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

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

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

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  1. 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

  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’ • Gas phase • Molecular H2 • Pressure • Temperature ‘Subsurface’ ? • Slow H2 outgassing from • Penetration through • vacuum chamber materials Pumping limitations vs. H2: TMP: rotor speed SIP: low sputtering efficiency => Best in UHV, XHV: NEG-Support (10-10 Pa)

  3. Important Applications of Hydrogen H2 O2 • Clean Energy: • Fuel cell (HOR) • Hydrogen storage • Catalysis: • NH3 synthesis: N2 + 3 H2 →2NH3 • Olefin (C=C) Hydrogenation: CnH2n → CnH2n+2

  4. H H2 gas-phase H2 dissociative adsorption kads surface-H z kpen 0 penetration absorption hydrogen-rich layer (hydride) 'subsurface'-H 進入 吸収 phase boundary 'bulk-dissolved’ hydrogen hydrogen-poor phase (a) Kdiff-a in-diffusion transition metal or alloy (Pd, Ni, Ti, Y, Zr, Mg ...) 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 capacitiesandcharge/release kinetics! Goal: Obtain atomic level understanding of absorption and desorption processes ! → Clarify the microscopic pathways of hydrogen penetration and recombination

  5. Outline: Hydrogen Absorption at Metal Surfaces • Introduction: Hydrogen and (Vacuum) Technology • Detection of Subsurface-H: Distinction from Surface-H • Formation of Subsurface-H: Absorption Mechanism • Role of near-surface absorbed H in Catalysis

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

  7. ‘Seeing’ Hydrogen is difficult ... ＋ • Standard chemical analysis (electron spectroscopy) fails: • (because H only has a single 1s electron …) Core hole relaxation Core ionization Particle emission → PIXE, … X-ray photon, ion, or electron → AES → XPS (ESCA) • Ion scattering (RBS) fails: • H-cross section small (σRBS∝Z2) • H-signal buried under large background from sample bulk e－ He+→ Ag/Si(100) (H) O Si Ag p+

  8. 昇温脱離分光法（TDS） • Mostly applied: Mass Spectroscopy 検出器 (質量分析器) 排気 H2 脱離スペクトルを測る 曝露温度 Te 気体に曝露 吸着･吸蔵 加熱 脱離速度 (Polanyi-Wigner式) r=νnθnexp(-E*/kT) Measurement of hydrogen desorption activation energies: • 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).

  9. Example: H Adsorption at Pd(100) 100 200 300 400 500 Temperature (K) Thermal desorption spectrum Pd(100) 4-fold hollow a，b 　？ • From where do the H states originate? H. Okuyama et al., Surf. Sci. 401 (1998) 344.

  10. H Resonant Nuclear Reaction Analysis (NRA) via 1H(15N,ag)12C Hydrogen Depth Profiling: Non-destructive ・ Quantitative ・ High-resolution 15N + 1H → 16O* →12C + a + g(4.43 MeV) Eres = 6.385 MeV g  [Hsurface] Ei=Eres H Experimental g  [Hbulk] N energy loss g-detector (BGO) Ei>Eres 15N2+ ion beam z → 0 z(Ei)= (Ei-Eres)/(dE/dz) stopping power (3.9 keV/nm for Pd) probing depth: • 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) K. Fukutani et al., PRL 88 (2002) 116101． M. Wilde et al., J. Appl. Phys. 98 (2005) 023503.

  11. Resonant nuclear reaction 1H(15N,ag)12C 15N+1H →12C+a+g（4.43MeV）Qm= 4.9656 MeV Res. Energy : ER = 6.385 MeV Res. Width : G=1.8 keV Cross section: 1650 mbarn J. Radioanal. Chemistry 77 (1983) 149.

