Chemische Funktionalität von größenselektierten Clustern auf Oberflächen mit Größen im nicht skalierbaren Bereich
Zusammenfassung der Projektergebnisse
As continuation of the first phase, the supported proposal aimed to further the understanding of the chemical functionality and activity of supported size selected clusters in the non-scalable size regime. The outcome of the project can be divided into three different major topics: – CO oxidation reaction on supported size-selected Au and Pd clusters – Chemistry of more complex molecules and reactions – Cluster reactivity under applied conditions Based on the experience of previous investigations, the CO oxidation reaction was used as a test reaction to further the understanding of fundamental aspects of this reaction. On Au clusters it could be representatively shown how the catalytic reactivity of the catalyst particle is strongly dependent on the support conditions. Tuning MgO support with respect to thickness and stoichiometry, and thereby changing the conditions on how the reactants adsorb on the cluster a way of controlling and manipulating heterogeneous catalysis is presented. Using Pd clusters insights into the temperature dependent oxidation state, underlying molecular symmetry and their influence on the activation barrier of the reaction were studies. Further, the influence of an oxygen pretreatment of the cluster catalysts was investigated and supported by means of first principle studies. An assignment of different reaction mechanisms to different temperature regimes, that relate to an better activation on catalyst interface sites, compared to other adsorption sites could be achieved. As a logical next step studies on the interaction of more complex molecules and reactions was started. First, an improved setup for Photoelectron Spectroscopy with ultimate surface sensitivity was integrated at the existing setup and could increase both resolution and signal intensities to complement the existing methods on the setup. As a test molecule Trichloroethylene (TCE), a common ground water pollutant with a distinct electronic structure was chosen to test the applicability towards cluster studies. A combination of methods allowed to compare adsorption of the molecule on different surfaces and gave an insight into the interaction strength of the molecule with the surface. Further the results quantified the sensitivity of the methods and indeed showed detection limits in the range of synchrotron based methods and thus low enough to study cluster based catalyst. Current efforts focus on the ethylene hydrogenation on Pt cluster catalysts on different support, a more complex reaction. The preliminary results show size and support dependent catalytic reactivity, further ongoing work is related to gain kinetic data. The last major topic was to explore the possibilities of cluster catalysts under more applied conditions (i.e. increased temperature and pressure, in liquids, etc.) and new applications. Under vacuum conditions it was achieved to investigate the CO oxidation on different sized Pd clusters under isothermal conditions and thereby discovering a different reactivity compared to a Pd single crystal surface. As a paramount achievement, cluster based catalysts were after preparation in the ultra high vacuum taken out to ambient conditions and consequently available for ex situ studies. In the liquid phase using electrochemical methods and local characterization the feasibility of this approach was shown and in addition first reactivity measurements on Pt clusters were performed. The influence of different electrochemical treatments was investigated and show beyond other results, that not only size but also the interparticle distance has a huge influence on the catalytic reactivity. In another, more applied approach, a novel hybrid cluster semiconductor material was created and established as a test platform. The catalyst materials were exploited towards understanding the coverage and size dependence on the photocatalytic hydrogen evolution reaction. The obtained results allow for a quantification of the minimum amount of catalyst necessary for maximum catalytic efficiency and further showed a size dependent catalytic reactivity, both effects being applicable to other reactions and thus a tool for future studies. Last, Pt clusters in gas phase micro-reactors are used to investigate fundamental reactivity, such as CO oxidation reaction and hydrogen splitting under elevated pressures. Ongoing efforts are focused on gaining an overview on the reactivity from very small particles to extended surfaces.
