Active sites of the Selective Catalytic Reduction of NO by NH3 over Fe-ZSM-5: Combining reaction kinetics with post-catalytic Mössbauer spectroscopy at cryogenic temperatures

57Fe-ZSM-5 catalysts were prepared by solid-state ion exchange of 57FeCl3 into H-ZSM-5, the exchange capacity of which had been blocked by Na+ or Ca2+ to ≅50 %, ≅25 %, and 0 %. Their Fe site structure was investigated by UV-Vis, EPR, and Mossbauer spectroscopy at different temperatures including cryogenic conditions, and their SCR activity was studied between 400 and 870 K. Mossbauer analysis was performed by establishing consistent models that well represent cryogenic measurements at low and high magnetic fields (10 mT and 5 T, respectively), but also low-field measurements at higher temperatures, which were performed in order to evaluate their analytical potential. It was found that a reliable analysis in particular of species in small structures (binary, oligomeric sites) requires modeling of cryogenic Mossbauer data with the Spin-Hamiltonian formalism, which needs to be combined with input from EPR spectroscopy for specifying/confining the type of paramagnetic FeIII sites to be included in the model. ...