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Steps at metal surfaces may influence energetics and kinetics of catalytic reactions in unexpected ways. Here, we report a significant reduction of the CO saturation coverage in Pd vicinal surfaces, which in turn is relevant for the light-off of the CO oxidation reaction. The study is based on a systematic investigation of CO adsorption on vicinal Pd(111) surfaces making use of a curved Pd crystal. A combined X-ray Photoelectron Spectroscopy and DFT analysis allows us to demonstrate that an entire row of atomic sites under Pd steps remains free of CO upon saturation at 300 K, leading to a step-density-dependent reduction of CO coverage that correlates with the observed decrease of the light-off temperature during CO oxidation in vicinal Pd surfaces., We acknowledge financial support from the Spanish Ministry of Science and Innovation (Grants MAT-2017-88374-P, PID2019-107338RB-C63, PID2020-116093RB-C44), the Basque Government (Grants IT-1255-19), the Knut and Alice Wallenberg foundation (DNR KAW 2015.0058 ”Atomistic Design of new Catalysts”), and the Swedish Research Council (DNR 349-2011-6491 “Catalysis on the Atomic Scale”). Financial support is acknowledged from the Swedish Research Council (2016-5234). The DFT calculations were performed at C3SE (Göteborg) via a SNIC grant. We acknowledge MAX IV Laboratory support, in particular Alexei Preobrajenski, for their help in synchrotron experiments, which were carried out at the FlexPES beamline under Proposal 20190717. Research conducted at MAX IV, a Swedish national user facility, is supported by the Swedish Research council under contract 2018-07152, the Swedish Governmental Agency for Innovation Systems under contract 2018-04969, and Formas under contract 2019-02496. Moreover, the research leading to this result has been supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020.

A methodology is proposed to investigate electron transfer reactions between redox-active biomolecular systems (e.g. a protein) and inorganic surfaces. The whole system is modelled at the atomistic level using classical molecular dynamics - making an extensive sampling of the system's configurations possible - and the energies associated with the redox-active complex reduction are calculated using a hybrid quantum/classical approach along the molecular dynamics trajectory. The non-adiabaticity is introduced a posteriori using a Monte Carlo approach based on the Landau-Zener theory extended to treat a metal surface. This approach thus allows us to investigate the role of the energy fluctuations, determined by the dynamical evolution of the system, as well as the role of non-adiabaticity in affecting the kinetic rate of the electron transfer reaction. Most notably, it allows us to investigate the two contributions separately, hence achieving a detailed picture of the mechanisms that determine the rate. The analysis of the system configurations also allows us to relate the estimated electronic coupling to the structural and dynamic properties of the system. As a test case, the methodology is here applied to study the electron transfer reaction between cytochrome c and a gold surface. The results obtained explain the different electron transfer rates experimentally measured for two different concentrations of proteins on the electrode surface.

Molecules self-assemble on surfaces forming a variety of patterns that depend on the relative strength between the intermolecular and molecule–surface interactions. In this study, the effect of the physisorption/chemisorption interplay on self-assembly is investigated using Monte Carlo simulations. The molecules are modeled as hexagonal tiles capable of assuming two distinct adsorption states, with different diffusion properties, on a hexagonal lattice. The self-assembled structures that emerge by tuning the molecule–surface and molecule–molecule interactions are systematically mapped out to develop understanding of their phase behavior. The resulting phase diagrams will guide the engineering of novel molecules to obtain desired collective structural properties for the development of innovative two-dimensional devices.

