Download - Near-surface behavior of hydrogen absorbed in palladium single crystals and nanoparticles
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Near-surface behavior of hydrogen absorbed in
palladium single crystals and nanoparticles
Markus Wilde
東京大学 生産技術研究所
日本真空協会 産学連携委員会
Tokyo
January 25, 2012
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Concept
HYDROGEN-IN (VACUUM) TECHNOLOGY
Bulk
• H-solubility• Phase transition• Lattice expansion• Diffusion• Embrittlement• Grain boundary• Vacancies• Defects
Surface
• Adsorption• Desorption• Reconstruction• Diffusion• Surface Reaction• Role of ‘Defects’
? ‘Subsurface’
Pumping limitations vs. H2:
TMP: rotor speed
SIP: low sputtering efficiency
Gas phase
• Molecular H2
• Pressure• Temperature
• Slow H2 outgassing from
• Penetration through
vacuum chamber materials
=> Best in UHV, XHV: NEG-Support (10-10 Pa)
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Clean Energy:
• Fuel cell (HOR)
• Hydrogen storage
Catalysis:
• NH3 synthesis: N2 + 3 H2 → 2NH3
• Olefin (C=C) Hydrogenation: CnH2n → CnH2n+2
Important Applications of Hydrogen
O2
H2
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Hydrogen Absorption/Recombination at Transition Metal Surfaces
Important Industrial Applications:
• Hydrogen Storage (in metal hydrides), Gettering and Purification
• Catalysis (of hydrogenation reactions: Olefins, Fuel Cell HOR)
=> Control of H-sorption capacities and charge/release kinetics!
→ Clarify the microscopic pathways of hydrogen penetration and recombination
Goal: Obtain atomic level
understanding
of absorption and
desorption
processes !
吸収 進入
dissociativeadsorption
hydrogen-richlayer (hydride)
surface-H
'subsurface'-H
in-diffusion
phase boundary
hydrogen-poorphase ()
kads
kpen
Kdiff-
gas-phase H2
penetration
transition metal or alloy (Pd, Ni, Ti, Y, Zr, Mg ...)
'bulk-dissolved’ hydrogen
absorption
H2
z0
H
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1. Introduction: Hydrogen and (Vacuum) Technology
2. Detection of Subsurface-H: Distinction from Surface-H
3. Formation of Subsurface-H: Absorption Mechanism
4. Role of near-surface absorbed H in Catalysis
Outline: Hydrogen Absorption at Metal Surfaces
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Abundance of Elements in the Universe
Atomic Number
75 % of all matter is Hydrogen !
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‘Seeing’ Hydrogen is difficult ...
• Ion scattering (RBS) fails:
H-cross section small (σRBS Z∝ 2)
H-signal buried under large
background from sample bulk
→ AES → XPS (ESCA)
X-ray photon, ion, or
electron
Core ionization Core hole relaxation
→ PIXE, …
+
e -
p+
Particle emission
• Standard chemical analysis (electron spectroscopy) fails: (because H only has a single 1s electron …)
He+ → Ag/Si(100)
O Si Ag(H)
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• Mostly applied: Mass Spectroscopy
• 異なるサイトの数と各サイトからの脱離の活性化エネルギー E* などが
測定可能 .
• H is desorbed during heating: => destructive.
• No information on H location (on / below the surface).
Measurement of hydrogen desorption activation energies:
粒子 H D HD H2 D2
m/e 1 2 3 2 4
昇温脱離分光法( TDS )
加熱
検出器( 質量分析器 )
気体に曝露 吸着 吸蔵・
脱離スペクトルを測る曝露温度 Te
排気H2
脱離速度 (Polanyi-Wigner 式 )
r=νnθnexp(-E*/kT)
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100 200 300 400 500
Temperature (K)
Example: H Adsorption at Pd(100)
, ,
Thermal desorption spectrum
H. Okuyama et al., Surf. Sci. 401 (1998) 344.
Pd(100)
4-fold hollow
• From where do the H states originate?
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Resonant Nuclear Reaction Analysis (NRA) via 1H(15N,)12C
Hydrogen Depth Profiling: Non-destructive ・ Quantitative ・ High-resolution
15N + 1H → 16O* → 12C + + (4.43 MeV) Eres = 6.385 MeV
Experimental
H
Ei=Eres
-detector (BGO)
N
probing depth:
Ei>Eres
z(Ei)= (Ei-Eres)/(dE/dz)z →
energy loss [Hbulk]
H15N2+ ion beam
stopping power (3.9 keV/nm for Pd)
0
K. Fukutani et al., PRL 88 (2002) 116101 . M. Wilde et al., J. Appl. Phys. 98 (2005) 023503.
[Hsurface]
Sensitivity:
Surface Coverages: 1% ML (~1013 cm-2)
Bulk concentrations: ~400 ppm (~1018 cm-3)
Depth resolution (limited by Doppler-broadening at the surface, by straggling in the bulk (>20 nm):
Near-surface: ~ 2-4 nm (standard: N.I.), < 1 nm (special case: grazing beam incidence)
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15N+1H →12C++ ( 4.43MeV ) Qm= 4.9656 MeV
Res. Energy : ER = 6.385 MeV
Res. Width : =1.8 keV
Resonant nuclear reaction 1H(15N,)12C
Cross section: 1650 mbarn
4)(
4)(
22
2
0
REE
E
J. Radioanal. Chemistry 77 (1983) 149.
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Experimental Setup for NRA
質量・
エネルギー分析器
(90o 偏向磁石 ): E = 3 keV
ExtractorIon Source (SNICS):
Cs+Ti15N+CC15N-
Inside the Accelerator Tank
Switching Magnet
Terminal: +2.48 MeV
5 MeV Van-de-Graaff Tandem Accelerator (MALT: AMS) (Univ. Tokyo)
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=> Combination of surface characterization and shallow H depth profiling (NRA) .
