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文部科学省科学研究費新学術領域研究 ・ 2014 年 3 月 11 日・東京大学. Materials Design through Computics Complex Correlation and Non-equilibrium Dynamics. 「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」 平成 25 年度 第 2 回研究会. Pd(110) 表面における水素吸収の機構. Markus Wilde ・ Satoshi Ohno ・ Katsuyuki Fukutani - PowerPoint PPT Presentation
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Pd(110) 表面における水素吸収の機構
Markus Wilde Satoshi Ohno Katsuyuki Fukutani
Institute of Industrial Science University of Tokyo
文部科学省科学研究費新学術領域研究 2014 年 3 月 11 日東京大学
Materials Design through ComputicsComplex Correlation and Non-equilibrium
Dynamics
「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」
平成 25 年度 第 2 回研究会
Hydrogen Absorption at Pd Surfaces
Industrial Importance
bull Hydrogen Storage (in hydrides)
bull Hydrogenation Catalysis
Objectives
bull Obtain atomic level understanding of the absorption mechanism
bull Model system H2 rarr Pd(110) (single crystal)
bull Influence of surface structure on absorption properties
=gt Clarify the microscopic pathways of hydrogen surface penetration
H2 z
0H
H2
HSurface
Subsurface
BulkH2
time
[5] Okuyama et al Surf Sci 401 (1998) 344[6] Ohno et al J Chem Phys submitted
Activation Energy Paradox
The actual reaction coordinate of H2 absorption
H
H2
Eabs lt 010 eV[5 6]
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705[2] Ferrin et al Surf Sci 606 679 (2012)[3] Nobuhara et al Surf Sci 566 703 (2004)[4] Ozawa et al J Phys Condens Matter 19 365214 (2007)
Prevailing H absorption model
Absorption activation
Experimental results
Emono = 03 ～ 06 eV[1-4]Monatomic in-diffusion
HPd
Chemi-sorption
Identify
R
Potential Energy
-05 eV
-02 eV-01 eV
Surface of particular interest Pd(110)
Pd(110) single crystal surface
Pd Well-known H absorbing metal
Excellent catalyst for olefin hydrogenation
(110) Single crystal Well-defined structure
Openness Surface atomic density ｰ 40 vs (111)
H-induced surface reconstruction ldquoProne to hydrogen absorptionrdquo[1]
[1] Christmann Prog Surf Sci 48 15 (1995)
_
[110]
[001]
H2 exposure
Pairing-row (P-R)reconstruction
Second-layer exposed Atomic step-like structure Lateral contraction in paired rows
Pd
(110)
Top view Side view
(1x2)
H2 D
eso
rptio
n s
ign
al
(TD
S)
600500400300200100
Temperature (K)
H2 Desorption signalCombine two hydrogen detection techniques
Experimental Approach TDS + NRA
① Thermal Desorption Spectroscopy (TDS)
rarr H2(D2) exposures at given Te desorption
rarr No of H species desorption activation energy
rarr lacks information on H location (onbelow surface)
H2 D
eso
rptio
n s
ign
al
(TD
S)
600500400300200100
Temperature (K)
Su
rface
ab
sorb
ed
H (N
RA
)
Surface-H signal (Hs) Subsurface-H signal (Hss)
Experimental
Ei=Eres -detector
N
probing depth
EigtEres
z(Ei)= (Ei-Eres)(dEdz)
[Habsorbed]
15N2+ ion beam
0
[Hsurface]
② Nuclear Reaction Analysis (NRA) via 1H(15N)12C (Eres=6385 MeV =18 keV)
rarr distinguishes surface-adsorbed from absorbed H (~2 nm depth resolution)
300 L H+H2 on Pd(100) at 100 K
15N ion energy [MeV]
637 638 639 640 641
ray
yiel
d [c
ts
C]
0
50
100
150
depth [nm]
-4 -2 0 2 4 6 8
15N ion energy (MeV)
Depth (nm)
-yi
eld
(ct
sC
)
M Wilde PRB 78 (2008) 114511
rarr achieves unambiguous TDS peak identifications
0 L (clean) 03 L 05 L
[1] Ledentu et al Surf Sci 411 (1998) 123 [2] Yoshinobu et al Phys Rev B 51 4529 (1995)
(1times1) (2times1) (1times2)
Surface Adsorption Phases (LEED amp TPD) HPd(110)
H2 exposure at Te = 130 K50 L ～
θ=15 MLθ=10 ML
_
[110]
[001] θ=0 ML
[1]
θ= ML
[2]
120
100
80
60
40
20
0Q
MS io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
120
100
80
60
40
20
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
β1 β2
α2Surface
1 L = 10-6 Torr s1 L = 10-6 Torr s
1 ML = 94 x 1014 atomscm2
1 ML = 94 x 1014 atomscm2
01 L03 L
08 L
03 L
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]400350300250200150
Temperature [K]
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
Surface
α1
α3
200
150
100
50
0
QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
600
500
400
300
200
100
0
-yi
eld
[co
un
ts
C]
20151050-5
Depth [nm]
64664464264063815
N ion energy [MeV]
10 L at 170 K 2000 L at 130 K 2000 L at 145 K
300
200
100
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
α1α3 α2
β2
β1
300
200
100
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
TPD
130 K
145 Kα3
NRA H Depth Distribution of Two Low-T TPD States
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
12 at
230 at
NRA
Absorbed hydrogenα1 Near surface α3 Bulk gt 50 nm
2000 L at Te = 130 K
α1 α3α2
β2
β1
surface
TPD feature Origin Depth extension
[H]avgVolume
ratioTe condition
α1 (170 K) Near surface hydride ～ 10 nm 23 at 30 lt 145 K
α3 (195 K) Bulk hydride gt 50 nm 12 at 2 lt 160 K
LEED NRA TPD Identification of H2Pd(110) desorption features
600
500
400
300
200
100
0
-yi
eld
[co
un
ts
C]
20151050-5
Depth [nm]
64664464264063815
N ion energy [MeV]
10 L at 170 K 2000 L at 130 K 2000 L at 145 K
=gt First revelation at Pd(110) TWO absorbed hydride states
300
200
100
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150Temperature [K]
TPD NRA
S Ohno M Wilde K Fukutani J Chem Phys submitted
NEW
pre postD2 10 L rarr H2 1000 L α1
α3
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
Te = 115 K
rArr Two separate absorption pathways exist ()
Near-surface hydride Bulk hydride
Investigation of the H2 Absorption Mechanism
Absorption experiments with isotope labeled surface hydrogen
Analysis of isotope populations (TPD)
=gt Clear difference between near-surface (α1) and bulk (α3) hydride (Also Different normal (H2gtD2) isotope effects in a1 and a3 population speeds)
=gt Absorption near minority sites (defects)
Isotope Population of the Absorbed Hydride States
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre 006 ML
Near-surface hydride (α1)
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
Bulk hydride (α3)
08
1
02
p=0
05
～ 4
Dominant transfer of pre-adsorbed H below the surface
(First observation)
=gt Absorption in regular terrace area ()
p=0~05
Only Pd(110) no lsquobypassingrsquo
p 1-p
lsquobypassingrsquo replacement
Recursive analysis of isotope composition
( )( 1) ( ) (1 )
( )( 1) ( ) (1 )
prepre pre
prepost post
N nN n N n p
N
N nN n N n p
N
Evaluation of lsquobypassingrsquo probability (p)
Stochastic Isotope Population Model for AbsorptionDesorption
(1)
(2)
(p) (1-p)
n+1th absorption eventpost
pre Npre(n)post Npost(n)
rarr uptake rarr desorption (microscopic reversibility)
Absorption mechanism Bypassing or Replacement
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
Bulk hydride (α3)
Near-surface hydride (α1)
08
1
02
05
08
1
02
p=0
05
p=0
Dominant absorption mechanism
Compatible Incompatible
p=0 Replacement p=1 Bypassing
Replacement
S Ohno M Wilde K Fukutani J Chem Phys submitted
What is the Rate Determining Step (RDS)
H2 absorption Eabs lt 01 eV[1 2]
H
H2
timesExperiment Prevailing model
Possible rate determining steps
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Ohno et al J Chem Phys submitted [3] Padama et al J Phys Soc Jpn 81 (2012) 114705 [4] Ferrin et al Surf Sci 606 (2012) 679
Emono = 03 ～ 06 eVMonatomic in-diffusion
HPd
Chemi-sorption
R
Potential Energy
-05 eV
-02 eV-01 eV
1)
2)
3)
RDS H2 Dissociation (at large H) or Concerted Penetration
[1] Rendulic Surf Sci 208 (1989) 404 [2] Groszlig ChemPhysChem 11 (2010) 1374 [3] Sakong ChemPhysChem 13 3467
bull H2 dissociation is non-activated (Ediss = 0) at bare Pd surfaces[1]
bull Dissociation becomes weakly activated (at high H-coverages)[2]
05 ML HPd(100)
Consider processes with activation energies compatible to Eabs (le01 eV)
rarr H2 dissociation (Ediss) concerted penetration (Ec-pen)
H2 dissociation at a H mono-vacancy
Ediss = 01 eV[23]
Excess H atom[2] (He)
Concerted penetration
Ec-pen asymp 006 eV[23]
He + Hs rarr Hs + Hss
bull The activation barrier of penetration can be drastically lowered when it occurs concertedly with stabilization of excess H atoms
Influence of Surface Structure on H2 dissociation (at large H)
Peculiarity of Pd(110) Terrace-related H2 absorption (not on Pd(111) and (100))[1 2]
- Possible explanation -
[1] Gdowski et al J Vac Sci Technol A 5 1103 (1987) [2] Okuyama et al Surf Sci 401 344 (1998)[3] Maringrtensson et al Phys Rev Lett 57 (1986) 2045 [4] Schmidt et al Phys Rev Lett 87 096103 (2001)[5] Ahmed et al Appl Surf Sci 257 10503 (2011) [6] Busnengo et al Phys Rev Lett 93 236103 (2004) (1x1)
constitute precursor statesfor H2 dissociation[4 5]
Top view Side view[5]
Step edge-like structures stabilize molecular H2 chemisorption states
step-likePd(322)
Ni(510)[3] Pd(210)[4] Pd(322)[5]
Theoretical prediction[6] H2 may exist at Pd(110)
bull H-vacancy-mediated dissociation vac = exp(-GsbkBT) = 2x10-8 at 145 K
bull Pabs max (Model) = Pdiss vac ltlt Pabs (Experiment) = Rabs2Zw = 5x10-4 at 145 K
bull =gt Direct gas phase H2 impact not sufficient =gt Involvement of mobile H2 precursors ()
Influence of Surface Structure on H vacancy generation
Defect-enhanced H2 absorption Terrace-related H2 absorption ( gt 24 x per site vs regular terraces) (peculiar vs Pd(100) (111) (311))
H2 dissociation may require H-vacancies
bull Rate of H-vacancy generation[1] Rvac = 1013 s-1 exp(-EssskBT) asymp 103 at 145 K
bull =gt enhanced at defects due to additional lsquoopennessrsquo May also stabilize H2
bull Widened penetration channels at defects and in troughs between paired Pd rows in
Pd(110)(1x2)-(PR)
Side view
(1x2)
Top view
Widened interstitial channels (in [001])
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705 Esss = 027 eV (110) cf Pd(111) (04 eV) Pd(100) (041 eV) (Ferrin)
Summary amp Conclusions
H2 absorption mechanism at Pd(110)-(1x2) (paired-row)
Two hydride states exist with different depth distributions
Two H absorption channels (defects + terrace Pd(110) only)
Hs is replaced (not bypassed) no simple in-diffusion Eabslt01 eV
RDS H2 dissociation (H-saturated Pd) or concerted penetration
Influence of Surface Structure H2 absorption enhanced by
ldquoOpenrdquo penetration channels (accelerate H-vacancy generation)
Stabilization of H2 precursors (at step edge-like structures)
D
H2H2
H
H2
α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
rarr Activation Energy H 1 003 eV
3 006 eV D 3 007 eV
Much smaller than expected for monatomic H surface-to-subsurface
diffusion (03~04 eV)
Arrhenius plot of1 3 population (Pa)
peak area vs exposure
lt01 eV
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
Hydrogen Absorption at Pd Surfaces
Industrial Importance
bull Hydrogen Storage (in hydrides)
bull Hydrogenation Catalysis
Objectives
bull Obtain atomic level understanding of the absorption mechanism
bull Model system H2 rarr Pd(110) (single crystal)
bull Influence of surface structure on absorption properties
=gt Clarify the microscopic pathways of hydrogen surface penetration
H2 z
0H
H2
HSurface
Subsurface
BulkH2
time
[5] Okuyama et al Surf Sci 401 (1998) 344[6] Ohno et al J Chem Phys submitted
Activation Energy Paradox
The actual reaction coordinate of H2 absorption
H
H2
Eabs lt 010 eV[5 6]
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705[2] Ferrin et al Surf Sci 606 679 (2012)[3] Nobuhara et al Surf Sci 566 703 (2004)[4] Ozawa et al J Phys Condens Matter 19 365214 (2007)
Prevailing H absorption model
Absorption activation
Experimental results
Emono = 03 ～ 06 eV[1-4]Monatomic in-diffusion
HPd
Chemi-sorption
Identify
R
Potential Energy
-05 eV
-02 eV-01 eV
Surface of particular interest Pd(110)
Pd(110) single crystal surface
Pd Well-known H absorbing metal
Excellent catalyst for olefin hydrogenation
(110) Single crystal Well-defined structure
Openness Surface atomic density ｰ 40 vs (111)
H-induced surface reconstruction ldquoProne to hydrogen absorptionrdquo[1]
[1] Christmann Prog Surf Sci 48 15 (1995)
_
[110]
[001]
H2 exposure
Pairing-row (P-R)reconstruction
Second-layer exposed Atomic step-like structure Lateral contraction in paired rows
Pd
(110)
Top view Side view
(1x2)
H2 D
eso
rptio
n s
ign
al
(TD
S)
600500400300200100
Temperature (K)
H2 Desorption signalCombine two hydrogen detection techniques
Experimental Approach TDS + NRA
① Thermal Desorption Spectroscopy (TDS)
rarr H2(D2) exposures at given Te desorption
