Blyholder Model Research Articles

A Blyholder mechanism in the chemisorption of N2O on Ni(111) – studied with Auger-photoelectron coincidence spectroscopy

In heterogeneous catalysis the surface-adsorbate bond strength is critical for the function of the system. Here we study a series consisting of multilayer, bilayer and monolayer N2O on Ni(111) and employ Auger-photoelectron coincidence spectroscopy (APECS) to study the interaction between the molecule and the substrate directly. We observe intensity in the nitrogen Auger spectra that arise from the interaction between molecule and surface (not observed in free molecules) whereas the oxygen spectra are thickness-independent. Since the two nitrogen atoms of N2O are chemically inequivalent we can assign the intensity present in the bilayer and monolayer cases to orbitals centered on the terminal nitrogen which is closest to the Ni(111) surface. Using ab initio, molecular dynamics and solid-state density functional theory calculations we infer a Blyholder model of the surface bond as consisting of donation from the terminal nitrogen lone-pair valence orbital with back-donation from the metal into the unoccupied orbitals on that nitrogen. This coincidence technique can readily be used to study substrate–adsorbate interactions directly with chemical and orbital specificity — this opens up prospects to study fundamental steps of molecular adsorption and heterogeneous catalysis with unprecedented detail.

Ab Initio modeling of Near-Edge EELS spectra for chemisorbed molecules

Electron energy loss spectroscopy (EELS) has recently been applied to probe chemisorbed molecules on metal nanostructures, but a fundamental understanding of the correlation between these spectra and the electronic structures of the adsorbates has been limited. We report here on the insights afforded by time-dependent density functional theory to decipher the energy loss near edge structure (ELNES) of EELS spectra associated with chemisorption. These first-principles calculations simulate the ELNES-EELS spectra for chemisorbed CO on various facets of Au and Pt. Computational predictions of key signatures such as the ‘red shift’ and reductions in the peak intensity of the 2π* and 6σ* peaks, as compared to free CO in the gas phase, are validated in comparison to experimentally collected EELS spectra. These signatures are revealed to arise from changes in the electronic structure in terms of unoccupied density of states associated with the chemisorption process. They are consistent with a Blyholder model that incorporates donation and back-donation of electrons. They are also characteristic of the chemisorption process, such as the choice of metal, site of adsorption and the coverage and distribution of adsorbates. Our simulations thus provide guidelines for the use of ELNES-EELS to characterize the atomic structure and adsorption property of nanostructured surfaces and facilitate the development of advanced nanomaterials for catalytic applications.

Influence of CO molecule on structure stability and electronic properties of silver nanotube

Influence of CO molecule on structure stability and electronic properties of silver nanotube (AgNT) has been systematically investigated by using first-principles calculations based on the density-functional theory. The results indicate that the top site is the most stable for CO adsorption, which is energetically favorable than the bridge site (B1) on the AgNT's surface. The AgNT undergoes a slight structural distortion after the CO adsorption. The binding mechanism between CO and AgNT can be described by the Blyholder's model, and the donation–backdonation process between the CO and Ag states leads to the formation of bonding/antibonding pairs, 5σb/5σa and 2πb*/2πa*. The quantum conductance of the AgNT decreases 1G0 after CO adsorption, which is promising for the application of CO sensor.

CO Adsorption on GaPd-Unravelling the Chemical Bonding in Real Space.

The electron localizability indicator-an efficient quantum chemical tool for analysis of chemical bonding-is applied to unveil the chemical bonding behind the CO adsorption on the (1‾ 1‾ 1‾ ) surface of the highly selective semi-hydrogenation catalyst GaPd. Refining the commonly applied Blyholder model, the obtained results are in excellent agreement with previous experimental and theoretical findings. The clean GaPd(1‾ 1‾ 1‾ ) surface presents unshielded negatively charged Pd centers and positively charged Ga species partially shielded by dangling bonds. The CO molecule adsorbs on-top of the Pd centers perperdicular to the surface, while no CO-Ga interaction is observed. The chemical bonding analysis results in deep understanding, thus enabling a cost efficient route to innovative materials by reverse engineering.

