Timo Jacob

Surface Chemistry of Ag Particles: Identification of Oxide Species by Aberration-Corrected TEM and by DFT Calculations

In the 1990s, by means of spectroscopic methods, Ertl et al. found surface and subsurface oxygen atoms on and in Ag catalysts. Three species of atomic oxygen with distinct structural and energetic properties were identified. According to their TDS behavior (TDS: thermal desorption spectroscopy), a surface atomic species was termed the a form, and a bulk-dissolved species of lower interaction energy the b form. Finally, g-oxygen was identified as strongly interacting atomic oxygen with high electron density, incorporated into the top atomic surface layer of Ag. As silver catalysts are used in many reactions, for example, hydrogenation of unsaturated aldehydes, partial oxidation of methanol to formaldehyde, and oxidative coupling of methane to ethane and ethylene, the discovery of surface and subsurface oxygen atoms in Ag is of great significance for understanding the catalytic reaction steps and mechanisms of silver catalysts. However, the location of surface and subsurface oxygen atoms remains an unanswered question in Ag catalysis and, perhaps more importantly, it is unclear whether nonmodel industrial catalysts exhibit the same surface chemistry. High-resolution transmission electron microscopy (HRTEM) has been widely used to study the morphology and structure of catalysts. It provides detailed information on the microand nanostructure of catalysts. By aligning the normal of a given surface perpendicular to the incident electron beam, the surface structure and its relationship to the underlying bulk structure can be investigated. However, due to the artefacts caused by spherical aberration of magnetic imaging lenses, conventional TEM is not optimally suited to obtaining readily interpretable images of catalyst surfaces. One major artefact is the delocalization of image details, which appears as an extension of the perimeter of a sample beyond the actual surface. In this study, we investigated an Ag/SiO2 catalyst using a TEM with a sphericalaberration corrector that can compensate for these problems and thus provide detailed information about the structure of the surfaces of a silver-based catalyst. With the support of DFT calculations, the positions of aand g-oxygen on the surfaces of Ag particles have been determined for the first time. Furthermore, the presence of local surface oxygen atoms or oxide was verified. To investigate Ag particles by a direct imaging technique, spatial frequencies in the band between 4.24 and 4.89 nm 1 representing Ag(111) and Ag(200) lattice-plane distances of 0.236 and 0.204 nm, respectively, must be transferred with the same contrast. Under the experimental conditions shown in Figure 1 the acquired high-resolution electron micrograph makes the surface terminations of the Ag particle clearly observable (Figure S1 and Figure 2). The internal crystalline structure of the Ag particles extends to the surface, where it is terminated abruptly in different ways. Steps consisting of one or two atom rows on the (111) facet are observed.

Molecular Dynamics Simulations of the Interactions between Platinum Clusters and Carbon Platelets

Molecular dynamics simulations have been performed with two reactive force fields to investigate the structure of a Pt100 cluster adsorbed on the three distinct sides of a carbon platelet. A revised Reax force field for the carbon-platinum system is presented. In the simulations, carbon platelet edges both with and without hydrogen termination have been studied. It is found that the initial mismatch between the atomic structure of the platelet egde and the adsorbed face of the Pt100 cluster leads to a desorption of a few platinum atoms from the cluster and the subsequent restructuring of the cluster. Consequently, the average Pt-Pt bond length is enlarged in agreement with experimental results. This change in the bond length is supposed to play an important role in the enhancement of the catalytic activity, which is demonstrated by studying the changes in the bond order of the platinum atoms. We found an overall shift to lower values as well as a loss of the well-defined peak structure in the bond-order distribution.

Theoretical predictions of adsorption behavior of elements 112 and 114 and their homologs Hg and Pb

Fully relativistic (four-component) density-functional theory calculations were performed for elements 112 and 114 and their lighter homologs, Hg and Pb, interacting with gold systems, from an atom to a Au(n) cluster simulating the Au(111) surface. Convergence of the adatom-metal cluster binding energies E(b) with cluster size was reached for n>90. Hg, Pb, and element 114 were found to preferably adsorb at the bridge position, while element 112 was found to preferably adsorb at a hollow site. Independently of the cluster size, the trend in E(b) is Pb>>114>Hg>112. The obtained E(b) for Pb and element 112 are in good agreement with the measured adsorption enthalpies of these elements on gold, while the Hg value is obviously underestimated, confirming the observation that adsorption takes place not on the surface but in it. A comparison of chemical bonding in various systems shows that element 114 should be more reactive than element 112: A relative inertness of the latter is caused by the strong relativistic stabilization of the 7s atomic orbital. On the contrary, van der Waals bonding in element 114 systems should be weaker than in those of element 112 due to its larger radius.

Carbon reaction and diffusion on Ni(111), Ni(100), and Fe(110): Kinetic parameters from x-ray photoelectron spectroscopy and density functional theory analysis

This paper investigates the reactivity of elemental carbon films deposited from the vapor phase with Fe and Ni substrates at room temperature. X-ray photoelectron spectroscopy (XPS) measurements are presented as a method for evaluating kinetic reaction data. Carbon films are deposited on different surface orientations representing geometries from a dense atom packing as in fcc (111) to an open surface structure as in fcc (100). During annealing experiments several reactions are observed (carbon subsurface diffusion, carbide formation, carbide decomposition, and graphite ordering). These reactions and the respective kinetic parameters are analyzed and quantified by XPS measurements performed while annealing at elevated temperatures (620-820 K). The resulting activation barriers for carbon subsurface diffusion are compared with calculated values using the density functional theory. The determined kinetic parameters are used to reproduce the thermal behavior of carbon films on nickel surfaces.

