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Quantum Nature of Hydrogen on Palladium

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Steven C. Erwin (Naval Research Laboratory) 3. Do we need quantum ... Our model explains clearly the inactivity of divacancy in hydrogen molecule dissociation. ... – PowerPoint PPT presentation

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Title: Quantum Nature of Hydrogen on Palladium


1
Quantum Nature of Hydrogen on Palladium
Seong-Gon Kim Physics Astronomy Mississippi
State University
2
Collaborators
  • Sungho Kim (Mississippi State University)
  • Steven C. Erwin (Naval Research Laboratory)

3
Do we need quantum mechanics?
Do we need quantum mechanics for nuclei?
H
Yes!
4
Hydrogen on metals
5
Experimental observations
T. Mitsui, M.K. Rose, E. Fomin, D.F. Ogletree,
M. Salmeron, Nature 422, 705 (2003)
6
Q1 Why vacancies have apparent height?
Low H coverage
High H coverage
T. Mitsui, et al Nature, 422, 705 (2003)
7
Q2 Why Three-lobed triangle?
T. Mitsui, et al Nature, 422, 705 (2003)
8
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)
9
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)
10
Q3 Why two-acancy is not an active site?
3V
2V
3VH2
2VH2
1V
active
inactive
Contrary to Langmuir adsorption kinetics
11
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
12
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?
13
System in Present Study
Scanning Tunnelling Microscope
Vacuum 6 atomic layers
Hydrogen atoms absorbed on Pd surface
Pd Slab 4 atomic layers
14
Why vacancy has apparent height?
Tunnelling current
STM
Palladium ? bright
Hydrogen ? dark
15
Hydrogen vacancy configurations
16
Site name convention
Pd
T
F
H
T
T
H
Fcc sites
F
F
T
T
Hcp sites
T
Top sites
17
2V(1H,3s)
18
Born-Oppenheimer Approx.
  • Density Functional Theory
  • Local Density Approximation
  • Ultrasoft Pseudopotentials
  • Vxc Ceperly and Alder

19
Potential Energy Surface
Minimum potential energy along z-direction
20
Zero-point energies
2E
1E 0.19 eV
Accidental degeneracy
2A1
1A1 0.18 eV
21
Wave functions for hydrogen
2A1
1A1
1E1
2E2
1E2
2E1
22
Mixed states
23
Localized state
2A1
1E1
1E2
24
Quantum vacancy geometries
  • Three-fold symmetry
  • Direct image

25
Larger vacancy geometries
  • All equilateral triangular shape
  • Size n(n1)/2 fcc sites
  • H atoms on n(n-1)/2 hcp site

26
Why divacancy is not active?
6V
3V(3H,6s)
3V(3H,6s)H2
2V(1H,3s)
Lopez, et al PRL 93, 146103 (2004)
27
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.
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