Hans-Joachim Freund

Metal deposits on well-ordered oxide films

Abstract Metal oxide interfaces, metal coatings or dispersed metals on oxide supports play an important part in many technological areas. Nevertheless, there is still a lack of fundamental knowledge about the essential properties of thin metal films and small metal particles on oxide supports, although a deeper understanding could help to improve the electronic, mechanical or catalytic performance of such systems. In the past, a number of different approaches have been proposed and explored aiming at the preparation of suitable model systems. In this review, we discuss the possibility to use thin, well-ordered oxide films as supports for the study of deposited metal particles. This approach offers the advantage to permit the unrestricted application of all experimental methods, which rely on a good electrical or thermal conductivity of the sample, like PES, LEED, STM or TDS. With the help of several examples taken from our own work on a thin alumina film, we show that it is feasible to characterise such systems on a microscopic level with respect to all relevant structural, electronic and adsorption properties. In this way, correlations between these features can be established helping to understand the particular chemistry and physics of small metal aggregates.

Surface chemistry of carbon dioxide

Abstract The review discusses how CO 2 surface chemistry has developed since the early 1950s. Emphasis is given to studies of well-characterized surfaces of metals, oxides and some more complex systems involving in particular alkali modified surfaces and also of coadsorbed molecules.

Formation of a well-ordered aluminium oxide overlayer by oxidation of NiAl(110)

We have investigated the electronic and geometric structure of a thin oxide film grown by oxidation on NiAl(110) using electron spectroscopic techniques, i.e., LEED, EELS, XPS and ARUPS. This film is inert to adsorption of, respectively reaction with many molecules up to temperatures of about 800 K. It is well ordered as deduced from the LEED pattern and covers the whole surface. We find that the oxide film is about 5 A thick, consisting of aluminium oxide as shown by EELS, XPS and ARUPS. It is most likely formed of two aluminium layers and two quasihexagonal oxygen layers with oxygen surface termination. Since the oxide film is rather thin it only shows a two-dimensional band structure which has been investigated using ARUPS. For the electronic levels of the oxide strong periodic dispersions are observed with bandwidths compatible to dispersion bandwidths calculated for the ΓX direction of α-Al2O3.

Clusters and islands on oxides: from catalysis via electronics and magnetism to optics

The study of metal deposits on oxides represents a field of wide interest with respect to applications as well as to basic science. The state of the art of the field is reviewed on the basis of examples from various research groups. An attempt is made to define and discuss a series of new experiments that could be undertaken to answer some key questions in the field.

Oxide ultra-thin films on metals: new materials for the design of supported metal catalysts

Ultrathin oxide films on metals offer new opportunities for the design of supported nanoclusters with potential use in catalysis. This requires a characterization at the atomistic level of the structure and composition of the thin film, of its morphology and defect structure. A proper selection of metal/oxide interface, film thickness, lattice mismatch, etc. makes it possible to prepare collections of supported metal particles with novel properties. This critical review describes some illustrative examples, emphasizes the role of the interplay between theory and experiment, and relates some recent findings related to the possibility to control the charge state of a supported nanoparticle on an ultrathin oxide film (211 references).

CO Oxidation as a Prototypical Reaction for Heterogeneous Processes

CO oxidation, although seemingly a simple chemical reaction, provides us with a panacea that reveals the richness and beauty of heterogeneous catalysis. The Fritz Haber Institute is a place where a multidisciplinary approach to study the course of such a heterogeneous reaction can be generated in house. Research at the institute is primarily curiosity driven, which is reflected in the five sections comprising this Review. We use an approach based on microscopic concepts to study the interaction of simple molecules with well-defined materials, such as clusters in the gas phase or solid surfaces. This approach often asks for the development of new methods, tools, and materials to prove them, and it is exactly this aspect, both, with respect to experiment and theory, that is a trade mark of our institute.

