Operando electron paramagnetic resonance (EPR) is a unique technique to trace, e. g., valence changes of transition metal ions, radicals and electron transfer in heterogeneous catalytic reactions. Nevertheless, it is still used by a few research groups only, meaning that its benefits for mechanistic studies might still be underestimated. In this keynote lecture, the potential of EPR as an in situ/operando technique for analyzing structure-reactivity relationships in heterogeneous catalysis under reaction conditions is critically evaluated. After a general overview on the state of the art, the opportunities and limitations of operando EPR are illustrated by three selected application examples: 1) Abatement of NOx from exhaust gases by NH3-SCR Fe sites in Fe-ZSM-5 active for fast SCR of NO/NO2 as well as special –O–Ce–O–V(=O)–O–M–O– moieties (M = Zr, Ti, Mn) in supported VOx/CeyM1-yO2 active for low-temperature SCR of NO are identified. 2) Charge separation and electron transfer during photocatalytic water splitting on supported Au/TiO2 catalysts SPR-mediated electron transfer from the metal conduction band into oxygen vacancies of the TiO2 support surface in close vicinity to the Au/TiO2 interphase has been directly detected under irradiation with light of different but distinct wavelength by in situ/operando EPR. 3) Butene dimerization on supported Ni/SiO2-Al2O3 catalysts under high pressure Single Ni(I)/Ni(II) shuttles are identified as active sites by combined operando EPR/X-ray absorption spectroscopy at 15 bar while inactive ferromagnetic Ni(0) particles form at ambient pressure. Conversion of initial EPR-active Ni(I) to EPR-silent Ni(II) by oxidative addition of butene is faster than the re-reduction of Ni(II) to Ni(I) by reductive elimination of the C8 product and requires XAS as an independent method for detecting Ni(II).

Operando EPR in heterogeneous catalysis – Step child or bright spot?

255th National Meeting (2018)

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