The adsorption, diffusion, and dissociation of pyridine, C5H5N, on Pt(111) are investigated with van derWaals-corrected density functional theory. An elaborate search for local minima in the adsorption potential energy landscape reveals that the intact pyridine adsorbs with the aromatic ring parallel to the surface. Piecewise interconnections of the local minima in the energy landscape reveal that the most favourable diffusion path for pyridine has a barrier of 0.53 eV. In the preferred path, the pyridine remains parallel to the surface while performing small single rotational steps with a carbon-carbon double bond hinged above a single Pt atom. The origin of the diffusion pathway is discussed in terms of the C-2-Pt pi-bond being stronger than the corresponding CN-Pt p-bond. The energy barrier and reaction enthalpy for dehydrogenation of adsorbed pyridine into an adsorbed, upright bound a-pyridyl species are calculated to 0.71 eV and 0.18 eV, respectively (both zero-point energy corrected). The calculations are used to rationalize previous experimental observations from the literature for pyridine on Pt(111). Published by AIP Publishing.

Journal article The self-assembly of molecules on surfaces into 2D structures is important for the bottom-up fabrication of functional nanomaterials, and the self-assembledstructure depends on the interplay between molecule-molecule interactions and molecule-surface interactions. Halogenated benzene derivatives on platinum have been shown to have two distinct adsorption states: a physisorbed state and a chemisorbed state, and the interplay between the two can be expected to have a profound effect on the self-assembly and phase behaviour of these systems. We developed a lattice model that explicitly includes both adsorption states, with representative interactions parameterised using density functional theory calculations. This model was used in Monte Carlo simulations to investigate pattern formation of hexahalogenated benzene molecules on the platinumsurface. Molecules that prefer the physisorbed state were found to self-assemble with ease, depending on the interactions between physisorbed molecules. In contrast, molecules that preferentially chemisorb tend to get arrested in disordered phases. However, changing the interactions between chemisorbed and physisorbed molecules affects the phase behaviour. We propose functionalising molecules in order to tune their adsorption states, as an innovative way to control monolayerstructure, leading to a promising avenue for directed assembly of novel 2D structures. Academia Nazionale dei Lincei; Royal Society of Edinburgh peer-reviewed

A simple direct mechanism for methane-to-methanol conversion has been investigated by first principles on a series of oxygen-precovered transition-metal surfaces. Energy barriers and reaction paths have been determined for three competing elementary processes by the nudged elastic band algorithm. Indicators of reactivity toward each elementary step have been identified, providing significant insight into a rational search for a suitable catalyst. The effect of chemical environment, local geometry, strain, and coadsorption have been addressed, and general guidelines have been identified. On the basis of this analysis, we suggest that upon suitable conditions O-dosed Ag surfaces could display considerable reactivity toward direct methane-to-methanol conversion.

The study of the molecular mechanism of chemical reactions occurring at solid surfaces is of primary importance to understand heterogeneous catalysis from a microscopic point of view. The present paper reviews the state of the art methods of electronic structure and the surface models currently used in this type of studies by making use of three different examples. Those are the decomposition of azomethane on Pt( 111), the study of the different selectivity of Cu( 111) and Ag( 111) towards ethene partial oxidation and the comparative study of NO dissociation on Rh( 111) and bimetallic RhCu( 111) surfaces. These examples illustrate the power of the electronic structure computational approaches to predict the structure and stability of different intermediates and to unravel the molecular mechanism of these surface reactions.

Using scanning tunneling microscopy, low-energy electron diffraction, and X-ray photoelectron spectroscopy. we studied the evolution of the structure and chemical state of a Rh(110) surface, modified by K adlayers and exposed to high O-2 doses at elevated temperatures, We find that oxygen coadsorption on the K-covered Rh(110) leads to massive reconstruction of the Rh(110) surface. Stable reconstructed (10 x 2) and (8 x 2) segmented phases with a local coverage of more than two oxygen atoms per surface Rh atom were observed. Formation of surface oxide, which coexists with the (10 x 2) and (8 x 2) segmented adsorption phases, is evidenced at the highest O-2 doses. The development of strongly reconstructed adsorption phases with oxide-like stoichiometry and surface oxide under UHV conditions is explained in terms of the stabilization of the (1 x 2) reconstruction and promotion of O-2 dissociation by the K adatoms.

