1. Pd(110) 表面における水素吸収の機構 Markus Wilde ・ Satoshi Ohno ・ Katsuyuki Fukutani Institute of Industrial Science, University of Tokyo 文部科学省科学研究費新学術領域研究 ・ 2014 年 3 月 11 日・東京大学 Materials Design through Computics Complex Correlation and Non-equilibrium Dynamics 「コンピューティクスによる物質デザイン： 複合相関と非平衡ダイナミクス」 平成 25 年度 第 2 回研究会

2. Hydrogen Absorption at Pd Surfaces Industrial Importance: Hydrogen Storage (in hydrides) Hydrogenation Catalysis Objectives: Obtain atomic level understanding of the absorption mechanism Model system: H 2 → Pd(110) (single crystal) Influence of surface structure on absorption properties => Clarify the microscopic pathways of hydrogen surface penetration H2H2 z 0 H H2H2 H Surface Subsurface Bulk H 2 time

3. [5] Okuyama et al., Surf. Sci. 401 (1998) 344. [6] Ohno et al., J. Chem. Phys., submitted. Activation Energy Paradox The actual reaction coordinate of H 2 absorption H H2H2 E abs < 0.10 eV [5, 6] * [1] Padama et al., J. Phys. Soc. Jpn. 81 (2012) 114705. [2] Ferrin et al., Surf. Sci. 606, 679 (2012). [3] Nobuhara et al., Surf. Sci. 566, 703 (2004). [4] Ozawa et al., J. Phys.: Condens. Matter 19, 365214 (2007). Prevailing H absorption model Absorption activation Experimental results E mono = 0.3 ～ 0.6 eV [1-4] * Monatomic in-diffusion H/Pd Chemi- sorption Identify: R Potential Energy -0.5 eV -0.2 eV -0.1 eV ?

4. Surface of particular interest: Pd(110) Pd(110) single crystal surface Pd ✓ Well-known H absorbing metal ✓ Excellent catalyst for olefin hydrogenation (110) ✓ Single crystal: Well-defined structure ✓ Openness: Surface atomic density ｰ 40% vs. (111) ✓ H-induced surface reconstruction: “Prone to hydrogen absorption” [1] [1] Christmann, Prog. Surf. Sci. 48, 15 (1995). H 2 exposure Pairing-row (P-R) reconstruction ・ Second-layer exposed ・ Atomic step-like structure ・ Lateral contraction in paired rows Pd (110) Top viewSide view (1x2)

5. Combine two hydrogen detection techniques: Experimental Approach: TDS + NRA 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) 114511. → achieves unambiguous TDS peak identifications

6. 0 L: (clean) 0.3 L0.5 L [1] Ledentu et al., Surf. Sci. 411 (1998) 123. [2] Yoshinobu et al., Phys. Rev. B 51, 4529 (1995). (1×1) (2×1) (1×2) Surface Adsorption Phases (LEED & TPD): H/Pd(110) H 2 exposure at T e = 130 K 50 L ～ θ=1.5 MLθ=1.0 ML θ=0 ML [1] θ=? ML [2] β1β1 β2β2 α2α2 Surface 1 L = 10 -6 Torr · s 1 ML = 9.4 x 10 14 atoms/cm 2 0.1 L 0.3 L 0.8 L 0.3 L Surface α1α1 α3α3

7. α1α1 α3α3 α2α2 β2β2 β1β1 TPD 130 K 145 K α3α3 H s : 1.7±0.3 ML NRA: H Depth Distribution of Two Low-T TPD States 1.2 at.% 23.0 at.% NRA Absorbed hydrogen α 1 : Near surface α 3 : Bulk > 50 nm

8. 2000 L at T e = 130 K α1α1 α3α3 α2α2 β2β2 β1β1 surface TPD featureOrigin Depth extension [H] avg Volume ratio T e condition α 1 (170 K) Near surface hydride ～ 10 nm 23 at.%30%< 145 K α 3 (195 K)Bulk hydride> 50 nm1.2 at.%2%< 160 K LEED, NRA, TPD: Identification of H 2 /Pd(110) desorption features => First revelation at Pd(110): TWO absorbed hydride states TPD NRA S. Ohno, M. Wilde, K. Fukutani, J. Chem. Phys., submitted. NEW:

