4 From Small Molecules to Complex Systems: A Survey of Chemical …
213
More and more understanding will be gathered by studying complex systems
which have several iron species like paramagnetic iron ions, agglomerates of a
small number of iron sites or even clusters with hundreds of iron atoms or more.
The evaluation of temperature and field dependent Mössbauer spectra by combining
different evaluation routes like e.g. simple lorentzian line shape analysis, fitting with
magnetic hyperfine field distributions and spin Hamiltonian analysis, the latter being
very important as far as paramagnetic iron centers are concerned have been used
to understand complex systems like e.g. heterogeneous catalysts. In these systems
the identification of active iron-sites is a demanding task. Such supported catalysts
contain iron in many phases, e.g. iron oxide nanoparticles, as single ions, but also
agglomerates of only a few iron centers.
Iron loaded zeolites (e.g. Fe–ZSM-5) are potent candidates for the catalytic abatement of nitrogen oxides from car exhaust, e.g. from Diesel engines. This catalytic
process, the reduction of nitrogen oxides (NO x ) to N 2 by NH 3 in presence of oxygen is
called “selective catalytic reduction, SCR”). Recently Fe–ZSM-5 catalysts prepared
via solid-state ion exchange have been investigated by using field dependent Mössbauer spectroscopy at low temperature in order to identify the different iron species
present in this type of catalyst in the fresh state and after use in catalysis or more
precisely before and after standard SCR (Fig. 4.27) [95]. Mössbauer spectroscopy
proves to be the key technique for a full understanding of the iron species, but due to
the complexity of structures, guidance by complimentary methods like EPR experiments and control by SQUID magnetometry were essential to prove reliability of
derived species distributions. The combination with complimentary spectroscopic
techniques like EPR spectroscopy allowed quantifying the amount of paramagnetic
iron sites in Fe–ZSM-5 and the detection of EPR-silent diamagnetic iron species like
μ-oxo-bridged dinuclear iron sites even in these highly complex systems. The simulations in Fig. 4.27 allow to identify and quantify the relative amount of the different
iron species: K1 is assigned to iron oxide/hydroxide nanoparticles, K2 represents
diamagnetic most probably μ-oxo bridged dinuclear iron(III) centers, K3 are paramagnetic iron(II) high spin sites and K4 and K5 are due to single high spin ferric sites.
A similar methodical approach has been used very recently in order to elucidate the
structural composition of an Fe–N–C catalyst related to fuel cell batteries [96].
Synchrotron based NFS will allow to obtain Mössbauer parameters of small
species like micro- and nanocrystals or even films of biological cells which have
been feeded with
57 Fe containing nutrients. Today the maximum foci which can be
achieved for e.g. 14.4 keV with meV energy resolution are in the range of some
micrometers. With the help of the now developed diffraction limited synchrotron
sources it can be expected that nanofocussing can be done. However, care must be
taken, because the investigation of single micro- and nano-objects can also be influenced by radiation damage which needs to be investigated expecially for biological
systems.
Synchrotron based NIS has added the possibility to trace down iron centred molecular modes in molecular switches like SCO materials and proteins. Only some few
examples have been discussed in this contribution. These applications will certainly
213
More and more understanding will be gathered by studying complex systems
which have several iron species like paramagnetic iron ions, agglomerates of a
small number of iron sites or even clusters with hundreds of iron atoms or more.
The evaluation of temperature and field dependent Mössbauer spectra by combining
different evaluation routes like e.g. simple lorentzian line shape analysis, fitting with
magnetic hyperfine field distributions and spin Hamiltonian analysis, the latter being
very important as far as paramagnetic iron centers are concerned have been used
to understand complex systems like e.g. heterogeneous catalysts. In these systems
the identification of active iron-sites is a demanding task. Such supported catalysts
contain iron in many phases, e.g. iron oxide nanoparticles, as single ions, but also
agglomerates of only a few iron centers.
Iron loaded zeolites (e.g. Fe–ZSM-5) are potent candidates for the catalytic abatement of nitrogen oxides from car exhaust, e.g. from Diesel engines. This catalytic
process, the reduction of nitrogen oxides (NO x ) to N 2 by NH 3 in presence of oxygen is
called “selective catalytic reduction, SCR”). Recently Fe–ZSM-5 catalysts prepared
via solid-state ion exchange have been investigated by using field dependent Mössbauer spectroscopy at low temperature in order to identify the different iron species
present in this type of catalyst in the fresh state and after use in catalysis or more
precisely before and after standard SCR (Fig. 4.27) [95]. Mössbauer spectroscopy
proves to be the key technique for a full understanding of the iron species, but due to
the complexity of structures, guidance by complimentary methods like EPR experiments and control by SQUID magnetometry were essential to prove reliability of
derived species distributions. The combination with complimentary spectroscopic
techniques like EPR spectroscopy allowed quantifying the amount of paramagnetic
iron sites in Fe–ZSM-5 and the detection of EPR-silent diamagnetic iron species like
μ-oxo-bridged dinuclear iron sites even in these highly complex systems. The simulations in Fig. 4.27 allow to identify and quantify the relative amount of the different
iron species: K1 is assigned to iron oxide/hydroxide nanoparticles, K2 represents
diamagnetic most probably μ-oxo bridged dinuclear iron(III) centers, K3 are paramagnetic iron(II) high spin sites and K4 and K5 are due to single high spin ferric sites.
A similar methodical approach has been used very recently in order to elucidate the
structural composition of an Fe–N–C catalyst related to fuel cell batteries [96].
Synchrotron based NFS will allow to obtain Mössbauer parameters of small
species like micro- and nanocrystals or even films of biological cells which have
been feeded with
57 Fe containing nutrients. Today the maximum foci which can be
achieved for e.g. 14.4 keV with meV energy resolution are in the range of some
micrometers. With the help of the now developed diffraction limited synchrotron
sources it can be expected that nanofocussing can be done. However, care must be
taken, because the investigation of single micro- and nano-objects can also be influenced by radiation damage which needs to be investigated expecially for biological
systems.
Synchrotron based NIS has added the possibility to trace down iron centred molecular modes in molecular switches like SCO materials and proteins. Only some few
examples have been discussed in this contribution. These applications will certainly