A simple approach based on (i) the glue model for gold, (ii) an empirical force field for C60 and (iii) the charge equilibration plus the Born-Mayer potential for their interaction shows that the two experimentally detected structures for C60 on Au(110) 6] are competitive from 100 K. The model further shows the different nature of the Au-Au and Au-C60 interactions inside the two unit cells, which makes one structure prevail over the other as a function of the experimental conditions. More specifically, the calyx-like structure of ref. [6] is favoured by metal-organic interactions, while the regular structure of ref. [5] is characterized by a remarkable stability of the Au surface. Poster ECSCD-8 18-21 July, 2004 (Segovia, Spain)

We study how quenched impurities affect the surface diffusion and ordering of strongly interacting adsorbate atoms on surfaces. To this end, we carry out Monte Carlo simulations for a lattice-gas model of O/W~110!, including small concentrations of immobile impurities which block their adsorption sites. We examine the behavior of the diffusion coefficients and order parameters as a function of coverage corresponding to various ordered phases at low temperatures. The effects of impurities are examined under both equilibrium and nonequilibrium conditions, and the results are compared to recent studies on a completely clean surface. We find that even minute impurity concentrations affect the diffusion behavior considerably in equilibrium. The effects are strongest in ordered phases and close to phase boundaries, where quenched impurities lead to a reduction of order, which in turn leads to significant changes in the collective diffusion and phase behavior. As the impurity concentration is increased to a level of a few percent of the total surface area, the reduction in order becomes particularly prominent at high coverages. Further studies under nonequilibrium conditions reveal that nonequilibrium effects are strong in the absence of impurities, while for surfaces covered by impurities the nonequilibrium effects are relatively weaker. © 2002 American Institute of Physics. @DOI: 10.1063/1.1505856#

We study the coverage dependence of surface diffusion for chainlike molecules by the fluctuating-bond model with a Monte Carlo dynamics. The model includes short-ranged excluded volume interactions between different chains as well as an intrachain bond angle potential to describe the chain stiffness. Our primary aim is to consider the role played by chain stiffness and the resulting memory effects in tracer diffusion, and in particular their role in the effective tracer diffusion barrier E-A(T) extracted from the well-known Arrhenius form. We show that the memory effects in tracer diffusion become more pronounced at an increasing coverage as a result of packing requirements. Increasing the chain flexibility furthermore has the same overall effect as increasing the chain length, namely, they both increase E-A(T). We then analyze the influence of memory effects on E-A(T) and find that, for a single diffusing chain, about 20% of E-A(T) arises from temperature variations in the memory effects, while only the remaining part comes from thermally activated chain segment movements. At a finite coverage, the memory contribution in E-A(T) is even larger and is typically about 20%-40%. Further studies with chains of different lengths lead to a conclusion that, for a single diffusing chain, the memory contribution in E-A(T) decreases along with an increasing chain length and is almost negligible in the case of very long chains. Finally, we close this work by discussing our results in light of recent experimental work as regards surface diffusion of long DNA molecules on a biological interface. (C) 2000 American Institute of Physics.

The tris-methylene bridged compound (NO)(4)Fe2Se(mu-CH2)(3) has been isolated. It has been characterised by IR and H-1, C-13, and Se-77 NMR spectroscopy. Its structure has been determined by single-crystal X-ray diffraction methods. The structure consists of a heavy atom triangle consisting of one Se and two Fe atoms. The Fe-Fe and the two Fe-Se edges are bridged by methylene groups. (C) 1997 Elsevier Science S.A.