  12. Experimental Setup for NRA 5 MeV Van-de-Graaff Tandem Accelerator (MALT: AMS) (Univ. Tokyo) Ion Source (SNICS): Cs+Ti15N+CC15N- Extractor Inside the Accelerator Tank Terminal: +2.48 MeV Switching Magnet 質量・ エネルギー分析器 (90o偏向磁石): DE = 3 keV

  13. AES 2.5 keV Ti(0001) Ultra-pure H2 viewport Pbase < 1 x 10-8 Pa H+H2 doser dN/dE [arb. units] ion gun UHV C(KLL) FC g-ray detector BGO S(KLL) Ti(LMM) deflector NRA ion beam 0 100 200 300 400 500 600 energy [eV] LEED 243 eV LEED / AES sample (80 - 1400 K) shielded QMS (RGA + TDS) Ti(0001) Ultra-High Vacuum System for Sample Preparation and in-situ NRA Chemical Composition Structural Order Reactivity towards H2, H. => Combination of surface characterization and shallow H depth profiling (NRA) .

  14. ② Nuclear Reaction Analysis (NRA) via 1H(15N,ag)12C: (Eres=6.385 MeV, G=1.8 keV) • → distinguishes surface-adsorbed from absorbed H (~2 nm depth resolution) Depth (nm) g  [Hsurface] Ei=Eres g-detector g-yield (cts/mC) Experimental N 15N2+ ion beam g  [Habsorbed] Ei>Eres 0 probing depth: z(Ei)= (Ei-Eres)/(dE/dz) 15N ion energy (MeV) Our Experimental Approach: TDS + NRA (@ 東京大学) M. Wilde, PRB 78 (2008) 114511. Combine two hydrogen detection techniques: • 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) 300 L H+H2 on Pd(100) at 100 K • → unambiguously identifies TDS features 15N + 1H→ 12C + a + g(4.43 MeV)

  15. Outline: Hydrogen Absorption at Metal Surfaces • Introduction: Hydrogen and (Vacuum) Technology • Detection of Subsurface-H: Distinction from Surface-H • Formation of Subsurface-H: Absorption Mechanism • Role of near-surface absorbed H in Catalysis

  16. Fundamental: Energy Topography of H near Metal Surfaces 固体内部　　　　表面　　　　気祖 • 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 * Pd(100) DHs: > 0 吸熱 < 0 発熱 B DHs ‘Reaction coordinate’ Side view Top view z Surface Subsurface Bulk H 0 Surface-adsorbed hydrogen is bound to low-coordinated metal surface atoms: ALWAYS energetically more stable than H absorbed in the bulk!

  17. Depth Extension of Subsurface H in Pd(100) H2 Thermal Desorption Spectrum NRA H-Depth Profile (T<130 K) a b 15N2+ • Hss in ~ 20 atomic layers • => ‘hydride’ phase • Hss desorbs before Hs ! 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. => ‘Subsurface’ Hydrogen is NOT necessarily confined to first layer sites!

  18. H Absorption at Metal Surfaces: The Microscopic Perspective Surface-adsorbed hydrogen is ALWAYS more strongly bound than in the bulk (absorbed H). ES (< ESS) < EB • Elementary steps of H-Absorption: • → 1.) H2 dissociation at the surface. • → 2.) Surface saturation (rapid). • → 3.) Penetration into the bulk (slow). DHs: > 0 endothermic < 0 exothermic B DE>0 DHs Side view Top view 4-fold hollow site H2 H2 z Surface Subsurface Bulk H H 0 H2 time => Hydrogen absorption (‘starting’ at the surface) is an activated process!

  19. With H2 Without H2 H2 H2 H H DT or ? Do surface to subsurface transitions of adsorbed H atoms occur? A seemingly ‘simple’ question: Does surface-adsorbed H participate in H absorption on a clean, perfectly flat surface? H/Pd(100) (fcc) z H 0 0.3 eV => Study the response of surface-adsorbed H atoms to DT w/o gaseous H2.

