Volume 223, Issue 1-2

The interaction of hydrogen with bimetallic surfaces is assessed and discussed using structurally well defined PdAu and PtRu single crystal surfaces as model systems. The description is based on a correlation between the hydrogen adsorption behavior and the presence and abundance of specific bimetallic nanostructures. The latter are determined quantitatively by high resolution scanning tunneling microscopy. Following the changes in the hydrogen adsorption behavior on surfaces with systematically varied composition and thus varying concentrations of specific structural elements and adsorption ensembles allows a direct correlation between structural and chemical properties and thus to identify the adsorption characteristics of individual adsorption ensembles. This provides a structural basis for parallel theoretical studies, which are essential for a systematic understanding of the complex changes in the adsorption properties.Based on results from experimental studies and calculations, the role of electronic strain and ligand effects, including lateral and vertical ligands, as well the influence of geometric ensemble effects on the hydrogen adsorption characteristics are discussed. The distinct differences between surfaces modified by a monolayer or monolayer islands of a second metal and mixed surfaces such as surface alloys or alloy surfaces are discussed in these terms.

Phenomena like transfer, expression and amplification of chirality in molecular monolayers are reviewed. Chirality can be bestowed onto achiral surfaces by adsorption of chiral molecules. This offers a good opportunity to study two-dimensional chiral crystallization phenomena, like lateral resolution of enantiomers or the transfer of handedness from single molecules into mesoscopic ensembles at high resolution with scanning probe microscopy. Induction of homochirality on surfaces via cooperatively amplified interactions in molecular monolayers is a new phenomenon of supramolecular surface chirality. Prochiral molecules will turn into either handedness upon adsorption, but doping with intrinsically chiral molecules breaks this symmetry and induces homochirality. A similar effect is induced by a small enantiomeric excess. The excess molecules provide the chiral bias that becomes amplified into single lattice chirality.

Recent years have seen rapid progress in the field of surface-confined coordination chemistry. Adsorbed metal complexes of tetrapyrroles (porphyrins, phthalocyanines, corroles) are especially interesting in this context, since they combine a planar structure-determining element with an active site. While earlier studies of adsorbed metallo-tetrapyrroles mainly addressed aspects of molecular self-assembly, the focus of interest has shifted gradually to electronic structure and chemical reactivity. This article gives an overview of recent advances in the field of surface chemistry with tetrapyrroles. In particular, the following aspects will be discussed: intramolecular conformation and supramolecular ordering, electronic interaction with the substrate, surface-confined synthesis, and ligand-related effects such as the surface trans effect.

The adsorption structure of hydrogen on ultrathin epitaxial nickel films in pseudomorphic arrangement on the unreconstructed Ir(100) surface is investigated by quantitative low-energy electron diffraction (LEED). This is highly interesting in the light of the fact that on Ni(100) adsorption is in hollow sites whilst on unreconstructed Ir(100) it is exclusively in bridge sites. On Ni films thicker than 1 monolayer (ML) the adsorption is in 4-fold coordinated hollow sites as it is on Ni(100). This is in spite of the considerable tensile strain of the Ni film (+8.9%) and is interpreted by the dominance of ensemble effects in the hydrogen bonding. In contrast, on the 1 ML Ni film on Ir(100), with iridium atoms being nearest neighbours to nickel atoms, there is occupation of both bridge and hollow sites of the nickel layer – a clear indication for the presence of ligand effects.

Catalytic CO oxidation in a ternary gas mixture of CO, NO and O 2 has been studied in the 10 –5 –10 –4 mbar range using a Pt(100) sample as catalyst. Measurements of the reactive sticking coefficients of CO and NO are used to decompose the overall CO oxidation rate into the contributions from oxidation by NO and from oxidation by O 2 . The experiments are complemented by simulations with a realistic 7-variable model. The surface phase transition hex ↔ 1×1 and the inhibition of NO dissociation and O 2 adsorption at high adsorbate coverages lead to significant hysteresis effects.

In this paper I outline the scope and success of the theory of equilibrium and nonequilibrium properties of surfaces and adsorbates Basic issues such as time and length scales lead to various levels of sophistication provided the proper assumptions are implimented successfully leading to a hierarchy of theories from the macroscopic level of nonequilibrium thermodynamics to Markov processes at the mesoscopic level and ultimately to a fully quantum mechanical description. Special ttention will be given to the role of detailed balance in sticking and desorption via direct and precursor-dominated channels.

We study the modification of the growth behaviour of iron on Ag(111) by pread sorbed monolayers of 2H-phthalocyanine (2HPc) and 2H-tetraphenylporphyrin (2HTPP) by scanning tunneling microscopy, upon Fe deposition in ultra-high vacuum at room temperature. The growth of iron on clean Ag(111) follows the scheme expected from reports of similar systems: For low coverages (up to ~0.3 ML) an almost exclusive agglomeration of iron clusters at step edges occurs, which continue to grow in a dendritic fashion at higher coverages. Upon deposition onto Ag(111) precovered with monolayers of 2HTPP and 2HPc, at coverages up to 0.037 ML no growth of Fe islands but complete metalation, i.e., incorporation of the Fe atoms in the macrocyle of the preadsorbed molecules is found. For higher amounts of deposited Fe (0.037 < θ Fe < 0.30 ML), the formation of Fe islands is observed, which occurs differently for 2HPC and 2HTPP. While on 2HTPP two-dimensional islands are formed presumably underneath a FeTPP monolayer, for 2HPc the Fe growth is dominated by the formation of small 3D clusters within a FePc monolayer. At θ Fe = 2.0 ML, both systems exhibit small three dimensional Fe islands. These results are discussed considering different aspects like the molecular conformation of FeTPP and FePc on Ag(111) and the diffusional properties of Fe on the clean and modified substrate.

