Single-atom catalysts (SACs) have recently attracted great attention due to their ultimate metal efficiency and the promise of novel properties. However, at the atomic level, little is known about their stability, interactions with the support, and mechanisms by which they operate. In hydrogenation reactions, the mechanisms of many critical steps such as hydrogen activation and spillover are far from understood. Here, we employ a combination of scanning tunneling microscopy and density functional theory and demonstrate that on a model SAC comprised of single Pd atoms on Fe3O4(001), H2 dissociates heterolytically between Pd and surface oxygen. The efficient hydrogen spillover allows for continuous hydrogenation to high coverages which ultimately leads to the lifting of the Fe3O4 reconstruction and Pd reduction and destabilization. Water plays an important role in reducing the hydrogen diffusion barrier, thereby facilitating the redistribution of hydroxyls away from Pd. Our study demonstrates a distinct H2 activation mechanism on single Pd atoms in contrast with Pd nanoparticles and corroborates the importance of charge transport on reducible support away from the active site. Computational studies yield detailed insight into the activity of different transition metals.

Heterolytic H2 cleavage and water-assisted hydrogen spillover on single palladium atoms supported on Fe3O4(001)

2019 Fall National Meeting

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