energy [eV]0 100 200 300 400 500 600
dN
/dE
[a
rb.
un
its]
S(KLL) Ti(LMM)
C(KLL)
AES 2.5 keV Ti(0001)
LEED 243 eV
Ti(0001)
shielded QMS
(RGA + TDS)
ion gun
LEED / AES
UHV
sample (80 - 1400 K)
BGO
FC
H+H2 doser
viewport
-ray detector
Ultra-pure H2
deflector
Pbase < 1 x 10-8 Pa
NRA
ion beam
Structural
Order
Chemical
Composition
Reactivity
towards
H2, H.
Ultra-High Vacuum System for Sample Preparation and in-situ NRA
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H2 D
eso
rptio
n s
ign
al
(TD
S)
600500400300200100
Temperature (K)
H2 Desorption signalCombine two hydrogen detection techniques:
Our Experimental Approach: TDS + NRA (@ 東京大学 )
① Thermal Desorption Spectroscopy (TDS):
→ H2(D2) exposures at given Te, desorption.
→ No. of H species, desorption activation energy
→ lacks information on H location (on/below 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:
Ei>Eres
z(Ei)= (Ei-Eres)/(dE/dz)
[Habsorbed]
15N2+ ion beam
0
[Hsurface]
② Nuclear Reaction Analysis (NRA) via 1H(15N,)12C: (Eres=6.385 MeV, =1.8 keV)
→ distinguishes surface-adsorbed from absorbed H (~2 nm depth resolution)
300 L H+H2 on Pd(100) at 100 K
15N ion energy [MeV]
6.37 6.38 6.39 6.40 6.41
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
s/C
)
M. Wilde, PRB 78 (2008) 114511.
→ unambiguously identifies TDS features
15N + 1H → 12C + + (4.43 MeV)
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1. Introduction: Hydrogen and (Vacuum) Technology
2. Detection of Subsurface-H: Distinction from Surface-H
3. Formation of Subsurface-H: Absorption Mechanism
4. Role of near-surface absorbed H in Catalysis
Outline: Hydrogen Absorption at Metal Surfaces
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Surface-adsorbed hydrogen is bound to low-coordinated metal surface
atoms: ALWAYS energetically more stable than H absorbed in the bulk!
Fundamental: Energy Topography of H near Metal Surfaces
Site-specific H-Energy
→ Surface: ES = -0.53 eV * 吸着エネルギー
→ Bulk: EB = -0.1 eV * 溶解エンタルピー
→ Subsurface: ESS = -0.19 eV *
In general: ES (< ESS) < EB
Top view
z
0H
Side view
Surface
Subsurface
BulkP
oten
tial e
nerg
y
R
2H2
1
SSS
0
≈ ≈
B
Hs: > 0 吸熱< 0 発熱
Hs
* Pd(100)
‘Reaction coordinate’
固体内部 表面 気祖
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70
60
50
40
30
20
10
0H2 d
eso
rptio
n s
ign
al
(10-1
0 A
)
600500400300200100
Temperature (K)
H-N
RA
-yield
(arb
.un
its)
H2 desorption H-NRA signal z=0 nm H-NRA signal z=6 nm
H2 Thermal Desorption Spectrum
15N ion energy [MeV]
6.37 6.38 6.39 6.40 6.41
ra
y yi
eld
[cts
/C
]
0
50
100
150
depth [nm]
-4 -2 0 2 4 6 8
NRA H-Depth Profile (T<130 K)
Surface and “Subsurface” H in Pd(100) after atomic H (+H2)
dosage (300 L) at 100 K.
M. Wilde et al., Surf. Sci. 482-485 (2001) 346.
Depth Extension of Subsurface H in Pd(100)
=> ‘Subsurface’ Hydrogen is NOT necessarily confined to first layer sites!
15N2+
• Hss in ~ 20 atomic layers => ‘hydride’ phase
• Hss desorbs before Hs !
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Surface-adsorbed hydrogen is ALWAYS more strongly bound than in the bulk (absorbed H).
H Absorption at Metal Surfaces: The Microscopic Perspective
Elementary steps of H-Absorption:
→ 1.) H2 dissociation at the surface.
→ 2.) Surface saturation (rapid).
→ 3.) Penetration into the bulk (slow).
Top view 4-fold hollow site
=> Hydrogen absorption (‘starting’ at the surface) is an activated process!
