In situ electron paramagnetic resonance: a unique tool for analyzing structure–reactivity relationships in heterogeneous catalysis

Electron Paramagnetic Resonance (EPR) offers widespread opportunities for monitoring catalytically relevant species that contain unpaired electrons under conditions close to those of heterogeneous catalytic gas and liquid phase reactions. In this tutorial review, after introducing basic theoretical and experimental principles of the technique, selected examples of typical applications are discussed that comprise (1) transition metal ions in paramagnetic valence states such as vanadium, (2) radical anions such as O˙− formed on oxide surfaces and (3) electrons in ferromagnetic particles such as nickel as well as in conduction bands of organic conductors such as polyaniline.