Magnetic resonance techniques, and in particularly electron spin resonance (ESR), are
very powerful and frequently used for probing the oxidation and coordination
environment of transition metal ions (TMI) in heterogeneous catalysts. There are a wide
variety of ESR techniques available, each with their particular advantages and
limitations. CW X-band ESR spectroscopy is the most popular technique because of its
availability and possibility to conduct in situ measurements, although there is now an
increasing use of more advanced techniques, such as ENDOR, ESEEM and high
frequency ESR.
All these ESR techniques can be applied at different levels of sophistication: from
merely detecting the presence of transition metal ions over the determination of the first
coordination sphere around this paramagnetic center up to a detailed description of its
electronic structure. Whatever level is being considered, it is important that the user
realises both the potential and the limitations of the particular technique.
Overinterpretation should certainly be avoided. This holds equally so for
underinterpretation, if with some extra effort (e.g., spectrum simulations), more
physically meaningful information can be extracted from the experimental data. The
systematic and intelligent application of ESR and its related techniques is therefore of
paramount important for a better understanding of the properties of TMI on surfaces in
the future
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