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