Pd(110) 表面における水素吸収の機構

1. 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 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

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: H2→ Pd(110) (single crystal) • Influence of surface structure on absorption properties H2 H2 z Surface Subsurface Bulk H H 0 H2 time => Clarify the microscopic pathways of hydrogen surface penetration

3. Activation Energy Paradox Potential Energy Prevailing H absorption model Experimental results H/Pd R H2 ? H -0.1 eV -0.2 eV Monatomic in-diffusion Absorption activation Chemi- sorption Emono = 0.3～0.6 eV[1-4] * * Eabs< 0.10eV[5, 6] Identify: The actual reaction coordinate of H2 absorption -0.5 eV [5] Okuyama et al., Surf. Sci. 401 (1998) 344. [6] Ohno et al., J. Chem. Phys., submitted. [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).

4. Pd (110) 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] Top view Side view H2 exposure Pairing-row (P-R) reconstruction (1x2) ・Second-layer exposed ・Atomic step-like structure ・Lateral contraction in paired rows [1] Christmann, Prog. Surf. Sci. 48, 15 (1995).

5. 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) Experimental Approach: TDS + NRA 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 → achieves unambiguous TDS peak identifications

6. Surface Adsorption Phases (LEED & TPD): H/Pd(110) H2 exposureat Te = 130 K 50 L～ 0 L: (clean) 0.3 L 0.5 L (1×1) (1×2) (2×1) [2] [1] θ=1.0 ML θ=1.5 ML θ=0 ML θ=? ML Surface Surface α2 α3 β1 1 L = 10-6 Torr ·s β2 α1 0.8 L 1 ML = 9.4 x 1014 atoms/cm2 0.3 L 0.3 L 0.1 L [1] Ledentu et al., Surf. Sci. 411 (1998) 123. [2] Yoshinobu et al., Phys. Rev. B 51, 4529 (1995).

7. NRA: H Depth Distribution of Two Low-T TPD States TPD NRA 130 K α3 α1 α2 β1 β2 23.0 at.% 145 K α3 1.2 at.% Absorbed hydrogen α1: Near surface α3: Bulk > 50 nm Hs: 1.7±0.3 ML

8. LEED, NRA, TPD: Identification of H2/Pd(110) desorption features 2000 L at Te = 130 K NRA TPD surface α1 α3 α2 β1 β2 NEW: => First revelation at Pd(110): TWO absorbed hydride states S. Ohno, M. Wilde, K. Fukutani, J. Chem. Phys., submitted.

9. Investigation of the H2 Absorption Mechanism Absorption experiments with isotope labeled surface hydrogen: Near-surface hydride Bulk hydride prepost D2 1.0 L →H2 1000 L Te = 115 K α1 Analysis of isotope populations (TPD): => Clear difference between near-surface (α1) and bulk (α3) hydride (Also: Different normal (H2>D2)isotope effects in a1 and a3 population speeds) α3 ⇒ Two separate absorption pathways exist (!)

10. Isotope Population of the Absorbed Hydride States Near-surface hydride (α1) Langmuir 2003, 19, 6750 Post p=0 p=0~0.5 0.2 Pre 0.5 ～4％ 0.06 ML AFM image of hydride grown on Pd thin film 0.8 1 => Absorption nearminority sites (defects) Bulk hydride (α3) Dominant transfer of pre-adsorbed H below the surface (First observation) => Absorption in regular terrace area (!)* *Only Pd(110): no ‘bypassing’!

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

12. Absorption mechanism: Bypassing or Replacement? Near-surface hydride (α1) p=0 p=0 p=0: Replacement p=1: Bypassing 0.2 0.2 0.5 0.8 0.5 0.8 1 1 Bulk hydride (α3) Compatible Incompatible Dominant absorption mechanism: Replacement! S. Ohno, M. Wilde, K. Fukutani, J. Chem. Phys., submitted.

13. What is the Rate Determining Step (RDS)? Potential Energy Experiment Prevailing model Monatomic in-diffusion Emono = 0.3 ～0.6 eV[3, 4] H2 absorption Eabs < 0.1 eV[1, 2] × R H2 H/Pd H 1) -0.1 eV -0.2 eV 2) Emono = 0.3～0.6 eV 3) Monatomic in-diffusion * Chemi- sorption Possible rate determining steps: -0.5 eV • H2 dissociation • Surface penetration • Bulk diffusion (inverse isotope effect (D2 faster than H2); Ediff > Eabs) [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.

