Introduction to X-ray Absorption Spectroscopy: an element sensitive method for local spatial and electronic structure K. Klementiev, Alba synchrotron - CELLS After this introductory lecture: 2. Introduction to theory 3. Data analysis (EXAFS, XANES) 4. Experiment (general and @CLÆSS) 5. Hands-on exercises 13-15.09.2012 K. Klementiev - Introduction to XAS 1 of 15 X-ray Absorption Spectra x-rays I0 sample transmission experiment I1 d I1 I0e XANES X-ray Absorption Near Edge Structure EXAFS measured at Alba/CLÆSS 13-15.09.2012 K. Klementiev - Introduction to XAS Extended X-ray Absorption Fine Structure 2 of 15 Qualitative Picture of XAFS M1–M5 L3 2p3/2 L2 2p1/2 L1 2s EF destructive interference – absorption minimum Here is where the interference is important! Ex-ray continuum E photoelectron constructive interference – absorption maximum K 1s 13-15.09.2012 K. Klementiev - Introduction to XAS 3 of 15 Theoretical Description In the MS theory, the expression for can be factored in terms of an atomic background and the oscillatory part In the photoelectron momentum space, k = [2m/ħ2(E–EF)]½, the function is parameterized as: f ( k ) 2 2 2 R / ( k ) 2 k j j ( k ) S Nsin( 2 kR ( k )) ee j j kR j 2 0 j 2 j j For each coordination shell j: Rj, Nj, 2j are the sought distance, coord. number and variance of distance fj(k)=|fj(k)|eij(k) is the scattering amplitude (calculated), λ is the electron free path (calculated), S02 accounts for many-electron excitations. The present theory cannot give reliable 0. • For XANES region this is a problem, because 0 there is a rapidly changing function with features comparable with . • In EXAFS region 0 is a smooth function and can be constructed empirically. Thus, XANES spectra are mostly interpreted, not analyzed. EXAFS spectra can be analyzed quantitatively. 13-15.09.2012 K. Klementiev - Introduction to XAS See more in the “Theory” lecture 4 of 15 What we can learn from XANES: a) Pre-edge Peak Dipole selection rule (only in central-symmetric case!): Consider K-absorption for transition metals: l = ±1 • initial state = 1s (l=0) • states near EF are formed by nd electrons (l=2) central-symmetry no resonance non-central symmetry resonance (pre-edge peak) from [F. Farges, G. E. Brown, J. J. Rehr, Phys. Rev. B 56 (1997) 1809] T. Ressler et al. J. Phys. Chem. B 104, 27 (2000) 6360-6370 13-15.09.2012 K. Klementiev - Introduction to XAS 5 of 15 What we can learn from XANES: b) Edge Shift T. Ressler, R. E. Jentoft, J. Wienold, M. M. Günter and O. Timpe: J. Phys. Chem. B 104, 27 (2000) 6360-6370 I. Arčon, B. Mirtič, A. Kodre, J. Am. Ceram. Soc. 81 (1998) 222–224 Why does it shift? 