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Modern Methods in Heterogeneous Catalysis Research: Theory and Experiment

Modern Methods in Heterogeneous Catalysis Research: Theory and Experiment. Photons : In situ spectroscopy in the soft X-ray energy range. Axel Knop-Gericke knop@fhi-berlin.mpg.de. Outline. Photons for the investigation of heterogeneous catalytic processes Synchrotron radiation

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Modern Methods in Heterogeneous Catalysis Research: Theory and Experiment

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  1. Modern Methods in Heterogeneous Catalysis Research: Theory and Experiment Photons: In situ spectroscopy in the soft X-ray energy range Axel Knop-Gericke knop@fhi-berlin.mpg.de

  2. Outline Photons for the investigation of heterogeneous catalytic processes Synchrotron radiation diffraction : single slit, double slit, grating in situ XAS in the soft energy range examples: methanol oxidation over copper n-butane oxidation over VPO catalysts

  3. Spectrum of electromagnetic radiation

  4. Other lectures in this field: UV-vis : A. Brückner 6.12.2002 Vibrational spectroscopy (IR spectroscopy, Raman: G. Rupprechter 13.12.2002 ) Electron spectroscopy: R. Schlögl 20.12.2002 X-ray diffraktion: I. Erran 10.1.2003 EXFAS/NEXAFS in the hard X-ray range: T.Ressler 17.1.2003

  5. Synchrotron radiation

  6. Synchrotron radiation

  7. Synchrotron radiation

  8. Synchrotron radiation

  9. Synchrotron radiation

  10. Synchrotron radiation

  11. Synchrotron radiation

  12. Synchrotron radiation

  13. Synchrotron radiation

  14. Undulator

  15. Synchrotron radiation

  16. Synchrotron radiation

  17. Single Slit Diffraction Princip of Huygens Minimum: g= k k=1,2,3,...<a/ sin k= g/a ; tan k =dk/l

  18. Diffraction Pattern

  19. Double Slit Diffraction Actually observed diffraction pattern Expected diffraction pattern:

  20. Double Slit Diffraction l>>a : waves normals related to Pk on the screen are parallel, sin k = g/a, tan k = dk/l k < 8 : sin k = tan k = dk/l = g/a  dk= gl/a Maximum (g= 0, , 2, 3,...) dk= kl/a k= 0,1,2,3.. Minimum (g= /2, 3/2 , 5/2 ,...) dk=(2k+1) l/2a k=0,1,2,3,..

  21. From single slit to grating: Diffraction pattern

  22. XAS in the soft energy range Soft energy range: 250 - 1000 eV which elements: C(1s), N(1s), O(1s), transition metal (2p).... • L edge / 2p XPS peaks of transition metal are sensitive to details of chemical • XAS is a local process not restricted to material with long range order • •surface sensitive when applied in electron yield mode • pellets can be investigated under reaction conditions • •orientation of molecules on single crystal surfaces can be estimated

  23. Spektroskopische Methode Prinzip der Röntgenabsorptionsspektroskopie (XAS) Auger-Elektron • Anregung von Rumpfniveauelektronen mittels Röntgenstrahlung • Relaxation mittels Fluoreszenzstrahlung oder durch Aussenden eines Auger-Elektrons (TEY, PEY, AEY) • Untersuchung der unbesetzten Zustände Fluoreszenz

  24. Mean free path of electron in solids

  25. Experimental Technique In situ methods are required to investigate heterogeneous catalytic reactions since the structure of a catalyst estimated ex situ might differ from the structure revealed by in situ studies Material gap single crystal vs real catalyst XAS in the soft energy range represent surface sensitive spectroscopic methods, which can be applied in the mbar pressure range Pressure gap UHV vs p > 1bar

  26. Transmission of 20 cm air

  27. manipulator mass spektrometer 1 5 butterfly-valve 0 m m 1 5 0 m process pump m sample gas inlet by MFC Plattenventil UHV-valve turbo pump f 100 mm Experimental Set-Up properties of the set-up heating • heating up to 900 K • pressure up to 20 mbar • batch- and flow-through-mode • angular dependent measurements

  28. In situ XAS Detector system Simultaneous detection of gas phase- and sample signal

  29. Gas phase subtraction Analysis of the Near Edge X-ray Absorption Fine Structure (NEXAFS) NEXAFS of the O K-edge • Total electron yield of the gas phase dominates all signals, therefore only small differences in the detector signals • Substraction allows to separate the absorption signal of the surface of the catalyst

  30. 2 CH3OH + O2 2 CH2O + 2 H2O 2 CH3OH + 3 O2 2 CO2 + 4 H2O CH3OH CH2O + H2 Methanol oxidation oxidative dehydrogenation dehydrogenation total oxidation

  31. Cu L3- NEXAFS NEXAFS at the Cu L3-edge Catalytic Activity Increased activity for gas flow ratios: O2 / CH3OH  0.5 Transition from an oxidic copper-phase to the metallic state Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

  32. NEXAFS at the O K-edge less active • NEXAFS of the active state is completly different from the NEXAFS of the known copper-oxides • 2 oxidic- and 1 suboxidic species can be distinguished very active

  33. Correlation between the SuboxideSpecies and CH2O Variation of temperature at O2 / CH3OH = 0.2 • Intensity of the suboxide species increases with increasing temperature • Intensity of the suboxide species is positively correlated to the yield of CH2O and CO

  34. Correlations between oxidic species and CO2 • Intensity of the oxidic species Oxsurf decreases with increasing CO2-yield • 2 areas of activity can be distinguished

  35. Model Proposed model of the copper surface under reaction conditions for methanol oxidation

  36. n-Butane Oxidation to MA by Vanadium Phosphorus Catalysts O VPO + 3,5 O + 4 H O 2 2 O 400 °C, 1 bar O Maleic Anhydride (MA) 1,5 Vol% air C H + 6,5 O 4 CO + 5 H O 4 10 2 2 2 C H + 4,5 O 4 CO + 5 H O 4 10 2 2 Active phase: highly ordered vanadyl pyrophosphate (VO)2P2O7)? Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

  37. V5 V valence V6 Details of the local chemical bonding V4 V3 Local geometric structure V2 V7 V1 The VPO V L3-NEXAFS Analysis of spectral shape by unconstrained least squares fit Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

  38. V4 V2O5 V3 V2 V1 V6 Å Å Interpretation of V L3 NEXAFS Experimental finding: V6* V5* V4* V3*  NEXAFS resonances appear in a sequence of V-O bond lengths V2* VPO V1*

  39. Proportion of int. intensity of V5 400°C RT 400°C RT RT V5 Spectra number Changes of NEXAFS while heating Relative spectral Intensity of V5 at V L3-edge Decrease while active MA Yield (a. u.) Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

  40. Interpretation of V L3 NEXAFS Identification of resonances (V5, V6): O(1a) V2O5 as model substance for VPO O(3) O(2) DFT calculation of DOS (V2O5 !)*: V V2O5: Close relationship between geometric and electronic structure at V L3-absorption edge O(3) O(2) O(1b)  main contributions to NEXAFS resonances appear in a sequence of V-O bond length  V6: O(1a)  V5: ? (estimated value of bond length between O(2) and O(1a): 1.72 Å) Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany

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