An Atomic-Scale View of Palladium Alloys and their Ability to Dissociate Molecular Hydrogen

Palladium and its alloys play a central role in a wide variety of industrially important applications such as hydrogen reactions, separations, storage devices, and fuel-cell components. Alloy compositions are complex and often heterogeneous at the atomic-scale and the exact mechanisms by which many of these processes operate have yet to be discovered. Herein, scanning tunneling microscopy (STM) has been used to investigate the atomic-scale structure of Pd-Au and Pd-Cu bimetallics created by depositing Pd on both Au(111) and Cu(111) single crystals at a variety of surface temperatures. We demonstrated that individual, isolated Pd atoms in an inert Cu matrix are active for the dissociation of hydrogen and subsequent spillover onto Cu sites. Our results indicated that H spillover was facile on Pd-Cu at 420 K but that no H was found under the same H-2 flux on a Pd-Au sample with identical atomic composition and geometry. In the case of Au, significant H uptake was only observed when larger ensembles of Pd were present in the form of nanoparticles. We report experimental evidence for hydrogen's ability to reverse the tendency of Pd to segregate into the Au surface at catalytically relevant temperatures and our STM images reveal a novel H-induced striped structure in which Pd atoms aggregated on top of the surface in regularly spaced rows. These results demonstrate the powerful influence an inert substrate has on the catalytic activity of Pd atoms supported in or on its surface and reveal how the atomic-scale geometry of Pd-Au alloys is greatly affected by the presence of hydrogen.