Herbert Over

Automated determination of complex surface structures by LEED

Conventional surface crystallography by low-energy electron diffraction (LEED) employs a trial-and-error search controlled at each step by human effort. This trial-and-error approach becomes very cumbersome and unreliable to solve complex surfaces with a large number of unknown structural parameters. We discuss automatic optimization procedures for LEED, which combine numerical search algorithms with efficient methods of determining the diffracted intensities for varying structures. Such approaches can reduce the computer time required for an entire structure determination by many orders of magnitude, while fitting many times more unknown structural parameters. Thereby, relatively complex structures, with typically 10 adjustable atoms (or 30 adjustable coordinates), can be readily determined on todays workstations. These include non-symmetrically relaxed structures, surface reconstructions and adsorbate-induced substrate distortions. We also address the theoretical and experimental requirements for an accurate structural determination.

Structure and stability of a high-coverage (1x1) oxygen phase on Ru(0001)

The formation of chemisorbed O phases on Ru(0001) by exposure to O{sub 2} at low pressures is apparently limited to coverages {Theta}{le}0.5. Using low-energy electron diffraction and density-functional theory we show that this restriction is caused by kinetic hindering and that a dense O overlayer ({Theta}=1) can be formed with a (1{times}1) periodicity. The structural and energetic properties of this new adsorbate phase are analyzed and discussed in view of attempts to bridge the so-called {open_quote}{open_quote}pressure gap{close_quote}{close_quote} in heterogeneous catalysis. It is argued that the identified system actuates the unusually high rate of oxidizing reactions at Ru surfaces under high oxygen pressure conditions. {copyright} {ital 1996 The American Physical Society.}

The atomic geometry of the O and CO + O phases on Rh(111)

Abstract The local adsorption geometries of the (2 × 2)-1O, (2 × 2)-2O, (2 × 2)-(O + CO) and (2 × 2)-(O + 2CO) phases on the Rh(111) surface have been investigated by analysing low-energy electron diffraction (LEED) intensity data. In all cases, the oxygen atoms were found to occupy the threefold fcc site and the topmost layer spacing d 12 of Rh(111) was found to be expanded by about 3%. Additional experiments with disordered oxygen overlayers prepared at low adsorption temperatures show a monotonic increase of d 12 with oxygen coverage. An O coverage of ϑ o ≈ 0.1 already lifts the contraction of 0.02 A of the clean surface. This finding is remarkable as for Ru(0001) a substantial expansion of the first layer spacing (≈ 3%) is observed only at very high O coverages, e.g. for the (1 × 1)-O structure. For the high-coverage Rh(111)-(2 × 2)-2O phase a honeycomb arrangement of the O atoms can clearly be ruled out. The (2 × 2)-2O has rather to be regarded as a (2 × 1)-1O phase. In the mixed (2 × 2)-(O + CO) overlayer the CO molecule occupies the on-top position which is also expected from a reduced back-donation of (substrate) electron charge density due to coadsorbed O atoms. The presence of CO molecules weakens the RhO bond. Altogether, the structural findings nicely reflect the competition of CO and O for electron charge density at the Rh(111) surface. In the (2 × 2)-(O + 2CO) phase 60% of available hcp sites and 100% of available on-top sites in the (2 × 2) unit cell are occupied.

Identification of RuO2 as the active phase in CO oxidation on oxygen-rich ruthenium surfaces

CO oxidation over ruthenium dioxide (RuO2) dominates the CO/CO2 conversion rate over the catalytically active oxygen-rich Ru(0001) surfaces. In sharp contrast, chemisorbed O overlayers on Ru(0001) (with and without dissolved oxygen) are virtually inactive with respect to CO oxidation.