1) Electrostatic: it is harder for the photoelectron to leave a positive (oxidized) atom 2) Shorter bonds at higher oxidation states Fermi energy is higher 13-15.09.2012 K. Klementiev - Introduction to XAS 6 of 15 L3 absorption edges for 5d metals: (transition 2p3/2 5d) 13-15.09.2012 …white line also depends on particle size and morphology: G. Meitzner, G. H. Via, F. W. Lytle, and J. H. Sinfelt, J. Phys. Chem. 96 (1992) 4960 Intensity is proportional to the number of free 5d states and also depends on valence state. But… K. Klementiev - Introduction to XAS from A. L. Ankudinov, J. J. Rehr, J. J. Low, and S. R. Bare, J. Chem. Phys. 116 (2002) 1911 a) Pt3 triangle (dashes), Pt4 tetrahedron (solid), Pt5 triangular bipyramid (dash-dot), and Pt6 octahedron (dash-dot-dot); (b) Pt7 and Pt4 clusters of different shape: planar ‘‘honeycomb,(dashes), D5h bipyramid (solid) , single-capped octahedron (long dashdot), Pt4 tetrahedron (dots), and Pt4 planar rhombus (dash-dot). What we can learn from XANES: c) White Line 7 of 15 What we can learn from EXAFS f ( k ) 2 2 2 R / ( k ) 2 k j ( 1 ), ( k ) S N sin( 2 kR ( k )) e e 0 j j kR j 2 0j 2 j j 2.5 Cu K-edge for Cu metal foil 4 2 d k 2 2.0 0d 1.5 0 -2 -4 1.0 -6 2 0.5 4 6 8 10 12 14 -1 photoelectron wave number (Å ) 8 9000 9200 9400 2.3 9600 photon energy (eV) |FT(k2)| 0.0 Unpleasant thing about EXAFS: FT positions are shifted towards small distances. More unpleasant: Each FT peak has its own shift. Because of the phase shifts, extracting the structural information from EXAFS requires curve fitting with assumed (calculated) phase shifts. 13-15.09.2012 2.55 (real RCu-Cu) 6 4 2 0 0 K. Klementiev - Introduction to XAS 2 4 6 8 distance (Å) 8 of 15 I2 reference (metal foil) Experimental Setup (a) LN2 I1 I0 optics heating 13-15.09.2012 K. Klementiev - Introduction to XAS source e+/- 9 of 15 Experimental Setup (b) 13-15.09.2012 K. Klementiev - Introduction to XAS 10 of 15 Example: Application of EXAFS to Pd,Pt/C Catalysts (a) Supported noble metal catalysts are used in a number of commercial chemical processes (Pd,Pt/C toluene and benzene hydrogenation). Pd,Pt/C catalyst • Support: graphite-like carbon (sibunit) • Preparation: mild oxidative treatment of the support followed by ion exchange with Pd or Pt amine complexes • Characterization: XPS, TPR, catalytic studies • Outcome: highly dispersed metal clusters with ~1.1 wt% of Pd and ~0.9 wt% of Pt XAFS measurements • X1 beamline at Hasylab/DESY with a Si 311 double crystal monochromator, transmission mode • Samples: non-pressed powders 13-15.09.2012 K. Klementiev - Introduction to XAS 11 of 15 Example: Application of EXAFS to Pd,Pt/C Catalysts (b) Reduction treatment: in 5%H2/He atmosphere, stepwise with T=50°C. Measurements at LN2 temperature in order not to have interference of two effects: (i) due to different temperature vibrations and (ii) due to different particle sizes. photelectronwavenumber k(Å-1) 2 6 8 10 12 14 PdKEXAFS distancer (Å) 16 0 1 2 3 EXAFS function k2 6 6 increasingreductionT (200–500°C, T=50°C, dehydridedinHeat the sameTafter reduction) 0 hydrided at 200°C Pd/Casprepared -2 increasingreductionT (200–500°C, T=50°C, de-4 hydridedinHeafter reduction) Pdmetal foil Pd/Chydrided 0 Pt metal foil PtO2 Pt/Cas prepared Pt/C350°CinH2 Pt/C350°CinHe Pt/C400, 450, 500°CinH2 Pt metal foil 0 Pt/Cas prepared Pt/C350°CinH2 Pt/C350°CinHe Pt/C400, 450, 500°CinH2 -4 2 Pd/Cas prepared Pt L3 EXAFS -2 4 PdO 2 6 4 2 b) Pt/C 0 2 4 6 8 10 12 14 photelectronwavenumber k(Å-1) 13-15.09.2012 5 metal foil a) Pd/C 2 4 4 |FT(k2)| 