Heiko Hamann

Bridging the pressure and materials gaps between catalysis and surface science: clean and modified oxide surfaces.

The preparation of model systems based on thin epitaxial oxide films and oxide single crystals is discussed. A variety of surface sensitive techniques has been applied to study the geometric and electronic properties of these systems. The findings are correlated with adsorption and reaction of probe molecules on the surfaces. Metal vapor deposition under controlled conditions leads to the formation of metal aggregates with narrow size distributions. Their properties have been characterized, establishing that we can begin to bridge the materials gap between catalysis and surface science. While mainly performed under UHV conditions, adsorption measurements can be pushed to ambient conditions using non-linear optical techniques such as sum frequency generation. Results for systems with deposited metal aggregates will be discussed.

Characterization of a model Ziegler-Natta catalyst for ethylene polymerization

Based on the work of the Somorjai group [Magni and Somorjai, Catal. Lett. 35, 205 (1995)] we have prepared a thin well ordered MgCl2(001) film by MgCl2 evaporation from a Knudsen cell. This film does not absorb TiCl4 at room temperature if it is not activated by increasing the defect density via electron or ion bombardment. The nature of some of the defects created is characterized by in situ ESR measurements and Auger spectroscopy. Paramagnetic surface defects are altered by the bonding of TiCl4 to the surface as observed by ESR spectroscopy. Ti3+ centers are detected if particularly severely defected MgCl2 layers are prepared. Reactivity studies show however, that these species are not correlated with polymerization activity. Interaction with aluminum alkyl leads to the formation of the active catalyst and we observe for the first time directly ethyl radicals formed from trimethyl-aluminum in an abstraction process which may be formulated as TiCl4/surface+AlMe3→Me−TiCl3/surface+AlMe2Cl, Me−TiCl3/surface...

Model in Heterogeneous Catalysis: Surface Science Quo Vadis?

Model catalyst systems have been prepared by growth of small metal aggregates on thin well ordered oxide films of alumina and silica. These systems lend themselves to structural and morphological characterization via scanning probe microscopies and transmission electron microscopy and bridge to a certain extent the materials gap between metal single crystal studies and the investigation of real catalyst samples. Recently, the classical surface science techniques applied under ultrahigh vacuum conditions have been augmented by non-linear optical techniques, such as sum frequency generation, which can also be applied under ambient gas pressures. Thus, the pressure gap between studies in surface science under realistic conditions can he bridged.

Investigation of the rotational motion of self-assembled fatty acid films: An electron paramagnetic resonance line shape analysis

We have investigated the electron paramagnetic resonance (EPR) of self-assembled stearic acid films adsorbed on an Al2O3-film. Doping the film with spin labels at different positions of the alkyl chain in order to make the films accessible for EPR spectroscopy provides an opportunity to investigate the rotational motion of the molecule along the alkyl chain. The temperature dependent EPR spectra show a strong dependence of the rotational motion of the molecules with variation of the location of the spin label along the chain. We study the rotational motion by means of the EPR line shape analysis.

Model systems for heterogeneous catalysis: Quo vadis surface science?

The preparation of model catalyst systems based on thin epitaxial oxide films and oxide single crystals is discussed. The effect of surface pre-treatment on the distribution of metal aggregates deposited by vapour deposition is investigated, and a range of surface sensitive techniques applied to study the geometric, optical and electronic properties of these systems. The findings are correlated with the adsorption and reaction of probe molecules on the surfaces studied using multiple molecular beams. While mainly performed under UHV conditions, adsorption measurements can be made under ambient conditions by means of non-linear optical techniques such as sum frequency generation. Particle size and shape are shown to have a profound effect on the thermal and photochemistry of the systems.

Molecules on Clean and Modified Oxide Surfaces

This paper reviews results obtained with a series of surface science methods on molecules adsorbed on well-ordered oxide surfaces. The experimental data are compared with calculations and it must be concluded that the understanding is far from satisfactory on a quantitative scale, although qualitatively some progress has been made. This is particularily demonstrated for photonstimulated adsorption of NO from NiO. In a second part oxide supported metal aggregates are studied with respect to morphology, geometric, electronic and magnetic structures. Their interaction with adsorbed molecules is a further topic. Size dependent properties such as CO dissociation probability and photonstimulated methan dissociation are discussed.