Dehydrogenation of Liquid Organic Hydrogen Carriers In-situ Spectroscopic Studies

Transcription

1 Dehydrogenation of Liquid Organic Hydrogen Carriers In-situ Spectroscopic Studies Chair of Physical Chemistry II C. Gleichweit, M. Amende, U. Bauer, S. Schernich, W. Zhao, M.P. A. Lorenz, O. Höfert, M. Laurin, J. Libuda, C. Papp, H.-P. Steinrück Institute of Chemical Reaction Engineering N. Brückner, P. Wasserscheid Department of Chemistry and Pharmacy Friedrich-Alexander-Universität Erlangen-Nürnberg

2 Outline Motivation Fundamentals & Experimental Reaction steps of dodecahydro-n-ethylcarbazole (H 12 -NEC) on single crystals Pt(111)/Pd(111) Reaction steps of H 12 -NEC on supported particles Vibrational splitting in XPS Dicyclohexylmethane (DHM) and diphenylmethane (DPM) on Pt(111)

3 Hydrogen as an alternative fuel for CO 2 free vehicle propulsion Storage requires either high pressures or cryogenic temperatures Alternative concept: Regenerable chemical hydrogen storage materials - liquid organic hydrogen carriers (LOHCs) Most studied example: Reversible hydrogenation of N-ethylcarbazole (NEC) to dodecahydro-n-ethylcarbazole (H 12 -NEC) H 12 -NEC Hydrogen loaded Motivation 6 H 2 NEC unloaded noble metal catalysts, e.g., Ru, Pt, and Pd. Surface science studies Are we able to optimize for efficient catalytic systems? Teichmann, D., et al., Energy Environ. Sci., 2011, 4, 2767, Sobota, M., et al., Chem. Eur. J. 2011, 17,

4 X-Ray Photoelectron Spectroscopy hν A E Kin B Atom B Atom A E Kin = hν - E B - hν = 500 ev Intensity [arb.u.] Fermi Edge Pos.: ev Binding Energy [ev] E B Gathered Information Elemental composition Oxidation state Chemical surrounding

5 Mean Free Path of Electrons e - I 0 I I = I 0 exp(-d/λ) λ = λ(e) electron mean free path: 5-30 Å (few layers of atoms) (strong interaction due to charge) Information gathered using PES stems from near the surface Choosing E kin to be ~100 ev gives highest surface sensitivity

6 Lab Source vs. Synchrotron Source Laboratory Setup Synchrotron Setup + Always available + Straightforward setup and use + Relatively non-sensitive towards disturbances (robust) - Low photon flux - Limited number of excitation energies available - Usually non-focused beam + High photon flux (adjustable!) + Continuously adjustable excitation energy ( ev at our beamline) + Focused beam (scan sample) - Beam time needed (proposal) - Experimentally demanding (chamber setup, software ) - Beam damage (system dependent)

7 Example: Pt(111) single crystal Make a survey (check for unexpected peaks) Scan suspicious core levels in detail Check LEED pattern Intensity [arb.u.] Fermi Edge Pos.: ev 10 sweeps, 10 min Comparison Is the sample clean? hν = ev Resolution 760 mev Binding Energy [ev] At BESSY II: set excitation energy to 150 ev, adjust flux using exit slit high resolution and surface sensitivity obtained check surface core level shift (SCS) Intensity [counts/s] Intensity [arb.u.] Fermi Edge Pos.: ev hν = 150 ev Resolution 150 mev Binding Energy [ev] 7x bulk Pt 4f 5/2 10 sweeps, 25 min surface bulk Pt 4f 7/ Binding Energy [ev] surface Intensity [counts/s] sweep, 10 s clean sample Pt 4f 5/2 dirty (covered) sample Pt 4f 7/ Binding Energy [ev]

8 HRXPS at BESSY II XPS Beamline U49/2-PGM1 at BESSY II Resolution set to 200 mev at hν = 380 ev (C 1s) 250 mev at hν = 500 ev (N 1s) High flux Fast measurements (~10 s/spec) High sensitivity (adsorbate coverages < ML) Evaporator for LOHCs in analysis chamber Supersonic molecular beam Variable pressure on sample (up to 10-6 mbar) Preparation chamber: LEED, sputter gun, evaporators.. Time resolved high resolution XP-spectra during adsorption and during heating (TPXPS) R. Denecke et al., Surf. Rev. Letters 9 (2002) 797

