新学術領域研究・研究報告会・2011年3月5日・東京大学
Materials Design through Computics
Complex Correlation and Non-equilibrium Dynamics

Pd(110)න⚿᥏⴫㕙䈮䈍䈔䉎᳓⚛ๆ෼䉻䉟䊅䊚䉪䉴䈫⸅ᇦ෻ᔕᕈ

Markus Wilde 䊶Satoshi Ohno 䊶Katsuyuki Fukutani
Institute of Industrial Science, University of Tokyo

Hydrogen Absorption at Pd Surfaces‘Subsurface’-absorbed Hydrogen:• H state located between surface and bulk, yet different from both
• Intermediate state in Hydrogen Storage (as metal hydrides)• Important Role in Hydrogenation Catalysis (C=C, Fuel cell: HOR, ORR, …)

3 Objectives:•

Characterize ‘subsurface H’(㸢 depth location)•Understand the absorption
mechanism•

Study its reactivity•

H2 㸢 Pd(110)

Surface
SubsurfaceBulkH2H2HHz0H2time=> Clarify the pathways of hydrogen surface penetration and recombination

The Conventional Picture of Hydrogen Absorption

H/Pd㸢 Surface-Subsurface Transition

Total Energy

Activation Energy Puzzle
Atomic H
Molecular H2HB

1H2 2SSRS
Es=Ͳ0.5 eV

Eb=Ͳ0.1 eV
Ess=Ͳ0.2 eV> 0.3 eVY. Fukai, The Metal Hydrogen System, Springer 2005.
Dong et al., Surf. Sci. 411 (1998) 123Conflicts Experiments:
Absorption Activation
Energy: =0.05 eV.
Okuyama et al.,Surf. Sci. 401 (1998) 344

H2(g) 㸤 Hs 㸤 Hss 㸤 Hbulk states linked by a 1-D reaction coordinate…

Experimental Approach: TDS + NRA
M. Wilde, PRB 78 (2008) 114511.

㸢 H2(D2) exposures at given Te, desorption.
㸢 No. of H species, desorption activation energy

Surface/absorbed H (NRA)㽲 Thermal Desorption Spectroscopy (TDS):

H2 Desorption signal

H2 Desorption signal (TDS)

Combine two hydrogen detection techniques:

Surface-H signal (Hs)Subsurface-H signal (Hss)300 L H+H2 on
Pd(100) at 100 K

100200300
400500Temperature (K)㸢 lacks information on H location (on/below surface)600

㽳 Nuclear Reaction Analysis (NRA) via 1H(15N,DJ)12C: (Eres=6.385 MeV, *=1.8 keV)㸢 distinguishes surface-adsorbed from absorbed H (~2 nm depth resolution)
㸢 achieves unambiguous TDS peak assignments

Depth(nm)depth [nm]Jv [Hsurface]

Experimen
15 2+NNtalion beam
Ei>EresJv [Habsorbed]-206.376.38

2466.396.406.41

150

J-yield(cts/PC)
J rayyield [cts/PC]

J-detector

Ei=Eres

-4

100

500

probing depth:z(Ei)= (Ei-Eres)/(dE/dz)

01515N ion
N ionenergy
energy [MeV](MeV)

8Outline (I)TDS H/Pd(110)

H2 → Pd(110):
Complex TD spectrum[1]Multiple
Unidentified HͲstates

䍃TDS Peak Assignment (㸢Depth Location)
䍃Investigate origin of unusually small penetration barrier
䍃Identify reactive H species in catalytic hydrogenations[1] Surf. Sci. 126 (1983) 382.

1.) Identify Surface Adsorption Phases: LEED & TDS㪇㩷㪣㪇㪅㪊㩷㪣㩿㪈㬍㪈㪀㪇㪅㪌㩷㪣㩿㪉㬍㪈㪀㪌㪇㩷㪣㩿㪈㬍㪉㪀㫊㫋㫉㪼㪸㫂㫐㩿㪈㬍㪉㪀[1][2]_[110][001]θ㪔㪇㩷㪤㪣θ㪔㪈㪅㪇㩷㪤㪣θ㪔㪈㪅㪌㩷㪤㪣⴫㕙㱍2 ⴫㕙

㱎1

㱍3
㱍1

㱎2

[1] Surf. Sci. 411 (1998) 123[2] Surf. Sci. 327 (1995) 505㪇㩷㪣㪇㪅㪊㩷㪣θ㪔㪖㩷㪤㪣㪇㪅㪌㩷㪣㪌㪇㩷㪣2.) Clarify Depth Range of Absorbed H States: NRA & TDS
NRA: H Depth Distribution

TDS (1,000 L H2)
㱍2

㱍1PdHx

X=0.14 (水素化物)
D1+D3
D3

X=0.007(固溶体)

㱍3

㱎1

㱎2

→ Complete Peak Assignment:

D2, E1, E2: SurfaceͲadsorbed H
D1 :NearͲsurface hydrideD3 :BulkͲdissolved H
First-time observation of two different absorbed H states in Pd(110)!

Outline (II)䍃TDS Peak Assignment (㸢Depth Location)
䍃Investigate origin of unusually small penetration barrier
䍃Identify reactive H species in catalytic hydrogenations

Absorption activation energy: A puzzle, indeed!
Arrhenius plot of
D1, D3 population (Pa)H2 TDS (Te=90 K)

㱍1 㱍3

㱍2
㱎1

㱎2

DD peak height vs. exposure㸢 Activation Energy
H D1 : 0.13 eV
D3 : 0.06 eV
D D3 : 0.17 eV

Much smaller than predictedby static potential energy
diagram (0.3 eV)!