Crystallographic study of interaction between adspecies on metal surfaces

The interaction among adsorbed atoms and molecules (adspecies) on metal surfaces plays a decisive role in catalytic reactions. Such interaction may cause structural changes of the local adsorption geometry which, together with spectroscopic and energetic data, may afford useful physical and chemical insights into the basic mechanisms of surface processes. When the adsorption geometry of a single adspecies is considered as a function of coverage, a deeper understanding of the nature of the adsorbate-substrate bonding can be obtained. Depending on the adsorbate coverage, the magnitude of adsorbate-induced relaxations and reconstructions vary widely. Occasionally, chemisorption systems transform gradually into adsorbate-substrate compounds, such as oxides, nitrides, hydrides, and sulfides. For the case of adsorption of different adspecies, coadsorption, structural data can make a vital contribution to our understanding of reaction intermediates, the promotion effect in heterogeneously catalyzed reactions, and the formation of ultra-thin compound films.

Heterogeneous oxidation catalysis on ruthenium: bridging the pressure and materials gaps and beyond

It is shown that both the materials and the pressure gaps can be bridged for ruthenium in heterogeneous oxidation catalysis using the oxidation of carbon monoxide as a model reaction. Polycrystalline catalysts, such as supported Ru catalysts and micrometer-sized Ru powder, were compared to single-crystalline ultrathin RuO2 films serving as model catalysts. The microscopic reaction steps on RuO2 were identified by a combined experimental and theoretical approach applying density functional theory. Steady-state CO oxidation and transient kinetic experiments such as temperature-programmed desorption were performed with polycrystalline catalysts and single-crystal surfaces and analyzed on the basis of a microkinetic model. Infrared spectroscopy turned out to be a valuable tool allowing us to identify adsorption sites and adsorbed species under reaction conditions both for practical catalysts and for the model catalyst over a wide temperature and pressure range. The close interplay of the experimental and theoretical surface science approach with the kinetic and spectroscopic research on catalysts applied in plug–flow reactors provides a synergistic strategy for improving the performance of Ru-based catalysts. The most active and stable state was identified with an ultrathin RuO2 shell coating a metallic Ru core. The microscopic processes causing the structural deactivation of Ru-based catalysts while oxidizing CO have been identified.

The adsorption of atomic nitrogen on Ru(0001): geometry and energetics

Abstract The local adsoprtion geometries of the (2 × 2)-N and (✓3 × ✓3)R30°-N phases on the Ru(0001) surface are determined by analyzing low-energy electron diffraction intensity data. For both phases, nitrogen occupies the threefold hcp site. The nitrogen sinks deeply into the top Ru layer resulting in a N-Ru interlayer distance of 1.05 and 1.10 A in the (2 × 2) and the (✓3 × ✓3)R30° unit cell, respectively. This result is attributed to a strong N binding to the Ru surface (Ru-N bond length = 1.93 A ) in both phases as also evidenced by an initio calculations which revealed binding energies of 5.82 and 5.59 eV, respectively.

Experimental and simulated STM images of stoichiometric and partially reduced RuO2(110) surfaces including adsorbates

We present experimental and DFT-simulated STM images of ultrathin RuO2(110) films on Ru(0001), including adsorbates such as oxygen and CO. We are able to identify the under-coordinated O atoms on the RuO2(110) surface with STM, i.e. the bridging O atoms and the on-top O atoms. The partial reduction of the RuO2(110) surface by CO exposure at room temperature leads to a surface where part of the bridging O atoms have been removed and some of the vacancies are occupied by bridging CO. When dosing 10 L of CO at room temperature, the RuO2(110) surface becomes fully mildly reduced in that all bridging 0 atoms are replaced by bridging CO molecules. Annealing the surface to 600 K produces holes on the terraces of such a mildly reduced RuO2(110) surface. These pits are not generated by the recombination of lattice O with CO, but rather these pits are assigned to a complex temperature-induced rearrangement of surface atoms in the topmost RuO2 double layer of RuO2(110). With this process the bridging O atoms are again populated and surplus Ru atoms agglomerate in small islands at the rims of the holes

Oxygen adsorption on the Ru(1010) surface: Anomalous coverage dependence

Oxygen adsorption on to Ru(101\ifmmode\bar\else\textasciimacron\fi{}0) results in the formation of two ordered overlayers, i.e., a

On the origin of the Ru-3d(5/2) satellite feature from RuO2(110)

c(2\ifmmode\times\else\texttimes\fi{}4)\ensuremath{-}2\mathrm{O}