rarr No of H species desorption activation energy
rarr lacks information on H location (onbelow surface)
H2 D
eso
rptio
n s
ign
al
(TD
S)
600500400300200100
Temperature (K)
Su
rface
ab
sorb
ed
H (N
RA
)
Surface-H signal (Hs) Subsurface-H signal (Hss)
Experimental
Ei=Eres -detector
N
probing depth
EigtEres
z(Ei)= (Ei-Eres)(dEdz)
[Habsorbed]
15N2+ ion beam
0
[Hsurface]
② Nuclear Reaction Analysis (NRA) via 1H(15N)12C (Eres=6385 MeV =18 keV)
rarr distinguishes surface-adsorbed from absorbed H (~2 nm depth resolution)
300 L H+H2 on Pd(100) at 100 K
15N ion energy [MeV]
637 638 639 640 641
ray
yiel
d [c
ts
C]
0
50
100
150
depth [nm]
-4 -2 0 2 4 6 8
15N ion energy (MeV)
Depth (nm)
-yi
eld
(ct
sC
)
M Wilde PRB 78 (2008) 114511
rarr achieves unambiguous TDS peak identifications
0 L (clean) 03 L 05 L
[1] Ledentu et al Surf Sci 411 (1998) 123 [2] Yoshinobu et al Phys Rev B 51 4529 (1995)
(1times1) (2times1) (1times2)
Surface Adsorption Phases (LEED amp TPD) HPd(110)
H2 exposure at Te = 130 K50 L ～
θ=15 MLθ=10 ML
_
[110]
[001] θ=0 ML
[1]
θ= ML
[2]
120
100
80
60
40
20
0Q
MS io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
120
100
80
60
40
20
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
β1 β2
α2Surface
1 L = 10-6 Torr s1 L = 10-6 Torr s
1 ML = 94 x 1014 atomscm2
1 ML = 94 x 1014 atomscm2
01 L03 L
08 L
03 L
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]400350300250200150
Temperature [K]
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
Surface
α1
α3
200
150
100
50
0
QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
600
500
400
300
200
100
0
-yi
eld
[co
un
ts
C]
20151050-5
Depth [nm]
64664464264063815
N ion energy [MeV]
10 L at 170 K 2000 L at 130 K 2000 L at 145 K
300
200
100
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
α1α3 α2
β2
β1
300
200
100
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
TPD
130 K
145 Kα3
NRA H Depth Distribution of Two Low-T TPD States
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
12 at
230 at
NRA
Absorbed hydrogenα1 Near surface α3 Bulk gt 50 nm
2000 L at Te = 130 K
α1 α3α2
β2
β1
surface
TPD feature Origin Depth extension
[H]avgVolume
ratioTe condition
α1 (170 K) Near surface hydride ～ 10 nm 23 at 30 lt 145 K
α3 (195 K) Bulk hydride gt 50 nm 12 at 2 lt 160 K
LEED NRA TPD Identification of H2Pd(110) desorption features
600
500
400
300
200
100
0
-yi
eld
[co
un
ts
C]
20151050-5
Depth [nm]
64664464264063815
N ion energy [MeV]
10 L at 170 K 2000 L at 130 K 2000 L at 145 K
=gt First revelation at Pd(110) TWO absorbed hydride states
300
200
100
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150Temperature [K]
TPD NRA
S Ohno M Wilde K Fukutani J Chem Phys submitted
NEW
pre postD2 10 L rarr H2 1000 L α1
α3
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
Te = 115 K
rArr Two separate absorption pathways exist ()
Near-surface hydride Bulk hydride
Investigation of the H2 Absorption Mechanism
Absorption experiments with isotope labeled surface hydrogen
Analysis of isotope populations (TPD)
=gt Clear difference between near-surface (α1) and bulk (α3) hydride (Also Different normal (H2gtD2) isotope effects in a1 and a3 population speeds)
=gt Absorption near minority sites (defects)
Isotope Population of the Absorbed Hydride States
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre 006 ML
Near-surface hydride (α1)
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
Bulk hydride (α3)
08
1
02
p=0
05
～ 4
Dominant transfer of pre-adsorbed H below the surface
(First observation)
=gt Absorption in regular terrace area ()
p=0~05
Only Pd(110) no lsquobypassingrsquo
p 1-p
lsquobypassingrsquo replacement
Recursive analysis of isotope composition
( )( 1) ( ) (1 )
( )( 1) ( ) (1 )
prepre pre
prepost post
N nN n N n p
N
N nN n N n p
N
Evaluation of lsquobypassingrsquo probability (p)
Stochastic Isotope Population Model for AbsorptionDesorption
(1)
(2)
(p) (1-p)
n+1th absorption eventpost
pre Npre(n)post Npost(n)
rarr uptake rarr desorption (microscopic reversibility)
Absorption mechanism Bypassing or Replacement
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
Bulk hydride (α3)
Near-surface hydride (α1)
08
1
02
05
08
1
02
p=0
05
p=0
Dominant absorption mechanism
Compatible Incompatible
p=0 Replacement p=1 Bypassing
Replacement
S Ohno M Wilde K Fukutani J Chem Phys submitted
What is the Rate Determining Step (RDS)
H2 absorption Eabs lt 01 eV[1 2]
H
H2
timesExperiment Prevailing model
Possible rate determining steps
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Ohno et al J Chem Phys submitted [3] Padama et al J Phys Soc Jpn 81 (2012) 114705 [4] Ferrin et al Surf Sci 606 (2012) 679
Emono = 03 ～ 06 eVMonatomic in-diffusion
HPd
Chemi-sorption
R
Potential Energy
-05 eV
-02 eV-01 eV
1)
2)
3)
RDS H2 Dissociation (at large H) or Concerted Penetration
[1] Rendulic Surf Sci 208 (1989) 404 [2] Groszlig ChemPhysChem 11 (2010) 1374 [3] Sakong ChemPhysChem 13 3467
bull H2 dissociation is non-activated (Ediss = 0) at bare Pd surfaces[1]
bull Dissociation becomes weakly activated (at high H-coverages)[2]
05 ML HPd(100)
Consider processes with activation energies compatible to Eabs (le01 eV)
rarr H2 dissociation (Ediss) concerted penetration (Ec-pen)
H2 dissociation at a H mono-vacancy
Ediss = 01 eV[23]
Excess H atom[2] (He)
Concerted penetration
Ec-pen asymp 006 eV[23]
He + Hs rarr Hs + Hss
bull The activation barrier of penetration can be drastically lowered when it occurs concertedly with stabilization of excess H atoms
Influence of Surface Structure on H2 dissociation (at large H)
Peculiarity of Pd(110) Terrace-related H2 absorption (not on Pd(111) and (100))[1 2]
- Possible explanation -
[1] Gdowski et al J Vac Sci Technol A 5 1103 (1987) [2] Okuyama et al Surf Sci 401 344 (1998)[3] Maringrtensson et al Phys Rev Lett 57 (1986) 2045 [4] Schmidt et al Phys Rev Lett 87 096103 (2001)[5] Ahmed et al Appl Surf Sci 257 10503 (2011) [6] Busnengo et al Phys Rev Lett 93 236103 (2004) (1x1)
constitute precursor statesfor H2 dissociation[4 5]
Top view Side view[5]
Step edge-like structures stabilize molecular H2 chemisorption states
step-likePd(322)
Ni(510)[3] Pd(210)[4] Pd(322)[5]
Theoretical prediction[6] H2 may exist at Pd(110)
bull H-vacancy-mediated dissociation vac = exp(-GsbkBT) = 2x10-8 at 145 K
bull Pabs max (Model) = Pdiss vac ltlt Pabs (Experiment) = Rabs2Zw = 5x10-4 at 145 K
bull =gt Direct gas phase H2 impact not sufficient =gt Involvement of mobile H2 precursors ()
Influence of Surface Structure on H vacancy generation
Defect-enhanced H2 absorption Terrace-related H2 absorption ( gt 24 x per site vs regular terraces) (peculiar vs Pd(100) (111) (311))
H2 dissociation may require H-vacancies
bull Rate of H-vacancy generation[1] Rvac = 1013 s-1 exp(-EssskBT) asymp 103 at 145 K
bull =gt enhanced at defects due to additional lsquoopennessrsquo May also stabilize H2
bull Widened penetration channels at defects and in troughs between paired Pd rows in
Pd(110)(1x2)-(PR)
Side view
(1x2)
Top view
Widened interstitial channels (in [001])
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705 Esss = 027 eV (110) cf Pd(111) (04 eV) Pd(100) (041 eV) (Ferrin)
Summary amp Conclusions
H2 absorption mechanism at Pd(110)-(1x2) (paired-row)
Two hydride states exist with different depth distributions
Two H absorption channels (defects + terrace Pd(110) only)
Hs is replaced (not bypassed) no simple in-diffusion Eabslt01 eV
RDS H2 dissociation (H-saturated Pd) or concerted penetration
Influence of Surface Structure H2 absorption enhanced by
ldquoOpenrdquo penetration channels (accelerate H-vacancy generation)
Stabilization of H2 precursors (at step edge-like structures)
D
H2H2
H
H2
α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
rarr Activation Energy H 1 003 eV
3 006 eV D 3 007 eV
Much smaller than expected for monatomic H surface-to-subsurface
diffusion (03~04 eV)
Arrhenius plot of1 3 population (Pa)
peak area vs exposure
lt01 eV
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
[5] Okuyama et al Surf Sci 401 (1998) 344[6] Ohno et al J Chem Phys submitted
Activation Energy Paradox
The actual reaction coordinate of H2 absorption
H
H2
Eabs lt 010 eV[5 6]
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705[2] Ferrin et al Surf Sci 606 679 (2012)[3] Nobuhara et al Surf Sci 566 703 (2004)[4] Ozawa et al J Phys Condens Matter 19 365214 (2007)
Prevailing H absorption model
Absorption activation
Experimental results
Emono = 03 ～ 06 eV[1-4]Monatomic in-diffusion
HPd
Chemi-sorption
Identify
R
Potential Energy
-05 eV
-02 eV-01 eV
Surface of particular interest Pd(110)
Pd(110) single crystal surface
Pd Well-known H absorbing metal
Excellent catalyst for olefin hydrogenation
(110) Single crystal Well-defined structure
Openness Surface atomic density ｰ 40 vs (111)
H-induced surface reconstruction ldquoProne to hydrogen absorptionrdquo[1]
[1] Christmann Prog Surf Sci 48 15 (1995)
_
[110]
[001]
H2 exposure
Pairing-row (P-R)reconstruction
Second-layer exposed Atomic step-like structure Lateral contraction in paired rows
Pd
(110)
Top view Side view
(1x2)
H2 D
eso
rptio
n s
ign
al
(TD
S)
600500400300200100
Temperature (K)
H2 Desorption signalCombine two hydrogen detection techniques
Experimental Approach TDS + NRA
① Thermal Desorption Spectroscopy (TDS)
rarr H2(D2) exposures at given Te desorption
rarr No of H species desorption activation energy
rarr lacks information on H location (onbelow surface)
H2 D
eso
rptio
n s
ign
al
(TD
S)
600500400300200100
Temperature (K)
Su
rface
ab
sorb
ed
H (N
RA
)
Surface-H signal (Hs) Subsurface-H signal (Hss)
Experimental
Ei=Eres -detector
N
probing depth
EigtEres
z(Ei)= (Ei-Eres)(dEdz)
[Habsorbed]
15N2+ ion beam
0
[Hsurface]
② Nuclear Reaction Analysis (NRA) via 1H(15N)12C (Eres=6385 MeV =18 keV)
rarr distinguishes surface-adsorbed from absorbed H (~2 nm depth resolution)
300 L H+H2 on Pd(100) at 100 K
15N ion energy [MeV]
637 638 639 640 641
ray
yiel
d [c
ts
C]
0
50
100
150
depth [nm]
-4 -2 0 2 4 6 8
15N ion energy (MeV)
Depth (nm)
-yi
eld
(ct
sC
)
M Wilde PRB 78 (2008) 114511
rarr achieves unambiguous TDS peak identifications
0 L (clean) 03 L 05 L
[1] Ledentu et al Surf Sci 411 (1998) 123 [2] Yoshinobu et al Phys Rev B 51 4529 (1995)
(1times1) (2times1) (1times2)
Surface Adsorption Phases (LEED amp TPD) HPd(110)
H2 exposure at Te = 130 K50 L ～
θ=15 MLθ=10 ML
_
[110]
[001] θ=0 ML
[1]
θ= ML
[2]
120
100
80
60
40
20
0Q
MS io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
120
100
80
60
40
20
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
β1 β2
α2Surface
1 L = 10-6 Torr s1 L = 10-6 Torr s
1 ML = 94 x 1014 atomscm2
1 ML = 94 x 1014 atomscm2
01 L03 L
08 L
03 L
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]400350300250200150
Temperature [K]
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
Surface
α1
α3
200
150
100
50
0
QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
600
500
400
300
200
100
0
-yi
eld
[co
un
ts
C]
20151050-5
Depth [nm]
64664464264063815
N ion energy [MeV]
10 L at 170 K 2000 L at 130 K 2000 L at 145 K
300
200
100
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
α1α3 α2
β2
β1
300
200
100
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
TPD
130 K
145 Kα3
NRA H Depth Distribution of Two Low-T TPD States
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
12 at
230 at
NRA
Absorbed hydrogenα1 Near surface α3 Bulk gt 50 nm
2000 L at Te = 130 K
α1 α3α2
β2
β1
surface
TPD feature Origin Depth extension
[H]avgVolume
ratioTe condition
α1 (170 K) Near surface hydride ～ 10 nm 23 at 30 lt 145 K
α3 (195 K) Bulk hydride gt 50 nm 12 at 2 lt 160 K
LEED NRA TPD Identification of H2Pd(110) desorption features
600
500
400
300
200
100
0
-yi
eld
[co
un
ts
C]
20151050-5
Depth [nm]
64664464264063815
N ion energy [MeV]
10 L at 170 K 2000 L at 130 K 2000 L at 145 K
=gt First revelation at Pd(110) TWO absorbed hydride states
300
200
100
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150Temperature [K]
TPD NRA
S Ohno M Wilde K Fukutani J Chem Phys submitted
NEW
pre postD2 10 L rarr H2 1000 L α1
α3
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
Te = 115 K
rArr Two separate absorption pathways exist ()
Near-surface hydride Bulk hydride
Investigation of the H2 Absorption Mechanism
Absorption experiments with isotope labeled surface hydrogen