A Molecular Perspective on the d-Band Model: Synergy Between Experiment and Theory

The d-band model of Hammer and Norskov (Nature 376:238, 1995 [3]) relating adsorption energies to the d-band position, and the adsorption energies to barriers in catalytic reactions, has been extremely successful in predicting reactivities and catalysts. In the present contribution we review recent combined experimental and theoretical work on chemical bond-formation at surfaces. We focus on the adsorbate and how the adsorbate electronic structure can be rehybridized through mixing with unoccupied states to generate the radical state, real or virtual, that can then form electron pairs with the metal d-states, as described by the d-band model. We discuss five important bonding situations: (i) atomic radical, (ii) diatomics with unsaturated π-systems (Blyholder model), (iii) unsaturated hydrocarbons (Dewar–Chatt–Duncanson model), (iv) lone–pair interactions, and (v) saturated hydrocarbons (physisorption). Where the d-band model predicts trends along the series of transition metals, the present work provides intuitive tools for predicting trends among different adsorbates.

Interaction of CO with an Au monatomic chain at different strains: Electronic structure and ballistic transport

We study the energetics, the electronic structure, and the ballistic transport of an infinite Au monatomic chain with an adsorbed CO molecule. We find that the bridge adsorption site is energetically favored with respect to the atop site, both at the equilibrium Au-Au spacing of the chain and at larger spacings. Instead, a substitutional configuration requires a very elongated Au-Au bond, well above the rupture distance of the pristine Au chain. The electronic structure properties can be described by the Blyholder model, which involves the formation of bonding/antibonding pairs of $5\ensuremath{\sigma}$ and $2{\ensuremath{\pi}}^{\ensuremath{\star}}$ states through the hybridization between molecular levels of CO and metallic states of the chain. In the atop geometry, we find an almost vanishing conductance due to the $5\ensuremath{\sigma}$ antibonding states giving rise to a Fano-like destructive interference close to the Fermi energy. In the bridge geometry, instead, the same states are shifted to higher energies and the conductance reduction with respect to pristine Au chain is much smaller. We also examine the effects of strain on the ballistic transport, finding opposite behaviors for the atop and bridge conductances. Only the bridge geometry shows a strain dependence compatible with the experimental conductance traces.

Nature of the Binding of a c(2×2)-CO Overlayer on Ag(001) and Surface Mediated Intermolecular Coupling

We present a first-principles study of the nature of the binding of a c(2×2)-CO overlayer on Ag(001) and of the origin of CO-CO interactions upon adsorption. Electronic structural changes induced by molecular adsorption provide an interpretation for earlier X-ray photoemission valence band spectra of CO/Ag(001). Our results establish that CO chemisorbs on clean Ag(001) and follows the Blyholder model of donation and back-donation between CO and metal orbitals. We analyze the origin of the dispersion of the C-O stretch mode and attest that it is caused by the metal-CO coupling. Specifically, the coupling of CO to Ag, although the weakest of those between it and transition and other noble metals, greatly enhances the intermolecular force constants. We also find that the response of the charge density around CO is much stronger and of longer range when the molecule stretches than when it rigidly vibrates against the surface. This difference explains why the C-O stretch mode disperses while the Ag-CO stretch mode does not.

Carbon Monoxide Adsorption on Silver Doped Gold Clusters

Well controlled gas phase experiments of the size and dopant dependent reactivity of gold clusters can shed light on the surprising discovery that nanometer sized gold particles are catalytically active. Most studies that investigate the reactivity of gold clusters in the gas phase focused on charged, small sized clusters. Here, reactivity measurements in a low-pressure reaction cell were performed to investigate carbon monoxide adsorption on neutral bare and silver doped gold clusters (Au(n)Ag(m); n = 10-45; m = 0, 1, 2) at 140 K. The size dependence of the reaction probabilities reflects the role of the electronic shells for the carbon monoxide adsorption, with closed electronic shell systems being the most reactive. In addition, the cluster's reaction probability is reduced upon substitution of gold atoms for silver. Inclusion of a single silver atom causes significant changes in the reactivity only for a few cluster sizes, whereas there is a more general reduction in the reactivity with two silver atoms in the cluster. The experimental observations are qualitatively explained on the basis of a Blyholder model, which includes dopant induced features such as electron transfer from silver to gold, reduced s-d hybrization, and changes in the cluster geometry.

