Adsorption Sites of Catalytic Promoters and Inhibitors on Transition Metals.

In order to understand the electronic mechanisms of catalytic promotion and inhibition it is essential to have detailed information about the adsorption site of the promoting (inhibiting) species. The promotion of molecular dissociation by alkali metal additives is conventionally explained by an electrostatic enhancement of backdonation into Pi* antibonding orbitals. This mechanism requires side-by-side adsorption of the molecule and the electropositive additive. The model is not applicable in the case of H2 dissociation, and it is subject to controversy in the case of alkali metal promoted N2 dissociation. We proposed recently an alternative mechanism based on surface state depletion caused by substitutionally adsorbed alkali metals. Up to now it is not clear, whether the alternative mechanism is actually relevant for catalytically active surfaces such as Pt(111), which is the most commonly used surface in catalytic model studies, or low-index Fe surfaces, which are of commercial interest for the Haber-Bosch NH3 synthesis. As a first step to evaluate the feasibility of our promotion mechanism we plan to investigate the surface phase diagram of alkali metals on these surfaces. For Pt(111) first results from our as well as other groups show that alkali metals are actually incorporated into the surface, but the existing studies do not agree with respect to the precise conditions for the alkali metal subsurface absorption nor has the driving force unambiguously been identified. Even less is known about alkali metal adsorption on low-index Fe surfaces. On these surfaces, not only a study of the alkali metal adsorption geometry, but also of the influence of the alkali metals on the electronic surface states is required. As a model system for catalytic inhibition we plan to study the adsorption of Bromine on Pt(110). Preliminary experimental and theoretical investigations indicate that the adsorption of Br causes a depletion of Pt d-state density at EF due to formation of low-lying bonding and entirely unoccupied antibonding orbitals. This is in line with the current thinking about halogen induced poisoning. However, in contrast to several other cases of halogen adsorption on metal surfaces, even submonolayer coverages induce a reconstruction of Pt(110). The precise nature of this reconstruction has yet to be determined, in particular the dimensionality of the reconstructed layer and its electronic properties. First scanning tunneling microscopy results seem to indicate a strongly anisotropic bonding within the layer. If this observation can be verified, the theory of one-dimensional electron states predicts interesting electronic properties for such a surface compound. This could have significant consequences for the catalytic activity. The experimental techniques used for determining the adsorption sites of poisons and promoters will comprise direct as well as reciprocal space methods. Scanning tunneling microscopy is used for surface imaging, ion backscattering spectroscopy will be used as chemically sensitive method to detect substitutional adsorption and incorporation of the species under study. Impact collision ion scattering spectroscopy will provide detailed information about the geometry of the adsorption sites. Low-energy electron diffraction and angle-resolved UV photoemission will be used to explore the reciprocal space properties. Measurements of the adsorption geometry will be complemented by measurements of the electronic structure, in particular the surface states, by means of angle-resolved high-resolution photoemission spectroscopy.

Catalysts are chemically active materials, which can turn on a chemical reaction, influence its rate and determine the route to different products without being consumed during the reaction. Most of the technically relevant chemical reactions today rely on the participation of such catalysts. It has been known for long that minute amounts of additives can have a decisive influence on the functionality and efficiency of catalysts. During the last thirty years the individual steps of many catalytic reactions could be identified and a detailed understanding of bond- breaking and -making emerged in several cases. However, the mechanisms by which small amounts of catalytic modifiers cause considerable and sometimes decisive changes of reaction probabilities and reaction mechanisms are still controversial. A major problem in this context is the determination of the adsorption sites of modifying atoms at or in the catalyst surface. A precise knowledge of the atomic structure, the bonding geometry and the environment of the modifying atoms is indispensable for an understanding of their effect on bond modification and bond formation. The present project aimed at an elucidation of the atomic geometry for some of these catalytic modifiers within a catalytically active surface. In some cases the task could be successfully completed. A combination of modern methods and international collaborations allowed a precise determination of atomic positions for all species involved. Accuracies of 0.03 Angstroms were achieved. Given this precision structure determination one could also determine, how the additives modify the choreography of bond-making and-breaking on the catalyst. Beyond the initial goal, however, a new effect was discovered, which may pave the way to other applications of the systems under investigation: A special kind of modifiers, the so-called halogens, are obviously able to confine the movement of electrons into one dimension. Electrons are the charged particles, which not only cooperate to form the chemical bonds, but also carry the electrical current in conducting wires and are responsible for most materials properties. If they are restricted to move in one dimension only, completely new and unexpected phenomena emerge. For instance, the atomic arrangement within a material, but also its electrical conductivity and other properties may become completely unstable towards minute changes in chemical composition. Small amounts of dopants may therefore cause phase transitions with dramatically altered properties. According to the present understanding even the superconductivity of high-temperature superconductors is linked to such a dimensional constraint of electronic movement. As a consequence of the results obtained within the project it may therefore be possible to develop a new class of chemical sensors based on halogen compounds. Furthermore, it appears possible to obtain new information on the mechanism of high-temperature superconductivity by studying the highly correlated electrons in halogen-modified metal surfaces. Such a line of reasoning is presently intensely pursued in the follow-up project.