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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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Presentation on theme: "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."— Presentation transcript:

1 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

2 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. H 2 : TMP: rotor speed SIP: low sputtering efficiency Gas phase Molecular H 2 Pressure Temperature Slow H 2 outgassing from Penetration through vacuum chamber materials => Best in UHV, XHV: NEG-Support ( Pa)

3 Clean Energy: Fuel cell (HOR) Hydrogen storage Catalysis: NH 3 synthesis: N H 2 → 2NH 3 Olefin (C=C) Hydrogenation: C n H 2n → C n H 2n+2 Important Applications of Hydrogen O 2 H 2

4 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 ! H2H2 z 0 H

5 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

6 Abundance of Elements in the Universe Atomic Number 75 % of all matter is Hydrogen !

7 ‘Seeing’ Hydrogen is difficult... Ion scattering (RBS) fails:  H-cross section small ( σ RBS ∝ 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-e- p+p+ Particle emission Standard chemical analysis (electron spectroscopy) fails:  (because H only has a single 1s electron …) He + → Ag/Si(100) OSi Ag (H)

8 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 HDHDH2H2 D2D2 m/e12324 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 T e 排気 H2H2 脱離速度 (Polanyi-Wigner 式 ) r=ν n θ n exp(-E * /kT)

9 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?

10 Resonant Nuclear Reaction Analysis (NRA) via 1 H( 15 N,  ) 12 C Hydrogen Depth Profiling: Non-destructive ・ Quantitative ・ High-resolution 15 N + 1 H → 16 O* → 12 C +  +  (4.43 MeV) E res = MeV Experimental H E i =E res  -detector (BGO) N probing depth: E i >E res z(E i )= (E i -E res )/(dE/dz) z → energy loss  [H bulk ] H 15 N 2+ ion beam stopping power (3.9 keV/nm for Pd) 0 K. Fukutani et al., PRL 88 (2002) . M. Wilde et al., J. Appl. Phys. 98 (2005)  [H surface ]  Sensitivity: Surface Coverages: 1% ML (~10 13 cm -2 ) Bulk concentrations: ~400 ppm (~10 18 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)

11 15 N+ 1 H → 12 C+  +  ( 4.43MeV ) Q m = MeV Res. Energy : E R = MeV Res. Width :  =1.8 keV Resonant nuclear reaction 1 H( 15 N,  ) 12 C Cross section: 1650 mbarn J. Radioanal. Chemistry 77 (1983) 149.

12 Experimental Setup for NRA 質量・ エネルギー分析器 (90 o 偏向磁石 ):  E = 3 keV Extractor Ion Source (SNICS): Cs +  Ti 15 N+C  C 15 N - Inside the Accelerator Tank Switching Magnet Terminal: MeV 5 MeV Van-de-Graaff Tandem Accelerator (MALT: AMS) (Univ. Tokyo)

13 => Combination of surface characterization and shallow H depth profiling (NRA). LEED 243 eV Ti(0001) Structural Order Chemical Composition Reactivity towards H 2, H. Ultra-High Vacuum System for Sample Preparation and in-situ NRA

14 Combine two hydrogen detection techniques: Our Experimental Approach: TDS + NRA 東京大学 ) ① Thermal Desorption Spectroscopy (TDS): → H 2 (D 2 ) exposures at given T e, desorption. → No. of H species, desorption activation energy → lacks information on H location (on/below surface) Experimen tal E i =E res  -detector N probing depth: E i >E res z(E i )= (E i -E res )/(dE/dz)  [H absorbed ] 15 N 2+ ion beam 0  [H surface ] ② Nuclear Reaction Analysis (NRA) via 1 H( 15 N,  ) 12 C: (E res =6.385 MeV,  =1.8 keV) → distinguishes surface-adsorbed from absorbed H (~2 nm depth resolution) 300 L H+H 2 on Pd(100) at 100 K 15 N ion energy (MeV) Depth (nm)  -yield (cts/  C) M. Wilde, PRB 78 (2008) → unambiguously identifies TDS features 15 N + 1 H → 12 C +  +  (4.43 MeV)

15 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

16 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: E S = eV * 吸着エネルギー → Bulk: E B = -0.1 eV * 溶解エンタルピー → Subsurface: E SS = eV * In general: E S (< E SS ) < E B Top view z 0 H Side view Surface Subsurface Bulk B Hs:Hs: > 0 吸熱 < 0 発熱 HsHs * Pd(100) ‘Reaction coordinate’ 固体内部 表面 気祖

17 H 2 Thermal Desorption SpectrumNRA H-Depth Profile (T<130 K) Surface and “Subsurface” H in Pd(100) after atomic H (+H 2 ) dosage (300 L) at 100 K. M. Wilde et al., Surf. Sci (2001) 346. Depth Extension of Subsurface H in Pd(100) => ‘Subsurface’ Hydrogen is NOT necessarily confined to first layer sites! 15 N 2+   H ss in ~ 20 atomic layers => ‘hydride’ phase H ss desorbs before H s !

