The sticking and scattering of O-2/Pt(111) has been studied by tight-binding molecular dynamics simulations based on an ab initio potential energy surface. We focus, in particular, on the sticking probability as a function of the angle of incidence and the energy and angular distributions in scattering. Our simulations provide an explanation for the seemingly paradox experimental findings that adsorption experiments suggest that the O-2/Pt(111) interaction potential should be strongly corrugated while scattering experiments indicate a rather small corrugation. The potential energy surface is indeed strongly corrugated which leads to a pronounced dependence of the sticking probability on the angle of incidence. The scattered O-2 molecules, however, experience a rather flat surface due to the fact that they are predominantly scattered at the repulsive tail of the potential.

Chemisorption of CO on the stepped Cu(211) surface is studied within ab initio density functional theory (DFT) and scanning tunneling microscopy (STM) imaging as well as manipulation experiments. Theoretically we focus on the experimentally observed ordered (2x1) and (3x1) CO phases at coverages Theta = 1/3, 1/2, and 2/3 monolayer (ML). Also, obtain information for isolated CO molecules that are found randomly distributed at low coverages, we performed calculations for a hypothetical (3x1) phase with Theta = 1/3 ML. The adsorption geometry, the stretching frequencies, the work functions, and adsorption energies of the CO molecules in the different phases are presented and compared to experimental data. Initially and up to a coverage of 1/2 ML, CO adsorbs upright on the on-top sites at step edge atoms. Determining the most favorable adsorption geometry for the 2/3 ML ordered phase turned out to be nontrivial, both from the experimental and the theoretical point of view. Experimentally, both top-bridge and top-top configurations were reported, whereby only the top-top arrangement was firmly established. The calculated adsorption energies and the stretching frequencies favor the top-bridge configuration. The possible existence of both configurations at 2/3 ML is critically discussed on the basis of the currently accessible experimental and theoretical data. In addition, we present observations of STM manipulation experiments and corresponding theoretical results, which show that CO adsorbed on top of a single Cu adatom, which is manipulated to a location close to the lower step edge, is more strongly bound than CO on top of a step edge atom.

Following our investigation of molecular NO adsorption (Gajdos et al 2006 J. Phys.: Condens. Matter 18 13), the dissociation of NO molecules on the close-packed surfaces of late transition (Co, Ni, Ru, Rh, Pd, Ir, Pt) and noble (Cu, Ag, An) metals has been studied using first-principles density functional calculations. The nudged-elastic-band method has been used for the determination of the transition states. Our results demonstrate that the transition-state energies show a linear dependence on the dissociative chemisorption energies according to the Bronsted-Evans-Polanyi rule. The validity of this linear relationship is shown to arise from the geometrical similarity of the transition states on all metals, which are very close to the final state geometries.

We have studied the trends in CO adsorption on close-packed metal surfaces: Co, Ni, Cu from the 3d row, Ru, Rh, Pd, Ag from the 4d row and Ir, Pt, Au from the 5d row using density functional theory. In particular, we were concerned with the trends in adsorption energy, geometry, vibrational properties and other parameters derived from the electronic structure of the substrate. The influence of specific changes in our set-up, such as choice of the exchange correlation functional, the choice of pseudopotential, size of the basis set and substrate relaxation, has been carefully evaluated. We found that, while the geometrical and vibrational properties of the adsorbate–substrate complex are calculated with high accuracy, the adsorption energies calculated with the gradient-corrected Perdew–Wang exchange–correlation energies are overestimated. In addition, the calculations tend to favour adsorption sites with higher coordination, resulting in the prediction of the wrong adsorption sites for the Rh, Pt and Cu surfaces (hollow instead of top). The revised Perdew–Burke–Erzernhof functional (RPBE) leads to lower (i.e. more realistic) adsorption energies for transition metals, but to the wrong results for noble metals—for Ag and Au, endothermic adsorption is predicted. The site preference remains the same. We discuss trends in relation to the electronic structure of the substrate across the periodic table, summarizing the state-of-the-art of CO adsorption on close-packed metal surfaces.

Thin epitaxial ZrO2 insulator films on a Pt(111) substrate have been studied by means of scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and density functional theory (DFT) calculations. The films have been prepared by Zr vapor deposition in an O2 atmosphere followed by post-annealing also in an O2 atmosphere. The process of film formation was investigated from the island state up to film thicknesses where the films completely cover the substrate. Continuous ZrO2(111) films of high quality with smooth surfaces and high structural order are obtained for film thicknesses above 3 monolayers (ML) and temperatures of deposition and post-annealing of 470 and 950 K, respectively. A carpet like growth mode is observed which corresponds to the relatively weak film/substrate interaction found in DFT calculations. The investigations reveal a clear p(1×1) surface structure, which rotates during annealing with respect to the substrate. Based on DFT calculations, the possibility of different surface terminations is discussed. Annealing the samples at temperatures >950 K induces a three-dimensional restructuring of the ZrO2 films which destroys the film continuity. Imaging discontinuous ZrO2 films at small tunneling voltages yield a characteristic contrast reversal for local film thicknesses >2 ML indicating the retarded formation of the insulator gap of ZrO2 during film growth which has been proven also by DFT calculations.

