NEXAFS. Axel Knop

Transcription

1 NEXAFS Modern Methods in Heterogeneous Catalysis Research..we are done in 2009! Axel Knop

2 Abbreviations XAS: X-ray Absorption Spectroscopy NEXAFS : Near Edge X-ray Absorption Fine Structure (preferentially used for low Z elements : O, N, C,..) XANES: X-ray Absorption Near Edge Structure EXAFS: Extended X-ray Absorption Fine Structure SEXAFS: Surface Extended X-ray Absorption Fine Structure

3 NEXAFS and EXAFS

4 S. Bare What is NEXAFS/XANES?

5 Valence- and Rydberg States in Molecules

6 Detection

7 Detection

8 Mean free path of electron in solids

9 XANES transitions

10 XANES interpretation

11 White Line

12 Local coordination environment S. Bare

13 Estimation of the oxidation state S. Bare

14 S. Bare XANES Analysis: Oxidation state of sulfur

15 What do we learn from XANES?

16 Advantages of NEXAFS vs. EXAFS

17 Time Evolution of XANES: Kinetics

18 Polarisation dependency

19 This is not completely right!

20 Synchrotronradiation Facilities in the soft X-ray range ALS (LBL, Berkeley CA USA) Astrid (Aarhus, Dänemark) BESSY (Berlin, Deutschland) Elettra (Trieste, Italy) Superaco (Orsay, France) Max-Lab (Lund, Schweden) NSLS (BNL, Brookhaven, NY USA) PAL (Pohang, Korea) Photon Factory at KEK (Japan) SLS (Villingen, Schweiz) Spring-8 (Japan) SRC (Staughton, WI USA) SRS (Daresbury, UK) SRRC (Hsinchu, Taiwan) SSRL (Stanford, USA)

21 Plane Grating Monochromator Beamline

22 Transmission von 20 cm air Transmission Energy (ev)

23 Experimental Set-Up manipulator properties of the set-up heating mass spektrometer sample 150 mm 150 mm process pump butterfly-valve gas inlet by MFC heating heatingup up to to K pressure pressureup up to to mbar mbar batch- batch-and and flow-through-mode flow-through-mode angular angulardependent dependentmeasurements Plattenventil UHV-valve turbo pump φ 100 mm

24 In situ XAS detector system Simultaneous Simultaneousdetection of of gas gas phase- phase-and and sample sample signal signal A. Knop-Gericke et al

25 Gas phase subtraction Analysis of the Near Edge X-ray Absorption Fine Structure (NEXAFS) O K-edge Intensity (a. u.) π* ev O 2 σ* ev ev ev ev CH 3 OH + O 2 CH 3 OH I gas I det I gas ( I det - ) * 3 NEXAFS of the O K-edge Total Total electron electronyield yieldof of the thegas gas phase phasedominates dominatesall all signals, signals, therefore thereforeonly onlysmall differences differencesin in the thedetector signals signals Substraction Substractionallows allowsto to separate separate the theabsorption absorptionsignal signal of of the thesurface surfaceof of the thecatalyst 0 Cu 2 O ev Photon Energy / ev A. Knop-Gericke et al

26 Methanol Oxidation 2 CH 3 OH + O 2 2 CH 2 O + 2 H 2 O CH 3 OH CH 2 O + H 2 2 CH 3 OH + 3 O 2 2 CO H 2 O oxidative dehydrogenation dehydrogenation total oxidation

27 Cu L 3 -NEXAFS NEXAFS at the Cu L 3 -edge Catalytic Activity Total Electron Yield (a. u.) Cu L 3 -edge Temperature 670 K 670 K 570 K Flow ratio O 2 / CH 3 OH Reference Cu 2 O Conversion CH 3 OH (%) 100 Increased Increasedactivity activityfor for gas gas flow flowratios: O 2 2 // CH CH 3 OH 3 OH < Flow ratio O 2 / CH 3 OH K Photon Energy / ev A. Knop-Gericke et al Transition Transitionfrom froman an oxidic oxidiccopper-phase copper-phaseto to the themetallic state state

28 NEXAFS at the O K-edge Total Electron Yield (a. u.) O K-edge Temperature 670 K 670 K 570 K 300 K ev ev ev Flow ratio O 2 / CH 3 OH Reference Cu 2 O less active very active NEXAFS NEXAFS of of the theactive state stateisis completly completly different different from fromthe the NEXAFS NEXAFS of of the theknown copper-oxides copper-oxides 22 oxidic- oxidic-and and 11 suboxidic suboxidicspecies speciescan canbe be distinguished distinguished Photon Energy / ev A. Knop-Gericke et al

29 Correlation between the SuboxideSpecies and CH 2 O-Yield Variation of temperature at O 2 / CH 3 OH = 0.2 Intensity Intensityof of the thesuboxide species speciesincreases increaseswith with increasing increasingtemperature Intensity Intensityof of the thesuboxide species speciesisispositively positively correlated correlatedto to the theyield yieldof of CH CH 2 2 O and and CO CO A. Knop-Gericke et al

30 Correlations between oxidic species and CO 2 Intensity Intensityof of the theoxidic oxidicspecies speciesox Ox surf surf decreases decreaseswith withincreasing increasingco CO 2 -yield 2 -yield 2 2 areas areasof of activity activitycan canbe bedistinguished A. Knop-Gericke et al

31 Model Proposed model of the copper surface under reaction conditions for methanol oxidation CO+H 2 CH OH 3 CO +H O 2 2 CH OH 3 CH OH 3 CH O+H 2 2 Ox surf Subox Cu 0 Ox bulk O+ O CH O+H 2 2 O O vol O 2 A. Knop-Gericke et al

32 n-butane Oxidation to MA by Vanadium Phosphorus Catalysts VPO + 3,5 O H 2 O 400 C, 1 bar O O O 1,5 Vol% air Maleic Anhydride (MA) C 4 H ,5 O 2 4 CO H 2 O C 4 H ,5 O 2 4 CO + 5 H 2 O Active phase: highly ordered vanadyl pyrophosphate (VO) 2 P 2 O 7 )?

