Marcus Bäumer

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.

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.

The application of infrared spectroscopy to probe the surface morphology of alumina-supported palladium catalysts

Five alumina-supported palladium catalysts have been prepared from a range of precursor compounds [palladium(II) nitrate, palladium(II) chloride, palladium(II) acetylacetonate, and tetraamminepalladium(II) tetraazidopalladate(II)] and at different metal loadings (1-7.3 wt %). Collectively, this series of catalysts provides a range of metal particle sizes (1.2-8.5 nm) that emphasize different morphological aspects of the palladium crystallites. The infrared spectra of chemisorbed CO applied under pulse-flow conditions reveal distinct groupings between metal crystallites dominated by low index planes and those that feature predominantly corner/edge atoms. Temperature-programmed infrared spectroscopy establishes that the linear CO band can be resolved into contributions from corner atoms and a combination of (111)(111) and (111)(100) particle edges. Propene hydrogenation has been used as a preliminary assessment of catalytic performance for the 1 wt % loaded catalysts, with the relative inactivity of the catalyst prepared from palladium(II) chloride attributed to a diminished hydrogen supply due to decoration of edge sites by chlorine originating from the preparative process. It is anticipated that refinements linking the vibrational spectrum of a probe molecule with surface structure and accessible adsorption sites for such a versatile catalytic substrate provide a platform against which structure/reactivity relationships can be usefully developed.

Size and support effects for CO adsorption on gold model catalysts

CO adsorption on gold particles deposited on well-ordered alumina and iron oxide films was studied with temperature-programmed desorption. Scanning tunneling microscopy was used to provide correlative structural characterization. The results show that the adsorption of CO on gold exhibits a size effect in that small particles adsorb CO more strongly. For a given particle size (∼3 nm), CO desorption temperature (at ∼170 K) is essentially independent of the supports studied. Therefore, support effects seen in CO oxidation on real catalytic systems must arise from the interaction of oxygen rather than CO with these catalysts.

Preparation and characterization of model catalysts: from ultrahigh vacuum to in situ conditions at the atomic dimension

Abstract In situ characterization and reaction studies of working catalytic systems are an issue of current interest. We present studies on well characterized model systems, i.e., deposited metal nanoparticles, applying a variety of experimental techniques in an attempt to bridge gaps between surface science and catalysis. In particular, we investigate methanol dehydrogenation and ethene hydrogenation under UHV as well as ambient conditions and apply nonlinear optical techniques. We use electron spin resonance to study intermediately formed radicals in Ziegler–Natta polymerization of ethene. It is concluded that there is a chance to transfer results from studies on model systems toward an understanding of catalysis.

Structure Sensitivity of CO Dissociation on Rh Surfaces

Using periodic self-consistent density functional calculations it is shown that the barrier for CO dissociation is ≃120 kJ/mol lower on the stepped Rh(211) surface than on the close-packed Rh(111) surface. The stepped surface binds molecular CO and the dissociation products more strongly than the flat surface, but the effect is considerably weaker than the effect of surface structure on the dissociation barrier. Our findings are compared with available experimental data, and the consequences for CO activation in methanation and Fischer–Tropsch reactions are discussed.

The structure of thin NiO(100) films grown on Ni(100) as determined by low-energy-electron diffraction and scanning tunneling microscopy

Abstract A Ni(100) surface exposing terraces of approximately 100 A width which are separated from each other by monatomic steps descending along the [010] direction has been oxidized above room temperature. Via intermediate formation of the well-known p(2 × 2) and c(2 × 2) chemisorbed phases, which are identified by LEED (low energy electron diffraction) and STM (scanning tunneling microscopy) in the present study, a thin film of 4–5 layers of NiO(100) builds up on the surface. The NiO layer consists of crystallites with a typical lateral extension of 50 A as revealed by the STM data. SPA-LEED (LEED spot profile analysis) measurements allowed us to determine that the crystallite surfaces are tilted preferentially along the [011] and [011] directions of the Ni(100) plane by an average angle of 8° with a half width of the angular distribution of 6°. We show that the development of the oxide islands most probably starts at the terrace edges of the metal surface. While the islands grow in size the strain between oxide and metal increases due to the large differences in the lattice constants of Ni and NiO. Part of the strain is compensated by a tilt of the islands induced via migration of Ni atoms from the step edges underneath the oxide islands. The generated NiO surface is characterized by two types of regions, namely the regions on the islands which are basically flat and contain regular NiO sites, covering 75–80% of the crystal surface, and the regions between the islands with many defect sites (20–25% of the surface area). The consequences of the structural properties of the NiO film on the adsorption of molecules, i.e., NO, are discussed in line with results of a previous study.