Room temperature reaction of (CO) 6 Fe 2 (μ-EE′) (E≠ E′; E, e′  S, Se, Te; E  E′  S, Se, Te) with diazoethane yielded two types of compound: (CO) 6 Fe 2 EE′{μ-C(H)CH 3 } 2 (1, 27%, E  E′  S; 2, 24%, E  S, E′  Se; 3, 21%, E  E′  Se; 5, 19%, E S, E′  Te; 7, 12%, E  Se, E ′  Te) and (CO) 6 Fe 2 {μ-EC(H)CH 3 )E′} (4, 13%, E  E′  Se; 16%, E  S, E′  Te; 8, 22%, E  Se, E′  Te; 9, 28%, E  E′  Te). Compounds 1–9 were characterized by IR 1 H, 13 C, 77 Se and 125 Te NMR spectroscopy. Structures of 1, 3, 5, 8 and 9 were established by crystallographic methods. Compounds 1, 3 and 5 are isomorphic and isostructural and their structure consists of a distorted rhombus in which Fe and E (E  S, Se, Te) occupy adjacent comers. Two C(H)(CH 3 ) moieties bridge opposite FeE bonds and give roughly a chair conformation. Compounds 8 and 9 are isomorphic and isostructural and their structure consists of an Fe 2 TeE (E  Se, Te) tetrahedrally core containing a C(H)(CH 3 ) group as bridge between two wing-tips of chalcogen atoms.

The adsorption of organic chiral molecules on metallic substrates is widely studied as a tool to obtain chiral surfaces. The chirality transfer from the molecules to the surface is strongly driven by the availability of hydrogen atoms, which guides a specific chiral self-assembled structure. In this paper we report, by combination of photoelectron spectroscopy, low-energy electron diffraction, and density functional theory calculations, on the adsorption of the D-enantiomer of alaninol on Cu(100) and on Cu(110) with the aim of revealing dehydrogenation in the formation of the molecular chiral superstructure. We show that, on both surfaces, at low coverage alaninol is dehydrogenated at the hydroxyl group, whereas at saturated coverage the substrate symmetry, in combination with intermolecular interactions, induces partial amino group dehydrogenation on Cu(100) or inhibits hydroxyl group dehydrogenation on Cu(110).

Fluorescence properties of submonolayers of rhodamine 6G have been measured as a function of distance to an aluminum mirror. For distances less than 5 nm (spacer layers of 1 nm thickness were used) a broadening of the emission spectrum is observed. The fluorescence lifetime has been measured for distances up to 6 nm. The broadening of the fluorescence spectra and the shortening of the lifetime observed when the sample is close to the mirror are attributed to efficient energy transfer from the monolayer to the mirror. The experiments clearly demonstrate that both volume and surface contributions must be considered in this energy-transfer process.

The growth of rubrene (C(42)H(28), 5,6,11,12-tetraphenylnaphthacene) multilayer islands up to a thickness of six layers on a Au(111) surface has been investigated by scanning tunneling microscopy. The molecules self-organize in parallel twin rows, forming mirror domains of defined local structural chirality. Each layer is composed of twin-row domains of the same structural handedness rotated by 120 degrees with respect to each other. Moreover, this structural chirality is transferred to all successive layers in the island, resulting in the formation of three-dimensional objects having a defined structural chirality. The centered rectangular surface unit cell differs from the one characteristic for the single-crystal orthorhombic phase.

Oxygen-induced Rh3d 5/2 surface core level shifts were used to probe the local electronic structure of first layer Rh atoms in the (2 x 2)pg reconstructed phase formed upon oxygen adsorption. By comparison of the computed shifts for differently reconstructed geometries with the experimental shifts obtained from high-energy resolution photoemission measurements, we confirm that the reconstructed phase is formed by Rh atoms single- and double-bonded to oxygen, yielding a shift from the bulk component of ―185 and +140 meV, respectively. We find that the core level shifts are affected by the local rhomboidal distortion of the surface lattice. Moreover, we show that, on Rh( 100), the oxygen-induced surface core level shifts are dominated by initial state effects and proportional to the calculated shift of the d-band center. The latter is therefore correlated to the local surface chemical reactivity, as already found for other adsorbate systems on transition metals.

