Hydrogen atom absorption inhydrogen pre-covered Pd(110) surface
水素吸着Pd(110)再構成表面における水素原子吸収過程	
Allan Abraham B. Padama1, Hiroshi Nakanishi1, Hideaki Kasai1,2
1Department of Applied Physics, Osaka University
2Center for Atomic and Molecular Technologies, Osaka University

Background of the study
Interaction of H with metal surface

dissocia-on	  	  H	   H	   and	  adsorp-on	  diﬀusion	  H	  METAL	  H	  absorp-on	  Hydrogen Absorption
Hydrogen Separation

Hydrogen Storage

Hydrogenation/Dehydrogenation

Background of the study
The H/Pd(110) System

H coverage of θ > 1 MLXY

(1X2) pairing-row(1X2) missing-row

The pairing-row structure is a metastable state [3].

Experiments suggest that “facile” absorption of H in the subsurface region of Pd(110)happens at high H coverage and when the surface is already reconstructed.[1-3]

In this work:
How does subsurface population of H in Pd(110) take place?
[1] R. Behm et al., J. Chem. Phys. 78 (1983) 7486.
[2] M. Cattania et al., Surf. Sci. 126 (1983) 382; G. Kleinle et al., Surf. Sci. 189–190 (1987) 177.
[3] J. Yoshinobu et al., Phys. Rev. B 51 (1995) 4529; K.H. Rieder et al. Phys. Rev. Lett. 51 (1983) 1799.

Computational detailsØØØØDensity functional theory
Lattice constants: 3.94 Å for Pd[1]Six atomic layers of Pd(110) facet with four atoms (2×2) in each layer
Three top-most layers of the surfaces is allowed to relax to account for surface relaxation.

Geometric structure of the optimized Pd(110) surface.
The changes in interlayer spacings:
d12 = ‒7.54%; d23= 3.37% .[2]The	  ac'va'on	  barriers	  are	  obtained	  from	  the	  constructed	  poten'al	  energy	  surface	  and	  by	  imlemen'ng	  CI-­‐NEB	  method	  calcula'ons.	  
[1] N. Ozawa et al., J. Appl. Phys. 101 (2007) 123530.
[2] M. Skottke et al., J. Chem. Phys. 87 (1987) 6191; V. Ledentu et al., Surf. Sci. 412–413 (1998) 518.

Results and discussion
Comparison of the H binding energies
Sites
Pseudo-threefold
Long bridge
Octahedral (1st ss)
Tetrahedral (1st ss)
Octahedral (2nd ss)
Tetrahedral (2nd ss)

Binding Energy(eV)-Relaxed-0.486-0.412-0.144-0.142-0.055-0.018Binding Energy (eV)-Frozen Lattice-0.452-0.391
differ by 〜0.02 – 0.07 eV-0.0780.331 eV more stable+0.188generally stronger binding energy in the relaxed calculation

P3F	  LB	  1OCT	  1TET	  2OCT	  2TET	  Pd atom moves in such a way that they provides enough spaceand effective interaction with the diffusing H atom.
The average interatomic distance of H with its nearest-neighbor Pd atoms is 1.774 Åin the relaxed calculation while it is 1.707 Å from frozen lattice calculation.

Results and discussion
Activation barriers of H in Pd(110)[1,2]stronger binding energy inrelaxed calculation

Substrate relaxation plays
important role in the absorptionof H atom.

comparable binding energy
The energetically increasing pathway when H moves from the surface to 2nd subsurface
layer suggests the tendency for the H to be trapped in the surface region[4].
[1] A.A.B. Padama et al., J. Phys. Soc. Jpn. 81 (2012) 114705.
[2] A.A.B. Padama et al., Int. J. Hydrogen Energy 38 (2013) 14715.
[3] P. Ferrin et al., Surf. Sci. 606 (2012) 679.
[4] H. Okuyama et al., Surf. Sci. 401 (1998) 344.	  Results and discussion
High coverage H on Pd(110)ridge	  site	  1.5	  ML	  of	  H	  	  ridge	  site	  1.5 ML of H

H-vacant ridge site

EB	  (HR):	  -­‐0.55	  eV	  	  EB	  (HT):	  -­‐0.44	  eV	  trough	  site	  trough	  site	  0.25	  ML	  of	  H	  	  XH-vacant trough site

EB:	  -­‐0.57	  eV	  EB:	  -­‐0.47	  eV	  Binding energies and electronic structure profile indicates thenegligible H – H interaction in the missing-row surface.

X[1] A.A.B. Padama et al., Int. J. Hydrogen Energy (2014) http://dx.doi.org/10.1016/j.ijhydene.2014.02.019.	  Results and discussion
Monoatomic absorption of H

0.54 eVRIDGE	  0.37 eVTROUGH	  The barrier for the absorption of H
atom (0.25 ML) is ~ 0.30 – 0.40 eV.

(top	  view)	  Monoatomic H absorption is not the appropriate process to explain the presence ofsubsurface H in Pd(110) surface.

Results and discussion
Diffusion of ridge-H and absorption of trough-H0.65 eV(top	  view)	  The geometric structures propose that simultaneous diffusion of ridge-H and absorption oftrough-H is not possible and sequential process is more preferred.

Results and discussion
Diffusion of ridge-H and absorption of trough-H
If the process is a “sequential”, there must be an additive properties of the activation barriers.
Eact1(diffusion+absorption) =Eact2(diffusion of ridge-H on vacant trough site) + Eact3(monoatomic absorption in trough site)eV	  Diﬀusion	  of	  ridge-­‐H	  to	  empty	  trough	  site	  0.65	  0.30 eV0.37	  0.30	  Eact1	  Eact2	  +	  Eact3	  Additive properties of the activation barriers also supports the “sequential process”
claimed for this process.

Results and discussion
H absorption through the assistance of H from vacuumnon-­‐ac-vated	  process	  (top	  view)	  Absorp-on	  of	  H	  occurs	  through	  the	  presence	  of	  adsorbing	  H	  atoms	  from	  vacuum.	  	  Results and discussion
H2 on Pd(110) missing row surface[1]The transition states for both cases are observed tooccur during the dissociation process in contrastwith the case of low H coverage in which theabsorption is the rate-limiting step [2,3].
[1] A.A.B. Padama et al., Int. J. Hydrogen Energy (2014) http://dx.doi.org/10.1016/j.ijhydene.2014.02.019[2] H. Dipojono et al., Jpn. J. Appl. Phys. 49 (2010) 115702.
[3] V. Ledentu et al., Surf. Sci. 412-413 (1998) 518.

Conclusion

The strong interaction of H with the surface Pd atoms suggests the occurrence ofsurface / near-surface H trapping.
For 1.5 monolayer H coverage in missing-row Pd(110), interaction among theadsorbed H atoms is negligible.
Neither the monoatomic absorption of H nor the assistance from the initiallyadsorbed H atom explains the experimentally observed presence of subsurface H in
Pd(110) at high H coverage. H absorption is realized through the assistance ofincoming H atoms from vacuum.
The absorption of H in the H-covered surface is non-activated while thedissociation of H2 is the rate-limiting process.

On-going task
Comparison	  of	  wavefunc'ons	  of	  H	  atom	  in	  low	  (unreconstructed)	  and	  high	  (missing-­‐row	  reconstructed)	  coverages	  on	  Pd(110)	  surface.	  	  Poten'al	  energy	  surface	  of	  H	  atom	  at	  low	  coverage	  ground	  state	  1st	  excited	  state	  Thank you for your attention.