  20. Pd(100): Surface to ‘subsurface’ transition H upon heating? H2 Thermal Desorption Spectrum NRA H-Depth Profile (T<130 K) a b 15N2+ ! Similar on Pd(110) and Pd(111) ! • Hss bypasses surface-H in desorption (no isotopic exchange)! S. Ohno, M. Wilde et al., in preparation, T. Stulen, JVSTA 5 (1983) . Okuyama et al., Surf. Sci. 401 (1998) 344. => Instead of moving into the bulk, surface and ‘subsurface’-H species desorb

  21. fcc hollow hcp hollow A comparison: H-Absorption of Surface-H into Ti(0001) (!) NRA H-Depth Profile (T=300 K) H2 Thermal Desorption Spectrum Ti-Bulk: [H]=500 ppm * Tdet=318±22 K * DHs = -0.47 eV/H (TDS: H2-saturated by 12000 L H2 at 100 K) NRA: Signal of surface hydrogen (qH = 0.4 ML at 200 K). H/a-Ti(0001) (hcp) M. Wilde and K. Fukutani, Phys. Rev. B 78, 115411 (2008). => Although H vanishes from the surface around 320 K, no H2 desorption occurs.

  22. Hs Hb H2 Hs Hb Opposite behavior of H on Pd(100) vs. Ti(0001) Absorption/Desorption of Surface Hydrogen H2 z H2 Hs Hs • 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? 0 Hss T = 330 K T = 180 K z 0 T ~ 320 K T >650 K

  23. * Pd: DHs = -0.10 eV/H Ti: DHs = -0.47 eV/H Absorption capacity for surface-H in the near-surface region => Consider possibility to dissolve the surface H atoms into the bulk by in-diffusion: LD(T, t) Tpen=318±22 K Tdes=340 K Phys. Rev. B 78, 115411 (2008) Dissolvable H coverage [ML] = Diffusion length (T) x H solubility (T) / (1/2 layer distance) → Near-surface H absorption involve both surface and bulk properties!

  24. Hydrogen Absorption Mechanism at Pd(110) Surf. Sci. 126 (1983) 382. TDS H/Pd(110) H2→ Pd(110): Complex TD spectrum Identify multiple H-states (→NRA) Solid solution (α phase) and hydride (β phase)of bulk Pd are well known. AFM image of Pd thin film surface H2 → Z. Phys. Chem. Neue Folge 64, 225 (1969) Clarify absorption pathways in the near-surface region (→TDS) Hydride evolves from surface point defects Langmuir 2003, 19, 6750

  25. 表面 表面 A) Identify Surface Adsorption Phases: LEED & TDS 0 L 0.3 L 0.5 L 50 L streaky (1×2) (1×1) (1×2) (2×1) [2] [1] θ=1.0 ML θ=1.5 ML θ=? ML θ=0 ML α2 β 1 α3 β 2 α1 [1] Surf. Sci. 411 (1998) 123 [2] Surf. Sci. 327 (1995) 505 0 L 0.3 L 0.5 L 50 L

  26. 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 Texp=90 K 0.5 – 2000 L α2 α1 α3 β1 β2 α1, α3 (never saturate) -> H in the Pd interior ☞ Surf. Sci. 126 (1983) 382. Surf. Sci. 195 (1988) L199. α3 Texp=145 K 0.5 – 2000 L α1 and α3 absorption depend on the exposure temperature (Texp) α2 β1 β2 α1 disappears at Texp≥ 145 K ☞ Pd(111); Surf. Sci. 181 (1987) L147. Pd(100); Surf. Sci. 401 (1998) 344.

  27. → Complete TDS Peak Assignment: a2, b1, b2: 表面水素 a1 : 表面近傍の水素化物 a3 : 固溶体祖の水素 B) Clarify Concentration Depth Distribution of α1 and α3 NRA Depth Profile Hydrogen concentration S (=α2, β1, β2) S, α1, α3 S, α3 α1; near surface hydride α3;bulk solid solution > 50 nm (TDS shows 3 ML of α3) 20.1% (hydride) 0.9% (solid solution) S. Ohno, M. Wilde, K. Fukutani, in preparation First-time observation of TWO different absorbed H states in Pd(110)!