CO adsorption on transition metal clusters: Trends from density functional theory

This work reports for the first time the trends for carbon monoxide (CO) chemisorption on transition metal clusters present in supported metal catalysts. In particular, the energetic, structural and infrared adsorption characteristics of linearly (atop) CO adsorbed on transition metal nano-clusters of less than 10Å in size were explored. Spin-unrestricted density functional theory (DFT) calculations were employed to explore the trends of CO adsorption energy (AM–CO) and C–O vibrational frequency (νCO) for clusters composed of Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt and Au.The effects of the transition metal electronic structure onto the adsorption energy of CO and the vibrational stretching frequency of C–O, and how these chemical parameters can be correlated to the catalytic activity of transition supported metal catalysts that involve the adsorption, surface diffusion, and C–O bond dissociation elementary steps in heterogeneous catalytic surface reactions, are discussed. Our findings show that an increase of the electronic d-shell occupancy and the principal quantum number (n) in transition metals causes an increase in the vibrational stretching frequency of the C–O bond. This trend is inconsistent with the classical Blyholder model for the metal–carbonyl bond.

The potential energy surface for dissociation of N 2 on W( [formula omitted

Electronic total energy methods have been used to compute the potential energy surface for the dissociation of nitrogen molecules on the (1 0 0) face of tungsten. The dissociation is found to be activated, but there are also deep molecular chemisorption wells into which molecules may steer and trap. The molecular geometries in these precursor states agree well with experimental assignments. The dominant well is for the adsorption of a molecule oriented vertically on the atop site. An analysis of the bonding there indicates that the orbitals break into components largely located on either the inner or the outer N atom, rather than the standard Blyholder model. Dissociation paths are suggested based on several steps, passing through at least two molecular precursor states.

Adsorption characteristics ofCOandN2onRuO2(110)

Low-energy electron diffraction and density-functional theory calculations are used to examine the adsorption properties of CO and ${\mathrm{N}}_{2}$ on ${\mathrm{RuO}}_{2}(110).$ Both molecules adsorb over the coordinatively unsaturated Ru sites (cus-Ru atoms) with their molecular axes normal to the surface plane. The chemisorption mechanism is well described within a donor-acceptor model, i.e., the Blyholder model. Since ${\mathrm{N}}_{2}$ is not reacting with lattice oxygen of ${\mathrm{RuO}}_{2},$ quite in contrast to CO, ${\mathrm{N}}_{2}$ may serve as a chemical, nondestructive probe to titrate but also to selectively block the cus-Ru atoms; recently, the cus-Ru atoms were shown to be the active centers for the chemisorption of molecules.

Total energy partitioning within a one-electron formalism: A Hamilton population study of surface–CO interaction in the c(2×2)-CO/ Ni(100) chemisorption system

A scheme for total electronic energy partitioning within the framework of a one-electron theory of the extended Hückel-type is presented, with a view to extending and augmenting the capabilities of existing theoretical electronic structure analysis tools, specifically overlap population analysis. A total electronic energy partitioning is developed first for molecular and subsequently extended materials. In constructing the partitioning, we define molecular orbital Hamilton populations (MOHP’s) for discrete systems, and Crystal Orbital Hamilton Populations (COHP’s) for extended systems. The various energy partitionings and overlap population analyses are exemplified and contrasted for HX (X=F,Cl,Br), ethane, and a [PtH4]2− polymer. The utility of energy partitioning is demonstrated by effecting a COHP partitioning of the surface–CO interaction for the c(2×2)-CO/Ni(100) chemisorption system. Aspects of the surface–CO interaction less amenable to overlap population analysis are addressed, specifically the role of energetically low-lying filled CO orbitals and the relative contributions of surface s, p, and d bands to surface–CO interaction. Hamilton population analysis leads to a CO (4σ, 5σ)–metal forward donation, metal–CO(2π*) backdonation model for the surface–CO interaction. The metal σ contribution to surface–CO bonding is described as sp dominated metal spd hybrid–CO bonding, modifying slightly the metal d–CO σ bonding model proposed by Blyholder. The metal d-2π* backdonation of the Blyholder model remains. The role of the CO(1π) orbitals is also discussed in the context of CO orbital mixing on binding CO to the Ni(100) surface.

