near-surface behavior of hydrogen absorbed in palladium single crystals and nanoparticles
DESCRIPTION
日本真空協会 産学連携委員会. Tokyo January 25, 2012. 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. Concept. HYDROGEN-IN (VACUUM) TECHNOLOGY. Bulk H-solubility Phase transition Lattice expansion Diffusion Embrittlement - PowerPoint PPT PresentationTRANSCRIPT
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
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)
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
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
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
Abundance of Elements in the Universe
Atomic Number
75 % of all matter is Hydrogen !
‘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)
• 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)
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?
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)
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.
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)
=> 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
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)
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
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
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 !
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
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
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) !
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
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
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
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→
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
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.
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
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)
→ 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
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
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
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
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
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
(≠)
• 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.
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
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
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)
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
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
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)
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).
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?
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
・ 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? ?
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
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)
・ 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
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
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
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**
・ 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)
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(!)
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
・ 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)
・ 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
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)
Nanocrystals
• Expose low-index facets to minimize surface energy
• Cuboctahedral shape
• Large surface area
~ 2 nm
(111) facet
(100) facet
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
)()( 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°
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.
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