Periodic Reporting for period 1 - ClusterDynamics (Characterising the dynamical properties of size-selected supported metal clusters)
Okres sprawozdawczy: 2017-03-20 do 2019-03-19
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.
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.
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.