18 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.) H 2 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! H2H2 z 0 H H2H2 H Side view Surface Subsurface Bulk B H 2 time Hs:Hs: > 0 endothermic < 0 exothermic HsHs E S (< E SS ) < E B  E>0

19 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 H 2. z 0 H H2H2 H or H2H2 ? With H 2 H TT Without H 2 H/Pd(100) (fcc) 0.3 eV

20 H 2 Thermal Desorption SpectrumNRA 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 15 N 2+   Okuyama et al., Surf. Sci. 401 (1998) 344. H ss bypasses surface-H in desorption (no isotopic exchange)! ! Similar on Pd(110) and Pd(111) !

21 A comparison: H-Absorption of Surface-H into Ti(0001) (!) M. Wilde and K. Fukutani, Phys. Rev. B 78, (2008). (TDS: H 2 -saturated by L H 2 at 100 K) NRA: Signal of surface hydrogen (  H = 0.4 ML at 200 K). T det =318±22 K H 2 Thermal Desorption Spectrum => Although H vanishes from the surface around 320 K, no H 2 desorption occurs. NRA H-Depth Profile (T=300 K) Ti-Bulk: [H]=500 ppm * hcp hollow fcc hollow H/  -Ti(0001) (hcp)  H s = eV/H

22 Pd(100): → Surface-H desorbs (at ~330 K): E s =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 H2H2 HsHs H ss HsHs HbHb H2H2 z 0 HsHs HsHs HbHb H2H2 T = 330 K T = 180 K T ~ 320 K T >650 K

23 Absorption capacity for surface-H in the near-surface region T pen =318±22 K T des =340 K => Consider possibility to dissolve the surface H atoms into the bulk by in-diffusion: Dissolvable H coverage [ML] = Diffusion length (T) x H solubility (T) / (1/2 layer distance) L D (T, t) Phys. Rev. B 78, (2008) → Near-surface H absorption involve both surface and bulk properties! * Pd:  H s = eV/H Ti:  H s = eV/H

24 Hydrogen Absorption Mechanism at Pd(110) Identify multiple H-states (→ NRA) H 2 → 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 surface H2H2 →

25 θ=1.5 MLθ=1.0 MLθ=? ML 0 L0.3 L0.5 L50 L θ=0 ML [1] Surf. Sci. 411 (1998) 123 [2] Surf. Sci. 327 (1995) 505 [1] [2] β 1 β 2 α2α2 α1α1 α3α3 0 L0.3 L0.5 L50 L (1×1) (2×1) (1×2) streaky (1×2) A) Identify Surface Adsorption Phases: LEED & TDS 表面

26 α1α1 α3α3 T exp =90 K 0.5 – 2000 L α2α2 β2β2 β1β1 TDS after large exposures :曝露温度依存性 β 2, β 1, α 2 (saturate at 0.5 L) -> H at the surface and in the first subsurface sites α 1, α 3 (never saturate) -> H in the Pd interior α 1 disappears at T exp ≥ 145 K ☞ Surf. Sci. 126 (1983) 382. Surf. Sci. 195 (1988) L199. α3α3 α2α2 β2β2 β1β1 T exp =145 K 0.5 – 2000 L α 1 and α 3 absorption depend on the exposure temperature (T exp ) ☞ Pd(111); Surf. Sci. 181 (1987) L147. Pd(100); Surf. Sci. 401 (1998) 344.

27 NRA Depth Profile 20.1% (hydride) Hydrogen concentration 0.9% (solid solution) S (=α 2, β 1, β 2 ) S, α 1, α 3 S, α 3 α 1 ; near surface hydride α 3 ; bulk solid solution > 50 nm (TDS shows 3 ML of α 3 )  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 α 1 and α 3

28 H/Pd 相図 H/Pd (Phase Diagram): α 相と β 相 α相α相 β相β相 α 相‥低水素濃度相,固溶体  相‥高水素濃度相,水素化物 相転移 Pd では α→β の相転移の際に 格子が広がり,系のエネルギー は下がる.水素の拡散は これにより遅くなる. α- 相 x ~  - 相 x ~ 0.65 (Extrapolation: Phase equilibrium at → p(H 2 ) = mmHg, 130 K) Wicke et al., Z. Phys. Chem. Neue Folge 64, 225 (1969)