Absorption pathway: Isotope-labeled hydrogen exposure→ Clarify participation of surfaceͲadsorbed H and gas phase H2Experiment:
1. Saturate Surface with D
2. PostͲdose H2
H2
D2

D2 1.25 L + H2 1,000 L @115 K

α1α3Result: D3 (+ surface species):→ complete isotopic scrambling.
 D1: Pure postͲdosed isotope→ no isotopic scrambling.

Different absorption
pathways exist for the D1
and D3 absorbed states!

D1 Formation mechanism: Direct molecular absorption involved?

α1Both H atoms of the postͲdosed H2enter the D1 state.

全系のエネルギーේሶ⁁ಽሶ⁁
12

Deep surface adsorption state isbypassed ! (H2として接近?)

E**REs

通常の解離吸着を経ているとは考えにくい

脱離のE*=0.30 eV(TDSより)Crossing point with atomic H state
PES is shifted towards the surface(E**=0.13 eV).
㸢 Molecular Precursor State?
cf.) H2/Pd(210) PRL 87 (2002) 096103

D3 Formation: Pathway via reconstructed surface?

α3
Isotopic scrambling w/ Ds.

全系のエネルギーේሶ⁁(⴫㕙ౣ᭴ᚑᓟ)Absorption path includes
surface adsorption state!

ಽሶ⁁R
Es=0.2 eV
Es=0.5 eV

EbEss

㸢 Small activation barrier?

Destabilized Hs state onreconstructed surface!

Outline (III)䍃TDS Peak Assignment (㸢Depth Location)
䍃Clarify origin of unusually small penetration barrier
䍃Identify reactive H species in catalytic hydrogenations

㸢 Proved recently:
Al2O3support

Olefin hydrogenation catalysis requires
Pd ClusterͲabsorbed H (!)

volumeModel Catalyst

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).

Hydrogenation catalysis: Butene C4H8 + H2 㸢 C4H10 Butane
1.) Prepare D1, D3 (1,000 L H2 @120 K)
2.) Dose Butene→Observe Butane desorption by TDS※ Butane/Pd(110) (w/o H):
㸢 Tdes = 167 K

Reaction during TDS!
→ Active H species?
Peak @ 190 K

C4H8C4H10C4H8C4H80.5 ML H/Pd(110) ਄䈪䊑䊁䊮㸢䊑䉺䊮䈱᳓⚛ൻ䈲起きないS. Katano et al., J. Phys. Chem. C 112 (2008) 17219.

C4H8

Identify active species in catalytic olefin hydrogenation
H2 1,000 L @115 K

D1 species from the nearͲsurface
hydride phase recombine anddesorb as H2 below 180 K.
No reaction w/ butene.
Subsurface hydride phase
is NOT necessary for thehydrogenation reaction.

C4H8Comparison of butane and D3 TDS

Butane desorption andD3 H2 peak overlap!

Hydrogenation reactivity
relates to HͲevolution
from the D3Ͳbulk H state!

Summary & Conclusions• First complete assignment of multiple H2 desorption peaks from Pd(110):㸢 Surface (E1, E2, D2), subsurface hydride (D1), bulk (D3).
• Identified new TDS feature (bulk H, “D3”).
• Unexpectedly small surface penetration barriers suggests existence of alternative reaction
coordinates that link gas phase molecular H2 and the subsurface/ bulk states without
including the chemisorption potential minimum.
• Different penetration and recombination pathways for the two absorbed HͲspecies (D1,D3) appear to exist. D1 does neither scramble with surfaceͲH nor react with adsorbed
butene, while D3 does both.
• Possible explanations:(a) D1 desorption temperature is below the reaction threshold.
(b) D1 Recombination pathway bypasses surfaceͲadsorbed species.
• Identified reactive H species in catalytic olefin hydrogenations as the D3 (bulk) state.
• The contribution of subsurface hydride to the catalytic hydrogenation activity seems thus
unlikely.

Dynamics and Reactivity of Absorbed H: Aspirations & Outlook
Single Crystals
㸢 Clarify the surface penetration mechanism in presence of gas phase H2.
㸢 Involvement of molecular precursor states? Dissociation: Where?

Nanoparticles
㸢 Clarify the origin of strong H-stabilization in nanocrystals.
㸢 Establish size-dependence of 'Hs.
㸢 Construct HRBS System.

Absorbed (‘Subsurface’) Hydrogen㸢 Role in Hydrogenation Catalysis?
㸢 Identify reactive species.

volume=> Computics-aided design of ideal hydrogen storage media and hydrogenation catalysts!

Summary & Conclusions

䍃TDS Peak Assignment (㸢Depth Location)
㸢 Surface (E1, E2, D2), subsurface hydride (D1), bulk (D3).
㸢 Identified new TDS feature (bulk H, “D3”).

䍃Origin of unusually small penetration barrier㸢 D1: Reaction coordinate that bypasses surface adsorption state!
㸢 D3: Reaction coordinate that includes surface adsorption state!

䍃 Reactive H species in catalytic hydrogenations
㸢 D3, D1: Different reactivity in butene (C4H8) hydrogenation!
㸢 Related to different recombination/’resurfacing’ reaction coordinates?