The establishment of a molecular view of heterogeneous catalysis has been hampered for a number of reasons. There are, however, recent developments, which show that we are now on the way towards reaching a molecular-scale picture of the way solids work as catalysts. By a combination of new theoretical methods, detailed experiments on model systems, and synthesis and in situ characterization of nano-structured catalysts, we are witnessing the first examples of complete atomic-scale insight into the structure and mechanism of surface-catalyzed reactions. This insight has already proven its value by enabling a rational design of new catalysts. We illustrate this important development in heterogeneous catalysis by highlighting recent examples of catalyst systems for which it has been possible to achieve such a detailed understanding. In particular, we emphasize examples where this progress has made it possible to propose entirely new catalysts, which have then been proven experimentally to exhibit improved performance in terms of catalytic activity or selectivity.

The electronic structure of a thin film of chlorine-substituted Zn-phthalocyanine (ZnPcCl 8 ) deposited on Ag(11) was studied by valence band photoemission and near-edge X-ray absorption fine structure spectroscopy. At the first stages of molecular adsorption the interface formation is accompanied by the promotion of intense interface states. A charge transfer from the substrate to lowest unoccupied molecular orbital (LUMO) of the molecules of the first layer appears as a new density of states close to the Fermi level in photoemission. The central role of the LUMO is corroborated by the reduction of the LUMO-derived transition observed in the low-coverage X-ray absorption spectrum taken at the N Is edge. However, the lack of a sizable density of states at E F as well as the photon energy dependence of the interface state peak intensity suggest that more complicated mechanisms of intra- or extramolecular charge redistribution may also play a role. The Ag 4d-derived valence band region also appears strongly modified upon molecular adsorption. In the paper it is shown that these modifications are merely due to the vanishing of the substrate surface states and the emerging of a bulk density of states at the interface. It is suggested that such behavior should be common for other organic monolayers adsorbed on noble metal surfaces. Finally, the study of the work function as a function of coverage is presented. Its behavior is interpreted in terms of coexistence of different structural phases and compared to a local-probe work function study on the same system.

We present the results of high-energy resolution core level photoelectron spectroscopy experiments paralleled by density functional theory calculations to investigate the electronic structure of highly undercoordinated Pt atoms adsorbed on Pt(111) and its correlation with chemical activity. Pt4f(7/2) core level binding energies corresponding to atoms in different configurations are shown to be very sensitive not only to the local atomic coordination number but also to the interatomic bond lengths. Our results are rationalized by introducing an indicator, the effective coordination, which includes both contributions. The calculated energy center of the valence 5d-band density of states, which is a well known depicter of the surface chemical reactivity, shows a noteworthy correlation with the Pt4f(7/2) core level shifts and with the effective coordination.

The adsorption of trimesic acid (TMA) on Cu(110) has been studied in the temperature range between 130 and 550 K and for coverages up to one monolayer. We combine scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), reflection absorption infrared spectroscopy (RAIRS), X-ray photoemission spectroscopy (XPS), and density functional theory (DFT) calculations to produce a detailed adsorption phase diagram for the TMA/Cu(110) system as a function of the molecular coverage and the substrate temperature. We identify a quite complex set of adsorption phases, which are determined by the interplay between the extent of deprotonation, the intermolecular bonding, and the overall energy minimization. For temperatures up to 280 K, TMA molecules are only partly deprotonated and form hydrogen-bonded structures, which locally exhibit organizational chirality. Above this threshold, the molecules deprotonate completely and form supramolecular metal-organic structures with Cu substrate adatoms. These structures exist in the form of single and double coordination chains, with the molecular coverage driving distinct phase transitions.