Intermetallic compounds of the heaviest elements: the electronic structure and bonding of dimers of element 112 and its homolog Hg

Abstract Fully relativistic (four-component) density-functional calculations were performed for the element 112 dimers (112)X (X = Pd, Cu, Ag and Au) and those of its lighter homolog, Hg. A relatively small decrease of about 15–20 kJ/mol in bonding was found from the HgX to (112)X compounds. Respectively, the bond lengths were increased by 0.06 A on the average. The Mulliken population analysis has shown this effect to be a result of a decreasing contribution of the relativistically stabilized 7s-AO of element 112 to bonding. The following trend in the binding energies was predicted for (112)X as a function of X: Pd >Cu>Au>Ag, exactly as the trend obtained experimentally for adsorption of Hg on the corresponding metal surfaces.

Adsorption of CO on cluster models of platinum Ñ111Ö: A four-component relativistic density-functional approach

We report on results of a theoretical study of the adsorption process of a single carbon oxide molecule on a Platinum (111) surface. A four-component relativistic density functional method was applied to account for a proper description of the strong relativistic effects. A limited number of atoms in the framework of a cluster approach is used to describe the surface. Different adsorption sites are investigated. We found that CO is preferably adsorbed at the top position.

First-principles studies on oxygen-induced faceting of Ir(210).

Density functional theory calculations were performed to obtain an atomistic understanding of facet formation on Ir(210). We determined geometries and energetics of clean and oxygen-covered surfaces of planar Ir(210) as well as Ir(311) and two types of Ir(110) surfaces, which are involved in faceting by forming three-sided nanopyramids. Using the energies together with the ab initio atomistic thermodynamics approach, we studied the stability of substrate and facets in the presence of an oxygen environment. Our results show that facets are stable over the entire temperature range at which oxygen is adsorbed on the surface at coverages >/=0.45 physical ML, supporting the picture of a thermodynamic driving force. We also investigated the dependence of the phase diagram on the choice of the exchange-correlation functional and obtained qualitatively the same behavior. Finally, this work helps to better understand reactivity and selectivity of O-covered planar and faceted Ir surfaces in catalysis.

Facet stability in oxygen-induced nanofaceting of Re(1231)

The stability of the various facets in oxygen-induced faceting of Re(1231) has been studied by low-energy electron diffraction, scanning tunneling microcopy, and synchrotron-based high-resolution X-ray photoemission spectroscopy. When Re(1231) is annealed at 800-1200 K in oxygen (10(-7) Torr), the surface becomes completely covered with nanometer-scale facets, and its morphology depends on the substrate temperature and oxygen exposure. Especially, the (1121) facet competes with the (1011) facet in determining the surface morphology, and the stability of each facet relies on oxygen coverage. Using density functional theory, the O-Re binding energies on the facets for various oxygen concentrations are calculated to explain how the oxygen coverage affects the anisotropy of surface free energy, which in turn determines the morphology of the faceted surface.

Theoretical predictions of trends in spectroscopic properties of gold containing dimers of the 6p and 7p elements and their adsorption on gold.

Fully relativistic, four-component density functional theory electronic structure calculations were performed for the MAu dimers of the 7p elements, 113 through 118, and their 6p homologs, Tl through Rn. It was shown that the M-Au bond strength should decrease from the 6p to 7p homologs in groups 13 and 14, while it should stay about the same in groups 15 through 17 and even increase in group 18. This is in contrast with the decreasing trend in the M-M bond strength in groups 15 through 17. The reason for these trends is increasingly important relativistic effects on the np AOs of these elements, particularly their large spin-orbit splitting. Trends in the adsorption energies of the heaviest elements and their homologs on gold are expected to be related to those in the binding energies of MAu, while sublimation enthalpies are closely connected to the binding energies of the MM dimers. Lack of a correlation between the MAu and MM binding energies means that no correlation can also be expected between adsorption enthalpies on gold and sublimation enthalpies in groups 15 through 17. No linear correlation between these quantities is established in the row of the 6p elements, as well as no one is expected in the row of the 7p elements.

Atomic-scale insight into the interactions between hydroxyl radicals and DNA in solution using the ReaxFF reactive force field

Cold atmospheric pressure plasmas have proven to provide an alternative treatment of cancer by targeting tumorous cells while leaving their healthy counterparts unharmed. However, the underlying mechanisms of the plasma–cell interactions are not yet fully understood. Reactive oxygen species, and in particular hydroxyl radicals (OH), are known to play a crucial role in plasma driven apoptosis of malignant cells. In this paper we investigate the interaction of OH radicals, as well as H2O2 molecules and HO2 radicals, with DNA by means of reactive molecular dynamics simulations using the ReaxFF force field. Our results provide atomic-scale insight into the dynamics of oxidative stress on DNA caused by the OH radicals, while H2O2 molecules appear not reactive within the considered time-scale. Among the observed processes are the formation of 8-OH-adduct radicals, forming the first stages towards the formation of 8-oxoGua and 8-oxoAde, H-abstraction reactions of the amines, and the partial opening of loose DNA ends in aqueous solution.