Design principles of operando ultraviolet-visible and electron paramagnetic resonance spectroscopy setups for active site characterization in ion-exchanged zeolites

Cu- and Fe-exchanged zeolites have been widely investigated for their applicability in selective partial oxidation of CH4 and abatement of environmentally harmful nitrogen oxides. However, the differentiation between spectator and active sites is cumbersome due to their dynamic co-existence, which aggravates the elucidation of the redox dynamics of the latter. Therefore, correlated multi-spectroscopic approaches carried out under operando conditions are needed in order to disentangle the inherent reactivity of specific species. Here we describe the conceptualization of complementary operando spectroscopic methods for the investigation of ion-exchanged zeolites employed in redox reactions. The potential and versatility of this approach are demonstrated by means of two exemplary case studies. First, we present the insight generated by two custom operando ultraviolet-visible and electron paramagnetic resonance spectroscopy setups operating in batch mode into CH4-to-CH3OH conversion over Cu-exchanged mordenite. In addition to recording the method-dependent spectroscopic fingerprints of distinct Cu centers, the overall CH4 consumption is simultaneously monitored in both setups. The extracted apparent activation energies of the reactive centers obtained from the two systems are in good agreement with each other allowing to link the bulk reactivity to the intrinsic site-specific kinetics of all Cu centers. Second, the EPR setup is equipped with a flow reactor designed to carry out modulated excitation experiments with phase-sensitive detection in order to improve the signal-to-noise ratio and time resolution. The cell is employed to study the conversion of NO via selective catalytic reduction over Fe-exchanged ferrierite. In combination with operando X-ray absorption spectroscopy, a small portion of active Fe species consisting of monomers in gamma-positions and oligomeric structures located in the main channel is identified, which are kept in a redox active state under relevant reaction conditions and thus sustain the chemical transformation of NO. [Display omitted] •Operando UV-Vis/EPR setups for batch CH4-to-CH3OH experiments using Cu-zeolite.•Operando UV-Vis/EPR yields complementary insights into Cu2+ speciation and site-specific kinetics.•EPR flow reactor for modulated excitation experiments in selective catalytic reduction of NO on Fe-zeolite.•Operando XANES/EPR reveals Fe2+/Fe3+ transitions and their site-selective fingerprints.