Watching catalysts at work, before work, and after work

Heterogeneous catalysts for selective catalytic reduction or oxidation often cycle between paramagnetic and diamagnetic states. Some catalysts for other processes, such as polymerization, may involve paramagnetic metal ions as well. EPR spectroscopy is often the technique that provides most information on the paramagnetic species, because it can detect them with high sensitivity while being insensitive to all diamagnetic background species. Identification of the active species may still be difficult, as paramagnetic bystander species may exist and heterogeneous catalysts can contain multiple active surface species. Therefore, EPR methods are required for recognizing active species. Complementary spectroscopic techniques are needed to detect EPR-silent active species. In favorable cases, active species can be recognized by variation of catalyst composition, as we show for a titanium-based Ziegler-Natta polymerization catalyst.1 Further insight can then be obtained by applying hyperfine spectroscopy ex situ at cryogenic temperature. In general, reliable identification of active species requires operando EPR, which is the acquisition of spectra under reaction conditions while simultaneously monitoring product composition. We present a versatile operando EPR setup that supports elevated pressure, stable EPR measurements in temperature ramps, and variation of the reaction gas stream on time scales matching the rates of the catalyzed reactions. By using digitally controlled mass flow controllers for reaction gases, we demonstrated modulation excitation (ME) EPR spectroscopy with phase-sensitive detection on the example of N2O decomposition over a commercial Fe-doped ferrierite catalyst with CO as a reductant. For this process, ME EPR confirms β-Fe sites as the active species. Analysis of the rise and decay of the β-Fe-site signal upon switching of gas composition provides rate constants for the oxidation and reduction half cycles. In another application of in situ EPR, we looked into CO2 hydrogenation to methanol over a Pd-In2O3-ZrO2 catalyst at 573 K. In this case, switching the reaction gas stream from H2 to CO2 revealed line width changes of a broad ferromagnetic signal. We could trace this signal back to exchange-coupled magnetic polarons that are bound to oxygen vacancies, whereas the signal from isolated oxygen vacancies did not change upon the gas composition change. We gratefully acknowledge our cooperation partners from the groups of Christophe Copéret, Javier Perez Ramírez, Jeroen von Bokhoven at ETH Zürich and Davide Ferri at Paul Scherrer Institute.