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Schönebaum, Simon ; Chen, Peirong ; Simböck, Johannes ; Rauch, Dieter ; Simons, Thomas ; Palkovits, Regina ; Moos, Ralf ; Simon, Ulrich:
Monitoring NH3 storage and conversion in Cu-ZSM-5 and Cu SAPO 34 catalysts for NH3-SCR by simultaneous impedance and DRIFT spectroscopy.
2017
Veranstaltung: 50. Jahrestreffen Deutscher Katalytiker
, 15. - 17. März 2017
, Weimar.
(Veranstaltungsbeitrag: Kongress/Konferenz/Symposium/Tagung
,
Poster
)
Abstract
The selective catalytic reduction of NOx with NH3 (NH3-SCR) utilizing metal promoted zeolites is a key technology to minimize NOx emissions from lean-burn engines in passenger cars and trucks. The development of more efficient zeolite NH3-SCR catalysts requires insights into both reaction mechanism and real-time state of the catalysts (e.g., the storage level of NH3). Here, we show that impedance spectroscopy (IS) can be applied to sense electrically the uptake of NH3 into protonconducting, metal-promoted zeolite catalysts, i.e., into Cu-ZSM-5 or Cu-SAPO-34. Comparative investigations indicated that Cu-SAPO-34, as compared to Cu-ZSM-5, shows a high sensitivity to NH3 concentration changes at a broader temperature range. While both zeolites performed similarly for direct monitoring of the SCR conversion of stored NH3 at temperatures above 350 °C, Cu-SAPO-34 performed better than Cu-ZSM-5 at lower temperatures. A simultaneous IS and diffuse reflection infrared Fourier transform spectroscopy (IS-DRIFTS) study revealed that NH4+ or NH4NO3 intermediates form on the two zeolite catalysts under SCR-related conditions and affect the proton conductivity at low temperatures, thus influencing significantly the monitoring of SCR conversion of stored NH3. Investigations on these and other materials, e.g. Fe-ZSM-5, showed also that the catalytic activity of zeolite catalysts can be correlated with the formation of NH4+ intermediates. Since the intermediates strongly influence the proton conductivity of the zeolite catalysts, such correlation opens the possibility to investigate the catalytic properties directly by analyzing their electrical behavior. The correlation of integral electrical responses with molecular processes, achieved by our simultaneous IS-DRIFTS studies, not only clarifies the origin of the sensing mechanism of zeolite catalysts at a molecular level, but also provides a new perspective to understand the NH3-SCR mechanism over metal-promoted zeolites at low reaction temperatures.