29 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% ×100 = 30% Hydride: ~ 65% H2H2 NRA: average [H] = 20% 15 N ion beam Solid solution phase: 0.009% Langmuir 19 (2003) (300 K, bar H 2 )

30 Conditions Enabling Hydride Nucleation Low temperature & high surface penetration rates (J pen >) Near-surface accumulation of absorbed (subsurface) H => 水素化物の核生成 H2H2 J pen サブサーフェス H の堆積 水素化物の核生成 (low T) H2H2 J pen (high T) J diff J diff ~ L D = (D  t) 1/2 = D 0 1/2 exp(-E diff /2k B T) J diff (100 K: L D = 10 nm in 100s)(145 K: L D = 640 nm in 100s) J pen > J diff J pen < J diff

31 H 2 is absorbed faster than D 2 : α 1 : 40 x α 3 : 2 x T exp = 115 K (Exposures ≤ 2000 L) H2H2 D2D2 C) Isotope effects in hydrogen absorption at Pd(110) → A normal isotope effect: H faster than D (opposite to bulk diffusion)! => The absorption rate is controlled at the surface! 内部水素 α1α1 α3α3 α1α1 α3α3 => Penetration pathways are different! S. Ohno, M. Wilde, K. Fukutani, in preparation

32 → TDS after isotope-labeled hydrogen exposure Experiment: 1. Saturate Surface with D 2. Post-dose H 2 D L + H 2 1,000 K α1α1 α3α3 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 H2H2 D2D2 α1α1 α3α3  1,  2,  2 D S. Ohno, M. Wilde, K. Fukutani, in preparation

33 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 ML (initially: 1.5 ML D) Hydride nucleation at a few specially active sites (T e <145 K) Isotopic Composition: Hydride Phase (  1 ) x x J pen J diff D H2H2 Cf: Pd thin film – AFM: Langmuir 19 (2003) S. Ohno, M. Wilde, K. Fukutani, in preparation

34 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 K α1 ・ θ α1 ≒ K α3 ・ θ α3 ∴ K α1 ≒ (θ α3 / θ α1 ) ・ K α3 >> K α3 Isotopic Composition: Solid Solution Phase (  3 ) => Gas-phase H 2 -assisted penetration of surface-adsorbed D (first observation at a Pd single crystal) S. Ohno, M. Wilde, K. Fukutani, in preparation

35 Hydride and Solid Solution Formation Mechanism α 1 contains 0.06 ML (4%) of prechemisorbed species: -> Nucleation only at ~ 4% of special surface sites. -> Fast penetration rate (J pen >) -> Surface diffusion toward the ‘entrance sites’ is prohibited (no isotope exchange with H s ) Pre-dosed surface isotope in α 3 increases together with post-dosed isotope. Complete isotopic exchange with H s during penetration. Slower penetration rate. S. Ohno, M. Wilde, K. Fukutani, in preparation hydride (  1 ) no J pen (3)(3) J diff yes

36 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

37 Olefin Hydrogenation Catalysis Concerted reaction is extremely unlikely in the gas phase Large activation energy barrier (E a ): → Small reaction rate: R = exp(-E a /RT) C 4 H 8 C 4 H 10 D 2 D2D2 Butene Butane-d2 EaEa H3CH3C CH 3 H H + D 2 Reactants  G R < 0 H3CH3C CH 3 H H D … D H3CH3C CH 3 H H D D Necessary elementary steps: D-D bond break (~4.5 eV, 430 kJ/mol) C=C  -bond break (~ 615 kJ/mol) C rehybridization: sp 2 → sp 3 new C-H bond formation (414 kJ/mol x2) Example: Butene Hydrogenation Product Transition state (hypothetical) ≠  S R << 0

38 (≠) Catalyst … drastically reduces activation energy barrier (E a ’ << E a ) … enables reaction at far lower temperature … itself is not consumed in the reaction. EaEa H3CH3C CH 3 H H + D 2 Reactants H3CH3C CH 3 H H D … D H3CH3C CH 3 H H D D Product Transition state ≠’ Olefin Hydrogenation Catalysis +D 2 D D D D -H cis-2-butene butyl intermediate trans-2-butene-d 1 butane-d 2 isomerization hydrogenation +D Pd surface Ea’Ea’ New, easier elementary steps: Olefin (C 4 H 8 ) adsorbs on catalyst, C=C  -bond opens. D 2 bond breaks spontaneously on Pd surface (dissociative adsorption) Coadsorbed D atoms easily attach to the intermediate; products desorb.

