Characterization. of solid catalysts. 5. Mössbauer Spectroscopy. Prof dr J W (Hans) Niemantsverdriet.

Transcription

1 Characterization of solid catalysts 5. Mössbauer Spectroscopy Prof dr J W (Hans) Niemantsverdriet Schuit Institute of Catalysis J.W. Niemantsverdriet, TU/e, Eindhoven, The Netherlands

2 How often are techniques used XRD Adsorption XPS TP Techniques Infrared TEM SEM UV-vis NMR Raman ESR EXAFS XANES EDX Mössbauer Calorimetry ISS / LEIS Neutron Scattering SIMS <0.1 <0.1 <0.1 Journals: Applied Catalysis A & B Catalysis Letters Journal of Catalysis Jan 2002 and Oct 2006 Total Number of Articles: 8112 J.W. Niemantsverdriet, TU/e, Eindhoven, The Netherlands percentage

3 Mössbauer Spectroscopy 57 Fe nuclear technique 14.4 kev nucleus feels environment via hyperfine interactions recoilfree emission and absorption of gamma rays by nucleus high penetrating power of gamma rays enables in situ investigations

4 Mössbauer Effect (1957) recoil E R emission spectrum Emission E o Absorption E o recoil E R absorption spectrum Resonant absorption only if nuclei (atoms) are fixed in a solid Recoil taken up by lattice vibrations (quantized) recoil energy < phonon energy: then some absorption events occur without recoil E o - E R E o E o + E R no resonant absorption in free atoms due to the (tiny!) energy loss by recoil f: recoil free fraction

5 Mössbauer Spectrometer detector I(v) min max min max velocity v single line emitter Doppler effect: E(v) = E o (1+v/c) absorber with 57 Fe gamma ray detector, transmitted intensity versus velocity

6 Mössbauer Spectroscopy in 57 Fe Decay of 57 Co to 57 Fe transition: kev natural linewidth: 4.6 nev hyperfine interactions: nev are easily resolved!

7 Mössbauer Intensity: Lattice Vibrations intensity = const x n Fe x f f = exp ( -k < x 2 > ) = f ( T,O D ) surface u bulk u

8 recoil-free fraction, f (T, D ) 1.0 D (K) f Debye Temperature, high for rigid lattice O D low for soft vibrations can be determined from 200 T-dependence intensity surface: ~50% of bulk value T (K)

9 Hyperfine Interactions Isomer Shift: oxidation state Quadrupole splitting: local symmetry Magnetic Splitting: nuclear magnetic field

10 Hyperfine Interactions Isomer Shift: I.S. Coulomb interaction nucleus - s-electrons Information on oxidation state

11 Hyperfine Interactions Quadrupole Splitting: Q.S. nuclear quadrupole moment electric field gradient (EFG) EFG: due to electrons and lattice Information on local symmetry

12 Hyperfine Interactions Magnetic Splitting: nuclear magnetic moment magnetic field at nucleus (Zeeman Effect) Information on magnetism

13 singlet quadrupole doublet = v 1 = (v 1 + v 2 )/ E = v 2 - v 1 The most common types of Mössbauer spectra from iron compounds v 1 magnetic sextuplet v 1 v v (mm/s) v (mm/s) magnetic + quadrupole interaction = (v 1 + v 6 )/2 = (v 1 + v 2 + v 5 + v 6 )/4 H v 6 - v 1 H v 6 - v 1 = (v 6 - v 5 - v 2 + v 1 )/ v 1 v 2 v 3 v 4 v 5 v 6 v 1 v 2 v 3 v 4 v 5 v v (mm/s) v (mm/s)

14 Isomer shift (mm/s) Isomer Shift depends on Temperature Second-order Doppler shift, (T, D ) FeOOH bulk material small particles D 600 K 500 K 400 K Temperature (K) J.W. Niemantsverdriet, C.F.J. Flipse, B. Selman, J.J. van Loef and A.M. van der Kraan, Phys. Lett. 100A (1984) 445.

15 Hyperfine Interactions 4 3 Fe 2+ E Q (mm/s) 2 Fe 2+ Fe 3+ low spin high spin 1 Fe 3+ Fe o Fe 2 O 3 alloys high spin α-fe (mm/s)

16 dispersed Fe 3+ dispersed Fe 3+ ; some larger iron oxide (6-line pattern) mostly reduced iron + unreduced residue Fe 3+ Fe 2+ iron converted to carbide + residual iron oxides part of carbide oxidized to dispersed iron oxide A.M. van der Kraan, R.C.H. Nonnekens, F. Stoop and J.W. Niemantsverdriet, Appl. Catal. 27 (1986) 285

17 contribution (%) rate (a.u.) Mössbauer Spectra -Fe during FTS Reaction rate and catalyst composition 0.5 h h 1 reaction rate 2.5 h Fe 6.5 h 24 h 48 h Fe x C -Fe 2.2 C Doppler velocity (mm/s) 0 -Fe 5 C synthesis time (h) J.W. Niemantsverdriet, A.M. van der Kraan, W.L. van Dijk, and H.S. van der Baan, J. Phys. Chem. 84 (1980) 3363

18 Superparamagnetic Fe on a carbon support Langevin equation: 2.5 nm Fe particles P.H. Christensen, S. Mørup and J.W. Niemantsverdriet, J. Phys. Chem. 89 (1985) 4898

19 Kinetics of solid state reactions Fe 2 N converts to metallic iron: single velocity experiment: follow intensity during reaction A.A. Hummel, A.P. Wilson and W.N. Delgass, J. Catal. 113 (1988) 236

20 Mössbauer spectroscopy during CO + H 2 reaction = FeIr alloy + oxide oxide peaks change = iron carbide J.W. Niemantsverdriet and W.N. Delgass, Topics Catal. 8 (1999) 133

21 CO Hydrogenation on FeIr/SiO 2 Schuit Institute of Catalysis iron oxide iron rich surface FeIr alloy silica Slow Activation: restructuring!! CO Hydrogenation iron carbide FeIr alloy silica Active State for MeOH Production weak CO adsorption more hydrogenation L.M.P. van Gruijthuijsen et al, J.Catal. 170 (1997) 331

22 transmission Mössbauer spectroscopy Au 0 Au I Au III Au III of Au catalysts Y. Kobayashi, S. Nasu, S. Tsubota and M. Haruta, Hyperfine Interactions 126 (2000) velocity (mm/s)

23 Mossbauer Emission Spectroscopy I(v) velocity v emitter with 57 Co single line absorber Doppler effect: E(v) = E o (1+v/c) gamma ray detector, transmitted intensity versus velocity

24 S Mørup, H Topsøe & coworkers, J. Catal. 68 (1981)433, and 453 Sulfided CoMo Catalysts: ppm Co on MoS 2

25 Mossbauer Spectroscopy in Catalysis Limited to characterization of catalysts (Materials Science of Catalysts) Great advantage: in situ application Highly relevant information on a small number of important catalysts

26 Download the handout for this lecture from Read more about Mössbauer spectroscopy in Chapter 5 of Spectroscopy in Catalysis: An Introduction, Third Edition J. W. Niemantsverdriet Copyright 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: gives many examples and references to the literature J.W. Niemantsverdriet, TU/e, Eindhoven, The Netherlands Version 2000