Martin Frank

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

Metal-oxide interaction for metal clusters on a metal-supported thin alumina film

Abstract The interaction between deposited metal clusters and a thin model alumina film grown on NiAl(110) have been studied using X-ray absorption spectroscopy (XAS) and core and valence photoelectron spectroscopy. A lower limit for the fundamental gap of the supported alumina film is determined, and found to be slightly lower than that of alumina surfaces. O 1s XAS shows that new states appear in the fundamental gap upon metal deposition. Al 2p X-ray photoelectron spectra from the alumina film are also sensitive to metal deposition, whereas spectra from Al atoms at the substrate–oxide interface appear unaffected. The present data demonstrate the existence of gap states in the pristine film, and we discuss the effects of these states for the properties of this film as a model oxide substrate.

Particle size dependent CO dissociation on alumina-supported Rh: a model study

Particle size dependent CO dissociation on alumina-supported Rh: a model study (vol 279, pg 92, 1997)

The influence of OH groups on the growth of rhodium on alumina: a model study

In order to investigate how the presence of surface hydroxyl groups on oxide surfaces affects the interaction with the supported metal, we have modified a well-ordered alumina film on NiAl(110) by Al deposition and subsequent exposure to water. This procedure yields a hydroxylated alumina surface as revealed by infrared and high-resolution electron energy loss spectroscopy. By means of scanning tunneling microscopy, we have studied the growth of rhodium on the modified film at 300 K. Clear differences in the particle distribution and density are observed in comparison to the clean substrate. While, in the latter case, decoration of domain boundaries as typical defects of the oxide film governs the growth mode, a more isotropic island distribution and a drastically increased particle density is found on the hydroxylated surface. From infrared data, it can be deduced that the growth is connected with the consumption of the hydroxyl groups due to the interaction between the metal deposit and the hydroxylated areas. This finding is in line with photoemission results published earlier.

Nucleation and growth of transition metals on a thin alumina film

We have studied the growth of various metals (Pd, Rh, Co and Ir) on a thin well-ordered alumina film in order to elucidate the influence of film structure and deposition conditions on nucleation and growth behaviour. All metals exhibit a three-dimensional growth mode in agreement with thermodynamic considerations. The nucleation is, however, dominated by the defects of the substrate. At a deposition temperature of 90 K, point defects are the primary nucleation centres. At 300 K, the situation is different for some metals (such as Pd and Rh) since decoration of steps and film domain boundaries is favoured under these conditions. This temperature dependence points to a stronger interaction of the diffusing metal atoms with the line defects which, however, can only play a role if the thermal energy is sufficiently high to reach them. Metals which are expected to interact more strongly with the support (such as Ir and Co) do not show such a diversity with respect to their nucleation behaviour.

Effect of carbon deposits on reactivity of supported Pd model catalysts

Alumina-supported Pd model catalysts were prepared by Pd evaporation onto a thin alumina film grown on a NiAl(110) substrate. Adsorption and co-adsorption of ethene, CO and hydrogen on Pd/Al2O3/NiAl(110) covered by carbon species, formed by ethene dehydrogenation at ∼550 K, was studied by temperature programmed desorption (TPD). TPD results show that carbon deposits do not prevent adsorption but inhibit dehydrogenation of di-σ bonded ethene. Carbon species suppress CO adsorption in the highly coordinated sites and also suppress the formation of hydrogen ad-atoms on the surface. The ethene hydrogenation reaction performed by co-adsorption of hydrogen and ethene is inhibited by the presence of carbon deposits. The inhibition is independent of particle size studied (1-3 nm). The effects are rationalized in terms of a site-blocking behavior of carbon species occupying highly coordinated sites on the Pd surface.

CO dissociation characteristics on size-distributed rhodium islands on alumina model substrates

The dissociation of CO on size-distributed Rh particles supported on a thin alumina film has been studied with high resolution X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS). Adsorbed CO dissociates upon heating to temperatures above 300 K. The dissociation activity is dependent on the island size, exhibiting a maximum for islands with around 1000 atoms. We have identified size-dependent changes in the C 1s photoelectron spectra for these CO–Rh systems occurring at temperatures lower than the onset of both the dissociation and desorption processes. These changes are interpreted as being due to adsorbed CO shifting into more highly coordinated sites. The dissociation activity is directly correlated to the availability of these sites, where the observed dissociation is proposed to occur. These results can be interpreted primarily in terms of the size and shape of the deposited Rh particles.

Vibrational spectroscopy of CO adsorbed on supported ultra-small transition metal particles and single metal atoms

Ultra-small rhodium, palladium and iridium particles with minimum average sizes of five atoms have been grown on a thin, well-ordered alumina film at temperatures of 90 K and below. Scanning tunneling microscopy (STM ) served to characterize the morphology of these deposits. In the infrared spectra of adsorbed carbon monoxide, characteristic features were observed by infrared reflection absorption spectroscopy (IRAS). These originate from CO adsorbed on single metal atoms and extremely small aggregates, most likely dimers or trimers. In this way, we have detected the atomic nuclei of heterogeneous rhodium nucleation at 90 K. Located at oxide point defects, these atoms form rhodium dicarbonyl species with a symmetric stretch frequency of 2117 cm’1, while for iridium dicarbonyl species a frequency of 2107 cm’1 was found. At 60 K, rhodium aggregates also grow elsewhere on the oxide film. Comparing the nucleation behaviour at low temperatures, we find an increasing metal‐oxide interaction strength in the order Pd

Growth and morphology of Rh deposits on an alumina film under UHV conditions and under the influence of CO

Adsorption of certain gases may alter the structure of small metal particles deposited on a surface. If the particles are prepared by vapour deposition, the morphology may also be changed by the presence of these gases during growth. We have investigated the growth and morphology of Rh under ultrahigh vacuum (UHV) conditions and under the influence of CO using a thin, well-ordered A1203 film as substrate. First, the growth modes at UHV pressures were characterized with the help of spot profile analysis low energy electron diffraction (SPA-LEED) and scanning tunnelling microscopy at two substrate temperatures: 90 and 300 K. At 300 K, we observe the decoration of domain boundaries between different antiphase domains of the film. The particles are mainly disordered, only a small fraction is crystalline. Keeping the substrate temperature at 90 K, smaller (disordered) aggregates form, which are mostly located inside the domains. If the deposits are exposed to CO, in both cases structural changes are detectable: SPA-LEED results point to a spreading of the particles, resulting in an expansion of their surface area. Exposing the sample to CO during deposition, on the other hand, leads to the formation of a carbonyl-like species in the case of large amounts of Rh. Its decomposition gives rise to a characteristic peak in the thermal desorption spectrum.