Keywords: Supramolecular Architecture at Surfaces Reference LNS-ARTICLE-2007-005doi:10.1002/cphc.200700370View record in Web of Science Record created on 2009-04-14, modified on 2017-05-12

We have studied the dissociative chemisorption and scattering of N-2 on and from Ru(0001), using a six-dimensional quasiclassical trajectory method. The potential energy surface, which depends on all the molecular degrees of freedom, has been built applying a modified Shepard interpolation method to a data set of results from density functional theory, employing the RPBE generalized gradient approximation. The frozen surface and Born-Oppenheimer [Ann. Phys. (Leipzig) 84, 457 (1927)] approximations were used, neglecting phonons and electron-hole pair excitations. Dissociative chemisorption probabilities are found to be very small even for translational energies much higher than the minimum reaction barrier, in good agreement with experiment. A comparison to previous low dimensional calculations shows the importance of taking into account the multidimensional effects of N-2 rotation and translation parallel to the surface. The new calculations strongly suggest a much smaller role of nonadiabatic effects than previously assumed on the basis of a comparison between low dimensional results and experiments [J. Chem. Phys. 115, 9028 (2001)]. Also in agreement with experiment, our theoretical results show a strong dependence of reaction on the initial vibrational state. Computed angular scattering distributions and parallel translation energy distributions are in good agreement with experiments on scattering, but the theory overestimates vibrational and rotational excitations in scattering. (c) 2006 American Institute of Physics.

Herein, a detailed investigation of the adsorption and dynamics of C-60 and C-70 fullerenes hosted in a self-assembled, two-dimensional, nanoporous porphyrin network on a solid Ag surface is presented. Time-resolved scanning tunneling microscopy (STM) studies of these supramolecular systems at the molecular scale reveal distinct host-guest interactions giving rise to a pronounced dissimilar mobility of the two fullerenes within the porphyrin network. Furthermore, long range coverage-dependant interactions between the all-carbon guests, which clearly affect their mobility and are likely mediated by a complex mechanism involving the Ag substrate and the flexible porphyrin host network, are observed. At increased fullerene coverage, this unprecedented interplay results in the formation of large fullerene chains and islands. By applying a lattice gas model with nearest-neighbour interactions and by evaluating the fullerence-pair distribution functions, the respective coverage-dependant guest-guest interaction energies are estimated.

We studied the growth of pentacene (C22H14) on the Au(110) surface by means of He atom scattering and Synchrotron X-ray photoemission. We found that two-dimensional commensurate growth only occurs in the monolayer range for a substrate temperature, T(s), higher than approximately 370 K. Larger amounts of deposited molecules forms three-dimensional uncorrelated clusters on the wetting layer. The desorption of second layer molecules occurs at T(s) > or = 420 K. The highest coverage ordered phase displays a (6 x 8) symmetry and corresponds to the saturation coverage at T(s) = 420 K. The (3 x 6) symmetry phase, recently reported for a multilayer planar film [Ph. Guaino, et al. Appl. Phys. Lett. 2004, 85, 2777], is only found at a coverage slightly lower than the (6 x 8) one. The (3 x 6) phase corresponds to the saturation coverage of the first layer at T(s) = 470 K.

The coadsorption of ethylene, C2H4, and atomic oxygen on Ag(100) was studied using density-functional theory. As for the adsorption of oxygen alone, the on-surface hollow sites are predicted to be the most stable adsorption sites at low coverage (or =1/2 ML). Above this coverage, mixed on-surface + subsurface oxygen configurations become more stable. The binding of ethylene to the clean Ag(100) is weak and little affected by oxygen when it is adsorbed on-surface. On the other hand, we find that the adsorption energy of C2H4 may increase considerably when oxygen is adsorbed into subsurface sites. Our results indicate that the increased reactivity of surface Ag atoms is because of their decreased coordination due to the push out effect of oxygen underneath, more than to their oxidation.

We have studied the adsorption of benzenethiol molecules on the Au(111) surface by using first principles total energy calculations. A single thiolate molecule is adsorbed at the bridge site slightly shifted toward the fcc-hollow site, and is tilted by 61 degrees from the surface normal. As for the self-assembled monolayer (SAM) structures, the (2 square root of 3 x square root of 3)R30 degrees herringbone structure is stabilized against the (square root 3 x square root 3)R30 degrees structure by large steric relaxation. In the most stable (2 square root 3 x square root 3)R30 degrees SAM structure, the molecule is adsorbed at the bridge site with the tilting angle of 21 degrees, which is much smaller compared with the single molecule adsorption. The van der Waals interaction plays an important role in forming the SAM structure. The adsorption of benzenethiolates induces the repulsive interaction between surface Au atoms, which facilitates the formation of surface Au vacancy.

