In nanocatalysis, a great challenge is to obtain truly homogenous and well-defined highly active nanostructures on surfaces. An emerging class of functional nanomaterials with atomic precision, well-defined molecular structure and intriguing molecular-like properties are the ligand protected metal nanoclusters. An enhancement of the catalytic properties by metal nanoclusters has been observed, when compared to classical nanoparticles of similar size. Their uniform and atomically precise nanostructure facilitates the correlation of structure with catalytic properties, providing a fundamental understanding.
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 on their stability.[1-3] 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 [1]. The redistribution and oxidation of the ligands modified the support surface, a factor that may alter its catalytic 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. 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 [4]. The mobility of the Au-S units can explained the different catalytic behaviour depending on the treatment also confirmed by in situ DRIFTS measurements [5].
Heterogeneous catalysis research of atomically precise gold nanoclusters is an emerging field because of the outstanding size control during cluster synthesis. It opens new opportunities for accurate studies of size-dependent properties, atomic structure effects and reaction mechanisms in catalysis.
References
[1] Zhang, B., A. Sels, G. Salassa, S. Pollitt, V. Truttmann, C. Rameshan, J. Llorca, W. Olszewski,
G. Rupprechter, T. Bürgi, and N. Barrabés, ChemCatChem, 2018. 10(23): p. 5372-5376.
[2] García, C., S. Pollitt, M. van der Linden, V. Truttmann, C. Rameshan, R. Rameshan, E. Pittenauer, G. Allmaier, P. Kregsamer, M. Stöger-Pollach, N. Barrabés, and G. Rupprechter, Catalysis Today, 2018. DOI: 10.1016/j.cattod.2018.12.013.
[3] Zhang, B., S. Kaziz, H. Li, M.G. Hevia, D. Wodka, C. Mazet, T. Burgi, and N. Barrabes, Journal of Physical Chemistry C, 2015. 119(20): p. 11193-11199.
[4] Zhang, B., O.V. Safonova, S. Pollitt, G. Salassa, A. Sels, R. Kazan, Y. Wang, G. Rupprechter, N. Barrabés, and T. Bürgi, Physical Chemistry Chemical Physics, 2018. 20(7): p. 5312-5318.
[5] S.Pollitt, V. Truttmann, C. Garcia, G. Salassa, A. Sels, W.Olszewski, J.Llorca, T.Bürgi, N. Barrabés, G. Rupprechter, in preparation