A new oxime-based hexanuclear Mn III complex containing the structural core [Mn 6 III ( μ 3 -O) 2 ] was synthesized through a general route. We describe the preparation and characterization of the manganese(III) complex [Mn 6 III ( μ 3 -O) 2 (Salox) 6 (CH 3 CH 2 OH) 4 {(CH 3 ) 3 CCOO} 2 ] by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). Therefore we offer the first core level XPS, and XAS reference spectra of polynuclear complexes comprising only Mn(III) ions in octahedral surrounding.

This mini-review focuses on the recently discovered nanomeshes and nanotemplates made from carbon, boron nitride and their mixtures with a thickness of just a single atomic layer. Typically they exist on some transition metal or semiconductor substrate, the interaction with it playing a crucial role in nanopattern formation. We review systems such as graphene/SiC(0001), graphene/Ru(0001), h-BN/Ru(0001), and h-BN/Rh(111), their atomistic models, synthesis routes, as well as possible applications as templates for nanoperiodic arrays of clusters and molecules. Scanning tunneling microscopy (STM), a technique with ultimate resolution in real space, is stressed as an indispensable tool for a comprehensive characterization of the given systems.

Scherrer formula is often applied to X-ray-diffraction profiles for the size determination of small size clusters. However, for small clusters this often leads to conflicting results in comparison to other methods. A series of Pd-clusters of different size is studied by X-ray diffraction analysis and transmission electron microscopy. The influence of size and structure on the results is presented and discussed in comparison with theoretical calculations. It will be shown that the different structure of small size systems are one main origin of the conflicts. The structure problem can be overcome by using Fourier Transform of the X-ray diffraction pattern. The importance of the knowledge of the cluster structure was demonstrated by showing its strong influence on the hydrogen solubility.

The oxidation of carbon monoxide over polycrystalline ruthenium dioxide (RuO 2 ) powder was studied in a packed-bed reactor and by bulk and surface analytical methods. Activity data were correlated with bulk phases in an in-situ X-ray diffraction (XRD) setup at atmospheric pressure. Ruthenium dioxide was pre-calcined in pure oxygen at 1073 K. At this stage RuO 2 is completely inactive in the oxidation of CO. After a long induction period in the feed at 503 K RuO 2 becomes active with 100% conversion, while in-situ XRD reveals no changes in the RuO 2 diffraction pattern. At this stage selective roughening of apical RuO 2 facets was observed by scanning electron microscopy (SEM). Seldom also single lateral facets are roughened. EDX indicated higher oxygen content in the following order: flat lateral facets > rough lateral facets > rough apical facets. Further, experiments in the packed bed reactor indicated oscillations in the CO 2 formation rate. At even higher temperatures in reducing feed (533–543 K) the sample reduces to ruthenium metal according to XRD. The reduced particles exhibiting lower ignition temperature are very rough with cracks and deep star-shaped holes. An Arrhenius plot of the CO 2 formation rate below the ignition temperature reveals the reduced samples to be significantly more active based on mass unit and shows lower apparent activation energy than the activated oxidized sample. Micro-spot X-ray photoelectron spectroscopy (XPS) and XPS microscopy experiments were carried out on a Ru(0001) single crystal exposed to oxygen at different temperature. Although low energy electron diffraction (LEED) images show a strong 1×1 pattern, the XPS data indicated a wide lateral inhomogeneity with different degree of oxygen dissolved in the subsurface layers. All these and the literature data are discussed in the context of different active states and transport issues, and the metastable nature of a phase mixture under conditions of high catalytic activity.

The adsorption of ethene (C 2 H 4 ) has been studied on Pd(111) and ordered Sn/Pd(111) surface alloys using temperature programmed desorption (TPD), ultraviolet photoelectron spectroscopy (UPS), high-resolution electron energy loss spectroscopy (HREELS), and low energy electron diffraction (LEED). Two surface alloys were prepared by thermal treatment of Sn-films, which were vapor deposited on Pd(111) at room temperature. Depending on the preparation conditions, surface alloys giving a p(2×2) or a (√3×√3)R30° LEED pattern were produced. Below 250 K ethene adsorbs on pure Pd(111) in an undissociated – but substantially distorted – form relative to the molecular structure in the gas phase: HREELS suggests an adsorption in the di-σ bonded state. A π-bonded ethene species was, however, found to coexist with this strongly rehybridized form, probably as a result of hydrogen coadsorbed from the residual gas. TPD and annealing experiments followed by UPS and HREELS indicated that most of the adsorbed ethene desorbs reversibly in the temperature range between 150 K and 350 K, while a small amount dehydrogenates. After adsorption at room temperature, ethylidyne (≡CCH 3 ) has been identified as the most important species. Alloying Pd(111) with Sn results in a decreasing ethene-substrate interaction with increasing Sn-content in the topmost layer of the substrate. Only π-bonded ethene was formed on both surface alloys and decomposition reactions were suppressed.