Analysis of isotope populations (TPD)
=gt Clear difference between near-surface (α1) and bulk (α3) hydride (Also Different normal (H2gtD2) isotope effects in a1 and a3 population speeds)
=gt Absorption near minority sites (defects)
Isotope Population of the Absorbed Hydride States
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre 006 ML
Near-surface hydride (α1)
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
Bulk hydride (α3)
08
1
02
p=0
05
～ 4
Dominant transfer of pre-adsorbed H below the surface
(First observation)
=gt Absorption in regular terrace area ()
p=0~05
Only Pd(110) no lsquobypassingrsquo
p 1-p
lsquobypassingrsquo replacement
Recursive analysis of isotope composition
( )( 1) ( ) (1 )
( )( 1) ( ) (1 )
prepre pre
prepost post
N nN n N n p
N
N nN n N n p
N
Evaluation of lsquobypassingrsquo probability (p)
Stochastic Isotope Population Model for AbsorptionDesorption
(1)
(2)
(p) (1-p)
n+1th absorption eventpost
pre Npre(n)post Npost(n)
rarr uptake rarr desorption (microscopic reversibility)
Absorption mechanism Bypassing or Replacement
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
Bulk hydride (α3)
Near-surface hydride (α1)
08
1
02
05
08
1
02
p=0
05
p=0
Dominant absorption mechanism
Compatible Incompatible
p=0 Replacement p=1 Bypassing
Replacement
S Ohno M Wilde K Fukutani J Chem Phys submitted
What is the Rate Determining Step (RDS)
H2 absorption Eabs lt 01 eV[1 2]
H
H2
timesExperiment Prevailing model
Possible rate determining steps
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Ohno et al J Chem Phys submitted [3] Padama et al J Phys Soc Jpn 81 (2012) 114705 [4] Ferrin et al Surf Sci 606 (2012) 679
Emono = 03 ～ 06 eVMonatomic in-diffusion
HPd
Chemi-sorption
R
Potential Energy
-05 eV
-02 eV-01 eV
1)
2)
3)
RDS H2 Dissociation (at large H) or Concerted Penetration
[1] Rendulic Surf Sci 208 (1989) 404 [2] Groszlig ChemPhysChem 11 (2010) 1374 [3] Sakong ChemPhysChem 13 3467
bull H2 dissociation is non-activated (Ediss = 0) at bare Pd surfaces[1]
bull Dissociation becomes weakly activated (at high H-coverages)[2]
05 ML HPd(100)
Consider processes with activation energies compatible to Eabs (le01 eV)
rarr H2 dissociation (Ediss) concerted penetration (Ec-pen)
H2 dissociation at a H mono-vacancy
Ediss = 01 eV[23]
Excess H atom[2] (He)
Concerted penetration
Ec-pen asymp 006 eV[23]
He + Hs rarr Hs + Hss
bull The activation barrier of penetration can be drastically lowered when it occurs concertedly with stabilization of excess H atoms
Influence of Surface Structure on H2 dissociation (at large H)
Peculiarity of Pd(110) Terrace-related H2 absorption (not on Pd(111) and (100))[1 2]
- Possible explanation -
[1] Gdowski et al J Vac Sci Technol A 5 1103 (1987) [2] Okuyama et al Surf Sci 401 344 (1998)[3] Maringrtensson et al Phys Rev Lett 57 (1986) 2045 [4] Schmidt et al Phys Rev Lett 87 096103 (2001)[5] Ahmed et al Appl Surf Sci 257 10503 (2011) [6] Busnengo et al Phys Rev Lett 93 236103 (2004) (1x1)
constitute precursor statesfor H2 dissociation[4 5]
Top view Side view[5]
Step edge-like structures stabilize molecular H2 chemisorption states
step-likePd(322)
Ni(510)[3] Pd(210)[4] Pd(322)[5]
Theoretical prediction[6] H2 may exist at Pd(110)
bull H-vacancy-mediated dissociation vac = exp(-GsbkBT) = 2x10-8 at 145 K
bull Pabs max (Model) = Pdiss vac ltlt Pabs (Experiment) = Rabs2Zw = 5x10-4 at 145 K
bull =gt Direct gas phase H2 impact not sufficient =gt Involvement of mobile H2 precursors ()
Influence of Surface Structure on H vacancy generation
Defect-enhanced H2 absorption Terrace-related H2 absorption ( gt 24 x per site vs regular terraces) (peculiar vs Pd(100) (111) (311))
H2 dissociation may require H-vacancies
bull Rate of H-vacancy generation[1] Rvac = 1013 s-1 exp(-EssskBT) asymp 103 at 145 K
bull =gt enhanced at defects due to additional lsquoopennessrsquo May also stabilize H2
bull Widened penetration channels at defects and in troughs between paired Pd rows in
Pd(110)(1x2)-(PR)
Side view
(1x2)
Top view
Widened interstitial channels (in [001])
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705 Esss = 027 eV (110) cf Pd(111) (04 eV) Pd(100) (041 eV) (Ferrin)
Summary amp Conclusions
H2 absorption mechanism at Pd(110)-(1x2) (paired-row)
Two hydride states exist with different depth distributions
Two H absorption channels (defects + terrace Pd(110) only)
Hs is replaced (not bypassed) no simple in-diffusion Eabslt01 eV
RDS H2 dissociation (H-saturated Pd) or concerted penetration
Influence of Surface Structure H2 absorption enhanced by
ldquoOpenrdquo penetration channels (accelerate H-vacancy generation)
Stabilization of H2 precursors (at step edge-like structures)
D
H2H2
H
H2
α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
rarr Activation Energy H 1 003 eV
3 006 eV D 3 007 eV
Much smaller than expected for monatomic H surface-to-subsurface
diffusion (03~04 eV)
Arrhenius plot of1 3 population (Pa)
peak area vs exposure
lt01 eV
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
Surface of particular interest Pd(110)
Pd(110) single crystal surface
Pd Well-known H absorbing metal
Excellent catalyst for olefin hydrogenation
(110) Single crystal Well-defined structure
Openness Surface atomic density ｰ 40 vs (111)
H-induced surface reconstruction ldquoProne to hydrogen absorptionrdquo[1]
[1] Christmann Prog Surf Sci 48 15 (1995)
_
[110]
[001]
H2 exposure
Pairing-row (P-R)reconstruction
Second-layer exposed Atomic step-like structure Lateral contraction in paired rows
Pd
(110)
Top view Side view
(1x2)
H2 D
eso
rptio
n s
ign
al
(TD
S)
600500400300200100
Temperature (K)
H2 Desorption signalCombine two hydrogen detection techniques
Experimental Approach TDS + NRA
① Thermal Desorption Spectroscopy (TDS)
rarr H2(D2) exposures at given Te desorption
rarr No of H species desorption activation energy
rarr lacks information on H location (onbelow surface)
H2 D
eso
rptio
n s
ign
al
(TD
S)
600500400300200100
Temperature (K)
Su
rface
ab
sorb
ed
H (N
RA
)
Surface-H signal (Hs) Subsurface-H signal (Hss)
Experimental
Ei=Eres -detector
N
probing depth
EigtEres
z(Ei)= (Ei-Eres)(dEdz)
[Habsorbed]
15N2+ ion beam
0
[Hsurface]
② Nuclear Reaction Analysis (NRA) via 1H(15N)12C (Eres=6385 MeV =18 keV)
rarr distinguishes surface-adsorbed from absorbed H (~2 nm depth resolution)
300 L H+H2 on Pd(100) at 100 K
15N ion energy [MeV]
637 638 639 640 641
ray
yiel
d [c
ts
C]
0
50
100
150
depth [nm]
-4 -2 0 2 4 6 8
15N ion energy (MeV)
Depth (nm)
-yi
eld
(ct
sC
)
M Wilde PRB 78 (2008) 114511
rarr achieves unambiguous TDS peak identifications
0 L (clean) 03 L 05 L
[1] Ledentu et al Surf Sci 411 (1998) 123 [2] Yoshinobu et al Phys Rev B 51 4529 (1995)
(1times1) (2times1) (1times2)
Surface Adsorption Phases (LEED amp TPD) HPd(110)
H2 exposure at Te = 130 K50 L ～
θ=15 MLθ=10 ML
_
[110]
[001] θ=0 ML
[1]
θ= ML
[2]
120
100
80
60
40
20
0Q
MS io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
120
100
80
60
40
20
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
β1 β2
α2Surface
1 L = 10-6 Torr s1 L = 10-6 Torr s
1 ML = 94 x 1014 atomscm2
1 ML = 94 x 1014 atomscm2
01 L03 L
08 L
03 L
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]400350300250200150
Temperature [K]
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
Surface
α1
α3
200
150
100
50
0
QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
600
500
400
300
200
100
0
-yi
eld
[co
un
ts
C]
20151050-5
Depth [nm]
64664464264063815
N ion energy [MeV]
10 L at 170 K 2000 L at 130 K 2000 L at 145 K
300
200
100
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
α1α3 α2
β2
β1
300
200
100
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
TPD
130 K
145 Kα3
NRA H Depth Distribution of Two Low-T TPD States
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
12 at
230 at
NRA
Absorbed hydrogenα1 Near surface α3 Bulk gt 50 nm
2000 L at Te = 130 K
α1 α3α2
β2
β1
surface
TPD feature Origin Depth extension
[H]avgVolume
ratioTe condition
α1 (170 K) Near surface hydride ～ 10 nm 23 at 30 lt 145 K
α3 (195 K) Bulk hydride gt 50 nm 12 at 2 lt 160 K
LEED NRA TPD Identification of H2Pd(110) desorption features
600
500
400
300
200
100
0
-yi
eld
[co
un
ts
C]
20151050-5
Depth [nm]
64664464264063815
N ion energy [MeV]
10 L at 170 K 2000 L at 130 K 2000 L at 145 K
=gt First revelation at Pd(110) TWO absorbed hydride states
300
200
100
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150Temperature [K]
TPD NRA
S Ohno M Wilde K Fukutani J Chem Phys submitted
NEW
pre postD2 10 L rarr H2 1000 L α1
α3
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
Te = 115 K
rArr Two separate absorption pathways exist ()
Near-surface hydride Bulk hydride
Investigation of the H2 Absorption Mechanism
Absorption experiments with isotope labeled surface hydrogen
Analysis of isotope populations (TPD)
=gt Clear difference between near-surface (α1) and bulk (α3) hydride (Also Different normal (H2gtD2) isotope effects in a1 and a3 population speeds)
=gt Absorption near minority sites (defects)
Isotope Population of the Absorbed Hydride States
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre 006 ML
Near-surface hydride (α1)
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
Bulk hydride (α3)
08
1
02
p=0
05
～ 4
Dominant transfer of pre-adsorbed H below the surface
(First observation)
=gt Absorption in regular terrace area ()
p=0~05
Only Pd(110) no lsquobypassingrsquo
p 1-p
lsquobypassingrsquo replacement
Recursive analysis of isotope composition
( )( 1) ( ) (1 )
( )( 1) ( ) (1 )
prepre pre
prepost post
N nN n N n p
N
N nN n N n p
N
Evaluation of lsquobypassingrsquo probability (p)
Stochastic Isotope Population Model for AbsorptionDesorption
(1)
(2)
(p) (1-p)
n+1th absorption eventpost
pre Npre(n)post Npost(n)
rarr uptake rarr desorption (microscopic reversibility)
Absorption mechanism Bypassing or Replacement
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
Bulk hydride (α3)
Near-surface hydride (α1)
08
1
02
05
08
1
02
p=0
05
p=0
Dominant absorption mechanism
Compatible Incompatible
p=0 Replacement p=1 Bypassing
Replacement
S Ohno M Wilde K Fukutani J Chem Phys submitted
What is the Rate Determining Step (RDS)
H2 absorption Eabs lt 01 eV[1 2]
H
H2
timesExperiment Prevailing model
Possible rate determining steps
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Ohno et al J Chem Phys submitted [3] Padama et al J Phys Soc Jpn 81 (2012) 114705 [4] Ferrin et al Surf Sci 606 (2012) 679
Emono = 03 ～ 06 eVMonatomic in-diffusion
HPd
Chemi-sorption
R
Potential Energy
-05 eV
-02 eV-01 eV
1)
2)
3)
RDS H2 Dissociation (at large H) or Concerted Penetration
[1] Rendulic Surf Sci 208 (1989) 404 [2] Groszlig ChemPhysChem 11 (2010) 1374 [3] Sakong ChemPhysChem 13 3467
bull H2 dissociation is non-activated (Ediss = 0) at bare Pd surfaces[1]
bull Dissociation becomes weakly activated (at high H-coverages)[2]
05 ML HPd(100)
Consider processes with activation energies compatible to Eabs (le01 eV)
rarr H2 dissociation (Ediss) concerted penetration (Ec-pen)
H2 dissociation at a H mono-vacancy
Ediss = 01 eV[23]
Excess H atom[2] (He)
Concerted penetration
Ec-pen asymp 006 eV[23]
He + Hs rarr Hs + Hss
bull The activation barrier of penetration can be drastically lowered when it occurs concertedly with stabilization of excess H atoms
Influence of Surface Structure on H2 dissociation (at large H)
Peculiarity of Pd(110) Terrace-related H2 absorption (not on Pd(111) and (100))[1 2]
- Possible explanation -
[1] Gdowski et al J Vac Sci Technol A 5 1103 (1987) [2] Okuyama et al Surf Sci 401 344 (1998)[3] Maringrtensson et al Phys Rev Lett 57 (1986) 2045 [4] Schmidt et al Phys Rev Lett 87 096103 (2001)[5] Ahmed et al Appl Surf Sci 257 10503 (2011) [6] Busnengo et al Phys Rev Lett 93 236103 (2004) (1x1)
constitute precursor statesfor H2 dissociation[4 5]
Top view Side view[5]
Step edge-like structures stabilize molecular H2 chemisorption states
step-likePd(322)
Ni(510)[3] Pd(210)[4] Pd(322)[5]
Theoretical prediction[6] H2 may exist at Pd(110)
bull H-vacancy-mediated dissociation vac = exp(-GsbkBT) = 2x10-8 at 145 K
bull Pabs max (Model) = Pdiss vac ltlt Pabs (Experiment) = Rabs2Zw = 5x10-4 at 145 K
bull =gt Direct gas phase H2 impact not sufficient =gt Involvement of mobile H2 precursors ()
