Download Practical application of Mössbauer Iron spectroscopy

Practical application of
Mössbauer Iron spectroscopy
By: Udo Bauer, Jan Hufschmidt,
Daniel Malko, Marius Piermeier,
Florian Späth and Patrick Uffinger
Table of Contents
• Mössbauer Spectroscopy in Fischer Tropsch catalysis
• Mössbauer Spectroscopy in Metal-Organic-Frameworks
• Spincrossover control via Mössbauer Spectroscopy
• Mössbauer studies on the oxidationstate
• Mössbauer studies on the geometry
• Mössbauer Spectroscopy in material science
Mössbauer Spectroscopy (MöS) in
Catalysis
• In the Fischer Tropsch
process all sorts row 8 – 10
metal catalysts are used
• Iron has the advantage to
be cheap and very active if
used right
• Different preparation
methods lead different
activities of the catalysts
• This is dependents on the
pH and the additions in
your solution
Measurements of fresh Catalysts
Changes during the Reaction
• Kinetic measurements show that high amount of α-Fe increase the
catalytic activity as does Iron carbide as 8.0 AH is most reactive
• This increased reactivity leads, however to reduced selectivity
• MöS was able to identify active species and helped tuning the
catalyst to enhance reactivity or selectivity
Another look an FT-Catalysts
U
n
r
e
d
u
c
e
d
R
e
d
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c
e
d
Another look at FT-Catalysts
• In the reduced form we now
have two species in different
amounts
• We have a lower signal to noise
ratio at the lower temperature
• At 4 K the spectra looks very
different
• This is due to low intensity,
hyperfine-splitting and because
the material is amorphous,
meaning it has only a chaotic
order in the long distance
MöS in Metal-Organic Freameworks
(MOF)
• MOFs may be used as catalyst carrier, as chirality inducing agents,
as spin crossover systems and in nonlinear optics
• Its behaviors depend on many parameters during formation (pH,
heat, [Fe], solvent)
Differences in the formation
• One can see that the
Fe-Ions have a
different surrounding
and very broad lines
due to randomly
scattered Iron
concentrations
within the polymer
• Overall not very
strong effects
• MöS turned out to
be not that helpful
Spincrossover studies via MöS
• Spincrossover, no matter how
it is induced always is affiliated
to a chance in bondlength and
thus MöS is ideal to study it
• The figure to the side shows the
frank condon principle for spin
transition
Light-induced excited spin state
trapping (LIESST)
I.S. = 0,11 mms-1
Q.S.= 3,08 mms-1
I.S. = 0,44 mms-1
Q.S. = 1,14 mms-1
• A spincrossover can be observed via MöS. I.S. tells us that the bonds are
indeed longer in the HS state and Q.S. tells us that we have a more
symmetrical electron distribution around the core in HS
• Two Iron centers, which seem to
be independent form each other
in switching behavior
• Left: HS: grey; LS: dark grey
• Although this is now a
Liquid Crystal system
and the shift form HS to
LS is more steady, MöS
look the same as most
of the time
[Fe(L)2](PF6)
Spincrossover studies via MöS
• dinuclear Fe(II)-compound:
[Fe(NCSe)(py)]2(bpypz)2
• magnetic measurements and Mössbauer
spectra to learn more about the behavior of
that compound
• unfilled circles show the μeff-value plotted
vs. T: T1/2 = 109 K, μeff = 5,3 ( for 300 - 150 K)
• and μeff = ~1,5 (for 100 – 25 K)
• abrupt HS-HS to LS-LS transition
• Mössbauer spectra of
[Fe(NCSe)(py)]2(bpypz)2]:
a)
b)
δ / mm s-1
1,00
ΔEQ / mm s-1
1,99
0,58
0,54
0,49
3,71
1,15
0,33
• a) quite normal values for such
compounds
• b) unusual lineshape, fitted to a sum
of three lines
• δ-values lower → LS-LS state (since
no change in oxidation number), no
antibonding orbitals filled, shorter
bonds lead to lower values
• further work is needed to
understand the whole mechanism
Dinitrosyl iron complexes (DNIC) with
imidazole bridging ligands
•
•
•
•
•
•
compound 1: [(imidazole)-Fe(NO)2]4
compound 2: [(2-isopropylimidazole)Fe(NO)2]4
compound 3: [(benzimidazole)Fe(NO)2]4
compounds forming tetramers
biological implication of DNICs
measurement of MöS of the complexes and of
some reference complexes
compound 1:
Fe (orange), O
(red), N(blue), C
(black)
tetramers
• isomer shifts nearly the
same for all 3 compounds →
nearly same oxidation state
for all iron centers [ Fe(III),
S=1/2, low spin] → also
nearly the same bond
distances of imidazolenitrogens to iron centers
• quadrupole splitting
parameters also nearly the
same for all 3 complexes →
all 3 complexes low spin d5
with nearly the same noncubic electron distribution
• A and C are reduced, B and D oxidized
forms, whereas D is most similar to the
tetramers
• A has two strongly σ-donating NHC
ligands, in comparison C has one CO as
weaker σ-donor but stronger π-acceptor
→ shorter bond of CO to Fe center →
lower isomer shift than A
• D has highest δ due to strong σ- and πdonating ligands
• interesting here: A and C have lower
isomer shifts than B and D although they
have the lower oxidation state this is due
to greater π-backbonding in A and C (3d
orbitals of Fe in reduced DNIC
energetically close to NO π* orbitals)
• all in all: tetramers have much higher δvalues since NHCs as bridging ligands
have less σ-donating ability than ligands in
D
reference complexes
Mössbauer study of Fe(Dioximato)nL2]
mixed coordination compounds
• Important in Biochemistry and Analytical Chemistry
• Two families: One Octahedral and one Planar
• The strong donor–
acceptor interactions
between the metal and
ligand ions
• empty 4s and 3d
orbitals of iron serve as
the main acceptors
• N-donated 4s electron
density increases the
total s electron density
and thus reducing δ
Octahedral
Planar
• As expected the quasi Octahedral
Structure is more Symmetric than
the Planar one
1-5 Octahedral
6-8 Planar
Metallurgical behavior of iron in brass
studied using MöS
• brass = Cu / Zn – alloy
• α-Fe (bcc structure,
stable below 910 °C,
ferromagnetic)
• γ-Fe (fcc structure,
stable between 910 °C 1390 °C, weakly
antiferromagnetic)
• γ-Fe undergoes
transition to α-Fe due
to plastic deformation
or aging thus meaning a
change in the
properties of the brass
material
• The amount of Fe
is proportional to
the peaksize
• Fe atoms with
only Cu / Zn
neighbours
• Fe with one Fe
atom as nearest
neighbour
• Fe with mostly Fe
atoms as
neighbours
Tuning material properties with MÖS
• Different annealing procedures lead to differently ordered Fe
impurities in our Brass and thus to different mechanical and
electrical properties
• For specific applications of your brass you want certain annealing
processes
Conclusion
• MöS is a very versatile spectroscopy
and applicable in a wide field
• MöS spectra may be easy to
interpret on the first look, may,
however, get more complicated if
you dive deeper in the use
• It is used to identify Spins,
Oxidationstats, Ligandsurroundings,
Crystalstructure and the
composition of your material