IDECAT WP3 Seminar. Prag November 28,2006

Transcription

1 IDECAT WP3 Seminar Prag November 28,2006 High pressure XPS: A tool for the investigation of heterogeneous catalytic processes A. Knop-Gericke, D.Teschner, H. Bluhm, M. Hävecker (knop@fhi-berlin.mpg.de) Fritz-Haber-Institut, Berlin, Department of Inorganic Chemistry 1

2 Why in situ XPS? Many processes cannot be investigated in UHV: Pressure Gap - environmental chemistry - catalysis - corrosion - electrochemistry -biologicalsamples water vapor pressure (torr) T m Very few methods can investigate the solid-gas interface at high pressures - non-linerar optics (SFG, SHG) - scanning probe microscopies - X-ray diffraction temperature (ºC) Photoelectron spectroscopy is very powerful Goal: XPS at pressures of at least 5 torr 2

3 In situ XPS: obstacles Fundamental limit: 30x10-21 O 2 exp. data from Schram et al. (1965) elastic and inelastic scattering of electrons in the gas phase Ionization cross section (m 2 ) in situ XPS extrapolation of Schram's data in situ EXAFS in situ SEM in situ TEM Electron kinetic energy (ev) Technical issues: - Differential pumping to keep analyzer in high vacuum - Sample preparation and control in a flow reactor 3

4 In situ XPS: basic concept Photons enter through a window Electrons and a gas jet escape through an aperture to vacuum 4

5 In situ XPS instruments: previous designs H. Siegbahn et al. (1973- ) M.W. Roberts et al. (1979) M. Faubel et al. (1987) M. Grunze et al. (1988) P. Oelhafen (1995) H. Siegbahn et al., J. Electron Spectrosc. Relat. Phenom., 319 (1973) 2 H. Siegbahn et al., J. Electron Spectrosc. Relat. Phenom. 24, 205 (1981) 5

6 In situ XPS using differentially pumped electrostatic lenses to pump to pump X-rays from synchrotron D.F. Ogletree, H. Bluhm, G. Lebedev, C.S. Fadley, Z. Hussain, M. Salmeron, Rev. Sci. Instrum. 73 (2002) 3872.

7 Close-up of sample-first aperture region p 0 hν Gas phase composition can be measured by XPS. gas phase signal: 1 torr mm ~ a few monolayers p/p z=2 z mm e z [d] d=1 mm gas

8 In situ XPS system X-rays enter the cell at 55 incidence through an SiN x window (thickness ~ 1000 Å) Hemisphercal electron analyzer (10-9 p 0 ) mass spectrometer and additional pumping Analyzer input lens Focal point of analyzer input lens Third differential pumping stage (10-8 p 0 ) Experimental cell supplied by gas lines (p 0 ) First differential pumping stage (10-4 p 0 ) 8 Second differential pumping stage (10-6 p 0 )

9 Introduction + H2 [ ] Literature carbon laydown similar catalysts selective hydrogenation different activity & selectivity (structure sensitivity?) Selectivity issue: what defines selectivity? 9

10 Summary 1. Subsurface H : effective for alkene-to-alkane but also for alkyne-to-alkane transformation 2. Surface H : could be selective (spillover) 3. Different reaction orders in the different selectivity regimes & Abrupt changes between regimes 4. C uptake is significantly more in the selective regime 10

11 Reaction in the mbar p region (in-situ XPS) + H2 5% Pd/CNT 3% Pd/Al 2 O 3 Pd foil Pd(111) Conversion [%] ~ 10 ~5 ~2.5 <1 Selectivity Pentene [%] ~95 ~80 ~ Selectivity Pentane [%] ~5 ~20 ~2 - Recation conditions: C5/H2 = 1:9, 1 mbar, 358 K 11

12 In-situ XPS: Pd 3d (hν: 720 ev) 12

13 In-situ XPS: Pd 3d depth profiling Not only adsorbate-induced surface core level shift! But on-top location! 13

14 In-situ XPS: C1s (Switching off experiments) 1: reaction mix. 2: C5 off 3: H2 off 4: C5 gas-phase 14 Teschner et al. J. Catal. 242 (2006) 26-37