Influence of Surface Structure on H vacancy generation
Defect-enhanced H2 absorption Terrace-related H2 absorption ( gt 24 x per site vs regular terraces) (peculiar vs Pd(100) (111) (311))
H2 dissociation may require H-vacancies
bull Rate of H-vacancy generation[1] Rvac = 1013 s-1 exp(-EssskBT) asymp 103 at 145 K
bull =gt enhanced at defects due to additional lsquoopennessrsquo May also stabilize H2
bull Widened penetration channels at defects and in troughs between paired Pd rows in
Pd(110)(1x2)-(PR)
Side view
(1x2)
Top view
Widened interstitial channels (in [001])
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705 Esss = 027 eV (110) cf Pd(111) (04 eV) Pd(100) (041 eV) (Ferrin)
Summary amp Conclusions
H2 absorption mechanism at Pd(110)-(1x2) (paired-row)
Two hydride states exist with different depth distributions
Two H absorption channels (defects + terrace Pd(110) only)
Hs is replaced (not bypassed) no simple in-diffusion Eabslt01 eV
RDS H2 dissociation (H-saturated Pd) or concerted penetration
Influence of Surface Structure H2 absorption enhanced by
ldquoOpenrdquo penetration channels (accelerate H-vacancy generation)
Stabilization of H2 precursors (at step edge-like structures)
D
H2H2
H
H2
α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
rarr Activation Energy H 1 003 eV
3 006 eV D 3 007 eV
Much smaller than expected for monatomic H surface-to-subsurface
diffusion (03~04 eV)
Arrhenius plot of1 3 population (Pa)
peak area vs exposure
lt01 eV
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
H2 D
eso
rptio
n s
ign
al
(TD
S)
600500400300200100
Temperature (K)
H2 Desorption signalCombine two hydrogen detection techniques
Experimental Approach TDS + NRA
① Thermal Desorption Spectroscopy (TDS)
rarr H2(D2) exposures at given Te desorption
rarr No of H species desorption activation energy
rarr lacks information on H location (onbelow surface)
H2 D
eso
rptio
n s
ign
al
(TD
S)
600500400300200100
Temperature (K)
Su
rface
ab
sorb
ed
H (N
RA
)
Surface-H signal (Hs) Subsurface-H signal (Hss)
Experimental
Ei=Eres -detector
N
probing depth
EigtEres
z(Ei)= (Ei-Eres)(dEdz)
[Habsorbed]
15N2+ ion beam
0
[Hsurface]
② Nuclear Reaction Analysis (NRA) via 1H(15N)12C (Eres=6385 MeV =18 keV)
rarr distinguishes surface-adsorbed from absorbed H (~2 nm depth resolution)
300 L H+H2 on Pd(100) at 100 K
15N ion energy [MeV]
637 638 639 640 641
ray
yiel
d [c
ts
C]
0
50
100
150
depth [nm]
-4 -2 0 2 4 6 8
15N ion energy (MeV)
Depth (nm)
-yi
eld
(ct
sC
)
M Wilde PRB 78 (2008) 114511
rarr achieves unambiguous TDS peak identifications
0 L (clean) 03 L 05 L
[1] Ledentu et al Surf Sci 411 (1998) 123 [2] Yoshinobu et al Phys Rev B 51 4529 (1995)
(1times1) (2times1) (1times2)
Surface Adsorption Phases (LEED amp TPD) HPd(110)
H2 exposure at Te = 130 K50 L ～
θ=15 MLθ=10 ML
_
[110]
[001] θ=0 ML
[1]
θ= ML
[2]
120
100
80
60
40
20
0Q
MS io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
120
100
80
60
40
20
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
β1 β2
α2Surface
1 L = 10-6 Torr s1 L = 10-6 Torr s
1 ML = 94 x 1014 atomscm2
1 ML = 94 x 1014 atomscm2
01 L03 L
08 L
03 L
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]400350300250200150
Temperature [K]
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
Surface
α1
α3
200
150
100
50
0
QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
600
500
400
300
200
100
0
-yi
eld
[co
un
ts
C]
20151050-5
Depth [nm]
64664464264063815
N ion energy [MeV]
10 L at 170 K 2000 L at 130 K 2000 L at 145 K
300
200
100
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
α1α3 α2
β2
β1
300
200
100
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
TPD
130 K
145 Kα3
NRA H Depth Distribution of Two Low-T TPD States
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
12 at
230 at
NRA
Absorbed hydrogenα1 Near surface α3 Bulk gt 50 nm
2000 L at Te = 130 K
α1 α3α2
β2
β1
surface
TPD feature Origin Depth extension
[H]avgVolume
ratioTe condition
α1 (170 K) Near surface hydride ～ 10 nm 23 at 30 lt 145 K
α3 (195 K) Bulk hydride gt 50 nm 12 at 2 lt 160 K
LEED NRA TPD Identification of H2Pd(110) desorption features
600
500
400
300
200
100
0
-yi
eld
[co
un
ts
C]
20151050-5
Depth [nm]
64664464264063815
N ion energy [MeV]
10 L at 170 K 2000 L at 130 K 2000 L at 145 K
=gt First revelation at Pd(110) TWO absorbed hydride states
300
200
100
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150Temperature [K]
TPD NRA
S Ohno M Wilde K Fukutani J Chem Phys submitted
NEW
pre postD2 10 L rarr H2 1000 L α1
α3
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
Te = 115 K
rArr Two separate absorption pathways exist ()
Near-surface hydride Bulk hydride
Investigation of the H2 Absorption Mechanism
Absorption experiments with isotope labeled surface hydrogen
Analysis of isotope populations (TPD)
=gt Clear difference between near-surface (α1) and bulk (α3) hydride (Also Different normal (H2gtD2) isotope effects in a1 and a3 population speeds)
=gt Absorption near minority sites (defects)
Isotope Population of the Absorbed Hydride States
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre 006 ML
Near-surface hydride (α1)
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
Bulk hydride (α3)
08
1
02
p=0
05
～ 4
Dominant transfer of pre-adsorbed H below the surface
(First observation)
=gt Absorption in regular terrace area ()
p=0~05
Only Pd(110) no lsquobypassingrsquo
p 1-p
lsquobypassingrsquo replacement
Recursive analysis of isotope composition
( )( 1) ( ) (1 )
( )( 1) ( ) (1 )
prepre pre
prepost post
N nN n N n p
N
N nN n N n p
N
Evaluation of lsquobypassingrsquo probability (p)
Stochastic Isotope Population Model for AbsorptionDesorption
(1)
(2)
(p) (1-p)
n+1th absorption eventpost
pre Npre(n)post Npost(n)
rarr uptake rarr desorption (microscopic reversibility)
Absorption mechanism Bypassing or Replacement
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
Bulk hydride (α3)
Near-surface hydride (α1)
08
1
02
05
08
1
02
p=0
05
p=0
Dominant absorption mechanism
Compatible Incompatible
p=0 Replacement p=1 Bypassing
Replacement
S Ohno M Wilde K Fukutani J Chem Phys submitted
What is the Rate Determining Step (RDS)
H2 absorption Eabs lt 01 eV[1 2]
H
H2
timesExperiment Prevailing model
Possible rate determining steps
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Ohno et al J Chem Phys submitted [3] Padama et al J Phys Soc Jpn 81 (2012) 114705 [4] Ferrin et al Surf Sci 606 (2012) 679
Emono = 03 ～ 06 eVMonatomic in-diffusion
HPd
Chemi-sorption
R
Potential Energy
-05 eV
-02 eV-01 eV
1)
2)
3)
RDS H2 Dissociation (at large H) or Concerted Penetration
[1] Rendulic Surf Sci 208 (1989) 404 [2] Groszlig ChemPhysChem 11 (2010) 1374 [3] Sakong ChemPhysChem 13 3467
bull H2 dissociation is non-activated (Ediss = 0) at bare Pd surfaces[1]
bull Dissociation becomes weakly activated (at high H-coverages)[2]
05 ML HPd(100)
Consider processes with activation energies compatible to Eabs (le01 eV)
rarr H2 dissociation (Ediss) concerted penetration (Ec-pen)
H2 dissociation at a H mono-vacancy
Ediss = 01 eV[23]
Excess H atom[2] (He)
Concerted penetration
Ec-pen asymp 006 eV[23]
He + Hs rarr Hs + Hss
bull The activation barrier of penetration can be drastically lowered when it occurs concertedly with stabilization of excess H atoms
Influence of Surface Structure on H2 dissociation (at large H)
Peculiarity of Pd(110) Terrace-related H2 absorption (not on Pd(111) and (100))[1 2]
- Possible explanation -
[1] Gdowski et al J Vac Sci Technol A 5 1103 (1987) [2] Okuyama et al Surf Sci 401 344 (1998)[3] Maringrtensson et al Phys Rev Lett 57 (1986) 2045 [4] Schmidt et al Phys Rev Lett 87 096103 (2001)[5] Ahmed et al Appl Surf Sci 257 10503 (2011) [6] Busnengo et al Phys Rev Lett 93 236103 (2004) (1x1)
constitute precursor statesfor H2 dissociation[4 5]
Top view Side view[5]
Step edge-like structures stabilize molecular H2 chemisorption states
step-likePd(322)
Ni(510)[3] Pd(210)[4] Pd(322)[5]
Theoretical prediction[6] H2 may exist at Pd(110)
bull H-vacancy-mediated dissociation vac = exp(-GsbkBT) = 2x10-8 at 145 K
bull Pabs max (Model) = Pdiss vac ltlt Pabs (Experiment) = Rabs2Zw = 5x10-4 at 145 K
bull =gt Direct gas phase H2 impact not sufficient =gt Involvement of mobile H2 precursors ()
Influence of Surface Structure on H vacancy generation
Defect-enhanced H2 absorption Terrace-related H2 absorption ( gt 24 x per site vs regular terraces) (peculiar vs Pd(100) (111) (311))
H2 dissociation may require H-vacancies
bull Rate of H-vacancy generation[1] Rvac = 1013 s-1 exp(-EssskBT) asymp 103 at 145 K
bull =gt enhanced at defects due to additional lsquoopennessrsquo May also stabilize H2
bull Widened penetration channels at defects and in troughs between paired Pd rows in
Pd(110)(1x2)-(PR)
Side view
(1x2)
Top view
Widened interstitial channels (in [001])
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705 Esss = 027 eV (110) cf Pd(111) (04 eV) Pd(100) (041 eV) (Ferrin)
Summary amp Conclusions
H2 absorption mechanism at Pd(110)-(1x2) (paired-row)
Two hydride states exist with different depth distributions
Two H absorption channels (defects + terrace Pd(110) only)
Hs is replaced (not bypassed) no simple in-diffusion Eabslt01 eV
RDS H2 dissociation (H-saturated Pd) or concerted penetration
Influence of Surface Structure H2 absorption enhanced by
ldquoOpenrdquo penetration channels (accelerate H-vacancy generation)
Stabilization of H2 precursors (at step edge-like structures)
D
H2H2
H
H2
α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
rarr Activation Energy H 1 003 eV
3 006 eV D 3 007 eV
Much smaller than expected for monatomic H surface-to-subsurface
diffusion (03~04 eV)
Arrhenius plot of1 3 population (Pa)
peak area vs exposure
lt01 eV
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
0 L (clean) 03 L 05 L
[1] Ledentu et al Surf Sci 411 (1998) 123 [2] Yoshinobu et al Phys Rev B 51 4529 (1995)
(1times1) (2times1) (1times2)
Surface Adsorption Phases (LEED amp TPD) HPd(110)
H2 exposure at Te = 130 K50 L ～
θ=15 MLθ=10 ML
_
[110]
[001] θ=0 ML
[1]
θ= ML
[2]
120
100
80
60
40
20
0Q
MS io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
120
100
80
60
40
20
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
β1 β2
α2Surface
1 L = 10-6 Torr s1 L = 10-6 Torr s
1 ML = 94 x 1014 atomscm2
1 ML = 94 x 1014 atomscm2
01 L03 L
08 L
03 L
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]400350300250200150
Temperature [K]
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
Surface
α1
α3
200
150
100
50
0
QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
600
500
400
300
200
100
0
-yi
eld
[co
un
ts
C]
20151050-5
Depth [nm]
64664464264063815
N ion energy [MeV]
10 L at 170 K 2000 L at 130 K 2000 L at 145 K
300
200
100
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
α1α3 α2
β2
β1
300
200
100
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
TPD
130 K
145 Kα3
NRA H Depth Distribution of Two Low-T TPD States
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
12 at
230 at
NRA
Absorbed hydrogenα1 Near surface α3 Bulk gt 50 nm
2000 L at Te = 130 K
α1 α3α2
β2
β1
surface
TPD feature Origin Depth extension
[H]avgVolume
ratioTe condition
α1 (170 K) Near surface hydride ～ 10 nm 23 at 30 lt 145 K
α3 (195 K) Bulk hydride gt 50 nm 12 at 2 lt 160 K
LEED NRA TPD Identification of H2Pd(110) desorption features
600
500
400
300
200
100
0
-yi
eld
[co
un
ts
C]
20151050-5
Depth [nm]
64664464264063815
N ion energy [MeV]
10 L at 170 K 2000 L at 130 K 2000 L at 145 K
=gt First revelation at Pd(110) TWO absorbed hydride states
300
200
100
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150Temperature [K]
TPD NRA
S Ohno M Wilde K Fukutani J Chem Phys submitted
NEW
pre postD2 10 L rarr H2 1000 L α1
α3
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
Te = 115 K
rArr Two separate absorption pathways exist ()
Near-surface hydride Bulk hydride
Investigation of the H2 Absorption Mechanism
Absorption experiments with isotope labeled surface hydrogen
Analysis of isotope populations (TPD)
=gt Clear difference between near-surface (α1) and bulk (α3) hydride (Also Different normal (H2gtD2) isotope effects in a1 and a3 population speeds)
=gt Absorption near minority sites (defects)
Isotope Population of the Absorbed Hydride States