H2 z
0H
H2
H
Side view
Surface
Subsurface
BulkP
oten
tial e
nerg
y
R
2H2
1
SSS
0
≈ ≈
B
H2
time
Hs: > 0 endothermic< 0 exothermic
Hs
ES (< ESS) < EB
E>0
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A seemingly ‘simple’ question: Does surface-adsorbed H participate in H absorption on a clean, perfectly flat surface?
Do surface to subsurface transitions of adsorbed H atoms occur?
=> Study the response of surface-adsorbed H atoms to T w/o gaseous H2.
z
0H
H2
H
or
H2
?
With H2
H
T
Without H2
H/Pd(100) (fcc)
Pot
enti
al e
nerg
y
R
2H2
1
SSS
0
≈ ≈
0.3 eV
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70
60
50
40
30
20
10
0H2 d
eso
rptio
n s
ign
al
(10-1
0 A
)
600500400300200100
Temperature (K)
H-N
RA
-yield
(arb
.un
its)
H2 desorption H-NRA signal z=0 nm H-NRA signal z=6 nm
H2 Thermal Desorption Spectrum
15N ion energy [MeV]
6.37 6.38 6.39 6.40 6.41
ra
y yi
eld
[cts
/C
]
0
50
100
150
depth [nm]
-4 -2 0 2 4 6 8
NRA H-Depth Profile (T<130 K)
S. Ohno, M. Wilde et al., in preparation,
T. Stulen, JVSTA 5 (1983) .
Pd(100): Surface to ‘subsurface’ transition H upon heating?
=> Instead of moving into the bulk, surface and ‘subsurface’-H species desorb
15N2+
Okuyama et al., Surf. Sci. 401 (1998) 344.
• Hss bypasses surface-H in desorption (no isotopic exchange)!
! Similar on Pd(110) and Pd(111) !
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50
40
30
20
10
0
-yi
eld
(ct
s/C
)
6.426.416.406.396.386.3715
N ion energy (MeV)
151050-5
Depth (nm)
x 5
A comparison: H-Absorption of Surface-H into Ti(0001) (!)
M. Wilde and K. Fukutani, Phys. Rev. B 78, 115411 (2008).
(TDS: H2-saturated by 12000 L H2 at 100 K) NRA: Signal of surface hydrogen (H = 0.4 ML at 200 K).
Tdet=318±22 K
H2 Thermal Desorption Spectrum
=> Although H vanishes from the surface around 320 K, no H2 desorption occurs.
NRA H-Depth Profile (T=300 K)
Ti-Bulk:
[H]=500 ppm *
70
60
50
40
30
20
10
0
H2
des
orp
tion
sig
nal
(1
0-1
0 A)
800700600500400300200
Temperature (K)
H-N
RA
-yield (arb.u
nits)
H2 desorption H-NRA signal (6.3912 MeV)
hcp hollow
fcc hollow
H/-Ti(0001) (hcp)
Hs = -0.47 eV/H
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Pd(100):
→ Surface-H desorbs (at ~330 K): Es=0.53 eV/H.
→ Subsurface-H bypasses surface-H in desorption at 180 K.
Ti(0001):
→ Surface-H is absorbed into the bulk (near 320 K).
→ Bulk-dissolved H desorbs from an empty surface!
How can we understand the difference?
Absorption/Desorption of Surface Hydrogen
Opposite behavior of H on Pd(100) vs. Ti(0001)
z
0
H2Hs
Hss
Hs
Hb
H2z
0
Hs
Hs
Hb
H2
T = 330 K T = 180 K
T ~ 320 K T >650 K
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Absorption capacity for surface-H in the near-surface region
10-6
10-5
10-4
10-3
10-2
10-1
100
101
102
Dis
solv
able
H c
ove
rag
e [M
L]
800700600500400300200100
Temperature [K]
Ti Pd
Tpen=318±22 K
Tdes=340 K
=> Consider possibility to dissolve the surface H atoms into the bulk by in-diffusion:
Tk
ESTH
p
ptD
dML
B
diffSsH
Lsolv
21
exp2
][2/1
00
2
Dissolvable H coverage [ML] = Diffusion length (T) x H solubility (T) / (1/2 layer distance)
LD(T, t)
Phys. Rev. B 78, 115411 (2008)
→ Near-surface H absorption involve both surface and bulk properties!
* Pd: Hs = -0.10 eV/H Ti: Hs = -0.47 eV/H
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Hydrogen Absorption Mechanism at Pd(110)
Identify multiple H-states (→ NRA)
H2 → Pd(110): Complex TD spectrum
TDS H/Pd(110)
Surf. Sci. 126 (1983) 382.
Solid solution (α phase) and hydride (β phase) of bulk Pd are well known.