15 In-situ XPS: Pd 3d (Switching off experiments) 1: reaction mix. 2: C5 off 3: H2 off 4: reaction; high T 523 K 15

16 In-situ XPS: Pd vs. C depth profiling 0.3 nm 1.4 nm 14 nm Carbon Content [%] Palladium Inelastic Mean Free Path [Å] A Experiment: 358 K Model: 358 K Model: 3.5 ML Experiment: C5 off Model: C5 off Carbon content [%] B 7.25 Electron Kinetic Energy [ev] Epxeriment: 523K Model: 523K Model: 4.2 ML Electron Kinetic Energy [ev] Carbon Inelastic Mean Free Path [Å] 16

17 Summary 1. Subsurface H : effective for alkene-to-alkane but also for alkyne-to-alkane transformation 2. Surface H : could be selective (spillover) 3. Different reaction orders in the different selectivity regimes & Abrupt changes between regimes 4. C uptake is considerably more in the selective regime 5. Pd-C surface phase forms in the early stage of selective pentyne hydrogenation & there is significant amount of subsurface C below of it 17

18 Model (during the reaction) 18

19 Summary 1. Subsurface H : effective for alkene-to-alkane but also for alkyne-to-alkane transformation 2. Surface H : could be selective (spillover) 3. Different reaction orders in the different selectivity regimes & Abrupt changes between regimes 4. C uptake is considerably more in the selective regime 5. Pd-C surface phase forms during selective hydrogenation of pentyne & there is significant amount of subsurface C below of it 6. Dynamic behaviour of Pd-C and subsurface C 19

20 Outlook: In situ XPS / XAS The future at BESSY ISISS: Innovative Station for In Situ Spectroscopy A project of BESSY and the Dep. Inorganic Chemistry, Fritz-Haber-Institut Installation of a beamline exclusively used for in situ spectroscopy in the soft X-ray range Installation of infrastructure optimized for these kind of experiments on site (e.g. chemical lab, gas supply, gas analytics) Later, further implementation of other in situ spectroscopy techniques: multi wavelength Raman, UV-Vis, fluorescence yield?! Start of user operation of the beamline: M. Hävecker, Surface Analysis, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

21 Outlook: In situ XPS / XAS The future at BESSY ISISS: Sketch of laboratory: dr. heinekamp, Berlin, Germany 21 M. Hävecker, Surface Analysis, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

22 Outlook: In situ XPS / XAS The future at BESSY flux (phot./s/0.1a/0.1%bw) 1,0E+11 1,0E+10 1,0E Resolution (E/dE) Photon Energy / ev Photon Energy / ev 600 l/mm, 80 μm 1200 l/mm, 80 μm 600 l/mm, 20 μm 1200 l/mm, 20 μm 1200 l/mm, 20 μm600 l/mm, μm l/mm, 80 μm 600 l/mm, 80 μm 22 M. Hävecker, Surface Analysis, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

23 Outlook: In situ XPS / XAS The future at BESSY 23 M. Hävecker, Surface Analysis, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

24 Outlook: In situ XPS / XAS The future at BESSY 24 M. Hävecker, Surface Analysis, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

25 Outlook: In situ XPS / XAS The future at BESSY 25 M. Hävecker, Surface Analysis, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

26 Outlook: In situ XPS / XAS The future at BESSY 26 M. Hävecker, Surface Analysis, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

27 Outlook: In situ XPS / XAS The future at BESSY 27 M. Hävecker, Surface Analysis, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

28 Thanks to: Detre Teschner, Elaine Vass, Michael Hävecker, Evgueni Kleimenov,,Spiros Zafeiratos, Péter Schnörch, Hermann Sauer, Robert Schlögl (FHI, Dept. AC) Mounir Chamam, Attila Wootsch (Institute of Isotops, Budapest) Arran S. Canning, Jonathan J. Gamman, S. David Jackson (Glasgow University) James McGregor, Lynn F. Gladden (Cambrigde University) Fred Senf, Rolf Follath, Walter Braun, Jens Blume, Jan Schmidt (BESSY) 28