Studying Reaction Mechanisms of Isolated Metal Clusters with Small Molecules

Bare clusters of a metal of interest are produced in a laser vaporization cluster source, where they can also be chemically modified by pulses of a reactant gas (e.g. by O₂ to obtain oxides). The charged clusters are then guided to a quadrupole mass filter, where the species of choice is separated from the unwanted rest. Only these clusters are then stored in a ring-electrode ion trap for a defined reaction time with the desired reactants (e.g., CH₄), which can be introduced in the desired concentration. After that, the reaction is quenched by the extraction of all charged reaction products into a reflectron time-of-flight mass spectrometer, allowing the identification of reaction products and intermediates and the determination of their abundance. By varying the reaction time in the ring-electrode ion trap, reaction compositions for different reaction times are obtained, allowing the determination of underlying reaction schemes and reaction kinetics. Finally, the variation of the reactant concentration, reaction temperature, changing the clusters’ charge state and their chemical modification (e.g., the oxidation) enables a comprehensive understanding of the chemistry of the metal cluster and the reaction, in particular when the data is compared with theoretical calculations (usually DFT).

We are part of the CRC 1441 TrackAct, in which we strongly collaborate with different groups from the Karlsruhe Institute of Technology (KIT) and the Deutsches Elektronen-Synchrotron (DESY). The project’s goal is to track the active sites in heterogeneous catalysis, utilizing the knowledge transfer from model systems to real catalytic systems by increasing the complexity of the systems step-by-step. Our part is to provide information about the driving forces, potential reaction pathways, and important relevant properties of potential catalysts, which include oxidation states of clusters, their sizes, charge states on a surface tuned by a support material, and important structural motifs of the catalyst, and reaction intermediates. These insights are then transferred to more realistic systems to improve the catalytic properties of the systems. This way, we are elucidating the activation of ammonia using platinum and platinum oxide clusters. Since ammonia is seen as a potential, carbon-free alternative fuel source preventing its emission and over-oxidation to NO_x will be important for running processes environmentally friendly in the future. Another reaction investigated is the activation of the greenhouse gas CH₄ by Pd-catalysts, which is difficult because of the molecule's peculiar properties. The goal for both reactions is to enable the targeted design of efficient and powerful catalysts, for which an understanding of the intrinisic reactvity of clusters ist key.