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Characterising the dynamical properties of size-selected supported metal clusters

Periodic Reporting for period 1 - ClusterDynamics (Characterising the dynamical properties of size-selected supported metal clusters)

Reporting period: 2017-03-20 to 2019-03-19

The ability of catalysts to facilitate chemical reactions and to make them selective is of immense importance for industrial production processes. A wide range of heterogeneous catalysts consists of active clusters (fewer than 100 atoms) or nanoparticles (several 100 atoms) supported on oxide surfaces. Such small particles are intrinsically metastable due to their high surface area: Clusters are often mobile on their supports which leads to sintering into larger particles, and they further exhibit internal dynamics by rearranging their atoms into different structural isomers. Moreover, clusters often exhibit special geometric structures and electronic properties, which can vary with the addition or removal of just a single atom. Clusters of different size can therefore often have vastly different catalytic properties. In order to fine-tune catalysts for maximum reactivity and selectivity, a key challenge is the fundamental understanding of particle mobility under reaction conditions. In particular, the understanding of these cluster dynamics at the atomic scale promises to allow us a more targeted design of efficient catalysts with long-term stability, however such dynamics are often not well understood.
The overall objective of project ClusterDynamics was to investigate cluster stability and dynamics experimentally on increasingly complex model systems and to establish an experimental tool box for future studies.
Specifically, we brought to maturity an electronic add-on module for scanning tunneling microscopes (STM) which accelerates standard instruments to video frame rates (20 fps) and beyond. Using this FastSTM method allowed us to monitor individual metal clusters on graphene and boron nitride thin films directly, giving us a picture of their dynamics in real space and real time. The dynamics of a more typical catalyst support, namely a magnetite surface, were further investigated by FastSTM. Finally, a so-called sniffer setup was implemented and tested for temperature programmed reaction (TPR) experiments. The results from this project provide fundamental knowledge within the framework addressing the societal challenges of “smart, green and integrated transport” and “climate action, environment, resource efficiency and raw materials” identified in Horizon 2020. In the long run, the new findings are expected to enable the more targeted design of efficient, highly selective and most importantly stable catalysts for applications ranging from car exhaust and exhaust gas after-treatment in industrial applications to more resource and energy efficient materials production.
The work for project ClusterDynamics was performed at the Technical University of Munich. We investigated the dynamics of model catalysts with a very high degree of definition, combining size-selected (i.e. defined to the exact number of atoms) metal clusters with single crystalline supports. Fast scanning tunneling microscopy (FastSTM) was used to monitor cluster diffusion, sintering into larger particles and internal restructuring on the atomic scale with a time resolution down to some tens of msec.
In a first step, Pd clusters, which are a very versatile model system thanks to their rich redox chemistry, were deposited on the moiré films of graphene and boron nitride grown on Rh(111). These well-defined models were used to test and further develop the so-called FAST module, an add-on electronics module which we connected to our standard STM instrument, to allow us to measure STM at up to 20 fps. We found that Pd clusters containing only a few atoms are highly mobile on boron nitride and identified motion ranging from long-range diffusion leading to sintering, over confined diffusion inside a boron nitride pore or the moiré structure, to the internal restructuring of clusters. To our surprise, the cluster dynamics did not appear to be influenced by adsorbate molecules.
In another part of the project, we investigated the stability of clusters when exposed to air for transportation between different vacuum instruments. We found that Pt clusters are highly stable on the native oxide film of a silicon wafer, SiO2. Scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS) showed that the clusters can be transported and stored in ambient conditions without loss of the size selection. Furthermore, this system proved highly sinter-resistant, even at elevated temperatures and in reactive environments.
After initial measurements on boron nitride and graphene films which established the FastSTM technique as a routine analysis tool in our lab, we investigated Pt clusters on a more relevant support material, Fe3O4(001). We found that even the support itself already exhibits rich surface dynamics, ranging from adsorbate diffusion to the dynamics the Fe atoms themselves depending on temperature. In contrast, Pt clusters adsorbed on the support are stabilized and do not sinter at temperatures up to 600 K. The temperature-dependence of the apparent cluster height and adsorption site and the influence of adsorbates on the clusters were investigated. In order to expose the clusters to a higher pressure environment (up to 10-3 mbar), a sniffer-TPR setup was installed and characterized.
Project ClusterDynamics represents a significant step forward in the understanding of the stability and dynamics of metal clusters on thin film and oxide supports. The studies provided new insights into the interplay of inter-cluster, cluster–support and cluster–adsorbate interactions. We have observed internally fluxional clusters, highly mobile systems as well as sinter-resistant ones, thus highlighting the importance of understanding the influence of support material, morphology and defect density, and the cluster material and size for catalyst design. The results further the understanding of cluster dynamics and are thus expected to have an impact on the fundamental knowledge in the scientific community. The results from this project have laid the groundwork for further scientific studies on more realistic catalyst systems, bringing us a significant step closer to research relevant to industrial catalyst applications. The investigation of size-selected metal clusters on oxide supports is now possible in relatively routine fashion with complementary microscopic and spectroscopic tools. We thus expect future measurements to be quicker and more efficient, thus moving us in the direction of materials screening. Moreover, we anticipate a more long-term impact on industrial applications which will be interesting for the chemical industry.
Finally, outreach activities – ranging from Open Days over lab visits by school children to public talks – brought the fundamental research on cluster dynamics closer to the general public. Specifically, our approach to the FastSTM technique was presented, some examples shown for illustration purposes and the importance of such fundamental investigations set forth. All outreach events have attracted great attention and been met with enthusiasm by the audience.
Schematic illustration of the diffusion of a Pd cluster in an h-BN pore