We study the nature of nonequilibrium effects in the collective diffusion coefficient D-C(theta) vs the coverage theta as extracted from Boltzmann-Matano analysis of spreading coverage profiles. We focus on the temporal behavior of the profiles and study how the corresponding nonequilibrium effects in D-C(theta) depend on the initial density gradient and the initial state from which the spreading starts. To this end, we carry out extensive Monte Carlo simulations for a lattice-gas model of the O/W(110) system. Studies of submonolayer spreading from an initially ordered p(2x1) phase at theta = 1/2 reveal that the spreading and diffusion rates in directions parallel and perpendicular to rows of oxygen atoms are significantly different within the ordered phase. Aside from this effect, we find that the degree of ordering in the initial phase has a relatively small impact on the overall behavior of D-C(theta). Also, although we find that nonequilibrium effects are clearly present in submonolayer spreading profiles, D-C(theta) determined from such data approaches its asymptotic equilibrium behavior much more rapidly than in the case of full spreading. Nevertheless, in both cases there are noticeable deviations from equilibrium results that persist even at very long times and are strongest in ordered phases and in the vicinity of phase boundaries. These conclusions are confirmed by complementary studies of the temporal behavior of the order parameter phi(theta). Finally, we use D-C(theta) and phi(theta) to determine the locations of phase boundaries and find such data to be clearly time dependent during full spreading. We conclude that nonequilibrium effects seem to be an inherent feature in profile evolution studies of surface diffusion in all cases where ordering plays a prominent role. This warrants particular care to be taken with profile spreading experiments. (C) 2001 American Institute of Physics.

Gradient corrected periodic density functional theory (DFT-GGA) slab calculations were used to examine the chemisorption of atomic hydrogen on various Pd-Re alloyed overlayers and uniformly alloyed surfaces. Adsorption was examined at 33% surface coverage, where atomic hydrogen preferred the three-fold fcc sites. The binding energy of atomic hydrogen is observed to vary by as much as 0.7 eV due to Pd-Re interactions. The computed adsorption energies were found to be between -2.35 eV [for monolayer Pd-on-Re, i.e., Pd-ML/Re(0001)] and -3.05 eV [for Pd-33 Re-66/Pd(111)]. A d-band weighting scheme was developed to extend the Hammer-Norskov surface reactivity model [Surf. Sci. 343, 211 (1995)] to the analysis of bimetallic Pd-Re alloyed systems. The hydrogen chemisorption energies are correlated linearly to the surface d-band center, which is weighted appropriately by the d-band coupling matrix elements for Pd and Re. The farther the weighted d-band center is shifted below the Fermi energy, the weaker is the interaction of atomic hydrogen with the alloyed Pd-Re surface. (C) 2000 American Institute of Physics.

We study the diffusive dynamics of adparticles in two model systems with strong interactions by considering the decay of the single-particle velocity correlation function φ(t). In accordance with previous studies, we find φ(t) to decay nonexponentially and follow a power-law φ(t)∼t−x at intermediate times t, while at long times there is a crossover to an exponential decay. We characterize the behavior of the decay exponent x in detail in various ordered phases and in the vicinity of phase boundaries. We find that within the disordered phase, the behavior of x can be rationalized in terms of interaction effects. Namely, x is typically larger than two in cases where repulsive adparticle–adparticle interactions dominate, while attractive interactions lead to x

