Quantum Nature of Hydrogen on Palladium

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Quantum Nature of Hydrogen on Palladium
Seong-Gon Kim Physics Astronomy Mississippi
State University
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Collaborators

• Sungho Kim (Mississippi State University)
• Steven C. Erwin (Naval Research Laboratory)

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Do we need quantum mechanics?
Do we need quantum mechanics for nuclei?
H
Yes!
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Hydrogen on metals
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Experimental observations
T. Mitsui, M.K. Rose, E. Fomin, D.F. Ogletree,
M. Salmeron, Nature 422, 705 (2003)
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Q1 Why vacancies have apparent height?
Low H coverage
High H coverage
T. Mitsui, et al Nature, 422, 705 (2003)
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Q2 Why Three-lobed triangle?
T. Mitsui, et al Nature, 422, 705 (2003)
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Annihilation of three-vacancy cluster
Mean adsorption time 2V Never (gt5.6h) 3V
330 seconds
NOTE size of the vacancy (6 sites for 3V)
T. Mitsui, et al Nature, 422, 705 (2003)
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Annihilation of four-vacancy cluster
Dissociative hydrogen adsorption on palladium
requires aggregates of three or more vacancies
T. Mitsui, et al Nature, 422, 705 (2003)
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Q3 Why two-acancy is not an active site?
3V
2V
3VH2
2VH2
1V
active
inactive
Contrary to Langmuir adsorption kinetics
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Q3 Why two-acancy is not an active site?
inactive
2V
6V
3V(3H,6s)
3V(3H,6s)H2
3V
active
Contrary to Langmuir adsorption kinetics
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Three Puzzles
Q1 Why vacancies have apparent height ?
2 V
Q2 Why divacances appear as three-lobed triangle
?
1 V
Q3 Why divacancy is not active in H2 adsorption?
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System in Present Study
Scanning Tunnelling Microscope
Vacuum 6 atomic layers
Hydrogen atoms absorbed on Pd surface
Pd Slab 4 atomic layers
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Why vacancy has apparent height?
Tunnelling current
STM
Palladium ? bright
Hydrogen ? dark
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Hydrogen vacancy configurations
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Site name convention
Pd
T
F
H
T
T
H
Fcc sites
F
F
T
T
Hcp sites
T
Top sites
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2V(1H,3s)
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Born-Oppenheimer Approx.

• Density Functional Theory
• Local Density Approximation
• Ultrasoft Pseudopotentials
• Vxc Ceperly and Alder

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Potential Energy Surface
Minimum potential energy along z-direction
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Zero-point energies
2E
1E 0.19 eV
Accidental degeneracy
2A1
1A1 0.18 eV
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Wave functions for hydrogen
2A1
1A1
1E1
2E2
1E2
2E1
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Mixed states
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Localized state
2A1
1E1
1E2
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Quantum vacancy geometries

• Three-fold symmetry
• Direct image

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Larger vacancy geometries

• All equilateral triangular shape
• Size n(n1)/2 fcc sites
• H atoms on n(n-1)/2 hcp site

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Why divacancy is not active?
6V
3V(3H,6s)
3V(3H,6s)H2
2V(1H,3s)
Lopez, et al PRL 93, 146103 (2004)
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Summary

• Ground-state structure of divacancy defect of H
  atom on Pd(111) surfaces
• Substantial quantum effects on H atoms in the
  vicinity of a divacancy
• Ground-state wave function is localized in the
  hcp site rather than the fcc site
• Quantum effect plays a major role in the reversal
  of the order of the most energetically favorable
  site
• Our model explains clearly the inactivity of
  divacancy in hydrogen molecule dissociation.