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O: Fachverband Oberflächenphysik

O 70: Supported nanoclusters: Structure, Reactions, Catalysis

O 70.3: Vortrag

Donnerstag, 8. September 2022, 15:45–16:00, H6

Ripening mechanism changes with cluster size: In situ observation of Pt cluster diffusion on Fe3O4(001) — •Sebastian Kaiser, Johanna Plansky, Ueli Heiz, Barbara A. J. Lechner, and Friedrich Esch — Technical University of Munich, Garching, Germany

Ripening of small oxide-supported metal clusters is a common deactivation mechanism in heterogeneous catalysis. We use scanning tunneling microscopy (STM) to follow the diffusion and ripening of size-selected Pt clusters on an Fe3O4(001) support. Thanks to a strong cluster-support bonding, ripening and coalescence only set in at elevated temperatures. Particle size analysis of our STM images reveals that Pt19 exhibits Ostwald ripening at temperatures above 800 K, i.e. with cluster growth by atom diffusion. In contrast, the smaller Pt5 and Pt10 clusters show Smoluchowski ripening, i.e. the diffusion of entire clusters, already in the temperature range of 600 to 800 K and Ostwald ripening >800 K. We not only observe the ripened clusters in STM, but could successfully follow the diffusion process in situ by STM movies. Surprisingly, temperature programmed desorption (TPD) measurements of CO molecules show that in this temperature range the clusters get concomitantly encapsulated by iron oxide via strong metal support interaction (SMSI). Cluster diffusion thus occurs despite a strong interaction with the magnetite support and the diffusing species is most likely a cluster with Fe and O atoms on top.