14. RDS: H2 Dissociation (at large qH) or Concerted Penetration Consider processes with activation energies compatible to Eabs (≤0.1 eV): → H2 dissociation (Ediss) / concerted penetration (Ec-pen) • H2 dissociation is non-activated (Ediss = 0) at bare Pd surfaces[1] • Dissociation becomes weakly activated (at high H-coverages)[2] Ediss = 0.1 eV[2,3] H2 dissociation at a H mono-vacancy* Concerted penetration: He + Hs→Hs + Hss Ec-pen≈0.06 eV[2,3] 0.5 ML H/Pd(100) Excess H atom[2] (He) • The activation barrier of penetration can be drastically lowered when it occurs concertedlywith stabilization of excess H atoms. [1] Rendulic, Surf. Sci. 208 (1989) 404. [2] Groß, ChemPhysChem 11 (2010) 1374. [3] Sakong, ChemPhysChem13, 3467.

15. Influence of Surface Structure on H2 dissociation (atlarge qH) Peculiarity of Pd(110): Terrace-related H2 absorption (not on Pd(111)and (100))[1, 2] • H-vacancy-mediated dissociation: qvac = exp(-DGs,b/kBT) = 2x10-8 at 145 K • Pabs, max (Model) = Pdissqvac << Pabs (Experiment) = Rabs/2Zw = 5x10-4 at 145 K • => Direct gas phase H2 impact not sufficient => Involvement of mobile H2 precursors (!) Step edge-like structures stabilize molecular H2 chemisorption states* - Possible explanation - Ni(510)[3] Pd(210)[4] Pd(322)[5] Top view [5] Side view Theoretical prediction[6]: H2 may exist at Pd(110) step-like Pd(322) *constitute precursor states for H2 dissociation[4, 5] [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)

16. Influence of Surface Structure on H vacancy generation Defect-enhanced H2 absorption Terrace-related H2 absorption ( > 24 x per site vs. regular terraces) (peculiar vs. Pd(100), (111), (311)) Top view Side view (1x2) Widened interstitial channels (in [001]) H2 dissociation may require H-vacancies: • Rate of H-vacancy generation[1]: Rvac = 1013 s-1 exp(-DEs,ss/kBT) ≈ 103 at 145 K • => enhanced at defects due to additional ‘openness’. May also stabilize H2. • Widened penetration channels at defects and in troughs between paired Pd rows in Pd(110)(1x2)-(PR). [1] Padama et al., J. Phys. Soc. Jpn. 81 (2012) 114705. DEs,ss = 0.27 eV (110); cf. Pd(111) (0.4 eV), Pd(100) (0.41 eV) (Ferrin)

17. Summary & Conclusions H2 H2 H2 D H H2 absorption mechanism at Pd(110)-(1x2) (paired-row): ・Two hydride states exist with different depth distributions ・TwoH absorption channels (defects + terrace, Pd(110) only) ・Hs is replaced (not bypassed), no simple in-diffusion, Eabs<0.1 eV ・RDS: H2dissociation (H-saturated Pd) or concerted penetration ・Influence of Surface Structure: H2 absorption enhanced by * “Open” penetration channels (accelerate H-vacancy generation) * Stabilization of H2 precursors (at step edge-like structures)

18. Activation energy for hydrogen absorption at Pd(110) Arrhenius plot of a1, a3 population (Pa) H2 TDS(Te=90 K) α2 α1 α3 β1 β2 a1, a3peak area vs. exposure → Activation Energy H a1 : 0.03 eV a3 : 0.06 eV D a3 : 0.07 eV <0.1eV Much smaller than expected for monatomic H surface-to-subsurface diffusion (0.3~0.4 eV)!

19. Isotope Population of the Near Surface Hydride (α1) Langmuir 2003, 19, 6750 Post ↓ Pre 0.06 ML AFM image of hydride grown on Pd thin film Only～4％ of surface area is affected by isotope exchange: → Absorption at minority sites → defects Pre Post α1

20. Isotope Population of the Bulk Hydride (α3) Pre ↓ Dominant transfer of pre-adsorbed H below the surface Post (First observation at a Pdsingle crystal surface)* H2 absorption takes place in the regular terrace area of Pd(110) (!) α3 * 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).