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

Unreduced

Reduced

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 • The figure to the side shows the

frank condon principle for spin transition

• Spincrossover, no matter how it is induced always is affiliated to a chance in bondlength and thus MöS is ideal to study it

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

[Fe(L)2](PF6)

• 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

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]:

δ / mm s-1 ΔEQ / mm s-1 a) 1,00 1,99 b) 0,58 3,71 0,54 1,15 0,49 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)

• 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 imidazole-nitrogens to iron centers

• quadrupole splitting parameters also nearly the same for all 3 complexes → all 3 complexes low spin d5

with nearly the same non-cubic electron distribution

tetramers

• 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 π- donating 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 δ- values 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

1-5 Octahedral 6-8 Planar

Octahedral 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

• As expected the quasi Octahedral

Structure is more Symmetric than the Planar one

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

• 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

Tuning material properties with MÖS

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