Structure and defects of an ordered alumina film on NiAl(110)

Abstract Via oxidation a well ordered Al 2 O 3 film may be grown on an ordered NiAl(110) surface. Its structure has been studied with SPA-LEED (spot-profile analysis) as well as with scanning tunneling microscopy (STM). The oxide overlayer grows strictly two-dimensional with a thickness of close to 5 A. Double diffraction spots have been observed but they are very weak, thus not excluding the existence of an interfacial layer between NiAl(110) and the oxide film. STM provides preliminary evidence for such a film and presents first clues to what the structure of the interface may be. The defect structure of the Al 2 O 3 film has been investigated. In addition to boundaries between two rotational domains constituting the Al 2 O 3 film, we also identify anti-phase domain boundaries through both the SPA-LEED as well as the STM measurements.

Catalysis and Surface Science

In 1835 the Swedish chemist Jons Jakob Berzelius coined the term “catalysis” to describe chemical reactions in which progress is affected by a substance that is not consumed in the reaction and hence is apparently not involved in the reaction. Both the term and the phenomenon were heavily debated throughout the rest of the 19th century until the German chemist Wilhelm Ostwald proposed a now generally accepted definition: “A catalyst is a substance that accelerates the rate of a chemical reaction without being part of its final products.” the catalyst acts by forming intermediate compounds with the molecules involved in the reaction, offering them an alternate, more rapid path to the final products.

Nanoparticles for Heterogeneous Catalysis: New Mechanistic Insights

Metallic nanoparticles finely dispersed over oxide supports have found use as heterogeneous catalysts in many industries including chemical manufacturing, energy-related applications and environmental remediation. The compositional and structural complexity of such nanosized systems offers many degrees of freedom for tuning their catalytic properties. However, fully rational design of heterogeneous catalysts based on an atomic-level understanding of surface processes remains an unattained goal in catalysis research. Researchers have used surface science methods and metal single crystals to explore elementary processes in heterogeneous catalysis. In this Account, we use more realistic materials that capture part of the complexity inherent to industrial catalysts. We assess the impacts on the overall catalytic performance of characteristics such as finite particle size, particle structure, particle chemical composition, flexibility of atoms in clusters, and metal-support interactions. To prepare these materials, we grew thin oxide films on metal single crystals under ultrahigh vacuum conditions and used these films as supports for metallic nanoparticles. We present four case studies on specifically designed materials with properties that expand our atomic-level understanding of surface chemistry. Specifically, we address (1) the effect of dopants in the oxide support on the growth of metal nanoclusters; (2) the effects of size and structural flexibility of metal clusters on the binding energy of gas-phase adsorbates and their catalytic activity; (3) the role of surface modifiers, such as carbon, on catalytic activity and selectivity; and (4) the structural and compositional changes of the active surface as a result of strong metal-support interaction. Using these examples, we demonstrate how studies of complex nanostructured materials can help revealing atomic processes at the solid-gas interface of heterogeneous catalysts. Among our findings is that doping of oxide materials opens promising routes to alter the morphology and electronic properties of supported metal particles and to induce the direct dissociation and reaction of molecules bound to the oxide surface. Also, the small size and atomic flexibility of metal clusters can have an important influence on gas adsorption and catalytic performance.

Adsorption on Ordered Surfaces of Ionic Solids and Thin Films

I Halide Ultrathin Films and Single Crystals: Structure and Adsorption.- Epitaxy of CaF2/Si(111) and LiF/Ge(100).- Adsorption of Water Vapor on NaCl(100) and KCl(100) without and with Defects.- Phases and Phase Diagrams of Xenon Adsorbed on Epitaxial NaCl(100) Films and on Ge(100).- Kinetic and Electronic Properties of Physisorbates on Epitaxial NaCl(100) Layers: SF6, Xe, and CO2.- Adsorption of Gases on Ionic Single Crystals: NaCl(100) and MgO(100).- Excitons and High Resolution Infrared Spectroscopy of Adlayers on Ionic Surfaces.- II Clean Surfaces of Metal Oxides: Geometric and Electronic Structure.- Characterisation of Single Crystal ?-Al2O3(0001) and (11