From atoms to crystallites: adsorption on oxide-supported metal particles

The properties of metal clusters and nanoparticles are attracting more and more attention in fundamental and applied research. The application of such structures in heterogeneous catalysis is a particularly important area. Nevertheless, there is only limited fundamental knowledge about the dependence of adsorption behaviour and catalytic activity on particle size. In this article, we describe an approach to tackle such questions on the basis of model systems which are prepared by metal vapour deposition under ultrahigh vacuum conditions. A thin alumina film grown on a metal substrate is used as support. This has the advantage that scanning tunneling microscopy and photoelectron spectroscopy can be applied without any charging problems. Firstly, structural data covering palladium, rhodium and iridium deposits demonstrate that a wide spectrum of particle sizes and morphologies may be obtained by taking advantage of kinetically controlled nucleation and growth processes. Subsequently, we discuss the adsorption of simple molecules on the particles, such as CO and ethene. These studies, which have been carried out using infrared and photoemission spectroscopy, illustrate possible variations in the adsorption and reaction behaviour as a function of particle size. Aspects which will be considered include: adsorption at facets and defects, size-dependent variation of adsorption sites, formation of surface complexes, decomposition of molecules as well as co-adsorption phenomena.

Interaction of rhodium with hydroxylated alumina model substrates

In order to investigate how metal growth and metal-oxide interaction depend on the chemical properties of oxide surfaces, we describe a modification procedure which allows the introduction of surface hydroxyl groups on a well-ordered Al2O3 film on NiAl(110). The modification — based on deposition of metallic Al and subsequent water exposure — is characterized using LEED spot-profile analysis (SPA-LEED) and high-resolution photoelectron spectroscopy (PES). Upon Al deposition, small aggregates are formed, which are oxidized completely in the final preparation step as verified via PES. The presence of OH-groups is supported by the appearance of additional Al 2p and O 1s surface features. The origin of oxide core and valence level binding energy shifts induced by the modification procedure is discussed. Growth and metal-substrate interaction of Rh deposited onto the hydroxylated Al2O3 film is compared to Rh growth on the non-modified oxide surface. It is shown that at 300 K nucleation preferentially occurs on modified oxide areas (SPA-LEED). Photoelectron spectroscopy of both oxide and rhodium core levels points to a direct chemical interaction between the metal and surface hydroxyl groups.

Interaction of oxygen with palladium deposited on a thin alumina film

The interaction of oxygen with Pd particles, vapor deposited onto a thin alumina film grown on a NiAl(1 1 0) substrate, was studied by STM, AES, LEED, XPS, TPD and molecular beam techniques. The results show that O2 exposure at 400–500 K strongly influences the oxide support. We suggest that the oxygen atoms formed by dissociation on the Pd surface can diffuse through the alumina film and react with the NiAl substrate underneath the Pd particles, thus increasing the thickness of the oxide film. The surface oxygen inhibits hydrogen adsorption, and readily reacts with CO at 300–500 K. For large and crystalline Pd particles, the system exhibits adsorption–desorption properties which are very similar to those of the Pd(1 1 1) single crystal surface. The molecular beam and TPD experiments reveal that, at low coverage, CO adsorbs slightly stronger on the smaller Pd particles, with an adsorption energy difference of � 5–7 kJ mol � 1 for 1 and 3–5 nm Pd particles studied. 2002 Elsevier Science B.V. All rights reserved.