Clarify absorption pathways in the near-surface region (→ TDS)
Z. Phys. Chem. Neue Folge 64, 225 (1969)
Langmuir 2003, 19, 6750
Hydride evolves from surface point defects
AFM image of Pd thin film surfaceAFM image of Pd thin film surface
H2→
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θ=1.5 MLθ=1.0 ML
_
[110]
[001] θ=? ML
0 L 0.3 L 0.5 L 50 L
θ=0 ML
[1] Surf. Sci. 411 (1998) 123[2] Surf. Sci. 327 (1995) 505
[1] [2]
β 1 β 2
α2
α1
α3
0 L 0.3 L 0.5 L 50 L
(1×1) (2×1) (1×2) streaky(1×2)
A) Identify Surface Adsorption Phases: LEED & TDS
表面 表面
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α1 α3
Texp=90 K0.5 – 2000 L
α2
β2
β1
TDS after large exposures :曝露温度依存性 β2, β1, α2 (saturate at 0.5 L)
-> H at the surface and in the first subsurface sites
α1, α3 (never saturate) -> H in the Pd interior
α1 disappears at Texp ≥ 145 K
☞ Surf. Sci. 126 (1983) 382. Surf. Sci. 195 (1988) L199.
α3
α2
β2β1
Texp=145 K0.5 – 2000 L
α1 and α3 absorption depend on the exposure temperature (Texp)
☞ Pd(111); Surf. Sci. 181 (1987) L147. Pd(100); Surf. Sci. 401 (1998) 344.
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NRA Depth Profile
20.1%(hydride)
Hydrogen concentration
0.9%(solid solution)
S (=α2, β1, β2)S, α1, α3
S, α3
α1; near surface hydrideα3; bulk solid solution > 50 nm (TDS shows 3 ML of α3)
2, 1, 2: 表面水素1 : 表面近傍の水素化物3 : 固溶体祖の水素
→ Complete TDS Peak Assignment:
S. Ohno, M. Wilde, K. Fukutani, in preparation
First-time observation of TWO different absorbed H states in Pd(110)!
B) Clarify Concentration Depth Distribution of α1 and α3
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Near-surface condition at 130 K
• Coexistence of solid solution (3) and hydride (1) phases
• Non-uniform lateral and in-depth distribution
• In-plane ratio of hydride ~ 30%2065
×100 = 30%
Hydride: ~ 65%
H2NRA: average [H] = 20%
15N ion beam
Solid solution phase: 0.009%
Langmuir 19 (2003) 6750.
(300 K, bar H2)
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→ TDS after isotope-labeled hydrogen exposure
Experiment:1. Saturate Surface with D2. Post-dose H2
D2 1.25 L + H2 1,000 L @115 K
α1
α3
Different absorption pathways exist for the 1 and 3
absorbed states!
Result: 3 (+ surface species): → complete isotopic scrambling. 1: Pure post-dosed isotope → no isotopic scrambling.
Evidence for 2 Absorption pathways leading to 1 and 3
H2
D2
α1
α3
1, 2, 2
D
S. Ohno, M. Wilde, K. Fukutani, in preparation
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0.6
0.4
0.2
0.0Isot
ope
com
posi
tion
[ML]
0.60.40.20.0Total amount of 1 [ML]
0.6
0.4
0.2
0.0Isot
ope
com
posi
tion
[ML]
0.60.40.20.0Total amount of 1 [ML]
0.6
0.4
0.2
0.0Isot
ope
com
posi
tion
[ML]
0.60.40.20.0Total amount of 1 [ML]
Pre-adsorbed
Post-dosed
• Pre-adsorbed D (1.5 ML) is involved only in the initial absorption stage.
• Only ~4% of surface area is active.
• High penetration rate at active sites.
• Hydride consists predominantly of H.
0.06 ML
(initially: 1.5 ML D)
Hydride nucleation at a few specially active sites (Te<145 K)
Isotopic Composition: Hydride Phase (1)
x
x
Jpen
Jdiff
D
H2
Cf: Pd thin film – AFM:
Langmuir 19 (2003) 6750.
S. Ohno, M. Wilde, K. Fukutani, in preparation
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1.0
0.8
0.6
0.4
0.2
0.0Isot
ope
com
posi
tion
[ML]
1.61.20.80.40.0Total amount of 3 [ML]
1.0
0.8
0.6
0.4
0.2
0.0Isot
ope
com
posi
tion
[ML]
1.61.20.80.40.0Total amount of 3 [ML]
1.0
0.8
0.6
0.4
0.2
0.0Isot
ope
com
posi
tion
[ML]
1.61.20.80.40.0Total amount of 3 [ML]
Pre-adsorbed
Post-dosed
• Simultaneous and continuous absorption of pre-adsorbed and post-dosed hydrogen isotopes.
• Effective exchange with surface-D, possibly at regular terrace sites.
Solid solution H absorption at sites different from that of hydride nucleation
※侵入の確率 K, サイト数 θ
Kα1 ・ θα1≒ Kα3 ・ θα3
∴ Kα1 ≒ (θα3 / θα1) ・ Kα3 >> Kα3
Isotopic Composition: Solid Solution Phase (3)
=> Gas-phase H2-assisted penetration of surface-adsorbed D (first observation at a Pd single crystal)
S. Ohno, M. Wilde, K. Fukutani, in preparation
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Hydride and Solid Solution Formation Mechanism
α1 contains 0.06 ML (4%) of prechemisorbed species: -> Nucleation only at ~ 4% of special surface sites. -> Fast penetration rate (Jpen>)
-> Surface diffusion toward the ‘ entrance sites’ is prohibited (no isotope exchange with Hs)
Pre-dosed surface isotope in α3 increases together with post-dosed isotope. Complete isotopic exchange with Hs during penetration. Slower penetration rate.