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre 006 ML
Near-surface hydride (α1)
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
Bulk hydride (α3)
08
1
02
p=0
05
～ 4
Dominant transfer of pre-adsorbed H below the surface
(First observation)
=gt Absorption in regular terrace area ()
p=0~05
Only Pd(110) no lsquobypassingrsquo
p 1-p
lsquobypassingrsquo replacement
Recursive analysis of isotope composition
( )( 1) ( ) (1 )
( )( 1) ( ) (1 )
prepre pre
prepost post
N nN n N n p
N
N nN n N n p
N
Evaluation of lsquobypassingrsquo probability (p)
Stochastic Isotope Population Model for AbsorptionDesorption
(1)
(2)
(p) (1-p)
n+1th absorption eventpost
pre Npre(n)post Npost(n)
rarr uptake rarr desorption (microscopic reversibility)
Absorption mechanism Bypassing or Replacement
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
Bulk hydride (α3)
Near-surface hydride (α1)
08
1
02
05
08
1
02
p=0
05
p=0
Dominant absorption mechanism
Compatible Incompatible
p=0 Replacement p=1 Bypassing
Replacement
S Ohno M Wilde K Fukutani J Chem Phys submitted
What is the Rate Determining Step (RDS)
H2 absorption Eabs lt 01 eV[1 2]
H
H2
timesExperiment Prevailing model
Possible rate determining steps
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Ohno et al J Chem Phys submitted [3] Padama et al J Phys Soc Jpn 81 (2012) 114705 [4] Ferrin et al Surf Sci 606 (2012) 679
Emono = 03 ～ 06 eVMonatomic in-diffusion
HPd
Chemi-sorption
R
Potential Energy
-05 eV
-02 eV-01 eV
1)
2)
3)
RDS H2 Dissociation (at large H) or Concerted Penetration
[1] Rendulic Surf Sci 208 (1989) 404 [2] Groszlig ChemPhysChem 11 (2010) 1374 [3] Sakong ChemPhysChem 13 3467
bull H2 dissociation is non-activated (Ediss = 0) at bare Pd surfaces[1]
bull Dissociation becomes weakly activated (at high H-coverages)[2]
05 ML HPd(100)
Consider processes with activation energies compatible to Eabs (le01 eV)
rarr H2 dissociation (Ediss) concerted penetration (Ec-pen)
H2 dissociation at a H mono-vacancy
Ediss = 01 eV[23]
Excess H atom[2] (He)
Concerted penetration
Ec-pen asymp 006 eV[23]
He + Hs rarr Hs + Hss
bull The activation barrier of penetration can be drastically lowered when it occurs concertedly with stabilization of excess H atoms
Influence of Surface Structure on H2 dissociation (at large H)
Peculiarity of Pd(110) Terrace-related H2 absorption (not on Pd(111) and (100))[1 2]
- Possible explanation -
[1] Gdowski et al J Vac Sci Technol A 5 1103 (1987) [2] Okuyama et al Surf Sci 401 344 (1998)[3] Maringrtensson et al Phys Rev Lett 57 (1986) 2045 [4] Schmidt et al Phys Rev Lett 87 096103 (2001)[5] Ahmed et al Appl Surf Sci 257 10503 (2011) [6] Busnengo et al Phys Rev Lett 93 236103 (2004) (1x1)
constitute precursor statesfor H2 dissociation[4 5]
Top view Side view[5]
Step edge-like structures stabilize molecular H2 chemisorption states
step-likePd(322)
Ni(510)[3] Pd(210)[4] Pd(322)[5]
Theoretical prediction[6] H2 may exist at Pd(110)
bull H-vacancy-mediated dissociation vac = exp(-GsbkBT) = 2x10-8 at 145 K
bull Pabs max (Model) = Pdiss vac ltlt Pabs (Experiment) = Rabs2Zw = 5x10-4 at 145 K
bull =gt Direct gas phase H2 impact not sufficient =gt Involvement of mobile H2 precursors ()
Influence of Surface Structure on H vacancy generation
Defect-enhanced H2 absorption Terrace-related H2 absorption ( gt 24 x per site vs regular terraces) (peculiar vs Pd(100) (111) (311))
H2 dissociation may require H-vacancies
bull Rate of H-vacancy generation[1] Rvac = 1013 s-1 exp(-EssskBT) asymp 103 at 145 K
bull =gt enhanced at defects due to additional lsquoopennessrsquo May also stabilize H2
bull Widened penetration channels at defects and in troughs between paired Pd rows in
Pd(110)(1x2)-(PR)
Side view
(1x2)
Top view
Widened interstitial channels (in [001])
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705 Esss = 027 eV (110) cf Pd(111) (04 eV) Pd(100) (041 eV) (Ferrin)
Summary amp Conclusions
H2 absorption mechanism at Pd(110)-(1x2) (paired-row)
Two hydride states exist with different depth distributions
Two H absorption channels (defects + terrace Pd(110) only)
Hs is replaced (not bypassed) no simple in-diffusion Eabslt01 eV
RDS H2 dissociation (H-saturated Pd) or concerted penetration
Influence of Surface Structure H2 absorption enhanced by
ldquoOpenrdquo penetration channels (accelerate H-vacancy generation)
Stabilization of H2 precursors (at step edge-like structures)
D
H2H2
H
H2
α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
rarr Activation Energy H 1 003 eV
3 006 eV D 3 007 eV
Much smaller than expected for monatomic H surface-to-subsurface
diffusion (03~04 eV)
Arrhenius plot of1 3 population (Pa)
peak area vs exposure
lt01 eV
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
600
500
400
300
200
100
0
-yi
eld
[co
un
ts
C]
20151050-5
Depth [nm]
64664464264063815
N ion energy [MeV]
10 L at 170 K 2000 L at 130 K 2000 L at 145 K
300
200
100
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
α1α3 α2
β2
β1
300
200
100
0
QM
S io
n cu
rren
t [a
u]
400350300250200150
Temperature [K]
TPD
130 K
145 Kα3
NRA H Depth Distribution of Two Low-T TPD States
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
12 at
230 at
NRA
Absorbed hydrogenα1 Near surface α3 Bulk gt 50 nm
2000 L at Te = 130 K
α1 α3α2
β2
β1
surface
TPD feature Origin Depth extension
[H]avgVolume
ratioTe condition
α1 (170 K) Near surface hydride ～ 10 nm 23 at 30 lt 145 K
α3 (195 K) Bulk hydride gt 50 nm 12 at 2 lt 160 K
LEED NRA TPD Identification of H2Pd(110) desorption features
600
500
400
300
200
100
0
-yi
eld
[co
un
ts
C]
20151050-5
Depth [nm]
64664464264063815
N ion energy [MeV]
10 L at 170 K 2000 L at 130 K 2000 L at 145 K
=gt First revelation at Pd(110) TWO absorbed hydride states
300
200
100
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150Temperature [K]
TPD NRA
S Ohno M Wilde K Fukutani J Chem Phys submitted
NEW
pre postD2 10 L rarr H2 1000 L α1
α3
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
Te = 115 K
rArr Two separate absorption pathways exist ()
Near-surface hydride Bulk hydride
Investigation of the H2 Absorption Mechanism
Absorption experiments with isotope labeled surface hydrogen
Analysis of isotope populations (TPD)
=gt Clear difference between near-surface (α1) and bulk (α3) hydride (Also Different normal (H2gtD2) isotope effects in a1 and a3 population speeds)
=gt Absorption near minority sites (defects)
Isotope Population of the Absorbed Hydride States
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre 006 ML
Near-surface hydride (α1)
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
Bulk hydride (α3)
08
1
02
p=0
05
～ 4
Dominant transfer of pre-adsorbed H below the surface
(First observation)
=gt Absorption in regular terrace area ()
p=0~05
Only Pd(110) no lsquobypassingrsquo
p 1-p
lsquobypassingrsquo replacement
Recursive analysis of isotope composition
( )( 1) ( ) (1 )
( )( 1) ( ) (1 )
prepre pre
prepost post
N nN n N n p
N
N nN n N n p
N
Evaluation of lsquobypassingrsquo probability (p)
Stochastic Isotope Population Model for AbsorptionDesorption
(1)
(2)
(p) (1-p)
n+1th absorption eventpost
pre Npre(n)post Npost(n)
rarr uptake rarr desorption (microscopic reversibility)
Absorption mechanism Bypassing or Replacement
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
Bulk hydride (α3)
Near-surface hydride (α1)
08
1
02
05
08
1
02
p=0
05
p=0
Dominant absorption mechanism
Compatible Incompatible
p=0 Replacement p=1 Bypassing
Replacement
S Ohno M Wilde K Fukutani J Chem Phys submitted
What is the Rate Determining Step (RDS)
H2 absorption Eabs lt 01 eV[1 2]
H
H2
timesExperiment Prevailing model
Possible rate determining steps
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Ohno et al J Chem Phys submitted [3] Padama et al J Phys Soc Jpn 81 (2012) 114705 [4] Ferrin et al Surf Sci 606 (2012) 679
Emono = 03 ～ 06 eVMonatomic in-diffusion
HPd
Chemi-sorption
R
Potential Energy
-05 eV
-02 eV-01 eV
1)
2)
3)
RDS H2 Dissociation (at large H) or Concerted Penetration
[1] Rendulic Surf Sci 208 (1989) 404 [2] Groszlig ChemPhysChem 11 (2010) 1374 [3] Sakong ChemPhysChem 13 3467
bull H2 dissociation is non-activated (Ediss = 0) at bare Pd surfaces[1]
bull Dissociation becomes weakly activated (at high H-coverages)[2]
05 ML HPd(100)
Consider processes with activation energies compatible to Eabs (le01 eV)
rarr H2 dissociation (Ediss) concerted penetration (Ec-pen)
H2 dissociation at a H mono-vacancy
Ediss = 01 eV[23]
Excess H atom[2] (He)
Concerted penetration
Ec-pen asymp 006 eV[23]
He + Hs rarr Hs + Hss
bull The activation barrier of penetration can be drastically lowered when it occurs concertedly with stabilization of excess H atoms
Influence of Surface Structure on H2 dissociation (at large H)
Peculiarity of Pd(110) Terrace-related H2 absorption (not on Pd(111) and (100))[1 2]
- Possible explanation -
[1] Gdowski et al J Vac Sci Technol A 5 1103 (1987) [2] Okuyama et al Surf Sci 401 344 (1998)[3] Maringrtensson et al Phys Rev Lett 57 (1986) 2045 [4] Schmidt et al Phys Rev Lett 87 096103 (2001)[5] Ahmed et al Appl Surf Sci 257 10503 (2011) [6] Busnengo et al Phys Rev Lett 93 236103 (2004) (1x1)
constitute precursor statesfor H2 dissociation[4 5]
Top view Side view[5]
Step edge-like structures stabilize molecular H2 chemisorption states
step-likePd(322)
Ni(510)[3] Pd(210)[4] Pd(322)[5]
Theoretical prediction[6] H2 may exist at Pd(110)
bull H-vacancy-mediated dissociation vac = exp(-GsbkBT) = 2x10-8 at 145 K
bull Pabs max (Model) = Pdiss vac ltlt Pabs (Experiment) = Rabs2Zw = 5x10-4 at 145 K
bull =gt Direct gas phase H2 impact not sufficient =gt Involvement of mobile H2 precursors ()
Influence of Surface Structure on H vacancy generation
Defect-enhanced H2 absorption Terrace-related H2 absorption ( gt 24 x per site vs regular terraces) (peculiar vs Pd(100) (111) (311))
H2 dissociation may require H-vacancies
bull Rate of H-vacancy generation[1] Rvac = 1013 s-1 exp(-EssskBT) asymp 103 at 145 K
bull =gt enhanced at defects due to additional lsquoopennessrsquo May also stabilize H2
bull Widened penetration channels at defects and in troughs between paired Pd rows in
Pd(110)(1x2)-(PR)
Side view
(1x2)
Top view
Widened interstitial channels (in [001])
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705 Esss = 027 eV (110) cf Pd(111) (04 eV) Pd(100) (041 eV) (Ferrin)
Summary amp Conclusions
H2 absorption mechanism at Pd(110)-(1x2) (paired-row)
Two hydride states exist with different depth distributions
Two H absorption channels (defects + terrace Pd(110) only)
Hs is replaced (not bypassed) no simple in-diffusion Eabslt01 eV
RDS H2 dissociation (H-saturated Pd) or concerted penetration
Influence of Surface Structure H2 absorption enhanced by
ldquoOpenrdquo penetration channels (accelerate H-vacancy generation)
Stabilization of H2 precursors (at step edge-like structures)
D
H2H2
H
H2
α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
rarr Activation Energy H 1 003 eV
3 006 eV D 3 007 eV
Much smaller than expected for monatomic H surface-to-subsurface
diffusion (03~04 eV)
Arrhenius plot of1 3 population (Pa)
peak area vs exposure
lt01 eV
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
2000 L at Te = 130 K
α1 α3α2
β2
β1
surface
TPD feature Origin Depth extension
[H]avgVolume
ratioTe condition
α1 (170 K) Near surface hydride ～ 10 nm 23 at 30 lt 145 K
α3 (195 K) Bulk hydride gt 50 nm 12 at 2 lt 160 K
LEED NRA TPD Identification of H2Pd(110) desorption features
600
500
400
300
200
100
0
-yi
eld
[co
un
ts
C]
20151050-5
Depth [nm]
64664464264063815
N ion energy [MeV]
10 L at 170 K 2000 L at 130 K 2000 L at 145 K
=gt First revelation at Pd(110) TWO absorbed hydride states
300
200
100
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150Temperature [K]
TPD NRA
S Ohno M Wilde K Fukutani J Chem Phys submitted
NEW
pre postD2 10 L rarr H2 1000 L α1
α3
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
Te = 115 K
rArr Two separate absorption pathways exist ()
Near-surface hydride Bulk hydride
Investigation of the H2 Absorption Mechanism
Absorption experiments with isotope labeled surface hydrogen
Analysis of isotope populations (TPD)
=gt Clear difference between near-surface (α1) and bulk (α3) hydride (Also Different normal (H2gtD2) isotope effects in a1 and a3 population speeds)