9. pre post D 2 1.0 L → H 2 1000 L α1α1 α3α3 T e = 115 K ⇒ Two separate absorption pathways exist (!) Near-surface hydride Bulk hydride Investigation of the H 2 Absorption Mechanism Absorption experiments with isotope labeled surface hydrogen: Analysis of isotope populations (TPD): => Clear difference between near-surface (α 1 ) and bulk (α 3 ) hydride (Also: Different normal (H 2 >D 2 ) isotope effects in a 1 and a 3 population speeds)

10. => Absorption near minority sites (defects) Isotope Population of the Absorbed Hydride States Langmuir 2003, 19, 6750 AFM image of hydride grown on Pd thin film Post Pre 0.06 ML Near-surface hydride (α 1 ) Bulk hydride (α 3 ) 0.8 1 0.2 p=0 0.5 ～4％～4％ Dominant transfer of pre-adsorbed H below the surface (First observation) => Absorption in regular terrace area (!)* p=0~0.5 *Only Pd(110): no ‘bypassing’!

11. p 1-p ‘bypassing’ replacement Recursive analysis of isotope composition Evaluation of ‘bypassing’ probability (p) Stochastic Isotope Population Model for Absorption/Desorption (1) (2) (p)(p) (1-p) n+1 th absorption event post pre: N pre (n) post: N post (n) → uptake → desorption (microscopic reversibility)

12. Absorption mechanism: Bypassing or Replacement? Bulk hydride (α 3 ) Near-surface hydride (α 1 ) 0.8 1 0.2 0.5 0.8 1 0.2 p=0 0.5 p=0 Dominant absorption mechanism: CompatibleIncompatible p=0: Replacementp=1: Bypassing Replacement! S. Ohno, M. Wilde, K. Fukutani, J. Chem. Phys., submitted.

13. What is the Rate Determining Step (RDS)? H 2 absorption E abs < 0.1 eV [1, 2] Monatomic in-diffusion E mono = 0.3 ～ 0.6 eV [3, 4] 1)H 2 dissociation 2)Surface penetration 3)Bulk diffusion (inverse isotope effect (D 2 faster than H 2 ); E diff > E abs ) H H2H2 × Experiment Prevailing model Possible rate determining steps: [1] Okuyama et al., Surf. Sci. 401 (1998) 344. [2] Ohno et al., J. Chem. Phys., submitted. [3] Padama et al., J. Phys. Soc. Jpn. 81 (2012) 114705. [4] Ferrin et al., Surf. Sci. 606 (2012) 679. E mono = 0.3 ～ 0.6 eV * Monatomic in-diffusion H/Pd Chemi- sorption R Potential Energy -0.5 eV -0.2 eV -0.1 eV 1) 2) 3)

14. RDS: H 2 Dissociation (at large  H ) or Concerted Penetration [1] Rendulic, Surf. Sci. 208 (1989) 404. [2] Groß, ChemPhysChem 11 (2010) 1374. [3] Sakong, ChemPhysChem 13, 3467. H 2 dissociation is non-activated (E diss = 0) at bare Pd surfaces [1] Dissociation becomes weakly activated (at high H-coverages) [2] 0.5 ML H/Pd(100) Consider processes with activation energies compatible to E abs (≤0.1 eV): → H 2 dissociation (E diss ) / concerted penetration (E c-pen ) H 2 dissociation at a H mono-vacancy* E diss = 0.1 eV [2,3] Excess H atom [2] (H e ) Concerted penetration: E c-pen ≈ 0.06 eV [2,3] H e + H s → H s + H ss The activation barrier of penetration can be drastically lowered when it occurs concertedly with stabilization of excess H atoms.

