Supported sub-nanometer clusters are interesting as catalysts because they expose essentially all their atoms to reactants, evolve structurally as adsorbates bind and desorb, and have properties that can be varied dramatically by adding a few atoms. Problems with such catalysts is that small clusters are relatively unstable with respect to sintering, and poison easily because of the small reactive surface areas. We have developed an approach to preparing atomically size- and composition-selected cluster catalysts, using self-limiting chemistry to add atoms to size-selected Ptn clusters on alumina or silica supports. Alkane dehydrogenation is used to test the model catalysts to see how alloying affects sintering and poisoning by carbon deposition at high temperatures. Spectroscopic properties, reactant binding geometries, and desorption, decomposition, and poisoning processes are studied in situ under surface science conditions. A new high pressure micro-reactor/molecular beam sampling instrument is used to study catalysis and poisoning at high pressures and temperatures. The effects of alloying small Pt clusters with Ge, B, and Sn will be compared, with analysis by a combination of in situ spectroscopy, imaging, and DFT calculations. The figure compares desorption of intact ethylene vs. hydrogen (a sign of carbon deposition) for Pt7/alumina and Pt7Ge2/alumina samples subjected to repeated cycles of ethylene adsorption and heating, showing how much more rapidly the pure Pt cluster deactivates due to sintering and carbon deposition, compared to the cluster with two added Ge atoms.

Selectively alloyed sub-nano clusters as catalysts resistant to sintering and poisoning

ACS Spring 2022

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