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