Monitoring NH3 storage and conversion in Cu-ZSM-5 and Cu-SAPO-34 catalysts for NH3-SCR by simultaneous impedance and DRIFT spectroscopy

Abstract

The development of more efficient zeolite catalysts for selective catalytic reduction of NOx by NH3 (NH3-SCR) requires insights into both reaction mechanism and real-time state of the catalyst (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 proton-conducting, copper-promoted zeolite catalysts, i.e., into Cu-ZSM-5 and Cu-SAPO-34. Comparative investigations indicate that Cu-SAPO-34, as compared to Cu-ZSM-5, showed high sensitivity to the change of NH3 concentration 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 showed a better performance 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. 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.