39 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 Al 2 O 3 support

40 Pd-Nanocluster-Specific Reactivity for Alkene Hydrogenation: C n H 2n + H 2 → C n H 2n+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 Al 2 O 3 Pd Single Crystal (n=5): (pentene) (pentane) [D 2 ]pentane (C 5 H 10 D 2 ) D 2 + pentene (C 5 H 10 ) H inside NC? D 2 -TDS D D NRA! TDS

41 Oxide-supported Pd nano-crystallites: Morphology K.H. Hansen et al., PRL 83 (1999) x65 nm 2. 2 ML 300 K Aspect ratio: h/w=0.18±0.03 (constant for w>5.5 nm) Shape of Pd nano-crystallites on Al 2 O 3 /NiAl(110)

42 In-situ Nanocrystal Preparation for H-NRA 1.) Al 2 O 3 /NiAl(110) substrate: → NiAl(110) cleaning + in-situ oxidation. 2.) 5.85 Å Pd 300 K 3.) NRA: 1 H( 15 N,  ) 12 C z(E i ) = (E i -E res )/[(dE/dz)cos(  i )] grazing ion incidence (  i =75 o ) beam collimation <2 mm (slits) UPH ( %) H 2 background (<2x10 -3 Pa) _ + NEC 5UD Tandem 17.5 nm x 17.5 nm

43 Hydrogen Absorption in Al 2 O 3 -supported Pd nanocrystals 4-fold enhanced depth resolution in 75 o 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 Al 2 O 3 at 90 K, 2·10 -5 Pa H nm x 17.5 nm  i =75 o Al 2 O 3 /NiAl(110) Pd h~2 nm 15 N H  50 nm x 50 nm M. Wilde et al., Phys. Rev. B 77, (2008).

44 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  H S 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 nm74%, i = 5 S-3 3 nm60%, i = 7 S-5 5 nm41%, i = 12 Proposed explanation: ‘subsurface sites’ (→ large surface/volume ratio)

45 p(H 2 )-dependent H-uptake in Pd nanocrystals on Al 2 O 3 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 Al 2 O 3 /NiAl(110) 1 ML (111) (100) 2x10 -5 mbar 6x10 -6 mbar 2x10 -7 mbar M. Wilde et al., Phys. Rev. B 77, (2008).

46 Reactivity Study of Olefin Conversion over Pd/Al 2 O 3 Model Cat NRA measurement under reaction conditions  i =75 o 15 N Al 2 O 3 /NiAl(110) H Pd Alumina-Supported Model Catalysts QMS Sample 4 Å Pd/Fe 3 O 4 /Pt(111) 4 Å Pd/Al 2 O 3 /NiAl(110) cis-2-butene beam (pulsed) M. Wilde, S. Schauermann et al., Angew. Chem. Int. Ed. 47, 9289 (2008). Molecular Beam Reactive Scattering D 2 beam (steady) 2-4x10 -6 mbar)  H S [eV] bulkNP 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?

47 D 2 -pressure dependent reactivity of hydrogenation Isomerization: → r ≠ f(p H2 ) hydrogenation → r = f(p H2 ) NRA MBRS +D D D D D -H cis-2-butene butyl intermediate trans-2-butene-d 1 butane-d 2 isomerization hydrogenation Pressure-independent: → linked to surface-adsorbed H. Pressure-dependent: → linked to volume-adsorbed H. Reaction Mechanism

48 ・ Absorbed H species are essential in hydrogenation catalysis (e.g. Butene → Butane conversion: C 4 H 8 + D 2 → C 4 H 8 D 2 ) ・ => 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, (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? Al 2 O 3 support volume Pd Modified surface electronic structure on hydride phase? Attack of butyl by absorbed (→ resurfacing) H? ?

49 NRA: H Depth Distribution 1+31+3 X=0.20 (Hydride) PdH x X=0.009 (solid solution) TDS (1,000 L H 2 ) α3α3 α1α1 α2α2 β1β1 β2β2 3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

50 Pd(110): Reactivity of Subsurface H in hydrogenation catalysis Compare Butane (C 4 H 10 ) and H 2 -  3 TDS: Butane product desorption and  3 H 2 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 H 2 below 180 K. No reaction w/ butene (C 4 H 8 ). Subsurface hydride phase is NOT necessary for the hydrogenation reaction. C 4 H 8 → C 4 H 10 ? C 4 H 10 α1α1 α3α3 S. Ohno, M. Wilde, K. Fukutani, in preparation Recall: H/Pd(110)-TDS (1000 L H K)