S. Ohno, M. Wilde, K. Fukutani, in preparation
hydride(1)
no noJpen
Jpen
(3)
Jdiff
yes
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1. Introduction: Hydrogen and (Vacuum) Technology
2. Detection of Subsurface-H: Distinction from Surface-H
3. Formation of Subsurface-H: Absorption Mechanism
4. Role of near-surface absorbed H in Catalysis
Outline: Hydrogen Absorption at Metal Surfaces
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Olefin Hydrogenation Catalysis
• Concerted reaction is extremely unlikely in the gas phase
• Large activation energy barrier (Ea): → Small reaction rate: R = exp(-Ea/RT)
C4H8 C4H10D2
D2
Butene Butane-d2
Ea
H3C
CH3H
H+ D2
Reactants
GR < 0
H3C
CH3H
H
D … D
H3C
CH3
H
H
D DNecessary elementary steps:
• D-D bond break (~4.5 eV, 430 kJ/mol)
• C=C -bond break (~ 615 kJ/mol)
• C rehybridization: sp2 → sp3
• new C-H bond formation (414 kJ/mol x2)
Example: Butene Hydrogenation
Product
Transition state(hypothetical)
≠
SR << 0
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(≠)
• Catalyst … drastically reduces activation energy barrier (Ea’ << Ea)
• … enables reaction at far lower temperature
• … itself is not consumed in the reaction.
Ea
H3C
CH3H
H+ D2
Reactants
H3C
CH3H
H
D … D
H3C
CH3
H
H
D D
Product
Transition state
≠’
Olefin Hydrogenation Catalysis
CH3CH3
H H
CH3
CH3D
HH
DCH3
H CH3
H
D DCH3
H CH3
+D
+D
-H
cis-2-butene butyl
trans-2-butene-d1
butane-d2
+D2
D
D
DD
-H
cis-2-butene
butyl intermediate
trans-2-butene-d1
butane-d2
isomerization
hydrogenation
+D
Pd surface
Ea’
New, easier elementary steps:
• Olefin (C4H8) adsorbs on catalyst, C=C -bond opens.
• D2 bond breaks spontaneously on Pd surface (dissociative adsorption)
• Coadsorbed D atoms easily attach to the intermediate; products desorb.
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Hydrogen Absorption inside Pd Nanocrystals?
Industrial Catalysts: Oxide-supported Pd Nanocrystals
Olefin hydrogenation catalysis:
• Enhanced Reactivity of Pd Nano-clusters (for) compared to Pd(111)
single crystals.
→ participation of absorbed H suspected.
Model catalyst:volume
Al2O3 support
![Page 37: Near-surface behavior of hydrogen absorbed in palladium single crystals and nanoparticles](https://reader036.vdocuments.mx/reader036/viewer/2022062301/56814577550346895db24813/html5/thumbnails/37.jpg)
Pd-Nanocluster-Specific Reactivity for Alkene Hydrogenation: CnH2n + H2 → CnH2n+2
Enhanced Reactivity of Pd-Nanoparticles in Olefin Hydrogenation
A.M. Doyle et al., Angew. Chem. Int. Ed. 42 (2003) 5240; Journal of Catalysis 223 (2004) 444.
Pd Nanocrystals on Al2O3
Pd Single Crystal
(n=5): (pentene) (pentane)
[D2]pentane
(C5H10D2)
D2 + pentene (C5H10)
H inside NC?D2-TDS
D
D
NRA!
TDS
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Oxide-supported Pd nano-crystallites: Morphology
K.H. Hansen et al., PRL 83 (1999) 4120.
65x65 nm2.
2 ML Pd @ 300 K
Aspect ratio:
h/w=0.18±0.03
(constant
for w>5.5 nm)
Shape of Pd nano-crystallites on Al2O3/NiAl(110)
![Page 39: Near-surface behavior of hydrogen absorbed in palladium single crystals and nanoparticles](https://reader036.vdocuments.mx/reader036/viewer/2022062301/56814577550346895db24813/html5/thumbnails/39.jpg)
In-situ Nanocrystal Preparation for H-NRA
1.) Al2O3/NiAl(110) substrate:
→ NiAl(110) cleaning + in-situ oxidation.
2.) 5.85 Å Pd deposition @ 300 K
3.) NRA:
1H(15N, )12C
z(Ei) = (Ei-Eres)/[(dE/dz)cos(i)]
grazing ion incidence (i=75o)
beam collimation <2 mm (slits)
UPH (99.99999%) H2 background (<2x10-3 Pa)
shielded QMS H2 TDS
ion gun (→ Ar+
sputtering) UHV
sample (90-1300 K)on liquid N2
cryostat manipulator
BGOFC
viewport
-ray detector
Pd evaporator
deflector
Pbase <1 x 10-8 Pa
Energy monochromatized NRA 15N2+ ion beam
(E = 3 keV, ~15 nA)
LEED / AES
75o
_
+
NEC 5UD Tandem
17.5 nm x 17.5 nm
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Hydrogen Absorption in Al2O3-supported Pd nanocrystals
• 4-fold enhanced depth resolution in 75o grazing incidence angle NRA.