  28. Near-surface condition at 130 K 15N ion beam H2 NRA: average [H] = 20% Langmuir 19 (2003) 6750. (300 K, bar H2) Hydride: ～65% Solid solution phase: 0.009% 20 ×100 = 30% • Coexistence of solid solution (a3) and hydride (a1) phases • Non-uniform lateral and in-depth distribution • In-plane ratio of hydride ～30% 65

  29. b1, b2, a2 α1 α3 Evidence for 2 Absorption pathways leading to a1 and a3 S. Ohno, M. Wilde, K. Fukutani, in preparation → TDS after isotope-labeled hydrogen exposure Experiment: 1. Saturate Surface with D 2. Post-dose H2 H2 D2 D • Result: • a3 (+ surface species): → complete isotopic scrambling. • a1: Pure post-dosed isotope →no isotopic scrambling. D2 1.25 L + H2 1,000 L @115 K α1 α3 Different absorption pathways exist for the a1 and a3 absorbed states!

  30. Cf: Pd thin film – AFM: Langmuir 19 (2003) 6750. Isotopic Composition: Hydride Phase (a1) S. Ohno, M. Wilde, K. Fukutani, in preparation Hydride nucleation at a few specially active sites (Te<145 K) Pre-adsorbed 0.06 ML H2 Post-dosed x D Jpen Jdiff (initially: 1.5 ML D) • 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. x

  31. Isotopic Composition: Solid Solution Phase (a3) S. Ohno, M. Wilde, K. Fukutani, in preparation Solid solution H absorption at sites different from that of hydride nucleation Pre-adsorbed Post-dosed => Gas-phase H2-assisted penetration of surface-adsorbed D (first observation at a Pd single crystal) • Simultaneous and continuous absorption of pre-adsorbed and post-dosed hydrogen isotopes. • Effective exchange with surface-D, possibly at regular terrace sites. ※侵入の確率 K, サイト数 θ Kα1･θα1≒ Kα3･θα3 ∴Kα1 ≒ (θα3 / θα1)･ Kα3 >> Kα3

  32. Hydride and Solid Solution Formation Mechanism α1contains 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 α3increases together with post-dosed isotope. Complete isotopic exchange with Hs during penetration. Slower penetration rate. Jpen yes no no Jpen hydride (a1) (a3) Jdiff S. Ohno, M. Wilde, K. Fukutani, in preparation

  33. Outline: Hydrogen Absorption at Metal Surfaces • Introduction: Hydrogen and (Vacuum) Technology • Detection of Subsurface-H: Distinction from Surface-H • Formation of Subsurface-H: Absorption Mechanism • Role of near-surface absorbed H in Catalysis

  34. Olefin Hydrogenation Catalysis H3C H Example: Butene Hydrogenation Transition state (hypothetical) DSR << 0 CH3 H D2 D … D C4H8 C4H10D2 Butane-d2 ≠ Butene Ea H H3C CH3 • Necessary elementary steps: • D-D bond break (~4.5 eV, 430 kJ/mol) • C=C p-bond break (~ 615 kJ/mol) • C rehybridization: sp2 → sp3 • new C-H bond formation (414 kJ/mol x2) H D D H3C H DGR < 0 + D2 H CH3 Product Reactants • Concerted reaction is extremely unlikely in the gas phase • Large activation energy barrier (Ea): → Small reaction rate: R = exp(-Ea/RT)

  35. butyl intermediate isomerization D trans-2-butene-d1 -H +D2 D D D +D butane-d2 cis-2-butene hydrogenation Olefin Hydrogenation Catalysis H3C H Transition state (≠) CH3 H D … D Pd surface Ea ≠’ • New, easier elementary steps: • Olefin (C4H8) adsorbs on catalyst, C=C p-bond opens. • D2 bond breaks spontaneously on Pd surface (dissociative adsorption) • Coadsorbed D atoms easily attach to the intermediate; products desorb. H H3C Ea’ CH3 H3C H H D D + D2 H CH3 Product Reactants • Catalyst … drastically reduces activation energy barrier (Ea’ << Ea) • … enables reaction at far lower temperature • … itself is not consumed in the reaction.

