Kinetic and Mechanistic Reaction Studies of Metal Clusters Under Multi-Collision Conditions

In order to control the outcome of reactions, e.g. in selective catalysis, it is imperative to understand the underlying reactions mechanisms. Providing well-defined conditions, gas-phase studies of ion-molecule reactions are ideally suited to gain these fundamental insights. Moreover, metal clusters and metal oxide clusters are model catalysts as they can be regarded as idealized active centers of surfaces, zeolites and enzymes. Studying the reactions under multi-collision conditions may therefore provide valuable information about catalytic processes on the nanoscale. In this project, clusters are generated in a laser vaporization source, mass-selected in a quadrupole mass filter (QMF) and transferred into a home-built cryogenic ring electrode ion trap (RET), in which the clusters are stored for milliseconds up to a few seconds. The clusters undergo many collisions with buffer gas atoms inside of the ion trap and are therefore constantly thermalized. When a mixture of reactive and buffer gas is used, consecutive reactions between clusters and reactive gas molecules are observed. The charged reaction products are subsequently analyzed with a reflectron time-of-flight mass spectrometer (ReTOF-MS) and the product intensities are recorded as a function of reaction time. The reaction kinetics, i.e. the reaction model and the corresponding rate constants, are obtained by fitting the result of kinetic simulations to the measured data. By changing the reaction temperature from 20 K to 300 K, the presence of reaction barriers is probed and the apparent activation energy of each reaction step is determined from the change in the respective rate constant. The experimental setup subsequently offers the capability to study clusters of various size, ranging from a single atom to about 100 atoms per cluster, and different charge states. In addition, recent modifications to the cluster source enable the composition-specific generation of metal oxide clusters, i.e. a definite amount of oxygen atoms may be added to the bare metal clusters. It is consequently possible to not only determine reaction mechanisms but also observe differences in intrinsic cluster properties that are likely to change on an atom per atom basis. Future plans include the use of spectroscopic techniques to investigate the geometric and electronic properties of clusters and cluster-adsorbate complexes. These properties may furthermore be unraveled with the help of ab-initio calculations, which are facilitated for gas-phase systems and performed in close collaborations with theoreticians. A special focus of this project is C-H bond activation, which is considered to be one of the holy grails in chemistry. This non-trivial process is a requirement for the selective transformation of methane and has received a lot of attention in the last decades. Our aim is to contribute to the fundamental understanding of these reactions in order to design catalysts and purposefully activate methane.