Paper SS-MoA9
Novel Insight Into the Formation Mechanism of Subsurface Hydrogen at Pd(110) Surfaces

Monday, October 29, 2012, 4:40 pm, Room 21

The present study investigates the microscopic hydrogen (H) absorption mechanism at palladium (Pd) (110) single crystal surfaces. The unique properties of H-Pd interactions such as non-activated H₂ dissociation and facile bulk absorption are well known, yet an accurate atomic-level understanding of the H transportation across the surface, i.e., penetration into the Pd interior and hydride formation during absorption as well as the H release into the gas phase has not been achieved. We apply ¹H(¹⁵N,αγ)¹²C resonant nuclear reaction analysis (NRA) for nondestructive H depth profiling in combination with thermal desorption spectroscopy (TDS) to quantitatively reveal the H-concentration-depth distribution and to unambiguously assign characteristic TDS features to surface-adsorbed and subsurface-absorbed H-species. To obtain additional insight into the release mechanism during thermal desorption of the H states absorbed in the Pd interior, the internal state distribution of desorbing H₂ molecules is characterized with a rovibrational state selective resonance-enhanced multi-photon ionization (REMPI) technique.

TDS experiments using isotope labeling for the surface-adsorbed and subsurface-absorbed H states reveal that two parallel absorption routes exist that lead to two distinctly different depth distributions of Pd hydride in the near-surface region which give rise to separate TDS signatures (α₁ at T=160 K and α₃ at T>190 K), respectively. The first absorption pathway involves only a few (~4%) minority sites (presumably defects) at which penetration is highly efficient and leads to in-plane localized nucleation of hydride that remains concentrated within a few nanometers below the surface (α₁ state). The second absorption route proceeds on regular terraces with a significantly slower penetration rate per site so that subsurface H diffusion leads to a more extended hydride depth distribution (α₃ state, several tens of nm below the surface). A clear difference between the two H absorption states (α₁, α₃) is also seen in the REMPI internal-state populations of desorbing H₂. Both absorption pathways critically require gas phase H₂, replace surface-adsorbed H atoms with the gas phase isotope, and have activation energies (< 100 meV) much smaller than expected for the isolated subsurface migration of chemisorbed H. The results are accounted for by a concerted mechanism, in which pre-adsorbed H transits into the subsurface while its vacated adsorption site is simultaneously refilled by a nearby ‘nascent’ H atom in a state of high potential energy, which is supplied through gas phase H₂ dissociation at vacancies in the chemisorption layer.