  36. Industrial Catalysts: Oxide-supported Pd Nanocrystals Hydrogen Absorption inside Pd Nanocrystals? • Olefin hydrogenation catalysis: • Enhanced Reactivity of Pd Nano-clusters (for) compared to Pd(111) single crystals. • → participation of absorbed H suspected. volume Model catalyst: Al2O3 support

  37. D D NRA! Enhanced Reactivity of Pd-Nanoparticles in Olefin Hydrogenation (pentane) (n=5): (pentene) Pd-Nanocluster-Specific Reactivity for Alkene Hydrogenation: CnH2n + H2→ CnH2n+2 D2 + pentene (C5H10) TDS [D2]pentane (C5H10D2) Pd Nanocrystals on Al2O3 D2-TDS H inside NC? Pd Single Crystal A.M. Doyle et al., Angew. Chem. Int. Ed. 42 (2003) 5240; Journal of Catalysis 223 (2004) 444.

  38. Oxide-supported Pd nano-crystallites: Morphology Shape of Pd nano-crystallites on Al2O3/NiAl(110) 65x65 nm2. 2 ML Pd @ 300 K Aspect ratio: h/w=0.18±0.03 (constant for w>5.5 nm) K.H. Hansen et al., PRL 83 (1999) 4120.

  39. In-situ Nanocrystal Preparation for H-NRA 17.5 nm x 17.5 nm Pd evaporator viewport Pbase <1 x 10-8 Pa ion gun (→ Ar+ sputtering) UHV FC Energy monochromatized NRA 15N2+ ion beam (DE = 3 keV, ~15 nA) 75o g-ray detector BGO deflector sample (90-1300 K) on liquid N2 cryostat manipulator LEED / AES shielded QMS  H2 TDS • 1.) Al2O3/NiAl(110) substrate: • → NiAl(110) cleaning + in-situ oxidation. • 2.)5.85 ÅPd deposition @ 300 K • 3.) NRA: • 1H(15N, )12C • z(Ei) = (Ei-Eres)/[(dE/dz)cos(ai)] • grazing ion incidence (ai=75o) • beam collimation <2mm(slits) • UPH (99.99999%) H2 background (<2x10-3 Pa) _ NEC 5UD Tandem +

  40. g ai=75o H 15N Pd h~2 nm Al2O3/NiAl(110) Hydrogen Absorption in Al2O3-supported Pd nanocrystals Analysis of H distribution in 5.85 Å (2.6 ML) Pd on Al2O3 at 90 K, 2·10-5 Pa H2. M. Wilde et al., Phys. Rev. B 77, 113412 (2008). 17.5 nm x 17.5 nm 50 nm x 50 nm • 4-fold enhanced depth resolutionin75o grazing incidence angle NRA. • NP-absorbed H(arrow)can be probed independently fromsurface-adsorbed H. • =>Pd-NP stabilizeabsorbed Hwith 2-3 fold higher heat of solution than bulk Pd. • ( → H-binding occurs inside the NP, is not a mere surface-adsorption effect!)

  41. Common Notion of Hydrogen Absorption in Nanoparticles Peculiar H-Absorption Properties of NP’s: Heat ofH-solution of Nanoparticles is size-dependent and different from bulk metals => oftenDHS 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, … Proposed explanation: ‘subsurface sites’ (→ large surface/volume ratio) Fraction of atoms in two outermost shellsfor a cluster with i shells.

  42. Al2O3/NiAl(110) p(H2)-dependent H-uptake in Pd nanocrystals on Al2O3 at 90 K Separate monitoring of surface H and nanocrystal-absorbed H uptake 2x10-5 mbar M. Wilde et al., Phys. Rev. B 77, 113412 (2008). 6x10-6 mbar 1 ML 2x10-7 mbar (111) (100) • 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!

