Characterization
of solid catalysts
5. Mössbauer Spectroscopy
Prof dr J W (Hans) NiemantsverdrietSchuit Institute of Catalysis
© J.W. Niemantsverdriet, TU/e, Eindhoven, The Netherlands
www.catalysiscourse.com
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percentage
XRDAdsorption
XPSTP Techniques
InfraredTEMSEM
UV-visNMR
RamanESR
EXAFSXANES
EDXMössbauer
CalorimetryISS / LEIS
Neutron ScatteringSIMS
Total Number of Articles:
8112
18.417.410.0
9.27.36.94.34.33.63.22.01.91.51.20.80.4
<0.1<0.1<0.1
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percentage
How often are techniques used
Journals:
Applied Catalysis A & B Catalysis Letters Journal of Catalysis Jan 2002 and Oct 2006
© J.W. Niemantsverdriet, TU/e, Eindhoven, The Netherlands
Mössbauer Spectroscopy
• nuclear technique
• 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
14.4 keV
Fe57
Mössbauer Effect (1957)
• 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
• f: recoil free fractionno resonant absorption in free atoms
due to the (tiny!) energy loss by recoil
Emission Absorption
recoil
ER
recoil
ER
Eo Eo
Eo- ER Eo Eo + ER
emission
spectrumabsorption
spectrum
Mössbauer Spectrometer
detector
• single line emitter• Doppler effect:E(v) = Eo (1+v/c)
• absorber with 57Fe • gamma ray detector, • transmitted intensity
versus velocity
velocity v
I(v)
1 2
1 2
min max min max
Mössbauer Spectroscopy in 57Fe
Decay of 57Co to 57Fe
• transition:
14.4. keV
• natural linewidth:
4.6 neV
• hyperfine interactions:
1-100 neV
are easily resolved!
u
u
surface
bulk
Mössbauer Intensity: Lattice Vibrations
intensity = const x nFe x f
f = exp ( -k < x2 > ) = f ( T,OD )
recoil-free fraction, f (T, D)
D (K)
470
300
250
200
150
10050
1.0
0.8
0.6
0.4
0.2
0.0
0 100 200 300
T (K)
f ODDebye Temperature,
• high for rigid lattice
• low for soft vibrations
• can be determined from
T-dependence intensity
• surface: ~50% of bulk value
Hyperfine Interactions
• Isomer Shift:
oxidation state
• Quadrupole splitting:
local symmetry
• Magnetic Splitting:
nuclear magnetic field
Hyperfine Interactions
Isomer Shift:
• Coulomb interactionnucleus - s-electrons
• Information on
oxidation state
I.S.
Hyperfine Interactions
Quadrupole Splitting:
• nuclear quadrupole moment
electric field gradient (EFG)
• EFG: due to electrons and lattice
• Information on
local symmetry
Q.S.
Hyperfine Interactions
Magnetic Splitting:
• nuclear magnetic moment
magnetic field at nucleus
(Zeeman Effect)
• Information on
magnetism
v1 v2
singlet quadrupole doublet
magnetic sextuplet magnetic + quadrupole interaction
1
= v1
v1
0 0
0 0
v1 v2 v3 v4 v5 v6 v1 v2 v3 v4 v5 v6
= (v1+ v2)/2
E = v2 - v1
1 2
= (v1+ v6)/2
H v6 - v1
= (v1+ v2+ v5+v6)/4
H v6 - v1
= (v6- v5- v2+v1)/4
v (mm/s) v (mm/s)
v (mm/s) v (mm/s)
1 2 3 4 5 6 1 2 3 4 5 6
The most common types
of Mössbauer spectra
from iron compounds
0 100 200 300 400
Temperature (K)
0.54
0.44
0.34
0.24
D600 K500 K400 KIs
om
er
shift
(mm
/s)
-FeOOH
bulk material
small particles
Second-order Doppler shift, (T, D)
Isomer Shift depends on Temperature
J.W. Niemantsverdriet, C.F.J. Flipse, B. Selman, J.J. van Loefand A.M. van der Kraan, Phys. Lett. 100A (1984) 445.
Hyperfine Interactions
Fe2+ Fe3+
low spin
Feo
alloys
Fe3+
high spin
Fe2+
high spin
-0.5 0.0 0.5 1.0 1.5 2.0
(mm/s)
4
3
2
1
0
EQ
(mm/s)
α-Fe
Fe2O3
A.M. van der Kraan, R.C.H. Nonnekens,F. Stoop and J.W. Niemantsverdriet, Appl. Catal. 27 (1986) 285
dispersed Fe3+
dispersed Fe3+; some
larger iron oxide
(6-line pattern)
mostly reduced iron +
unreduced residue Fe3+ Fe2+
iron converted to carbide
+ residual iron oxides
part of carbide oxidized to
dispersed iron oxide
-8 -4 0 4 8 12
Doppler velocity (mm/s)
0 1 2 3 4 5 6
synthesis time (h)
2
1
0100
0100
0100
0100
0
Reaction rate and catalyst composition
reaction rate
-Fe
FexC
’-Fe2.2C
-Fe5C2
Mössbauer Spectra-Fe during FTS
0.5 h
1.1 h
2.5 h
6.5 h
24 h
48 h
contr
ibution (
%)
ra
te (
a.u
.)
J.W. Niemantsverdriet, A.M. van der Kraan, W.L. van Dijk, and H.S. van der Baan,
J. Phys. Chem. 84 (1980) 3363
P.H. Christensen, S. Mørup and J.W. Niemantsverdriet, J. Phys. Chem. 89 (1985) 4898
Superparamagnetic Fe on a carbon support
Langevin equation:
2.5 nm Fe particles
A.A. Hummel, A.P. Wilson
and W.N. Delgass,
J. Catal. 113 (1988) 236
Kinetics of solid state reactions
‘single velocity’ experiment:
follow intensity during reaction
Fe2N converts to metallic iron:
J.W. Niemantsverdriet
and W.N. Delgass,
Topics Catal. 8 (1999) 133
Mössbauer spectroscopy
during CO + H2 reaction
= iron carbide
= FeIr alloy + oxide
oxide peaks
change
Schuit Institute of Catalysis
CO Hydrogenation on FeIr/SiO2
L.M.P. van Gruijthuijsen et al, J.Catal. 170 (1997) 331
iron oxide
iron rich surface
FeIr alloy
FeIr alloy
iron carbide
silica
silica
CO Hydrogenation
Slow Activation:
• restructuring !!
Active State for
MeOH Production
• weak CO adsorption
• more hydrogenation
AuI
AuIII
AuIII
Au0
transm
issio
n
-10 -5 0 5 10
velocity (mm/s)
Y. Kobayashi,
S. Nasu,
S. Tsubota
and M. Haruta,
Hyperfine Interactions
126 (2000) 95
Mössbauer spectroscopy
of Au catalysts
Mossbauer Emission Spectroscopy
• single line absorber• Doppler effect:E(v) = Eo (1+v/c)
• emitter with 57Co • gamma ray detector, • transmitted intensity
versus velocity
velocity v
I(v)
1 2
1 2
S Mørup, H Topsøe & coworkers, J. Catal. 68 (1981)433, and 453
ppm Co on MoS2
Sulfided CoMo Catalysts:
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
Read more about Mössbauerspectroscopy
in Chapter 5 of
Spectroscopy in Catalysis: An Introduction, Third Edition
J. W. Niemantsverdriet
Copyright 2007 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-31651-9
Version 2000© J.W. Niemantsverdriet, TU/e, Eindhoven, The Netherlands
Download the handout for this lecture from
www.catalysiscourse.com
gives many examples and
references to the literature