We present an extension of the charge-dipole model for the description of periodic systems. This periodic charge-dipole electrostatic model (PCDEM) allows one to describe the linear response of periodic structures in terms of charge- and dipole-type gaussian basis functions. The long-range electrostatic interaction is efficiently described by means of the continuous fast multipole method. As a first application, the PCDEM method is applied to describe the polarizability of silver slabs. We find that for a correct description of the polarizability of the slabs both charges and dipoles are required. However a continuum set of parametrizations, i.e., different values of the width of charge- and dipole-type gaussians, leads to an equivalent and accurate description of the slabs polarizability but a completely unphysical description of induced charge-density inside the slab. We introduced the integral squared density measure which allows one to obtain a unique parametrization which accurately describes both the polarizability and the induced density profile inside the slab. Finally the limits of the electrostatic approximations are also pointed out.

Ab initio band-structure calculations within a density function formalism were performed to computer the binding energy curves of atomic hydrogen with the high-symmetry adsorption sites of the (111) surface of copper. For a two-layer slab of Cu atoms and H coverage equal to 0.25, the binding energies are 2.25, 3.12, and 3.24 eV, for on-top bridge, and threefold sites, so that the chemisorption of H{sub 2} on Cu(111) is exothermic for threefold and bridge sites, but endothermic for on-top sites. Starting from these results, an LEPS potential for the interaction of H{sub 2} with the Cu(111) surface was built. In this model potential, the most favored approaches correspond to a H{sub 2} molecule parallel to the Cu surface, and for them, the activation barrier is located at the corner between the entrance and the exit channels of the reaction, and it lowest value is 0.6 eV. The LEPS potential was used in quasi-classical trajectories calculations to simulate the adsorption of a beam of H{sub 2} molecules on Cu(111). The results show that (a) when H{sub 2} is in the ground vibrational state the dissociative adsorption probability P{sub a} increases from 0-90 along a roughly sigmoidal curve by increasing the collisionmore » kinetic energy from 0.4 to 1.3 eV, and (b) the vibrational energy can be as effective as the translation one in promoting dissociative chemisorption, in agreement with the experimental results. 39 refs., 8 figs., 1 tab.« less

International audience; In the present paper we study the reactivity of model Pt nanoparticles supported on glassy carbon. The particle size effect is rationalized for CO monolayer oxidation exploring electrochemical methods (stripping voltammetry and chronoamperometry) and modelling. Significant size effects are observed in the particle size interval from ca. 1 to 4 nm, including the positive shift of the CO stripping peak with decreasing particle size and a pronounced asymmetry of the current transients at constant potential. The latter go through a maximum at low COads conversion and exhibit tailing, which is the longer the smaller the particle size. Neither mean field nor nucleation & growth models give a coherent explanation of these experimental findings. We, therefore, suggest a basic model employing the active site concept. With a number of reasonable simplifications a full analytical solution is obtained, which allows a straightforward comparison of the theory with the experimental data. A good correspondence between experiment and theory is demonstrated. The model suggests restricted COads mobility at Pt nanoparticles below ca. 2 nm size, with the diffusion coefficient strongly dependent on the particle size, and indicates a transition towards fast diffusion when the particle size exceeds ca. 3 nm. Estimates of relevant kinetic parameters, including diffusion coefficient, reaction constant etc. are obtained and compared to the literature data for extended Pt surfaces.

We investigate the dynamical features of the adsorption of diphenylalanine molecules on the Cu(I 10) surface and of their assembling into supramolecular structures by a combination of quantum and classical atomistic modeling with dynamic scanning tunneling microscopy and spectroscopic experiments. Our results reveal a self-assembling mechanism in which isolated adsorbed molecules change their conformation and adsorption mode as a consequence of their mutual interactions. In particular, the formation of zwitterions after proton transfer between initially neutral molecules is found to be the key event of the assembling process, which stabilizes the supramolecular structures. Because of the constraints on the intermolecular bonds exerted by the surface-molecule interactions, the assembly process is strictly stereoselective, And may suggest a general model for patterning and functionalization of bare metal surfaces with short chiral peptides.