Ultraviolet/high resolution electron energy loss spectroscopy of CO on Pt(110) and benzene on Pt(110)

Recently there has been some controversy about how CO and benzene adsorb on platinum. With CO, there is disagreement over whether there is back donation of electrons from the metal to the CO 2π* orbital. With benzene, there is disagreement, over whether benzene adsorbs in a distorted state. In this article we use a standard high resolution electron energy loss (HREELS) spectrometer with modified electronics to measure the equivalent of a ultraviolet (UV) spectrum for two different systems: CO on Pt(110) and benzene on Pt(110) and benzene on Pt(110). In the case of CO, the UV spectrum shows peaks at 5.6 and 8.2 eV that may shift slightly with coverage. By comparison, gas-phase CO shows peaks at 6.2 and 8.3 eV. The difference between the gas-phase peak positions and those on the surface are indicative of the antibonding orbitals being stabilized, as one would expect from the Blyholder model. With benzene we observe two different spectra: a first monolayer spectrum with a broad peak centered at 4.9 eV, and a multilayer spectrum with peaks at 0.75, 3.82, 4.71, 6.20, and 6.84 eV. The multilayer spectrum matches the spectrum of condensed benzene, while the first monolayer spectrum is quite different and resembles that of a diene. Together, these results show that UV/HREELS spectroscopy provides useful information about adsorbates on surfaces.

UV/HREELS measurements of the excited states of adsorbed CO: benchmarks for ab initio calculations

Recently there has been some controversy about CO adsorption on platinum, with some researchers supporting the Blyholder model and others saying that corrections are needed. We have used UV/HREELS to examine the electronic structure of adsorbed CO to distinguish between the various models. We have found that the 5σ to 2π ∗ triplet peak shifts from 6.2 to 5.6 eV upon adsorption while the 5σ to 2π ∗ singlet peak shifts from 8.3 to 8.2 eV. Both peaks also broaden, as expected from the Blyholder model. A comparison of these results to previous calculations shows that only Hückel calculations predict a shift that is within 0.5 eV of the experimental shift. Most of the previous calculations predict shifts that are in the wrong direction.

Nature of the surface chemical bond inN2on Ni(100) studied by x-ray-emission spectroscopy andab initiocalculations

The electronic structure of the system ${\mathrm{N}}_{2}/\mathrm{N}\mathrm{i}(100)$ has been studied by means of angle-resolved x-ray-emission spectroscopy (XES) and ab initio calculations. XES allows a symmetry-resolved decomposition of the $2p$ density of states projected on each N atom. The calculations reproduce the experimental spectra well. Our results show that it is necessary to use an atom-specific description rather than treating the molecule and substrate as separate units. Hence a model of the surface chemical bond for this system is presented, in which the N${}_{2}$ $1\ensuremath{\pi}$--Ni $3d$ interaction is important for the bond of N${}_{2}$ to the Ni(100) surface. The weakening of the internal $\ensuremath{\pi}$ is seen as the appearance of a nonbonding orbital whose character is essentially Ni $3d$ with a contribution of N $2p$ lone pair on the outer nitrogen atom. The $\ensuremath{\sigma}$ system strongly polarizes in order to minimize the Pauli repulsion with the Ni $4sp$ states in the substrate. The traditional picture of the Blyholder model, which in a frontier orbital framework involves $\ensuremath{\sigma}$ donation and $\ensuremath{\pi}$ backdonation with more or less unperturbed orbitals, is not in agreement with the experimental data and is not supported by the calculations. In order to create the adsorbate orbitals we need to involve the whole original $\ensuremath{\pi}$ system of the free molecule, i.e., both the $1\ensuremath{\pi}$ and $2{\ensuremath{\pi}}^{*}$ orbitals.