=gt Absorption near minority sites (defects)
Isotope Population of the Absorbed Hydride States
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre 006 ML
Near-surface hydride (α1)
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
Bulk hydride (α3)
08
1
02
p=0
05
～ 4
Dominant transfer of pre-adsorbed H below the surface
(First observation)
=gt Absorption in regular terrace area ()
p=0~05
Only Pd(110) no lsquobypassingrsquo
p 1-p
lsquobypassingrsquo replacement
Recursive analysis of isotope composition
( )( 1) ( ) (1 )
( )( 1) ( ) (1 )
prepre pre
prepost post
N nN n N n p
N
N nN n N n p
N
Evaluation of lsquobypassingrsquo probability (p)
Stochastic Isotope Population Model for AbsorptionDesorption
(1)
(2)
(p) (1-p)
n+1th absorption eventpost
pre Npre(n)post Npost(n)
rarr uptake rarr desorption (microscopic reversibility)
Absorption mechanism Bypassing or Replacement
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
Bulk hydride (α3)
Near-surface hydride (α1)
08
1
02
05
08
1
02
p=0
05
p=0
Dominant absorption mechanism
Compatible Incompatible
p=0 Replacement p=1 Bypassing
Replacement
S Ohno M Wilde K Fukutani J Chem Phys submitted
What is the Rate Determining Step (RDS)
H2 absorption Eabs lt 01 eV[1 2]
H
H2
timesExperiment Prevailing model
Possible rate determining steps
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Ohno et al J Chem Phys submitted [3] Padama et al J Phys Soc Jpn 81 (2012) 114705 [4] Ferrin et al Surf Sci 606 (2012) 679
Emono = 03 ～ 06 eVMonatomic in-diffusion
HPd
Chemi-sorption
R
Potential Energy
-05 eV
-02 eV-01 eV
1)
2)
3)
RDS H2 Dissociation (at large H) or Concerted Penetration
[1] Rendulic Surf Sci 208 (1989) 404 [2] Groszlig ChemPhysChem 11 (2010) 1374 [3] Sakong ChemPhysChem 13 3467
bull H2 dissociation is non-activated (Ediss = 0) at bare Pd surfaces[1]
bull Dissociation becomes weakly activated (at high H-coverages)[2]
05 ML HPd(100)
Consider processes with activation energies compatible to Eabs (le01 eV)
rarr H2 dissociation (Ediss) concerted penetration (Ec-pen)
H2 dissociation at a H mono-vacancy
Ediss = 01 eV[23]
Excess H atom[2] (He)
Concerted penetration
Ec-pen asymp 006 eV[23]
He + Hs rarr Hs + Hss
bull The activation barrier of penetration can be drastically lowered when it occurs concertedly with stabilization of excess H atoms
Influence of Surface Structure on H2 dissociation (at large H)
Peculiarity of Pd(110) Terrace-related H2 absorption (not on Pd(111) and (100))[1 2]
- Possible explanation -
[1] Gdowski et al J Vac Sci Technol A 5 1103 (1987) [2] Okuyama et al Surf Sci 401 344 (1998)[3] Maringrtensson et al Phys Rev Lett 57 (1986) 2045 [4] Schmidt et al Phys Rev Lett 87 096103 (2001)[5] Ahmed et al Appl Surf Sci 257 10503 (2011) [6] Busnengo et al Phys Rev Lett 93 236103 (2004) (1x1)
constitute precursor statesfor H2 dissociation[4 5]
Top view Side view[5]
Step edge-like structures stabilize molecular H2 chemisorption states
step-likePd(322)
Ni(510)[3] Pd(210)[4] Pd(322)[5]
Theoretical prediction[6] H2 may exist at Pd(110)
bull H-vacancy-mediated dissociation vac = exp(-GsbkBT) = 2x10-8 at 145 K
bull Pabs max (Model) = Pdiss vac ltlt Pabs (Experiment) = Rabs2Zw = 5x10-4 at 145 K
bull =gt Direct gas phase H2 impact not sufficient =gt Involvement of mobile H2 precursors ()
Influence of Surface Structure on H vacancy generation
Defect-enhanced H2 absorption Terrace-related H2 absorption ( gt 24 x per site vs regular terraces) (peculiar vs Pd(100) (111) (311))
H2 dissociation may require H-vacancies
bull Rate of H-vacancy generation[1] Rvac = 1013 s-1 exp(-EssskBT) asymp 103 at 145 K
bull =gt enhanced at defects due to additional lsquoopennessrsquo May also stabilize H2
bull Widened penetration channels at defects and in troughs between paired Pd rows in
Pd(110)(1x2)-(PR)
Side view
(1x2)
Top view
Widened interstitial channels (in [001])
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705 Esss = 027 eV (110) cf Pd(111) (04 eV) Pd(100) (041 eV) (Ferrin)
Summary amp Conclusions
H2 absorption mechanism at Pd(110)-(1x2) (paired-row)
Two hydride states exist with different depth distributions
Two H absorption channels (defects + terrace Pd(110) only)
Hs is replaced (not bypassed) no simple in-diffusion Eabslt01 eV
RDS H2 dissociation (H-saturated Pd) or concerted penetration
Influence of Surface Structure H2 absorption enhanced by
ldquoOpenrdquo penetration channels (accelerate H-vacancy generation)
Stabilization of H2 precursors (at step edge-like structures)
D
H2H2
H
H2
α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
rarr Activation Energy H 1 003 eV
3 006 eV D 3 007 eV
Much smaller than expected for monatomic H surface-to-subsurface
diffusion (03~04 eV)
Arrhenius plot of1 3 population (Pa)
peak area vs exposure
lt01 eV
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
pre postD2 10 L rarr H2 1000 L α1
α3
120
100
80
60
40
20
0QM
S io
n cu
rren
t [1
0-12 A
]
400350300250200150
Temperature [K]
Te = 115 K
rArr Two separate absorption pathways exist ()
Near-surface hydride Bulk hydride
Investigation of the H2 Absorption Mechanism
Absorption experiments with isotope labeled surface hydrogen
Analysis of isotope populations (TPD)
=gt Clear difference between near-surface (α1) and bulk (α3) hydride (Also Different normal (H2gtD2) isotope effects in a1 and a3 population speeds)
=gt Absorption near minority sites (defects)
Isotope Population of the Absorbed Hydride States
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre 006 ML
Near-surface hydride (α1)
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
Bulk hydride (α3)
08
1
02
p=0
05
～ 4
Dominant transfer of pre-adsorbed H below the surface
(First observation)
=gt Absorption in regular terrace area ()
p=0~05
Only Pd(110) no lsquobypassingrsquo
p 1-p
lsquobypassingrsquo replacement
Recursive analysis of isotope composition
( )( 1) ( ) (1 )
( )( 1) ( ) (1 )
prepre pre
prepost post
N nN n N n p
N
N nN n N n p
N
Evaluation of lsquobypassingrsquo probability (p)
Stochastic Isotope Population Model for AbsorptionDesorption
(1)
(2)
(p) (1-p)
n+1th absorption eventpost
pre Npre(n)post Npost(n)
rarr uptake rarr desorption (microscopic reversibility)
Absorption mechanism Bypassing or Replacement
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
Bulk hydride (α3)
Near-surface hydride (α1)
08
1
02
05
08
1
02
p=0
05
p=0
Dominant absorption mechanism
Compatible Incompatible
p=0 Replacement p=1 Bypassing
Replacement
S Ohno M Wilde K Fukutani J Chem Phys submitted
What is the Rate Determining Step (RDS)
H2 absorption Eabs lt 01 eV[1 2]
H
H2
timesExperiment Prevailing model
Possible rate determining steps
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Ohno et al J Chem Phys submitted [3] Padama et al J Phys Soc Jpn 81 (2012) 114705 [4] Ferrin et al Surf Sci 606 (2012) 679
Emono = 03 ～ 06 eVMonatomic in-diffusion
HPd
Chemi-sorption
R
Potential Energy
-05 eV
-02 eV-01 eV
1)
2)
3)
RDS H2 Dissociation (at large H) or Concerted Penetration
[1] Rendulic Surf Sci 208 (1989) 404 [2] Groszlig ChemPhysChem 11 (2010) 1374 [3] Sakong ChemPhysChem 13 3467
bull H2 dissociation is non-activated (Ediss = 0) at bare Pd surfaces[1]
bull Dissociation becomes weakly activated (at high H-coverages)[2]
05 ML HPd(100)
Consider processes with activation energies compatible to Eabs (le01 eV)
rarr H2 dissociation (Ediss) concerted penetration (Ec-pen)
H2 dissociation at a H mono-vacancy
Ediss = 01 eV[23]
Excess H atom[2] (He)
Concerted penetration
Ec-pen asymp 006 eV[23]
He + Hs rarr Hs + Hss
bull The activation barrier of penetration can be drastically lowered when it occurs concertedly with stabilization of excess H atoms
Influence of Surface Structure on H2 dissociation (at large H)
Peculiarity of Pd(110) Terrace-related H2 absorption (not on Pd(111) and (100))[1 2]
- Possible explanation -
[1] Gdowski et al J Vac Sci Technol A 5 1103 (1987) [2] Okuyama et al Surf Sci 401 344 (1998)[3] Maringrtensson et al Phys Rev Lett 57 (1986) 2045 [4] Schmidt et al Phys Rev Lett 87 096103 (2001)[5] Ahmed et al Appl Surf Sci 257 10503 (2011) [6] Busnengo et al Phys Rev Lett 93 236103 (2004) (1x1)
constitute precursor statesfor H2 dissociation[4 5]
Top view Side view[5]
Step edge-like structures stabilize molecular H2 chemisorption states
step-likePd(322)
Ni(510)[3] Pd(210)[4] Pd(322)[5]
Theoretical prediction[6] H2 may exist at Pd(110)
bull H-vacancy-mediated dissociation vac = exp(-GsbkBT) = 2x10-8 at 145 K
bull Pabs max (Model) = Pdiss vac ltlt Pabs (Experiment) = Rabs2Zw = 5x10-4 at 145 K
bull =gt Direct gas phase H2 impact not sufficient =gt Involvement of mobile H2 precursors ()
Influence of Surface Structure on H vacancy generation
Defect-enhanced H2 absorption Terrace-related H2 absorption ( gt 24 x per site vs regular terraces) (peculiar vs Pd(100) (111) (311))
H2 dissociation may require H-vacancies
bull Rate of H-vacancy generation[1] Rvac = 1013 s-1 exp(-EssskBT) asymp 103 at 145 K
bull =gt enhanced at defects due to additional lsquoopennessrsquo May also stabilize H2
bull Widened penetration channels at defects and in troughs between paired Pd rows in
Pd(110)(1x2)-(PR)
Side view
(1x2)
Top view
Widened interstitial channels (in [001])
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705 Esss = 027 eV (110) cf Pd(111) (04 eV) Pd(100) (041 eV) (Ferrin)
Summary amp Conclusions
H2 absorption mechanism at Pd(110)-(1x2) (paired-row)
Two hydride states exist with different depth distributions
Two H absorption channels (defects + terrace Pd(110) only)
Hs is replaced (not bypassed) no simple in-diffusion Eabslt01 eV
RDS H2 dissociation (H-saturated Pd) or concerted penetration
Influence of Surface Structure H2 absorption enhanced by
ldquoOpenrdquo penetration channels (accelerate H-vacancy generation)
Stabilization of H2 precursors (at step edge-like structures)
D
H2H2
H
H2
α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
rarr Activation Energy H 1 003 eV
3 006 eV D 3 007 eV
Much smaller than expected for monatomic H surface-to-subsurface
diffusion (03~04 eV)
Arrhenius plot of1 3 population (Pa)
peak area vs exposure
lt01 eV
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
=gt Absorption near minority sites (defects)
Isotope Population of the Absorbed Hydride States
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre 006 ML
Near-surface hydride (α1)
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
Bulk hydride (α3)
08
1
02
p=0
05
～ 4
Dominant transfer of pre-adsorbed H below the surface
(First observation)
=gt Absorption in regular terrace area ()
p=0~05
Only Pd(110) no lsquobypassingrsquo
p 1-p
lsquobypassingrsquo replacement
Recursive analysis of isotope composition
( )( 1) ( ) (1 )
( )( 1) ( ) (1 )
prepre pre
prepost post
N nN n N n p
N
N nN n N n p
N
Evaluation of lsquobypassingrsquo probability (p)
Stochastic Isotope Population Model for AbsorptionDesorption
(1)
(2)
(p) (1-p)
n+1th absorption eventpost
pre Npre(n)post Npost(n)
rarr uptake rarr desorption (microscopic reversibility)
Absorption mechanism Bypassing or Replacement
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
Bulk hydride (α3)
Near-surface hydride (α1)
08
1
02
05
08
1
02
p=0
05
p=0
Dominant absorption mechanism
Compatible Incompatible
p=0 Replacement p=1 Bypassing
Replacement
S Ohno M Wilde K Fukutani J Chem Phys submitted
What is the Rate Determining Step (RDS)
H2 absorption Eabs lt 01 eV[1 2]
H
H2
timesExperiment Prevailing model
Possible rate determining steps
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Ohno et al J Chem Phys submitted [3] Padama et al J Phys Soc Jpn 81 (2012) 114705 [4] Ferrin et al Surf Sci 606 (2012) 679
Emono = 03 ～ 06 eVMonatomic in-diffusion
HPd
Chemi-sorption
R
Potential Energy
-05 eV
-02 eV-01 eV
1)
2)
3)
RDS H2 Dissociation (at large H) or Concerted Penetration
[1] Rendulic Surf Sci 208 (1989) 404 [2] Groszlig ChemPhysChem 11 (2010) 1374 [3] Sakong ChemPhysChem 13 3467
bull H2 dissociation is non-activated (Ediss = 0) at bare Pd surfaces[1]
bull Dissociation becomes weakly activated (at high H-coverages)[2]
05 ML HPd(100)
Consider processes with activation energies compatible to Eabs (le01 eV)
rarr H2 dissociation (Ediss) concerted penetration (Ec-pen)