33 The VPO V L 3 -NEXAFS Analysis of spectral shape by unconstrained least squares fit Total Electron Yield (norm. u.) V L 3 -edge V1 V2 V3 V4 V5 V6 V7 V valence Details of of the the local local chemical bonding Local geometric structure Photon Energy / ev M. Hävecker et al.

34 Interpretation of V L 3 NEXAFS Experimental finding: NEXAFS resonances appear in in a sequence of of V-O V-O bond bond lengths Relative Resonance Position / ev Relative Resonance Position / ev V 4 V 3 V 2 V 1 V 2 O 5 V Bond Length / Å Bond Length / Å M. Hävecker et al.

35 Changes of NEXAFS while heating Decrease while active Proportion of int. intensity of V5 0,55 0,50 0,45 0,40 0,35 0,30 Relative spectral Intensity of V 5 at V L 3 -edge RT 400 C RT 400 C RT Spectra number Total Electron Yield (norm. u.) MA Yield (a. u.) V L 3 -edge Photon Energy / ev V 5 Electronic Structure, Dept. AC, Fritz-Haber-Institut (MPG), Berlin, Germany M. Hävecker et al.

36 Interpretation of V L 3 NEXAFS Identification of resonances (V5, V6): O(1a) V 2 O 5 as model substance for VPO O(3) DFT O(2) DFT calculation of of DOS DOS (V (V 2 O 2 5!)*: 5!)*: V V 2 O 2 5 : 5 : Close relationship between geometric and and O(3) electronic O(2) structure at at V L 3 -absorption 3 edge edge O(1b) main main contributions to to NEXAFS resonances appear in in a sequence of of V6: V6: O(1a) V-O V-O bond bond length V5: V5:? (estimated value of of bond bond length between O(2) O(2) and and O(1a): Å) Å)

37 (SBA-15) System: Objective: VO x on silica support (SBA-15) What is the molecular structure of dehydrated VO x species (monomer vs. polymer)? Methodology: In situ NEXAFS in combination with DFT cluster calculations Question: Spectroscopic evidence for V-O-V V bonds?

38 Differentiation between gas phase and surface contribution by proper selection of the Auger-window

39 Simulation of O K-NEXAFS K for various VOx clusters on silica V-O-V bond no signature of Si-O-Si bonds M. Cavalleri et al., J. Catal. 262 (2009) 215

40 Experimental: In situ soft X-ray X NEXAFS Electron yield technique: (secondary) de-excitation process (Auger decay) as measure for absorption cross section σ : N abs = c*(1-e -σρ ) c*σρ σ N hole σ I EY I EY N abs Si 3 N 4 membrane X-ray (UHV) e spectrometer aperture e sample e e e e e e e electron spectrometer Auger Electron Yield +108V Total Electron Yield

41 Experimental O K-NEXAFS K of Mg vanadate reference compounds Crystal structure V-O-V bond no V-O-V bond V2p V3d O1s O2p

42 Fingerprint of V-O-V V V bonds O K-edge detail fingerprint of V-O-V oxygen

43 Experimental O K-NEXAFS K of VOx/SBA-15 in O 2 gas at 400 C (after correction for gas phase transmission) O-V bonds O-V bonds increasing V loading O-Si bonds

44 Comparison of experimental O K-NEXAFS K with theoretical cluster simulation Comparison with theory allows to differentiate between different V-O bonding configurations 8 wt% V A distribution of vanadia species is present including non-monomeric species M. Hävecker et al., pss(b) 246 (2009) 1459 M. Cavalleri et al., J. Catal. 262 (2009) 215

45 Why Auger Electron Yield? Total Electron Yield O 2 pressure varied vacuum Auger Auger Electron Electron Yield Yield 0.8 mbar O 2

46 Distortion by gas phase absorption TEY of O 2 -gas Simulation of photon absorption of O 2 (24mm, 0.5mbar, 20 C )

47 Outlook / Future plans How relevant are results obtained in 0.5 mbar reaction atmosphere for the real world? M. Bron et al. D. Teschner et al. Selective hydrogenation of acrolein to allyl alcohol over Ag Selective hydrogenation of Pentyne over Pd

48 Outlook / Future plans How relevant are results obtained in 0.5 mbar reaction atmosphere for the real world? Strategies: verify gas atmosphere composition during experiments (selectivity!) construction of fixed bed reactor at 1<p<1000mbar to perform kinetic measurements construction of soft-xas cell operated in electron yield mode at 1<p<1000mbar Theory to explore extended regime of µ(t,p)

49 Outlook / Future plans sample sapphire sapphire IR hν e - 1st aperture

50 Outlook / Future plans sapphire sample e - hν reaction cell gas IN

51 Literature: Joachim Stöhr: "NEXAFS Spectroscopy", Springer 1992 Jochen Haase: "SEXAFS und NEXAFS Röntgen-Absorptionsspektroskopie an Adsorbat-bedeckten Oberflächen", Chemie in unserer Zeit, 26. Jg 1992, Nr. 5, pp