15. Influence of Surface Structure on H 2 dissociation (at large  H ) Peculiarity of Pd(110): Terrace-related H 2 absorption (not on Pd(111) and (100)) [1, 2] - Possible explanation - [1] Gdowski et al, J. Vac. Sci. Technol. A 5, 1103 (1987). [2] Okuyama et al, Surf. Sci. 401, 344 (1998). [3] Mårtensson et al, Phys. Rev. Lett. 57 (1986) 2045. [4] Schmidt et al, Phys. Rev. Lett. 87, 096103 (2001). [5] Ahmed et al., Appl. Surf. Sci. 257, 10503 (2011). [6] Busnengo et al, Phys. Rev. Lett. 93, 236103 (2004). (1x1) *constitute precursor states for H 2 dissociation [4, 5] Top view Side view [5] Step edge-like structures stabilize molecular H 2 chemisorption states* step-like Pd(322) Ni(510) [3] Pd(210) [4] Pd(322) [5] Theoretical prediction [6] : H 2 may exist at Pd(110) H-vacancy-mediated dissociation:  vac = exp(-  G s,b /k B T) = 2x10 -8 at 145 K P abs, max (Model) = P diss  vac << P abs (Experiment) = R abs /2Z w = 5x10 -4 at 145 K => Direct gas phase H 2 impact not sufficient => Involvement of mobile H 2 precursors (!)

16. Influence of Surface Structure on H vacancy generation Defect-enhanced H 2 absorption Terrace-related H 2 absorption ( > 24 x per site vs. regular terraces) (peculiar vs. Pd(100), (111), (311)) H 2 dissociation may require H-vacancies: Rate of H-vacancy generation [1] : R vac = 10 13 s -1  exp(-  E s,ss /k B T) ≈ 10 3 at 145 K => enhanced at defects due to additional ‘openness’. May also stabilize H 2. Widened penetration channels at defects and in troughs between paired Pd rows in Pd(110)(1x2)-(PR). Side view (1x2) Top view Widened interstitial channels (in [001]) [1] Padama et al., J. Phys. Soc. Jpn. 81 (2012) 114705.  E s,ss = 0.27 eV (110); cf. Pd(111) (0.4 eV), Pd(100) (0.41 eV) (Ferrin)

17. Summary & Conclusions H 2 absorption mechanism at Pd(110)-(1x2) (paired-row): ・ Two hydride states exist with different depth distributions ・ Two H absorption channels (defects + terrace, Pd(110) only) ・ H s is replaced (not bypassed), no simple in-diffusion, E abs <0.1 eV ・ RDS: H 2 dissociation (H-saturated Pd) or concerted penetration ・ Influence of Surface Structure: H 2 absorption enhanced by * “Open” penetration channels (accelerate H-vacancy generation) * Stabilization of H 2 precursors (at step edge-like structures) D H2H2 H2H2 H H2H2

18. α1α1 α3α3 α2α2 β1β1 β2β2 H 2 TDS (T e =90 K) Activation energy for hydrogen absorption at Pd(110) → Activation Energy H  1 : 0.03 eV  3 : 0.06 eV D  3 : 0.07 eV Much smaller than expected for monatomic H surface-to-subsurface diffusion (0.3~0.4 eV)! Arrhenius plot of  1,  3 population (P a )      peak area vs. exposure <0.1 eV

19. Only～ 4 ％ of surface area is affected by isotope exchange: → Absorption at minority sites → defects Isotope Population of the Near Surface Hydride (α 1 ) Langmuir 2003, 19, 6750 AFM image of hydride grown on Pd thin film Post Pre Post Pre 0.06 ML ↓ α1α1

20. ↓ Post Pre H 2 absorption takes place in the regular terrace area of Pd(110) (!) Isotope Population of the Bulk Hydride (α 3 ) Dominant transfer of pre-adsorbed H below the surface * cf.) Pre-adsorbed H remains intact on Pd(111) [1] and (100) [2] ( → “Bypassing”) [1] Okuyama et al., Surf. Sci. 401 (1998) 344. [2] Gdowski et al., J. Vac. Sci. Technol. A 5, 1103 (1987). (First observation at a Pd single crystal surface)* α3α3