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Model Catalysis on Nanoclusters

The idea of these studies is to study a highly defined system that allows to investigate the fundamental surface chemistry on metal clusters. On lattice mismatched substrates, the spatially varying interaction strengths between graphene and the substrate leads to a Moiré pattern.Such corrugated graphene sheets are outstanding templates for the fabrication of nanocluster arrays, with a narrow size distribution of the clusters.
To understand catalytic processes on the atomic level, it is necessary to find suitable model systems. An appropriate model system must be simple enough to analyze it in detail yet complex enough to accurately reproduce the features of a real catalyst, which is given on for these nanocluster arrays.

On these clusters we determined the adsorption sites, site occupation and site preferences in situ in a quantitative way, to elucidate the adsorption properties. Also poisoning of the catalysts by e.g. by sulfur can be investigated in details. Even the reaction of carbon monoxide with oxygen is accessible, yielding significantly lower activation energies than for the flat single crystal surfaces.

These data become possible due to a small size distribution of the homogeneous cluster arrays, as seen in typical STM data from Pt clusters on h-BN / Rh(111). The h-BN Moire serves as a template and leads to well separated clusters that are stable even at reaction temperatures.


Typical XPS data of such a reaction experiment is shown below, for the isothermal reaction of adsorbed oxygen with carbon monoxide.
It turns out that in comparison to the single crystalline surfaces the reaction order changes from a ~0.5 order due tot he reaction at the edges of islands to a pseudo first order reaction as no extended islands are formed on the clusters. The reaction rate is higher with an activation energy of only ~13 kJ/mol.

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