectroscopy based on STM (STM-IETS) and scanning tunneling spectroscopy was used to identify the vibrational and electronic state of a single formate molecule adsorbed at different sites. The STM-IETS spectrum of formate adsorbed at the short-bridge site exhibited a broad peak at 80 mV and a sharp resonance peak at 360 mV. The former peak is assigned to the electronic states generated by the metal−molecule hybridization; the latter peak is assigned to the C−H stretching mode. The STM-IETS spectrum of formate adsorbed at the long-bridge site showed features similar to those of formate adsorbed at the short-bridge site, but the intensity of the broad peak appearing at near the Fermi level was markedly suppressed, indicating a weak metal−molecule interaction.

ADSORPTION OFLiONMo(110) SURFACE: A FIRST-PRINCIPLES STUDY

Periodic, self-consistent, density functional theory calculations have been performed to investigate Li adsorption on Mo (110) surface. It turns out that the long-bridge site is the most stable site and the Li – Mo surface alloy forms easily at high coverage with the substitution of Mo by Li atoms in the outermost layer. Work function analysis showed that the work function decreases dramatically as the coverage from 0 to 0.5 ML, and finally increases again at the coverage of 1 ML, which agrees well with the experimental finding of Kroger et al. (Surf. Sci.449 (2000) 227–235). Vibrational properties, diffusion barrier of Li along the Mo (110) surface, and the energy of formation of the surface have also been investigated for Li adsorption at various coverages.

First-principles studies of the oxygen adsorption on unreconstructed and reconstructed Ni(1 1 0) surfaces

First-principles calculations have been performed to investigate the adsorption of oxygen on unreconstructed and reconstructed Ni(1 1 0) surfaces. The energetics, structural, electronic and magnetic properties are given in detail. For oxygen adsorption on unreconstructed surface, ( n × 1 ) ( n = 2 , 3 ) substrate with oxygen atom on short-bridge site is found to be the most stable adsorption configuration. Whereas energetically most favorable adsorption phase of reconstructed surface is p ( n × 1 ) substrate with oxygen atom located at long-bridge site. Our calculations suggest that the surface reconstruction is induced by the oxygen adsorption. We also find there are redistributions of electronic structure and electron transfer from the substrate to adsorbate. Our calculations also indicate surface magnetic moment is enhanced on clean surfaces and oxygen atoms are magnetized weakly after oxygen adsorption. Interestingly, adsorption on unreconstructed surface does not change surface magnetic moment. However, adsorbate leads to reduction of surface magnetic moment in reconstructed system remarkably.

Remarkably long-ranged repulsive interaction between adsorbed CO molecules on Pt modified Ge(001)

We have studied the adsorption of CO molecules on a Pt modified Ge(001) surface at 77 K. The CO molecules preferentially adsorb on the self-organized Pt chains rather than on the terraces. We have identified two different adsorption sites at 77 K: one either on the short-bridge site or on top of one of the atoms of a buckled Pt dimer, and the other one on the long-bridge site in between two Pt dimers. The CO molecules are mobile at room temperature but immobile at 77 K. A statistical analysis of the nearest-neighbor spacing of the CO molecules reveals that adsorbed CO molecules interact repulsively along the Pt-dimer chains. This repulsive interaction, composed of electrostatic terms and a strain-mediated term, gradually fades away over, for simple molecules, a remarkably long distance of 3--4 nm.

The housing and neighbourhood impacts of knowledge-based economic development following industrial closure

Economic development initiatives following large-scale industrial closures often seek to regenerate the local economy through investment in technology and knowledge-intensive activities. The resulting changes in the make-up of the local workforce are in turn likely to generate new forms of demand for housing; demand that will not necessarily be met by the residential offer of the neighbourhoods worst affected by the initial closure. This paper explores these processes through a study of the proposals for a science park as a component of the programme to redevelop the Longbridge site in Birmingham in the UK, following the closure of the Rover automotive plant in 2005. The paper examines the capacity of local workers to take up the anticipated high-technology jobs; the likely configuration of an incoming workforce; and the fit between the housing requirements of these new workers and the residential offer of neighbourhoods in the Longbridge area. It concludes that there is likely to be a poor match between the housing and residential characteristics of neighbourhoods most closely associated with the plant closure and the requirements of an incoming high-tech workforce. This in turn raises a policy dilemma. On the one hand there is a necessity to secure economic diversification for the local economy as a whole, whilst on the other is the requirement to address the specific needs of the discrete neighbourhoods most affected by the closure and whose quality of place offer is often furthest from the requirements of the incoming workforce. A number of policy implications are discussed, drawing on the experience of recent housing and regeneration policy.