The adsorption of ethylene at 100 K on clean and oxygen precovered Pd(111) surfaces and the thermal evolution of the ethylene adsorbate layers have been investigated experimentally by high-resolution electron energy loss spectroscopy (HREELS), high-resolution X-ray photoelectron spectroscopy with synchrotron radiation, thermal desorption spectroscopy and theoretically by ab initio density functional theory (DFT) calculations. On the clean Pd(111) surface at 100 K ethylene is adsorbed in a di-σ bonding state, whereas on the oxygen precovered Pd(111)2×2-O surface the π-bonded configuration is more stable; this has been established both experimentally and theoretically. Upon adsorption at room temperature ethylidyne adspecies are formed on both surfaces, but neither di-σ nor π-bonded ethylene transform into ethylidyne on heating from low temperature up to 450 K. Complete molecular desorption of ethylene is observed in both cases, with no signs of dehydrogenation. The spectroscopic data recorded during the thermal evolution of the low temperature adsorbate phase have been analysed with the help of DFT and indicate that π-bonded ethylene adsorption states may become populated upon heating the low temperature adlayer to 350 K.

Tight-binding molecular dynamics simulations of the adsorption of O2/Pt(111) have been performed based on an ab initio potential energy surface. We demonstrate that, contrary to common belief, in this system the whole adsorption probability as a function of the kinetic energy can be understood in terms of trapping into chemisorbed molecular precursor states. This provides a novel unified picture of the trapping process which is relevant for the general understanding of adsorption. Furthermore, by simple steric arguments we are able to explain why O2 does not dissociate on cold Pt(111) surfaces even at kinetic energies that are much greater than the dissociation barrier.

Ab initio density-functional investigations of the gradual adsorption of increasing amounts of CO on partially precovered Pt(111) surfaces are presented. Our calculations show that up to precoverages as high as 0.5 monolayer (ML) CO, the adsorption energy is minimally influenced by lateral interactions and that adsorption of additional CO molecules remains an unactivated process. The saturation coverage is estimated to be similar to0.67 ML. Beyond this limit the adsorption energies are strongly reduced and substantial barriers against further adsorption are built up. For the high-coverage limit, we have examined several adsorption geometries proposed in the literature as well as a novel configuration. Energetic considerations, the calculated adsorption geometries, and the analysis of the calculated frequency spectra all favor a model with the c(root3x3) periodicity as proposed on the basis of the experimental data.

The different adsorption possibilities of thiophene (C4H4S) on the Ni(1 1 0) surface have been studied using first principle local-density-functional calculations, with the Vienna ab initio simulation package, which is based on a plane wave basis set and projector augmented wave potentials. For each configuration, a geometric optimisation has been performed. A detailed analysis of the structural and electronic properties of the molecule and the surface in the most stable conformations is presented, showing the combined roles of the molecular distortion and the interactions between the molecule and the surface. Three structures with comparatively large adsorption energies are identified, all with the molecule plane parallel to the surface.Starting from these stabilised structures, various scenarios for the desulfurisation process have been envisaged. While, for the most stable structure, the formation of an adsorbed thiol is an activated process, with an energetic barrier of 0.70 eV, the two structures which are just a bit less stable can dissociate to a C4H4 species and a sulfur atom with barriers as low as 0.07 eV. A description of the different transition states and a kinetic analysis of the desulfurisation reaction is also presented. (C) 2003 Elsevier B.V. All rights reserved.

The translational energy (ET) dependence of CO chemisorption has been investigated on Pd(110). A steep increase from 0.61 to 0.93 is observed in the initial sticking probability S0 with increasing ET from 27 to 93 meV. Comparison to density functional theory calculations suggests a steering-mediated adsorption channel, driving slow molecules towards less favorable surface sites. This long-range interaction between CO and the Pd d-orbitals screens the potential influence of the surface structural corrugations on the CO adsorption and thus provides an explanation to the well-known structure insensitivity of CO adsorption on Pd surfaces. For ET>93 meV the adsorption reverts back to the direct chemisorption mechanism found on other Group VIII metals.

The adsorption of oxygen on the Ag(100) is investigated by means of density functional techniques. Starting from a characterization of the clean silver surfaces oxygen adsorption in several modifications (molecularly, on-surface, sub-surface, Ag2O) for varying coverage was studied. Besides structural parameters and adsorption energies also work-function changes, vibrational frequencies and core level energies were calculated for a better characterization of the adsorption structures and an easier comparison to the rich experimental data.

A density-functional-theory- (DFT-) based method for the description of metal-oxide interfaces is presented. By superposition of the potential energy surface (PES) of an isolated metal atom on the oxide surface, predictions can be made for the optimal adsorption mode of arbitrary metal structures. The scheme is tested first for the adsorption of small Pt clusters on a tetragonal zirconia surface. There good agreement with a DFT-calculated PES for the cluster is achieved. The same cluster configuration is investigated for yttrium-stabilized zirconia, too, for which stronger bonding is predicted. Further, the scheme is applied for a Pt film on the same surface, where a complicated root7xroot7 Pt structure on a c(2x2) oxide structure is predicted, in excellent agreement with experimental data.