• NP-absorbed H (arrow) can be probed independently from surface-adsorbed H.
=> Pd-NP stabilize absorbed H with 2-3 fold higher heat of solution than bulk Pd.
( → H-binding occurs inside the NP, is not a mere surface-adsorption effect!)
Analysis of H distribution in 5.85 Å (2.6 ML) Pd on Al2O3 at 90 K, 2·10-5 Pa H2.
17.5 nm x 17.5 nm
i=75o
Al2O3/NiAl(110)
Pdh~2 nm
15N H
50 nm x 50 nm
M. Wilde et al., Phys. Rev. B 77, 113412 (2008).
250
200
150
100
50
0
-yi
eld
[co
un
ts/
C]
6.416.406.396.386.37
15N ion energy [MeV]
86420-2-4
depth z [nm]
Experiment Surface H Absorbed H
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Common Notion of Hydrogen Absorption in Nanoparticles
Peculiar H-Absorption Properties of NP’s:
Heat of H-solution of Nanoparticles is size-dependent and different from bulk metals => often HS is more negative.(→ larger H-absorption capacity)
Controversy on responsible factors:
• Large surface/volume ratio → adsorption ?
• Electronic structure → only for <100 atoms
• Lattice distortions, strain, interface effects, …
Fraction of atoms in two outermost shells for a cluster with i shells.
Cluster size (Sub)Surface atom fraction
S-2 2 nm 74%, i = 5
S-3 3 nm 60%, i = 7
S-5 5 nm 41%, i = 12
Proposed explanation: ‘subsurface sites’ (→ large surface/volume ratio)
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1050
700
350
0
-y
ield
[co
un
ts/
C]
6.416.406.396.386.3715
N ion energy [MeV]
86420-2-4
penetration depth z [nm] (i=75o)
c) 2x10-3
Pa
b) 6x10-4
Pa
a) 2x10-5
Pa
p(H2)-dependent H-uptake in Pd nanocrystals on Al2O3 at 90 K
• Below 1x10-4 mbar: Surface adsorption saturates (at 1 ML) (profile height at z=0).
Substantial H-uptake into the interior of the Pd nanocrystals!
• Above ~1x10-4 Pa: Absorption continues, absorbed H exceeds surface-adsorbed amount!
Separate monitoring of surface H and nanocrystal-absorbed H uptake
Al2O3/NiAl(110)
1 ML
(111)
(100)
2x10-5 mbar
6x10-6 mbar
2x10-7 mbar
5
4
3
2
1
0
H q
uant
ity/
ML
20x10-6
151050
H2 pressure/mbar
0.8
0.6
0.4
0.2
0.0
H:P
d ratio (absorb
ed H
)
Surface H Absorbed H
b)
M. Wilde et al., Phys. Rev. B 77, 113412 (2008).
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Reactivity Study of Olefin Conversion over Pd/Al2O3 Model Cat
NRA measurement under reaction conditions
i=75o
15N
Al2O3/NiAl(110)
H
Pd
Alumina-Supported Model Catalysts
QMSSample
4 Å Pd/Fe3O4/Pt(111) 4 Å Pd/Al2O3/NiAl(110)cis-2-butene beam (pulsed)
M. Wilde, S. Schauermann et al., Angew. Chem. Int. Ed. 47, 9289 (2008).
Molecular Beam Reactive Scattering
D2 beam (steady)
2-4x10-6 mbar)
HS [eV] bulk NP
Pd -(0.1…0.15) -0.28±0.02
(@ H/Pd<0.2)
• Does Pd Cluster-absorbed H play a role in
olefin (cis-2-butene) hydrogenation?
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D2-pressure dependent reactivity of hydrogenation
5
4
3
2
1
0
H q
uan
tity
/ML
20x10-6
151050
H2 pressure/mbar
0.8
0.6
0.4
0.2
0.0
H:P
d ratio (ab
sorbed
H)
Surface H Absorbed H
b)
Isomerization: → r ≠ f(pH2)
hydrogenation → r = f(pH2)
NRAMBRS
CH3CH3
H H
CH3
CH3D
HH
DCH3
H CH3
H
D DCH3
H CH3
+D
+D
-H
cis-2-butene butyl
trans-2-butene-d1
butane-d2
+D
D
D
DD+D
-H
cis-2-butene butyl intermediate
trans-2-butene-d1
butane-d2
isomerization
hydrogenation
Pressure-independent: → linked to surface-adsorbed H.
Pressure-dependent: → linked to volume-adsorbed H.
Reaction Mechanism
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・ Absorbed H species are essential in hydrogenation catalysis
(e.g. Butene → Butane conversion: C4H8 + D2 → C4H8D2 )
・ => Reactive species: Surface-adsorbed or subsurface-H ?
Catalytic Reactivity of Subsurface-Absorbed Hydrogen
M. Wilde, K. Fukutani, M. Naschitzki, H.-J. Freund, Phys. Rev. B 77, 113412 (2008).
M. Wilde, S. Schauermann et al., Angew. Chem. Int. Ed. 47, 9289 (2008).
→ What is the role of Pd Nanocrystal-absorbed H in
olefin hydrogenation catalysis?