H2 dissociation at a H mono-vacancy
Ediss = 01 eV[23]
Excess H atom[2] (He)
Concerted penetration
Ec-pen asymp 006 eV[23]
He + Hs rarr Hs + Hss
bull The activation barrier of penetration can be drastically lowered when it occurs concertedly with stabilization of excess H atoms
Influence of Surface Structure on H2 dissociation (at large H)
Peculiarity of Pd(110) Terrace-related H2 absorption (not on Pd(111) and (100))[1 2]
- Possible explanation -
[1] Gdowski et al J Vac Sci Technol A 5 1103 (1987) [2] Okuyama et al Surf Sci 401 344 (1998)[3] Maringrtensson et al Phys Rev Lett 57 (1986) 2045 [4] Schmidt et al Phys Rev Lett 87 096103 (2001)[5] Ahmed et al Appl Surf Sci 257 10503 (2011) [6] Busnengo et al Phys Rev Lett 93 236103 (2004) (1x1)
constitute precursor statesfor H2 dissociation[4 5]
Top view Side view[5]
Step edge-like structures stabilize molecular H2 chemisorption states
step-likePd(322)
Ni(510)[3] Pd(210)[4] Pd(322)[5]
Theoretical prediction[6] H2 may exist at Pd(110)
bull H-vacancy-mediated dissociation vac = exp(-GsbkBT) = 2x10-8 at 145 K
bull Pabs max (Model) = Pdiss vac ltlt Pabs (Experiment) = Rabs2Zw = 5x10-4 at 145 K
bull =gt Direct gas phase H2 impact not sufficient =gt Involvement of mobile H2 precursors ()
Influence of Surface Structure on H vacancy generation
Defect-enhanced H2 absorption Terrace-related H2 absorption ( gt 24 x per site vs regular terraces) (peculiar vs Pd(100) (111) (311))
H2 dissociation may require H-vacancies
bull Rate of H-vacancy generation[1] Rvac = 1013 s-1 exp(-EssskBT) asymp 103 at 145 K
bull =gt enhanced at defects due to additional lsquoopennessrsquo May also stabilize H2
bull Widened penetration channels at defects and in troughs between paired Pd rows in
Pd(110)(1x2)-(PR)
Side view
(1x2)
Top view
Widened interstitial channels (in [001])
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705 Esss = 027 eV (110) cf Pd(111) (04 eV) Pd(100) (041 eV) (Ferrin)
Summary amp Conclusions
H2 absorption mechanism at Pd(110)-(1x2) (paired-row)
Two hydride states exist with different depth distributions
Two H absorption channels (defects + terrace Pd(110) only)
Hs is replaced (not bypassed) no simple in-diffusion Eabslt01 eV
RDS H2 dissociation (H-saturated Pd) or concerted penetration
Influence of Surface Structure H2 absorption enhanced by
ldquoOpenrdquo penetration channels (accelerate H-vacancy generation)
Stabilization of H2 precursors (at step edge-like structures)
D
H2H2
H
H2
α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
rarr Activation Energy H 1 003 eV
3 006 eV D 3 007 eV
Much smaller than expected for monatomic H surface-to-subsurface
diffusion (03~04 eV)
Arrhenius plot of1 3 population (Pa)
peak area vs exposure
lt01 eV
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
p 1-p
lsquobypassingrsquo replacement
Recursive analysis of isotope composition
( )( 1) ( ) (1 )
( )( 1) ( ) (1 )
prepre pre
prepost post
N nN n N n p
N
N nN n N n p
N
Evaluation of lsquobypassingrsquo probability (p)
Stochastic Isotope Population Model for AbsorptionDesorption
(1)
(2)
(p) (1-p)
n+1th absorption eventpost
pre Npre(n)post Npost(n)
rarr uptake rarr desorption (microscopic reversibility)
Absorption mechanism Bypassing or Replacement
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
Bulk hydride (α3)
Near-surface hydride (α1)
08
1
02
05
08
1
02
p=0
05
p=0
Dominant absorption mechanism
Compatible Incompatible
p=0 Replacement p=1 Bypassing
Replacement
S Ohno M Wilde K Fukutani J Chem Phys submitted
What is the Rate Determining Step (RDS)
H2 absorption Eabs lt 01 eV[1 2]
H
H2
timesExperiment Prevailing model
Possible rate determining steps
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Ohno et al J Chem Phys submitted [3] Padama et al J Phys Soc Jpn 81 (2012) 114705 [4] Ferrin et al Surf Sci 606 (2012) 679
Emono = 03 ～ 06 eVMonatomic in-diffusion
HPd
Chemi-sorption
R
Potential Energy
-05 eV
-02 eV-01 eV
1)
2)
3)
RDS H2 Dissociation (at large H) or Concerted Penetration
[1] Rendulic Surf Sci 208 (1989) 404 [2] Groszlig ChemPhysChem 11 (2010) 1374 [3] Sakong ChemPhysChem 13 3467
bull H2 dissociation is non-activated (Ediss = 0) at bare Pd surfaces[1]
bull Dissociation becomes weakly activated (at high H-coverages)[2]
05 ML HPd(100)
Consider processes with activation energies compatible to Eabs (le01 eV)
rarr H2 dissociation (Ediss) concerted penetration (Ec-pen)
H2 dissociation at a H mono-vacancy
Ediss = 01 eV[23]
Excess H atom[2] (He)
Concerted penetration
Ec-pen asymp 006 eV[23]
He + Hs rarr Hs + Hss
bull The activation barrier of penetration can be drastically lowered when it occurs concertedly with stabilization of excess H atoms
Influence of Surface Structure on H2 dissociation (at large H)
Peculiarity of Pd(110) Terrace-related H2 absorption (not on Pd(111) and (100))[1 2]
- Possible explanation -
[1] Gdowski et al J Vac Sci Technol A 5 1103 (1987) [2] Okuyama et al Surf Sci 401 344 (1998)[3] Maringrtensson et al Phys Rev Lett 57 (1986) 2045 [4] Schmidt et al Phys Rev Lett 87 096103 (2001)[5] Ahmed et al Appl Surf Sci 257 10503 (2011) [6] Busnengo et al Phys Rev Lett 93 236103 (2004) (1x1)
constitute precursor statesfor H2 dissociation[4 5]
Top view Side view[5]
Step edge-like structures stabilize molecular H2 chemisorption states
step-likePd(322)
Ni(510)[3] Pd(210)[4] Pd(322)[5]
Theoretical prediction[6] H2 may exist at Pd(110)
bull H-vacancy-mediated dissociation vac = exp(-GsbkBT) = 2x10-8 at 145 K
bull Pabs max (Model) = Pdiss vac ltlt Pabs (Experiment) = Rabs2Zw = 5x10-4 at 145 K
bull =gt Direct gas phase H2 impact not sufficient =gt Involvement of mobile H2 precursors ()
Influence of Surface Structure on H vacancy generation
Defect-enhanced H2 absorption Terrace-related H2 absorption ( gt 24 x per site vs regular terraces) (peculiar vs Pd(100) (111) (311))
H2 dissociation may require H-vacancies
bull Rate of H-vacancy generation[1] Rvac = 1013 s-1 exp(-EssskBT) asymp 103 at 145 K
bull =gt enhanced at defects due to additional lsquoopennessrsquo May also stabilize H2
bull Widened penetration channels at defects and in troughs between paired Pd rows in
Pd(110)(1x2)-(PR)
Side view
(1x2)
Top view
Widened interstitial channels (in [001])
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705 Esss = 027 eV (110) cf Pd(111) (04 eV) Pd(100) (041 eV) (Ferrin)
Summary amp Conclusions
H2 absorption mechanism at Pd(110)-(1x2) (paired-row)
Two hydride states exist with different depth distributions
Two H absorption channels (defects + terrace Pd(110) only)
Hs is replaced (not bypassed) no simple in-diffusion Eabslt01 eV
RDS H2 dissociation (H-saturated Pd) or concerted penetration
Influence of Surface Structure H2 absorption enhanced by
ldquoOpenrdquo penetration channels (accelerate H-vacancy generation)
Stabilization of H2 precursors (at step edge-like structures)
D
H2H2
H
H2
α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
rarr Activation Energy H 1 003 eV
3 006 eV D 3 007 eV
Much smaller than expected for monatomic H surface-to-subsurface
diffusion (03~04 eV)
Arrhenius plot of1 3 population (Pa)
peak area vs exposure
lt01 eV
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
Absorption mechanism Bypassing or Replacement
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
Bulk hydride (α3)
Near-surface hydride (α1)
08
1
02
05
08
1
02
p=0
05
p=0
Dominant absorption mechanism
Compatible Incompatible
p=0 Replacement p=1 Bypassing
Replacement
S Ohno M Wilde K Fukutani J Chem Phys submitted
What is the Rate Determining Step (RDS)
H2 absorption Eabs lt 01 eV[1 2]
H
H2
timesExperiment Prevailing model
Possible rate determining steps
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Ohno et al J Chem Phys submitted [3] Padama et al J Phys Soc Jpn 81 (2012) 114705 [4] Ferrin et al Surf Sci 606 (2012) 679
Emono = 03 ～ 06 eVMonatomic in-diffusion
HPd
Chemi-sorption
R
Potential Energy
-05 eV
-02 eV-01 eV
1)
2)
3)
RDS H2 Dissociation (at large H) or Concerted Penetration
[1] Rendulic Surf Sci 208 (1989) 404 [2] Groszlig ChemPhysChem 11 (2010) 1374 [3] Sakong ChemPhysChem 13 3467
bull H2 dissociation is non-activated (Ediss = 0) at bare Pd surfaces[1]
bull Dissociation becomes weakly activated (at high H-coverages)[2]
05 ML HPd(100)
Consider processes with activation energies compatible to Eabs (le01 eV)
rarr H2 dissociation (Ediss) concerted penetration (Ec-pen)
H2 dissociation at a H mono-vacancy
Ediss = 01 eV[23]
Excess H atom[2] (He)
Concerted penetration
Ec-pen asymp 006 eV[23]
He + Hs rarr Hs + Hss
bull The activation barrier of penetration can be drastically lowered when it occurs concertedly with stabilization of excess H atoms
Influence of Surface Structure on H2 dissociation (at large H)
Peculiarity of Pd(110) Terrace-related H2 absorption (not on Pd(111) and (100))[1 2]
- Possible explanation -
[1] Gdowski et al J Vac Sci Technol A 5 1103 (1987) [2] Okuyama et al Surf Sci 401 344 (1998)[3] Maringrtensson et al Phys Rev Lett 57 (1986) 2045 [4] Schmidt et al Phys Rev Lett 87 096103 (2001)[5] Ahmed et al Appl Surf Sci 257 10503 (2011) [6] Busnengo et al Phys Rev Lett 93 236103 (2004) (1x1)
constitute precursor statesfor H2 dissociation[4 5]
Top view Side view[5]
Step edge-like structures stabilize molecular H2 chemisorption states
step-likePd(322)
Ni(510)[3] Pd(210)[4] Pd(322)[5]
Theoretical prediction[6] H2 may exist at Pd(110)
bull H-vacancy-mediated dissociation vac = exp(-GsbkBT) = 2x10-8 at 145 K
bull Pabs max (Model) = Pdiss vac ltlt Pabs (Experiment) = Rabs2Zw = 5x10-4 at 145 K
bull =gt Direct gas phase H2 impact not sufficient =gt Involvement of mobile H2 precursors ()
Influence of Surface Structure on H vacancy generation
Defect-enhanced H2 absorption Terrace-related H2 absorption ( gt 24 x per site vs regular terraces) (peculiar vs Pd(100) (111) (311))
H2 dissociation may require H-vacancies
bull Rate of H-vacancy generation[1] Rvac = 1013 s-1 exp(-EssskBT) asymp 103 at 145 K
bull =gt enhanced at defects due to additional lsquoopennessrsquo May also stabilize H2
bull Widened penetration channels at defects and in troughs between paired Pd rows in
Pd(110)(1x2)-(PR)
Side view
(1x2)
Top view
Widened interstitial channels (in [001])
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705 Esss = 027 eV (110) cf Pd(111) (04 eV) Pd(100) (041 eV) (Ferrin)
Summary amp Conclusions
H2 absorption mechanism at Pd(110)-(1x2) (paired-row)
Two hydride states exist with different depth distributions
Two H absorption channels (defects + terrace Pd(110) only)
Hs is replaced (not bypassed) no simple in-diffusion Eabslt01 eV
RDS H2 dissociation (H-saturated Pd) or concerted penetration
Influence of Surface Structure H2 absorption enhanced by
ldquoOpenrdquo penetration channels (accelerate H-vacancy generation)
Stabilization of H2 precursors (at step edge-like structures)
D
H2H2
H
H2
α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
rarr Activation Energy H 1 003 eV
3 006 eV D 3 007 eV
Much smaller than expected for monatomic H surface-to-subsurface
diffusion (03~04 eV)
Arrhenius plot of1 3 population (Pa)
peak area vs exposure
lt01 eV
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
What is the Rate Determining Step (RDS)
H2 absorption Eabs lt 01 eV[1 2]
H
H2
timesExperiment Prevailing model
Possible rate determining steps
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Ohno et al J Chem Phys submitted [3] Padama et al J Phys Soc Jpn 81 (2012) 114705 [4] Ferrin et al Surf Sci 606 (2012) 679
Emono = 03 ～ 06 eVMonatomic in-diffusion
HPd
Chemi-sorption
R
Potential Energy
-05 eV
-02 eV-01 eV
1)
2)
3)
RDS H2 Dissociation (at large H) or Concerted Penetration
[1] Rendulic Surf Sci 208 (1989) 404 [2] Groszlig ChemPhysChem 11 (2010) 1374 [3] Sakong ChemPhysChem 13 3467
bull H2 dissociation is non-activated (Ediss = 0) at bare Pd surfaces[1]
bull Dissociation becomes weakly activated (at high H-coverages)[2]
05 ML HPd(100)
Consider processes with activation energies compatible to Eabs (le01 eV)
rarr H2 dissociation (Ediss) concerted penetration (Ec-pen)
H2 dissociation at a H mono-vacancy
Ediss = 01 eV[23]
Excess H atom[2] (He)
Concerted penetration
Ec-pen asymp 006 eV[23]
He + Hs rarr Hs + Hss
bull The activation barrier of penetration can be drastically lowered when it occurs concertedly with stabilization of excess H atoms
