Near-surface behavior of hydrogen absorbed in palladium single crystals and nanoparticles

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)






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


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



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


=> 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


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]


0.6

0.4

0.2

0.0Isot

ope　

com

posi

tion　

[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.


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]


1.0

0.8

0.6

0.4

0.2

0.0Isot

ope　

com

posi

tion　

[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)


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.


hydride(1)

no noJpen

Jpen

(3)

Jdiff

yes






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


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)


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).


→ 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


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:



Absorption kinetics are surface-controlled

･ Investigate the surface penetration mechanism:

• Activation energy → ‘puzzle’ in 1-D scheme

• Involvement of gas phase H2

• Absorption site


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





･ Investigate the surface penetration mechanism:

• Activation energy → ‘activation energy puzzle’

• Role of gas phase H2 → Exchange with surface D

• Absorption site






･ 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


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

http://upload.wikimedia.org/wikipedia/commons/b/b0/Palladium_1.jpg

Pivotal role of absorbed hydrogen in hydrogenation catalysis

volume

Al2O3 support


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)


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

Documents

Near-surface behavior of hydrogen absorbed in palladium single crystals and nanoparticles