4 4 16 0 1 2 3 4 5 6 distancer (Å) K. Klementiev - Introduction to XAS 12 of 15 Example: Application of EXAFS to Pd,Pt/C Catalysts (c) Reference compound: metallic Pd foil 2 N 12 fcc= N 12.4(9) fit= raw·k2 EXAFS results for Pd metal (measured at LN2 temperature) Path r (Å) rcrysta (Å) CN CNcryst 2 (10-3Å2) E (eV) Pd-Pd 2.742(4) 2.751 12.4(9) 12 3.4(5) 6.2(7) a at room temperature [Kittel] 4 0 FT(·k2) EXAFS function ·k2 4 fitted 2 filtered& 0 Checking your amplitudes and phases with reference spectra must be always the first step in every EXAFS study! -4 data fit -2 2 0 4 6 8 10 12 14 16 -1) w avenum berk(Å 0 1 2 3 distancer(Å ) 4 The difference between a simply reduced Pd/C sample (hydrided) and one subsequently blown out by He flow (dehydrided) raw·k2 hydrided -1 0 -1 2 d tte i f d& e r filte 4 dehydrided 6 8 10 12 14 16 -1) w avenum berk(Å 13-15.09.2012 2 1 0 0 hydrided 1 dehydrided 0 0 EX A FSresultsforPd/Csam pleduringthereductionprocess 2 reduction Path r(Å ) C N 2(10-3Å ) E(eV ) tem perature, °C 200, hydrided Pd-Pd 2.83(1) 5.5(8) 6(1) 6(1) 200, dehydrided Pd-Pd2.733(6)5.2(5) 4.4(7) 5.3(7) 250, dehydrided Pd-Pd2.732(6)5.9(5) 4.5(6) 5.4(7) 300, dehydrided Pd-Pd2.733(5)6.6(6) 4.2(6) 5.5(7) 350, dehydrided Pd-Pd2.735(5)7.4(6) 3.8(9) 5.5(7) 400, dehydrided Pd-Pd2.738(4)8.0(6) 3.2(5) 5.7(7) 450, dehydrided Pd-Pd2.740(4)8.4(6) 2.9(4) 6.0(7) 500, dehydrided Pd-Pd2.742(8)8.7(6) 2.8(7) 6.2(7) data fit 3 FT(·k2) EXAFS function ·k2 1 1 2 3 distancer(Å ) 4 K. Klementiev - Introduction to XAS 13 of 15 Example: Application of EXAFS to Pd,Pt/C Catalysts (d) [M.Borowski, J. Phys. IV 7, C2-259 (1997)] 2 8 3 =1 cluster radius R, Å 7 6 5 4 3 2 7.5(5)Å 1 N=Nfcc·(1- 4 63) +1 rPd-Pd rPt-Pt = R or R 500°C 6.8(5)Å 450°C 6.1(4)Å 400°C 5.5(7 )Å reduced 5.3(4)Å 350°C Pt/C 4.4(3)Å 300°C 3.9(2)Å 250°C 3.4(2)Å 200°C Pd/C Å ) 3 Pt/C 2.5( aspreparedPt/C 3 4 5 6 7 8 support 9 apparent coordinationnum ber N Catalytic properties: • The catalytic activity in toluene and benzene hydrogenation was found to exceed the activity of conventional Pd/Al2O3 and Pt/Al2O3 fourfold for Pd/C and almost ten-fold for Pt/C. • If the reduction temperature exceeds 350oC the activity of both catalysts sharply decreases. The EXAFS study made it possible to exclude intense metal sintering, or Pd carbide formation as possible reasons of the activity drop. Stakheev et al., Russ.Chem.Bull.Int.Ed. 53 (2004) 528 13-15.09.2012 K. Klementiev - Introduction to XAS 14 of 15 Conclusions • XAFS (XANES and EXAFS) spectroscopies: • suitable under reaction conditions • does not require long-range order • element specific • XANES spectroscopy for: • symmetry information from the pre-edge peaks • valence state from the edge shift • analysis of mixtures using basis spectra • XANES is experimentally simpler than EXAFS • signal is stronger (one can measure faster and at lower concentrations) • does not depend on T (if without phase transitions, of course) • EXAFS gives: • inter-atomic distances and coordination numbers • identification of neighbor atoms 13-15.09.2012 K. Klementiev - Introduction to XAS 15 of 15