Influence of Surface Structure on H2 dissociation (at large H)
Peculiarity of Pd(110) Terrace-related H2 absorption (not on Pd(111) and (100))[1 2]
- Possible explanation -
[1] Gdowski et al J Vac Sci Technol A 5 1103 (1987) [2] Okuyama et al Surf Sci 401 344 (1998)[3] Maringrtensson et al Phys Rev Lett 57 (1986) 2045 [4] Schmidt et al Phys Rev Lett 87 096103 (2001)[5] Ahmed et al Appl Surf Sci 257 10503 (2011) [6] Busnengo et al Phys Rev Lett 93 236103 (2004) (1x1)
constitute precursor statesfor H2 dissociation[4 5]
Top view Side view[5]
Step edge-like structures stabilize molecular H2 chemisorption states
step-likePd(322)
Ni(510)[3] Pd(210)[4] Pd(322)[5]
Theoretical prediction[6] H2 may exist at Pd(110)
bull H-vacancy-mediated dissociation vac = exp(-GsbkBT) = 2x10-8 at 145 K
bull Pabs max (Model) = Pdiss vac ltlt Pabs (Experiment) = Rabs2Zw = 5x10-4 at 145 K
bull =gt Direct gas phase H2 impact not sufficient =gt Involvement of mobile H2 precursors ()
Influence of Surface Structure on H vacancy generation
Defect-enhanced H2 absorption Terrace-related H2 absorption ( gt 24 x per site vs regular terraces) (peculiar vs Pd(100) (111) (311))
H2 dissociation may require H-vacancies
bull Rate of H-vacancy generation[1] Rvac = 1013 s-1 exp(-EssskBT) asymp 103 at 145 K
bull =gt enhanced at defects due to additional lsquoopennessrsquo May also stabilize H2
bull Widened penetration channels at defects and in troughs between paired Pd rows in
Pd(110)(1x2)-(PR)
Side view
(1x2)
Top view
Widened interstitial channels (in [001])
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705 Esss = 027 eV (110) cf Pd(111) (04 eV) Pd(100) (041 eV) (Ferrin)
Summary amp Conclusions
H2 absorption mechanism at Pd(110)-(1x2) (paired-row)
Two hydride states exist with different depth distributions
Two H absorption channels (defects + terrace Pd(110) only)
Hs is replaced (not bypassed) no simple in-diffusion Eabslt01 eV
RDS H2 dissociation (H-saturated Pd) or concerted penetration
Influence of Surface Structure H2 absorption enhanced by
ldquoOpenrdquo penetration channels (accelerate H-vacancy generation)
Stabilization of H2 precursors (at step edge-like structures)
D
H2H2
H
H2
α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
rarr Activation Energy H 1 003 eV
3 006 eV D 3 007 eV
Much smaller than expected for monatomic H surface-to-subsurface
diffusion (03~04 eV)
Arrhenius plot of1 3 population (Pa)
peak area vs exposure
lt01 eV
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
RDS H2 Dissociation (at large H) or Concerted Penetration
[1] Rendulic Surf Sci 208 (1989) 404 [2] Groszlig ChemPhysChem 11 (2010) 1374 [3] Sakong ChemPhysChem 13 3467
bull H2 dissociation is non-activated (Ediss = 0) at bare Pd surfaces[1]
bull Dissociation becomes weakly activated (at high H-coverages)[2]
05 ML HPd(100)
Consider processes with activation energies compatible to Eabs (le01 eV)
rarr H2 dissociation (Ediss) concerted penetration (Ec-pen)
H2 dissociation at a H mono-vacancy
Ediss = 01 eV[23]
Excess H atom[2] (He)
Concerted penetration
Ec-pen asymp 006 eV[23]
He + Hs rarr Hs + Hss
bull The activation barrier of penetration can be drastically lowered when it occurs concertedly with stabilization of excess H atoms
Influence of Surface Structure on H2 dissociation (at large H)
Peculiarity of Pd(110) Terrace-related H2 absorption (not on Pd(111) and (100))[1 2]
- Possible explanation -
[1] Gdowski et al J Vac Sci Technol A 5 1103 (1987) [2] Okuyama et al Surf Sci 401 344 (1998)[3] Maringrtensson et al Phys Rev Lett 57 (1986) 2045 [4] Schmidt et al Phys Rev Lett 87 096103 (2001)[5] Ahmed et al Appl Surf Sci 257 10503 (2011) [6] Busnengo et al Phys Rev Lett 93 236103 (2004) (1x1)
constitute precursor statesfor H2 dissociation[4 5]
Top view Side view[5]
Step edge-like structures stabilize molecular H2 chemisorption states
step-likePd(322)
Ni(510)[3] Pd(210)[4] Pd(322)[5]
Theoretical prediction[6] H2 may exist at Pd(110)
bull H-vacancy-mediated dissociation vac = exp(-GsbkBT) = 2x10-8 at 145 K
bull Pabs max (Model) = Pdiss vac ltlt Pabs (Experiment) = Rabs2Zw = 5x10-4 at 145 K
bull =gt Direct gas phase H2 impact not sufficient =gt Involvement of mobile H2 precursors ()
Influence of Surface Structure on H vacancy generation
Defect-enhanced H2 absorption Terrace-related H2 absorption ( gt 24 x per site vs regular terraces) (peculiar vs Pd(100) (111) (311))
H2 dissociation may require H-vacancies
bull Rate of H-vacancy generation[1] Rvac = 1013 s-1 exp(-EssskBT) asymp 103 at 145 K
bull =gt enhanced at defects due to additional lsquoopennessrsquo May also stabilize H2
bull Widened penetration channels at defects and in troughs between paired Pd rows in
Pd(110)(1x2)-(PR)
Side view
(1x2)
Top view
Widened interstitial channels (in [001])
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705 Esss = 027 eV (110) cf Pd(111) (04 eV) Pd(100) (041 eV) (Ferrin)
Summary amp Conclusions
H2 absorption mechanism at Pd(110)-(1x2) (paired-row)
Two hydride states exist with different depth distributions
Two H absorption channels (defects + terrace Pd(110) only)
Hs is replaced (not bypassed) no simple in-diffusion Eabslt01 eV
RDS H2 dissociation (H-saturated Pd) or concerted penetration
Influence of Surface Structure H2 absorption enhanced by
ldquoOpenrdquo penetration channels (accelerate H-vacancy generation)
Stabilization of H2 precursors (at step edge-like structures)
D
H2H2
H
H2
α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
rarr Activation Energy H 1 003 eV
3 006 eV D 3 007 eV
Much smaller than expected for monatomic H surface-to-subsurface
diffusion (03~04 eV)
Arrhenius plot of1 3 population (Pa)
peak area vs exposure
lt01 eV
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
Influence of Surface Structure on H2 dissociation (at large H)
Peculiarity of Pd(110) Terrace-related H2 absorption (not on Pd(111) and (100))[1 2]
- Possible explanation -
[1] Gdowski et al J Vac Sci Technol A 5 1103 (1987) [2] Okuyama et al Surf Sci 401 344 (1998)[3] Maringrtensson et al Phys Rev Lett 57 (1986) 2045 [4] Schmidt et al Phys Rev Lett 87 096103 (2001)[5] Ahmed et al Appl Surf Sci 257 10503 (2011) [6] Busnengo et al Phys Rev Lett 93 236103 (2004) (1x1)
constitute precursor statesfor H2 dissociation[4 5]
Top view Side view[5]
Step edge-like structures stabilize molecular H2 chemisorption states
step-likePd(322)
Ni(510)[3] Pd(210)[4] Pd(322)[5]
Theoretical prediction[6] H2 may exist at Pd(110)
bull H-vacancy-mediated dissociation vac = exp(-GsbkBT) = 2x10-8 at 145 K
bull Pabs max (Model) = Pdiss vac ltlt Pabs (Experiment) = Rabs2Zw = 5x10-4 at 145 K
bull =gt Direct gas phase H2 impact not sufficient =gt Involvement of mobile H2 precursors ()
Influence of Surface Structure on H vacancy generation
Defect-enhanced H2 absorption Terrace-related H2 absorption ( gt 24 x per site vs regular terraces) (peculiar vs Pd(100) (111) (311))
H2 dissociation may require H-vacancies
bull Rate of H-vacancy generation[1] Rvac = 1013 s-1 exp(-EssskBT) asymp 103 at 145 K
bull =gt enhanced at defects due to additional lsquoopennessrsquo May also stabilize H2
bull Widened penetration channels at defects and in troughs between paired Pd rows in
Pd(110)(1x2)-(PR)
Side view
(1x2)
Top view
Widened interstitial channels (in [001])
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705 Esss = 027 eV (110) cf Pd(111) (04 eV) Pd(100) (041 eV) (Ferrin)
Summary amp Conclusions
H2 absorption mechanism at Pd(110)-(1x2) (paired-row)
Two hydride states exist with different depth distributions
Two H absorption channels (defects + terrace Pd(110) only)
Hs is replaced (not bypassed) no simple in-diffusion Eabslt01 eV
RDS H2 dissociation (H-saturated Pd) or concerted penetration
Influence of Surface Structure H2 absorption enhanced by
ldquoOpenrdquo penetration channels (accelerate H-vacancy generation)
Stabilization of H2 precursors (at step edge-like structures)
D
H2H2
H
H2
α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
rarr Activation Energy H 1 003 eV
3 006 eV D 3 007 eV
Much smaller than expected for monatomic H surface-to-subsurface
diffusion (03~04 eV)
Arrhenius plot of1 3 population (Pa)
peak area vs exposure
lt01 eV
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
Influence of Surface Structure on H vacancy generation
Defect-enhanced H2 absorption Terrace-related H2 absorption ( gt 24 x per site vs regular terraces) (peculiar vs Pd(100) (111) (311))
H2 dissociation may require H-vacancies
bull Rate of H-vacancy generation[1] Rvac = 1013 s-1 exp(-EssskBT) asymp 103 at 145 K
bull =gt enhanced at defects due to additional lsquoopennessrsquo May also stabilize H2
bull Widened penetration channels at defects and in troughs between paired Pd rows in
Pd(110)(1x2)-(PR)
Side view
(1x2)
Top view
Widened interstitial channels (in [001])
[1] Padama et al J Phys Soc Jpn 81 (2012) 114705 Esss = 027 eV (110) cf Pd(111) (04 eV) Pd(100) (041 eV) (Ferrin)
Summary amp Conclusions
H2 absorption mechanism at Pd(110)-(1x2) (paired-row)
Two hydride states exist with different depth distributions
Two H absorption channels (defects + terrace Pd(110) only)
Hs is replaced (not bypassed) no simple in-diffusion Eabslt01 eV
RDS H2 dissociation (H-saturated Pd) or concerted penetration
Influence of Surface Structure H2 absorption enhanced by
ldquoOpenrdquo penetration channels (accelerate H-vacancy generation)
Stabilization of H2 precursors (at step edge-like structures)
D
H2H2
H
H2
α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
rarr Activation Energy H 1 003 eV
3 006 eV D 3 007 eV
Much smaller than expected for monatomic H surface-to-subsurface
diffusion (03~04 eV)
Arrhenius plot of1 3 population (Pa)
peak area vs exposure
lt01 eV
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
Summary amp Conclusions
H2 absorption mechanism at Pd(110)-(1x2) (paired-row)
Two hydride states exist with different depth distributions
Two H absorption channels (defects + terrace Pd(110) only)
Hs is replaced (not bypassed) no simple in-diffusion Eabslt01 eV
RDS H2 dissociation (H-saturated Pd) or concerted penetration
Influence of Surface Structure H2 absorption enhanced by
ldquoOpenrdquo penetration channels (accelerate H-vacancy generation)
Stabilization of H2 precursors (at step edge-like structures)
D
H2H2
H
H2
α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
rarr Activation Energy H 1 003 eV
3 006 eV D 3 007 eV
Much smaller than expected for monatomic H surface-to-subsurface
diffusion (03~04 eV)
Arrhenius plot of1 3 population (Pa)
peak area vs exposure
lt01 eV
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
rarr Activation Energy H 1 003 eV
3 006 eV D 3 007 eV
Much smaller than expected for monatomic H surface-to-subsurface
diffusion (03~04 eV)
Arrhenius plot of1 3 population (Pa)
peak area vs exposure
lt01 eV
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
Only ～ 4 of surface area is affected by isotope exchange rarr Absorption at minority sites rarr defects
Isotope Population of the Near Surface Hydride (α1)
Langmuir 2003 19 6750
AFM image of hydride grown on Pd thin film
003
002
001
000
Isot
ope
com
posi
tion
[ML]
005004003002001000Total amount of 1 [ML]
06
04
02
00Isot
ope
com
posi
tion
[ML]
06040200
Total amount of 1 [ML]
Post
Pre
Post
Pre
006 ML
darrα1
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
06
04
02
00Isot
ope
com
posi
tion
[ML]
0806040200Total amount of 3 [ML]
darr
Post
Pre
H2 absorption takes place in the regular terrace area of Pd(110) ()
Isotope Population of the Bulk Hydride (α3)
Dominant transfer of pre-adsorbed H below the surface
cf) Pre-adsorbed H remains intact on Pd(111)[1] and (100)[2] (rarr ldquoBypassingrdquo)
[1] Okuyama et al Surf Sci 401 (1998) 344 [2] Gdowski et al J Vac Sci Technol A 5 1103 (1987)
(First observation at a Pd single crystal surface)
α3
![2050年までのスギ林の炭素吸収量を予測するtC] 0 5 10 15 20 炭素吸収量 [百万 tC/ 年] 炭素蓄積量 400 [炭素吸収量 森林データベース 気候値](https://img.pdfslide.tips/doc/110x75/5f8b5ebd8ad13c7d220e9166/2050cce-tc-0-5-10-15-20-cce.jpg)
![へ．吸収、分布、代謝、排泄 目 次...174 動物における成績 吸収： ラットにイベルメクチンの2成分、すなわち[3H] イベルメクチンB1a又は[](https://img.pdfslide.tips/doc/110x75/60e65fd54b2c9640015393c3/ife-c-174-cc.jpg)