Al2O3 support
volume
Pd
Modified surface electronic structure on hydride phase?
Attack of butyl by absorbed (→ resurfacing) H? ?
![Page 46: Near-surface behavior of hydrogen absorbed in palladium single crystals and nanoparticles](https://reader036.vdocuments.mx/reader036/viewer/2022062301/56814577550346895db24813/html5/thumbnails/46.jpg)
NRA: H Depth Distribution
1+3
X=0.20 (Hydride)PdHx
X=0.009 (solid solution)
TDS (1,000 L H2)
α3
α1α2
β1 β2
3
→ Does catalytic reactivity depend on subsurface depth distribution…?
Recall: Two ‘Subsurface’-Absorbed H States in Pd(110): 1 & 3
2, 1, 2: 表面水素1 : 表面付近水素化物3 : 固溶体祖の水素
→ Peak Assignment:
LEED & TDS: 表面水素
S. Ohno, M. Wilde, K. Fukutani, in preparation
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Pd(110): Reactivity of Subsurface H in hydrogenation catalysis
Compare Butane (C4H10) and H2-3 TDS:
Butane product desorption and 3 H2 peak neatly overlap!
Hydrogenation reactivity relates to H-evolution from the
3-bulk H state!
1 species from the near-surface hydride phase recombine and desorb as H2 below 180 K.
No reaction w/ butene (C4H8).
Subsurface hydride phase is NOT necessary for the
hydrogenation reaction.
C4H8 → C4H10 ?
C4H10
α1
α3
S. Ohno, M. Wilde,
K. Fukutani, in preparation
Recall: H/Pd(110)-TDS (1000 L H2@115 K)
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・ TDS/NRA → identified 2 absorbed H species :
1 → near-surface hydride phase
3 → bulk-dissolved H
・ Surface penetration mechanism:
• Activation energy → no simple Hs → Hss transition
• Absorption of Hs involves (requires) gas-phase H2
• 2 locally separated types of absorption sites, differ in probabilities for absorption and surface-H exchange
• Only bulk-dissoved H (3) active in catalysis!
Hydrogen Absorption Mechanism and Catalysis at Pd(110)
Summary & Conclusions
hydride(1)
no noJpen
Jpen
(3)
Jdiff
yes
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Acknowledgements
Thank you for your attention!
Institute of Industrial Science, University of Tokyo
K. Fukutani, Y. Murata, Y. Fukai, S. Ohno, K. Namba
Fritz-Haber Institute, Max-Planck Society, Berlin, Germany
S. Schauermann, S. Shaikhutdinov, H.-J. Freund
Dear audience:
MALT Tandem Accelerator, RCNST, University of Tokyo
H. Matsuzaki, C. Nakano
Contact: [email protected]
Supported by… CREST-JST, NEDO, MEXT, IIS
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α1 α3
α2
β1 β2
H2 TDS (Te=90 K)
Activation energy for hydrogen absorption at Pd(110)
→ Activation Energy H 1 : 0.13 eV
3 : 0.06 eV D 3 : 0.17 eV
Much smaller than predicted by the 1-D potential energy diagram
(0.3 eV)!
Arrhenius plot of 1, 3 population (Pa) ~ exp(-Ea/kBT)
peak height vs. exposure
~0.1 eV
内部水素
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Hydrogen Absorption: The Conventional Picture is too simple!
Dong et al., Surf. Sci. 411 (1998) 123
H/Pd Total Energy
RS
SSB
Eb=-0.1 eVEss=-0.2 eV
Es=-0.5 eV
Atomic HMolecular H2
1
2 H2
H
Conflicts Experiments:
Absorption Activation Energy: ~0.1±0.05 eV.
• Okuyama et al., Surf. Sci. 401 (1998) 344• S. Ohno, M. Wilde, K.
Fukutani, in preparation
H2(g) ↔ Hs ↔ Hss ↔ Hbulk states linked by a 1-D reaction coordinate…
> 0.3 eV
→ Surface-Subsurface Transition
Activation Energy Puzzle
H2
H
E**
![Page 53: Near-surface behavior of hydrogen absorbed in palladium single crystals and nanoparticles](https://reader036.vdocuments.mx/reader036/viewer/2022062301/56814577550346895db24813/html5/thumbnails/53.jpg)
・ TDS/NRA → identified 2 absorbed H species:
1 → near-surface hydride phase
3 → bulk-dissolved H
Absorption kinetics are surface-controlled
・ Investigate the surface penetration mechanism:
• Activation energy → ‘puzzle’ in 1-D scheme
• Involvement of gas phase H2
• Absorption site
Hydrogen Absorption Mechanism at Pd(110)
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Pd(110):
→ Gas-phase H2 elicits surface-adsorbed D-atoms to penetrate the surface!
Gas/Surface Hydrogen Exchange upon Absorption:
H-Absorption Mechanism at Pd(110): Isotope-labeled TDS
80x10-12
60
40
20
0
QM
S io
n cu
rren
t [A
]
400350300250200150
Temperature [K]
1.25 L D2 + 1000 L H2 (both at 130 K) H2 HD D2
-> D2 is included in alpha-1 peak (!)