  43. Reactivity Study of Olefin Conversion over Pd/Al2O3 Model Cat • Does Pd Cluster-absorbedH play a role in olefin (cis-2-butene) hydrogenation? M. Wilde, S. Schauermann et al., Angew. Chem. Int. Ed. 47, 9289 (2008). Molecular Beam Reactive Scattering Alumina-Supported Model Catalysts NRA measurement under reaction conditions D2 beam (steady) 2-4x10-6 mbar) ai=75o H 15N Sample QMS Pd cis-2-butene beam (pulsed) Al2O3/NiAl(110) 4 Å Pd/Fe3O4/Pt(111) 4 Å Pd/Al2O3/NiAl(110)

  44. isomerization D trans-2-butene-d1 -H +D D D +D D butane-d2 cis-2-butene butyl intermediate hydrogenation D2-pressure dependent reactivity of hydrogenation Reaction Mechanism Pressure-independent: → linked to surface-adsorbed H. Pressure-dependent: → linked to volume-adsorbed H. MBRS NRA Isomerization: → r ≠ f(pH2) hydrogenation → r = f(pH2)

  45. Modified surface electronic structure on hydride phase? Attack of butyl by absorbed (→ resurfacing) H? ? Al2O3 support volume Catalytic Reactivity of Subsurface-Absorbed Hydrogen ･Absorbed H species are essential in hydrogenation catalysis (e.g. Butene → Butane conversion: C4H8 + D2→ C4H8D2 ) ･ => Reactive species: Surface-adsorbed or subsurface-H ? Pd → What is the role ofPd Nanocrystal-absorbedH in olefin hydrogenation catalysis? 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).

  46. α2 α1 β1 β2 α3 → Peak Assignment: a2, b1, b2: 表面水素 a1 : 表面付近水素化物 a3 : 　　固溶体祖の水素 Recall: Two ‘Subsurface’-Absorbed H States in Pd(110): a1 & a3 LEED & TDS: 表面水素 TDS (1,000 L H2) NRA: H Depth Distribution PdHx X=0.20 (Hydride) a1+a3 a3 S. Ohno, M. Wilde, K. Fukutani, in preparation X=0.009 (solid solution) →　Does catalytic reactivity depend on subsurface depth distribution…?

  47. Pd(110): Reactivity of Subsurface H in hydrogenation catalysis Recall: H/Pd(110)-TDS (1000 L H2@115 K) a1species from the near-surface hydride phase recombine and desorb as H2below 180 K. α1 No reaction w/ butene (C4H8). α3 Subsurface hydride phase is NOT necessary for the hydrogenation reaction. C4H8 →　C4H10 ? Compare Butane (C4H10) and H2-a3 TDS: S. Ohno, M. Wilde, K. Fukutani, in preparation C4H10 Butane product desorption and a3 H2peak neatly overlap! Hydrogenation reactivity relates to H-evolution from the a3-bulk H state!

  48. Hydrogen Absorption Mechanism and Catalysis at Pd(110) Summary & Conclusions • ･ TDS/NRA→ identified 2 absorbed H species : • a1→ near-surface hydride phase • a3→ 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 (a3) active in catalysis! Jpen yes no no Jpen hydride (a1) (a3) Jdiff

  49. Acknowledgements Institute of Industrial Science, University of Tokyo K. Fukutani, Y. Murata, Y. Fukai, S. Ohno, K. Namba MALT Tandem Accelerator, RCNST, University of Tokyo H. Matsuzaki, C. Nakano Fritz-Haber Institute, Max-Planck Society, Berlin, Germany S. Schauermann, S. Shaikhutdinov, H.-J. Freund Supported by… CREST-JST, NEDO, MEXT, IIS Dear audience: Thank you for your attention! Contact: wilde@iis.u-tokyo.ac.jp

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