9 Transportable Setup

10 Infrared Reflection Absorption Spectroscopy (IRAS) Done by group of Jörg Libuda

11 Temperature Programmed Desorption dθ dt E RT m 0 d = km θ = km exp( ) Polanyi-Wigner Equation θ m θ m k m E d k m0 Surface coverage Desorption order Desorption speed Desorption energy Preexponential factor (about /s) TPD machine: LEED, Feulner Cup (QMS), X-ray gun Important tool to study desorbing species Used here mostly to identify hydrogen release temperatures of LOHCs

12 N-Ethyl-Carbazole (NEC) Loaded carrier liquid, unloaded carrier solid at RT Hydrogen-storage capacity of 5.8 wt % H 12 -NEC Hydrogen loaded NEC unloaded 6 H 2 noble metal catalysts, e.g., Ru, Pt, and Pd. Simple systems: Pt(111) and Pd(111) single crystals More realistic model system: supported Pt/Pd particles on Al 2 O 3 /NiAl Teichmann, D., et al., Energy Environ. Sci., 2011, 4, 2767, Sobota, M., et al., Chem. Eur. J. 2011, 17,

13 Experimental procedure

14 Das Bild kann zurzeit nicht angezeigt werden. NEC on Pt(111) NEC unloaded Synchrotron data TPXPS Lab source Intensity [arb.u.] N 1s Exposure: 2 L hν= ev C 1s Binding Energy (ev) Intensity [arb.u.] Uptake Binding Energy [ev]

15 Temperature [K] Exposure [L] Intensity [arb.u.] H 12 -NEC Hydrogen loaded (e) N 1s (c) 600 K 440 K 360 K 260 K 200 K 5.1 L 2.8 L 6 (a) Binding Energy [ev] G F E D C B A G F E D C B A Temperature [K] Exposure [L] K 500 K 400 K 300 K 200 K 2 L 1 L N 1s NEC unloaded Binding Energy [ev] G F E D C B A G F E D C B A

16 Das Bild kann zurzeit nicht angezeigt werden H NEC monolayer HR-XPS Data - N 1s C Coverage/ML C Coverage/ML Exposure/L (a) 1 (b) 1 Multilayer Adsorption Exposure/L H 12 -NEC Multi H 12 -NEC Mono NEC Carbazole Decomp 2 2 N 1s H 12 -NEC multilayers (c) Temperature/K Multilayer C Desorption 1s (d) Temperature/K 5 H 12 -NEC monolayer NEC monolayer 3 Decomposition Das Bild kann zurzeit nicht angezeigt werden. Pt(111) C and N fragments Gleichweit et al., ChemSusChem 2013, 6, Carbazole HNEC Coverage/ML Dehydrogenation Dealkylation HNEC Coverage/ML

17 Das Bild kann zurzeit nicht angezeigt werden. Das Bild kann zurzeit nicht angezeigt werden. Das Bild kann zurzeit nicht angezeigt werden. Das Bild kann zurzeit nicht angezeigt werden. HR-XPS Data - C 1s C. Gleichweit et al., ChemSusChem 2013, 6,

18 HR-XPS Data - C 1s K G Intensity [arb.u.] 440 K 360 K 260 K 200 K F E D C Intensity [arb.u.] 600 K 440 K 360 K 260 K 200 K G F E D C Temperature [K] Exposure [L] (e) N 1s (c) 5.1 L 2.8 L 6 (a) 5 B 4 3 A Binding Energy [ev] B A G F E D C Temperature [K] Exposure [L] C 1s (f) (d) (b) Max Binding Energy [ev] Min 5.1 L 3.0 L B A G F E C D B A Gleichweit et al., ChemSusChem 2013, 6,

19 IRAS Data Figure: IR spectra taken during dosing of H 12 -NEC onto Pt(111) under isothermal conditions at various temperatures (exposure time: 30 min). (Done by group of Jörg Libuda) M. Amende et al., ACS Catalysis, submitted 2013

20 TPD Experiments Pt(111) Mass 2: associative hydrogen desorption NEC H 12 -NEC Intensity [arb.u.] 2 K/s 6.3 L Intensity [arb.u.] 2 K/s 0.4 L 0.8 L 1.3 L 1.7 L 2.6 L Temperature [K] Temperature [K]

21 H 12 -NEC on Pd(111) - HRXPS N 1s Comparison to Pt(111) in the N 1s core level: Monolayer peak at different position Important: dealkylation earlier thermally not that stable Amende et al., Chemistry A European Journal 2013, 19,

