[Zurück]

N. Barrabés-Rabanal, S. Pollitt, V. Truttmann, C. Garcia Yago, G. Rupprechter:
"ON THE REACTIVITY OF Au (SR) NANOCLUSTERS CATALYSTS: LIGAND, STRUCTURE AND SUPPORT EVOLUTION";
Vortrag: Vienna young Scientists Symposium (VSS) 2019, Wien; 13.06.2019 - 14.06.2019; in: "VSS Vienna Young Scientists Symposium - 2019", (2019), ISBN: 978-3-9504017-9-0; 81 S.

INTRODUCTION
In nanocatalysis, a great challenge is to obtain truly homogenous and well-defined highly active nanostructures on surfaces. Ligand protected metal nanoclusters is an emerging class of functional nanomaterials consisting of less than 100 atoms, with atomic precision, well-defined molecular structure. These properties are also sensitive to the metal composition, which allows tuning properties by removing, adding or replacing (doping) one or more au atoms with heteroatoms. We successfully doped different cluster sizes by metal as Pd, Pt, Cu or Ag, producing bimetallic and trimetallic nanoclusters with atomic precision. Insights in the structural and properties changes were obtained by several spectroscopic studies (XAFS, XPS, MALDI...) correlated with DFT calculations[1, 2].
Heterogeneous catalytic research of atomically precise gold nanoclusters is an emerging field opening new opportunities for accurate studies of size-dependent properties, atomic structure effects and reaction mechanisms in catalysis. An enhancement of the catalytic properties by metal nanoclusters has been observed, when compared to classical nanoparticles of similar size. By in situ spectroscopic techniques we study the influence of the cluster structure and support material in their stability and reactivity, during pretreatment and in different gas and liquid phase reactions
RESULTS AND DISCUSSION
Generally, the supported nanoclusters undergo ligand removal treatments, in order to increase the active surface accessible for reaction. We studied the structural changes of different atomic sizes of Aux(SC2H4Ph)y (x=25,38 and 144) supported on various metal oxides (CeO2, Al2O3 and SiO2) upon thermal treatments and reaction conditions. In situ XAFS measurements revealed the strong influence of the cluster structure and support material in their stability.[3-5] Whereas high stability of the cluster is obtained with CeO2 and SiO2 supports, Al2O3 leads to an increase in particle size. These studies confirmed that the cluster core structure was stable upon deposition and post-treatment, but focused only on the Au kernel. However, the fate of the thiolates (ligands) during deposition of the clusters on an oxide support and post-treatments was not considered. Up to now, it was believed that the ligands of cluster catalysts "disappeared" (into the gas phase) upon activation. The field is in fact moving towards applications and many involve the deposition of clusters on solid surfaces (supports). This motivated us to study the fate of the thiolate ligands upon supporting clusters on surfaces. S K-edge measurements revealed for first time ligand migration from the gold clusters to the support, manifested by formation of unexpected oxidized sulfur species on the support.[3] The redistribution and oxidation of the ligands modified the support surface, a factor that may alter its properties.

In situ XAFS Au L3-edge studies under CO oxidation reaction conditions show the high stability of the main core cluster structure but a reversible mobility of the Au-S units[6]. The flexibility of the cluster structure and the mobility of atoms was observed in our previous work by in situ QXAFS of metal exchange reaction between Au38 and AgxAu38-x clusters[7]. The mobility of the Au-S units can explained the different catalytic behaviour depending on the treatment also confirmed by in situ DRIFTS measurements.
CONCLUSION
Insights into the interaction between the monolayer protected nanocluster and the oxide support are obtained leading to a better understanding on the different catalytic behavior of this class of nanocatalysts. In summary, we have shown that upon supporting thiolate protected gold clusters, the thiolates are redistributed between cluster and support, leading to oxidized sulfur species that alter the electronic and adsorption properties of the support. This effect has completely been neglected up to now but we believe that it must be taken into account to fully understand the complex selectivity patterns of supported thiolate-protected clusters in catalysis.
REFERENCES
1. Sels, A., et al., Nanoscale, 2016. 8(21): p. 11130-11135.
2. Barrabés, N., B. Zhang, and T. Bürgi, JACS, 2014. 136(41): p. 14361-14364.
3. Zhang, B., et al.,. ChemCatChem, 2018. 10(23): p. 5372-5376.
4. García, C., et al.,. Catalysis Today, 2019, in press
5. Zhang, B., et al.,. Journal of Physical Chemistry C, 2015. 119(20): p. 11193-11199.
6. Pollitt, S., et al,. 2019: in preparation.
7. Zhang, B., et al., Physical Chemistry Chemical Physics, 2018. 20(7): p. 5312-5318.