新学術領域「コンピューティクスによる物質デザイン：複合相関と非平衡ダイナミクス」
計画研究「A02-7 班: プロトン・ミューオンで探る新物性と量子ダイナミクス」

パラジウム(110)表面における水素吸収の機構

Mechanism of Hydrogen Absorption at the Palladium (110) Surface
ビルデマーカス、大野哲、福谷克之

Markus Wilde, Satoshi Ohno, and Katsuyuki Fukutani
東京大学生産技術研究所
Institute of Industrial Science, The University of Tokyo, Komaba 4-6-1, Meguro-ku,
Tokyo 153-8505
Despite its enormous technological relevance for hydrogen purification, storage, andhydrogenation catalysis, the exact atomic-level transportation mechanism of hydrogen(H) from the gas phase across the surface into the metal interior during H2 absorption atpalladium (Pd) surfaces has still not been clarified. The actual penetration mechanism
and the role of surface-adsorbed hydrogen in the absorption process remain disputed.
The decisive elementary step in the H2 absorption mechanism has often been envisioned
as monatomic diffusion of chemisorbed H atoms into interstitial sites just underneaththe Pd surface. Ab-initio (DFT) calculations, however, indicate that the activation
barrier for this monatomic diffusion is much (>0.3 eV) larger than the activation energyof H2 absorption (<0.1 eV), ruling out monatomic in-diffusion of chemisorbed H as therate-limiting step. A second issue is that chemisorbed D on Pd(100) and (111) surfaces
appears to be ‘bypassed’ during H2 absorption and subsequent desorption of absorbed H,
which has been attributed to exclusive absorption at defect sites. A microscopic
explanation for the small activation energy has yet been lacking. We therefore
investigated the H2 absorption mechanism at a structurally well-characterized Pd(110)
surface with H depth profiling via 15N nuclear reaction analysis and thermal desorption
spectroscopy with isotope (H,D) labeled surface hydrogen.
We thus discovered that on Pd(110) two distinct H absorption pathways exist that at low
temperatures (<145 K) populate two hydride states of distinctly different depth
distributions: Near-surface hydride nucleates after localized H2 absorption at minority
sites (defects), whereas nucleation of bulk Pd hydride results from absorption in theregular Pd(110) terrace area. The latter is observed for the first time at a Pd single
crystal and suggests that the absorption process is strongly structure-sensitive. Unlike
the previously suggested ‘bypassing’ scenario, both absorption pathways effectivelytransfer chemisorbed H below the surface and replace it by the gas phase isotope in aprocess that requires only very little (< 100 meV) activation energy.
We discuss these findings in the framework of a novel absorption mechanism, in which
H2 dissociates at vacancies in the H chemisorption layer and thereby generates ‘excess’
H atoms, which exist in a state of high potential energy on the H-saturated Pd surface.
Such excess H atoms may elicit penetration of pre-adsorbed H into the subsurface while
simultaneously reoccupying the chemisorption sites that the latter vacate. Theoreticalcalculations suggest that either this concerted penetration or the H2 dissociation on theH-saturated Pd surface may be the rate-limiting step of the H2 absorption process [1].
[1] S. Ohno, M. Wilde, K. Fukutani, J. Chem. Phys., submitted.