22 H 12 -NEC on Pd(111) - IRAS Figure: IR spectra taken during dosing of H 12 -NEC onto Pd(111) under isothermal conditions at various temperatures (exposure time: 30 min). (Done by group of Jörg Libuda) Amende et al., Chemistry A European Journal 2013, 19,

23 Summary H 12 -NEC on single crystals Analysis of surface reaction using XPS, IRAS and TPD Complete dehydrogenation on Pt(111) up to T = 380 K Dealkylation after complete dehydrogenation at T > 390 K on Pt(111) and T > 350 K on Pd(111) -6 H 2 Dealkylation Decomposition Pt(111)

24 Single crystals vs. Supported Particles Systems more realistic (oxide films used in applications) Contributions also from molecules on the support in XPS spectra Complete analysis of surface species might be hard Changes in N 1s region detectable (e.g. dealkylation)

25 More detailed analysis Complicated structure if many carbon atoms involved Most structures contain aromatic rings Building blocks of LOHCs are of interest Intensity [arb.u.] hν=380 ev, saturated measured values Figure: Adsorption sites of benzene on Pt(111) Figure: Saturation spectrum of benzene at 200 K on Pt(111) Vibrational finestructure observed Zhang R. et al., Physical Chemistry Chemical Physics, in press 2013

26 CH 4 /Pt(111) Adsorption Experiment E kin = 0.65 ev T N = 700 C Seeding 5% CH 4 in He (8 sec / spectrum) intensity [arb. u.] Formation of CH 3 Vibrational fine structure Fuhrmann et al., NJP 7 (2005) 107. Fuhrmann et al., NJP (2005), 7, 107

27 Vibrational Fine Structure Franck-Condon principle Linear coupling model (Cederbaum) I(0 S v) I (0 = I (0 = e v -S S 1) 0) v!

28 Methyle / Pt(111) Vibrational splitting CH 3 hν C-H = 400 ±4 mev S-factor: 0.50 ±0.04 CD 3 hν C-D = 294 ±5 mev S-factor: 0.71 ±0.04 hν Das Bild kann zurzeit nicht angezeigt werden. C-H =1.36 hνc-d

29 Summary Principals of synchrotron-based PES presented Surface reaction steps of H 12 -NEC/NEC on Pt(111) analyzed using HR- XPS, IRAS and TPD Comparison to Pd(111) lower dealkylation temperature Supported particle systems smaller size leads to lower stability Vibrational splitting analysis to disentangle complicated spectra Data for DHM/DPM on Pt(111) presented

30 Outlook Further investigations of DHM/DPM (IRAS, supported particles) Studying other LOHC candidates Studying other smaller building blocks (Toluene, Pyrrolidine, ) Mo 2 C as alternative catalyst to Pt(111) [Mo 2 C/Mo(100)] Building up NEXAFS setup for our machine

31 Acknowledgements Prof H.-P. Steinrück (HR)XPS, TPD.. Physical Chemistry II Prof Jörg Libuda IRAS, MB Experiments Sync-Team Dr. Christian Papp Oliver Höfert Karin Gotterbarm Wei Zhao Udo Bauer Florian Späth Carina Bronnbauer MOBY-Team Max Amende Stefan Schernich Theoretical Chemistry Prof. Andreas Görling DFT calculations Chemical Reaction Engineering Prof. Peter Wasserscheid Nicole Brückner

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33 H 12 -NEC - Continuous Adsorption 550 K/6.8 L Exposure [L] Intensity [arb.u.] K/5.7 L 450 K/4.9 L 350 K/4.1 L 250 K/3.1 L (d) (b) Max Min ev 2 Exp Binding Energy [ev] ev C 1s T Temperature [K] Exposure [L] Intensity [arb.u.] K/5.8 L 350 K/5.0 L 250 K/3.0 L (c) (a) N 1s T Exp Binding Energy [ev] Temperature [K] M. Amende et al., ACS Catalysis, submitted 2013

34 a) C Coverage [ML] N 1s Exp. Temperature [K] T H x -NEC Coverage [ML] b) C Coverage [ML] C 1s Exp. Exposure [L] Temperature [K] T H 12 -NEC NEC Carbazole Decomposition products H x -NEC Coverage [ML] Exposure [L] 0.00 M. Amende et al., ACS Catalysis, submitted 2013