Atomic structure of graphene supported heterogeneous model catalysts

Graphene on Ir(111) forms a moire structure with well defined nucleation centres. Therefore it can be utilized to create hexagonal metal cluster lattices with outstanding structural quality. At diffraction experiments these 2D surface lattices cause a coherent superposition of the moire cell structure factor, so that the measured signal intensity scales with the square of coherently scattering unit cells. This artificial signal enhancement enables the opportunity for x-ray diffraction to determine the atomic structure of small nano-objects, which are hardly accessible with any experimental technique. The uniform environment of every metal cluster makes the described metal cluster lattices on graphene/Ir(111) an attractive model system for the investigation of catalytic, magnetic and quantum size properties of ultra-small nano-objects. In this context the use of x-rays provides a maximum of flexibility concerning the possible sample environments (vacuum, selected gases, liquids, sample temperature) and allows in-situ/operando measurements.In the framework of the present thesis the structure of different metal clusters grown by physical vapor deposition in an UHV environment and after gas exposure have been investigated. On the one hand the obtained results will explore many aspects of the atomic structure of these small metal clusters and on the other hand the presented results will proof the capabilities of the described technique (SXRD on cluster lattices).For iridium, platinum, iridium/palladium and platinum/rhodium the growth on graphene/Ir(111) of epitaxial, crystalline clusters with an ordered hexagonal lattice arrangement has been confirmed using SXRD. The clusters nucleate at the hcp sites of the moire cell and bind via rehybridization of the carbon atoms ($\text{sp}^2\rightarrow \text{sp}^3$) to the Ir(111) substrate. This causes small displacements of the substrate atoms, which is revealed by the diffraction experiments. All metal clusters exhibit a fcc structure, whereupon the Ir clusters show equal ratio of ABC and ACB stacking. Pt/Rh clusters show mainly ($\sim 75$ %) ABC stacking and Ir/Pd clusters show exclusively ABC stacking - note the Ir(111) substrate stacking is chosen to be ABC-like. During gas exposure the clusters gain in height, while simultaneously the ordering of the cluster lattice decreases. The order of ACB stacked Pt/Rh clusters decreases more rapidly, so that a weakening of the cluster binding due to ACB stacking is proposed. In addition a size reduction of the lowest cluster layer to a hexagonal arrangement of 19 atoms was observed, which seems to be the energetically favoured interface area between graphene/Ir(111) and the investigated metal clusters.Platinum clusters containing fewer than 40 atoms have been investigated with surface sensitive x-ray diffraction. The cluster height could be determined and a reduced atom distance among the cluster was observed. CO exposure decreased the in-plane atom distance even more. This effect could be reverted by a subsequent oxygen exposure ($2\text{CO} + \text{O}_2 \rightarrow 2\text{ CO}_2$).For Pt/Rh clusters with fewer than 60 atoms the element distribution was investigated and revealed an enrichment of Rh at the two topmost cluster layers. This element distribution was found for co-deposition and sequential deposition of the material and is therefore independent of the order of deposition. Oxygen exposure led to the formation of a rhodium surface oxide, which could be prevented by preceding CO exposure.In case of Ir/Pd clusters containing $\sim 60$ atoms we found preferences for a core/shell structure with iridium atoms at core sites. Exposure to $\text{p}(\text{H}_2) = 1 \text{ bar}$ did not result in the formation of the palladium $\beta$-phase, so that a reduced hydrogen solubility of graphene/Ir(111) supported palladium clusters is assumed. The reduced hydrogen solubility is probably a consequence of substrate induced strain among the cluster atoms