Desorption Temperature Control of Palladium-Dissolved Hydrogen through Surface Structural Manipulation

Abstract

To assess the possibility of controlling the desorption temperature of palladium-absorbed hydrogen (Habs) through surface structural manipulation, we investigated coadsorption systems of H and CO on Habs-charged Pd(110) surfaces through temperature-programmed desorption, low-energy electron diffraction, and H-depth profiling by nuclear reaction analysis (NRA). A CO coverage of 0.5 ML lifts the H-induced (1 × 2) pairing-row (PR) reconstruction on Habs precharged Pd(110), and, as on clean Pd(110), heating Pd(110) (1 × 1) holding 0.3–1.0 ML of CO gives rise to a missing-row (MR) structure. Whereas Habs desorbs through surface defects of clean, PR-reconstructed Pd(110) at 160 K, CO coadsorption onto Habs-loaded Pd(110) gives rise to three new high-temperature shifted Habs desorption modes at 200, 270, and 375 K that are assigned to different exit sites for resurfacing Habs atoms at regular terraces of the individual Pd(110) structures, i.e., the (1 × 2) PR, bulk-terminated (1 × 1), and (1 × 2) MR reconstruction, respectively. Our results thus manifest the ability to control the Habs desorption temperature through surface restructuring in well-defined CO coverage regimes. Furthermore, the long-speculated transfer of chemisorbed H into the Pd interior upon CO coadsorption is confirmed directly by NRA, revealing also that all Habs diffuses into the Pd bulk at 200 K.

Nuclear Dynamics and Electronic Effects of Hydrogen on Solid Surfaces

“…With these CO post‐dosages, the PR reconstruction is gradually lifted. Concomitantly the γ 1 desorption intensity gradually decreases with increasing CO exposure and instead a new peak grows at 270 K designated as γ 2 as shown in Figure (c) . This indicates that the γ 2 state at 270 K corresponds to desorption of absorbed hydrogen from the (1 × 1) phase of Pd(110), and that the hydrogen desorption pathway is converted from the PR surface to the (1 × 1) surface.…”

“…For larger CO dosages between 1.2 ‐ 2 L, another type of modification of the hydrogen release is demonstrated by the further surface structure change as shown in Figure (d) . The series of H 2 TPD spectra shows a decrease in the γ 2 feature and the simultaneous appearance of a new desorption peak at 375 K designated as γ 3 .…”

“…This clearly indicates that hydrogen cannot be stably stored in Pd under a vacuum at room temperature, which is thermodynamically reasonable . However, it might be possible to kinetically stabilize absorbed hydrogen by modifying the Pd surface, which is demonstrated through a CO‐controlled surface structural change ,. Upon hydrogen dosage at low temperature, the Pd(110) surface reconstructs into the pairing‐row (PR) type (1 × 2) superstructure where the rows of surface Pd atoms form pairs as schematically shown in Figure (a) .…”

“…However, it might be possible to kinetically stabilize absorbed hydrogen by modifying the Pd surface, which is demonstrated through a CO‐controlled surface structural change ,. Upon hydrogen dosage at low temperature, the Pd(110) surface reconstructs into the pairing‐row (PR) type (1 × 2) superstructure where the rows of surface Pd atoms form pairs as schematically shown in Figure (a) . When CO adsorbs on this PR surface, the PR reconstruction is lifted and the surface changes to (1 × 1) at a low CO coverage.…”

“…Figure (b) displays TPD of H 2 after exposing Pd(110) to 1000 L H 2 and subsequently to small doses ( < 0.4 L) of CO at 120 K . With these CO doses, the PR reconstruction remains intact.…”