1. Preadsorb Ds
2. Post-dose H2 D2Hs
DssHss
H2 HD
→ S. Ohno, M. Wilde, K. Fukutani,
(in preparation)
NRA: Absorbed H
130 K
80 s
→ Absorbed H states contain D(!)
![Page 55: Near-surface behavior of hydrogen absorbed in palladium single crystals and nanoparticles](https://reader036.vdocuments.mx/reader036/viewer/2022062301/56814577550346895db24813/html5/thumbnails/55.jpg)
Pd(110):
Without gas-phase H2, adsorbed H-atoms simply stay on the surface.
→ Absorption of pre-adsorbed surface H requires interaction with gas-phase H2!
Role of H2 gas in Absorption Mechanism:
Pd(110): No surface-subsurface transition of H without H2 gas!
1. Hs
Hs
no Hss (!)
H2
→ S. Ohno, M. Wilde, K. Fukutani,
(in preparation)
140x10-12
120
100
80
60
40
20
0
QM
S io
n cu
rren
t [A
]
400350300250200150
Temperature [K]
0.8 L H2 at 130 K quenched to 85 K kept 80 sec at 130 K
no H2
130 K
80-10000 s
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・ TDS/NRA → identified 2 absorbed H species :
1 → near-surface hydride phase
3 → bulk-dissolved H
Absorption kinetics are surface-controlled
・ Investigate the surface penetration mechanism:
• Activation energy → ‘activation energy puzzle’
• Role of gas phase H2 → Exchange with surface D
• Absorption site
Hydrogen Absorption Mechanism at Pd(110)
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・ TDS/NRA → identified 2 absorbed H species :
1 → near-surface hydride phase
3 → bulk-dissolved H
Absorption kinetics are surface-controlled
・ Surface penetration mechanism:
• Activation energy → no simple Hs → Hss transition
• Absorption of Hs involves (requires) gas-phase H2
• 2 locally separated types of absorption sites, differ in
probabilities for absorption and surface-H exchange
Hydrogen Absorption Mechanism at Pd(110)
Summary & Conclusions
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Single crystal surfaces
• Crystallographic orientation (hkl) determines the structure.
• Atomic density (~ 1015 cm-2): (110) < (100) < (111)
• Surface energy (J/m2): (110) > (100) > (111)
2 unit cells of the close-packed, face-centered-cubic (fcc) lattice structure (Pd, Pt).
y
x
z[111][100][110]
a (~ 4 Å)
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Nanocrystals
• Expose low-index facets to minimize surface energy
• Cuboctahedral shape
• Large surface area
~ 2 nm
(111) facet
(100) facet
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Pivotal role of absorbed hydrogen in hydrogenation catalysis
volume
Al2O3 support
M. Wilde, S. Schauermann et al., Angew. Chem. Int. Ed. 47, 9289 (2008).
Olefin hydrogenation catalysis requires
Pd-Nanoparticle-absorbed H (!)
Model CatalystC4H8 C4H10
H2
Pd/Al2O3
INVITED TALK (DSL-2010, Paris)
Role of Subsurface Hydrogen Diffusion in Hydrocarbon Conversions on Supported Model Catalysts
Dr. Swetlana Schauermann
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
5
4
3
2
1
0
H q
uant
ity/
ML
20x10-6
151050
H2 pressure/mbar
0.8
0.6
0.4
0.2
0.0
H:P
d ratio (absorbed H)
Surface H Absorbed H
b)
Isomerization: → r ≠ f(pH2)
hydrogenation → r = f(pH2)
NRAMBRSH3C
CH3H
D
Isomerization
Hydrogenation+
butene butane
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)()( 0 zCzC
)0,()( 000 EgkNCEI
0000 )2
()( CkNdEEI
(1) : H only on the surface
-ra
y yi
eld
(arb
. uni
ts)
6.406.396.386.37
Energy (MeV)
Example: Si(111)-H
If k is known, C0 can be obtained.
=0°
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Cf.: Thermal Equilibrium of H-Absorption in Bulk Pt
Tk
H
k
S
p
px
B
s
B
sHH expexp
2/1
2
M + x ½ H2 MHx
Van’t Hoff equation for equilibrium H-concentration in a metal hydride (MHx)
Ss = -7 kB
Hs = +0.48 eV
p(H2) = 6x10-3 Pa
Po = 105 Pa
• T = 100 K → xH = 1.4x10-31
• T = 200 K → xH = 1.8x10-19
(→ NRA detection limit: ~10-4 (100 ppm)
Entropy change upon absorption
Heat of solution (strongly endothermic)!
H2 pressure
Standard pressure
=> H-concentration in Pt-NP exceeds that of bulk Pt by many orders of magnitude!
Rough estimation of H-uptake by the interior of the Pt-nanocrystals:
→ 10-20 at. % (!)
Clausius-Clapeyron Eq.
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α3 Post
Pre
Post
Pre
α1Isotope Labeled TDS 1. Cover surface with D (H) 2. Expose to H2 (D2)
Isotope Exchange with Hsurf in 1 and 3 formation
α1
α3
D2 1.25 L -> H2 1000 L
α1; Mainly post-dosed isotopeα3; Both pre- and post-dosed isotopes