First-principles study of CO bonding to Pt(111): validity of the Blyholder model

The chemisorption of CO molecules on Pt(111) is studied by first-principles calculations based on the local density functional formalism with a slab model to represent the extended metal surface. We have developed a population analysis scheme which is applicable to calculations with plane wave basis sets. By applying the scheme to CO Pt(111) , the 4σ and 1π orbitals of CO are found to be completely filled, showing that they do not play a role in the bonding of CO to the surface. On the other hand, the calculated populations of the 5σ and 2π orbitals indicate that there is substantial 5σ donation and 2π backdonation, supporting the Blyholder model of CO chemisorption, the validity of which has recently been questioned. These results also suggest that a molecular-orbital based picture such as the Blyholder model is appropriate for describing simply the chemisorption bond, in spite of the fact that the molecular orbitals of CO rehybridize with each other upon adsorption, as has been shown recently by X-ray emission spectroscopy.

The interaction of carbon monoxide with the pure and potassium-promoted Cu(332) surface

The adsorption behaviour of the KCO coadsorbate system has been studied at an adsorption temperature of 95 K on the vicinal Cu(332) surface. Thermal desorption spectroscopy experiments show that the K adatoms disturb the electrostatic potential induced by the steps of the vicinal surface, and therefore the CO molecules adsorbed on step sites are the first to be influenced by the coadsorbed alkali. With K present on the surface in a metal-like form, a strong KCO interaction is evidenced by a stabilization of the adlayer up to temperatures >500 K. Work-function change measurements reveal a change in the bonding of CO towards the precovered surface: the results cannot be interpreted using a simple Blyholder model, but imply a direct interaction of potassium with the 1π orbital of CO. The existence of this direct interaction gives rise to an abolition of the (1π + 5σ) degeneration in the CO-induced states in the UV-photoelectron spectra.

Local probing of the surface chemical bond using X-ray emission spectroscopy

and CO on Ni(100), benzene on Ni(100) and Cu(110), and glycine adsorbed on Cu(110). New types of molecular states are observed which are directly related to the surface chemical bond. The long-accepted Blyholder model which is based on a frontier orbital concept cannot explain our results for N2 and CO chemisorption. We find it necessary to offer a new picture where changes in the whole molecular orbital framework have to be considered. We show that both π and σ type interactions are important in describing the bonding in benzene to metal surfaces. The future prospect is illustrated by the adsorption of the simplest amino acid, glycine, on Cu(110). The adsorbate has four different atomic centers where X-ray emission spectra are obtained, providing a unique view of the local electronic structure.

CO chemisorption on Ni n, Pd n and Pt n clusters

Vibrationally resolved photoelectron spectra of mass-selected negatively charged Ni n − ( n = 1–3), Pd n − ( n = 2, 3) and Pt n − ( n = 1–4) clusters are compared with the corresponding spectra of these clusters ligated with m CO molecules ( m = 1–8). The spectra of the Pt n (CO) m −, species reveal part of the valence orbitals, which actually form the chemisorption bond. The data are in good agreement with the Blyholder model for CO chemisorption (σ-donation- π-backdonation scheme) and indicate that saturation corresponds to the formation of a closed electronic shell of the neutral. The strength of the π-backdonation is found to be larger for small particles compared with the corresponding single crystal surfaces, which may be related to the catalytic properties of small particles. The spectra of the unsaturated Ni n (CO) m − show fundamental differences compared with the ones of Pt n(CO) m − clusters owing to the high degree of localization of the Ni 3d orbitals. The spectra of the Pd n (CO) m − species show a rather irregular behaviour.

THE STRUCTURE OF REACTED METAL CLUSTERS: PHOTOELECTRON SPECTRA OF ${\rm{Pt}}_n^-$ CLUSTERS REACTED WITH CO

[Formula: see text] clusters produced by a laser-vaporization source are reacted with CO. The various reaction products [Formula: see text] are studied systematically using photoelectron spectroscopy of negatively charged clusters. First results are interpreted in terms of the Blyholder model. The spectra indicate an open electronic shell structure of the unsaturated and a closed shell structure of the saturated Pt carbonyls. For [Formula: see text] emission from the 2π/5d hybrid orbital is directly observed. The assignment is supported by the measured vibrational fine structure.