51 ・ TDS/NRA → identified 2 absorbed H species :  1 → near-surface hydride phase  3 → bulk-dissolved H ・ Surface penetration mechanism: Activation energy → no simple H s → H ss transition Absorption of H s involves (requires) gas-phase H 2 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 J pen (3)(3) J diff yes

52 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: Supported by… CREST-JST, NEDO, MEXT, IIS

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54 α1α1 α3α3 α2α2 β1β1 β2β2 H 2 TDS (T e =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 (P a ) ~ exp(-E a /k B T)      peak height vs. exposure ~0.1 eV 内部水素

55 Hydrogen Absorption: The Conventional Picture is too simple! Dong et al., Surf. Sci. 411 (1998) 123 H/Pd Total Energy R S SS B E b =-0.1 eV E ss =-0.2 eV E s =-0.5 eV Atomic H Molecular H 2 H2H2 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 H 2 (g) ↔ H s ↔ H ss ↔ H bulk states linked by a 1-D reaction coordinate… > 0.3 eV → Surface-Subsurface Transition Activation Energy Puzzle H2H2 H E**

56 ・ 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 H 2 Absorption site Hydrogen Absorption Mechanism at Pd(110)

57 Pd(110): → Gas-phase H 2 elicits surface-adsorbed D-atoms to penetrate the surface! Gas/Surface Hydrogen Exchange upon Absorption: H-Absorption Mechanism at Pd(110): Isotope-labeled TDS 1. Preadsorb D s 2. Post-dose H 2 D2D2 HsHs D ss H ss H2H2 HD → S. Ohno, M. Wilde, K. Fukutani, (in preparation) NRA: Absorbed H 130 K 80 s → Absorbed H states contain D(!)

58 Pd(110): Without gas-phase H 2, adsorbed H-atoms simply stay on the surface. → Absorption of pre-adsorbed surface H requires interaction with gas-phase H 2 ! Role of H 2 gas in Absorption Mechanism: Pd(110): No surface-subsurface transition of H without H 2 gas! 1. H s HsHs no H ss (!) H2H2 → S. Ohno, M. Wilde, K. Fukutani, (in preparation) no H K s

59 ・ 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 H 2 → Exchange with surface D Absorption site Hydrogen Absorption Mechanism at Pd(110)

60 ・ 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 H s → H ss transition Absorption of H s involves (requires) gas-phase H 2 2 locally separated types of absorption sites, differ in probabilities for absorption and surface-H exchange Hydrogen Absorption Mechanism at Pd(110) Summary & Conclusions

61 Single crystal surfaces Crystallographic orientation (hkl) determines the structure. Atomic density (~ cm -2 ): (110) < (100) < (111) Surface energy (J/m 2 ): (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)

62 Nanocrystals Expose low-index facets to minimize surface energy Cuboctahedral shape Large surface area ~ 2 nm (111) facet (100) facet

63 Pivotal role of absorbed hydrogen in hydrogenation catalysis volum e Al 2 O 3 support M. Wilde, S. Schauermann et al., Angew. Chem. Int. Ed. 47, 9289 (2008). Olefin hydrogenation catalysis requires Pd-Nanoparticle-absorbed H (!) Model Catalyst C 4 H 8 C 4 H 10 H 2 Pd/Al 2 O 3 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 Isomerization: → r ≠ f(p H2 ) hydrogenation → r = f(p H2 ) NRA MBRS H3CH3C CH 3 H D Isomerization Hydrogenation + butene butane

64 (1) : H only on the surface Example: Si(111)-H If k is known, C 0 can be obtained.  =0°

65 Cf.: Thermal Equilibrium of H-Absorption in Bulk Pt M + x ½ H 2 MH x Van’t Hoff equation for equilibrium H-concentration in a metal hydride (MH x )  S s = -7 k B  H s = eV p(H 2 ) = 6x10 -3 Pa P o = 10 5 Pa T = 100 K → x H = 1.4x T = 200 K → x H = 1.8x (→ NRA detection limit: ~10 -4 (100 ppm) Entropy change upon absorption Heat of solution (strongly endothermic)! H 2 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: → at. % (!) Clausius-Clapeyron Eq.

66 α3α3 Post Pre Post Pre α1α1 Isotope Labeled TDS 1. Cover surface with D (H) 2. Expose to H 2 (D 2 ) Isotope Exchange with H surf in  1 and  3 formation α1α1 α3α3 D L -> H L α 1 ; Mainly post-dosed isotope α 3 ; Both pre- and post-dosed isotopes


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