Exposure of Rh(100) to hydrogen (deuterium) in atomic form leads to the population of adsorption sites, not attainable with molecular species. Quantitative thermal desorption spectroscopy (TDS), high resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) calculations have been applied to investigate these new adsorption sites. In addition to the fourfold hollow sites (1 ML), which can be populated by dissociative adsorption, occupation of subsurface sites and the population of additional surface sites (for deuterium) have been observed (maximum coverage 3.4 ML). In TDS individual adsorption states show up in the form of three different peaks: Recombination of H (D) atoms from hollow sites around 300 K, desorption of subsurface species between 150–200 K, and recombinative desorption via a molecular precursor at about 120 K (for deuterium only). The exposure of the Rh(100) surface to atomic H (D) leads to a pronounced roughening of the surface, as evidenced in the HREELS spectra. Zero point corrected adsorption energies, activation barriers for adsorption, desorption, and diffusion into the subsurface sites, as well as vibrational energies have been calculated by DFT for a variety of adsorbate configurations of H and D and compared with the experimental data.

Density functional theory calculations have been used to investigate the adsorption of carbon monoxide on the (111) surface of nickel. For various surface coverages between Θ=1/12 and 1 ML with CO occupying different adsorption sites geometric structure, adsorption energy, CO stretch frequency (and intensities) and O1s and C1s binding energies have been calculated. By comparison to experimental data from the literature, the site occupation with coverage could be traced: at low coverage (Θ<0.5 ML) CO occupies mainly bridge sites, accompanied by small patches of hollow (for Θ<0.2 ML) and on-top (for Θ>0.2 ML) adsorbed species. The relatively high-dynamic dipole moment of the linear bond species (especially at low coverage) leads to a distinct signal even for very small amounts. At Θ=0.5 the well established c(2×4) structure with a mixed occupation of fcc and hcp adsorbed molecules is formed. In this structure strong dipole interaction between the adsorbates shift the CO stretch frequency to a value that is traditionally assigned to bridging species. For even higher coverage more complicated structures within 7×7 (Θ=4/7 ML) and c(23×4)rect (Θ=5/8 ML) cells are discussed. A further fundamental observation of this study is a very pronounced coverage dependence of the dynamic dipole moment for the CO stretch frequency and hence of the corresponding peak intensities in vibrational spectroscopy.

At low tunneling resistance, scanning tunneling microscopy (STM) images of a Rh(100) surface with adsorbed hydrogen reproducibly show protrusions in all bridge sites of the surface, leading to a naive interpretation of all bridge sites being occupied with H atoms. Using quantitative low-energy electron diffraction and temperature programmed desorption we find a much lower H coverage, with most H atoms in fourfold hollow sites. Density functional theory calculations show that the STM result is due to the influence of the tip, attracting the mobile H atoms into bridge sites. This demonstrates that STM images of highly mobile adsorbates can be strongly misleading and underlines the importance of additional analysis techniques.

In this study, density function theory calculations are applied to the simulation of CO oxidation reactions over platinum, palladium and rhodium surfaces. On the basis of these calculations alone the detailed reaction scenario together with activation energies, pre-factors and rate constants can be derived. Such studies allow a systematic analysis of trends due to exactly identical conditions. The comparison with observed reaction rates demonstrates that such an approach gives reliable results and provides further insight into the reaction mechanism.

The catalytic reduction of NO in combination with the oxidation of CO over a catalytic surface has been analyzed using density functional theory (DFT) calculations. In the light of this prototypical reaction the modeling of a complex catalytical reaction on the basis of DFT calculations is demonstrated. In particular activation energies, pre-factors and rate constants for all reaction steps are obtained. Reaction barriers and rates agree well with experimental estimates, providing a basis for kinetic modeling studies.

The adsorption dynamics of H2 on Pd(1 1 1) alloyed with V has been studied using molecular beam experiments (MB) and ab initio density functional theory (DFT) calculations. The experiments show that dissociation on pure Pd(1 1 1) is dominated by dynamical steering and therefore shows strong rotational hindering. The alloy surface exhibits a sticking probability independent of rotational excitation and an isotropic distribution of inelastically scattered molecules, attributed to a dynamical precursor. DFT calculations of the potential energy surface and ab initio simulations of scattering events demonstrate that the different behavior of Pd and Pd/V surfaces arises from the existence of a flat physisorption state.

Zirconia is of great industrial importance as support for catalysts and as ion conductor. Especially when mechanical stability is needed in environments with varying temperature the tetragonal modification is superior, because of its high resistivity against physical shocks and thermal stresses. In this study density functional theory is used to investigate the interaction between Y impurities and oxygen vacancies and to develop in this way a structural model for Y-doped zirconia. For every two Y defects an O vacancy is formed at a distance of about $4.11 \AA{}$ to each dopant. An activation energy for ion conductivity of 0.87 eV is calculated in very good agreement with experimental findings.