Formation and Effect of NH₄⁺ Intermediates in NH₃–SCR over Fe-ZSM-5 Zeolite Catalysts

Abstract

With the help of a technique combining in situ electrical impedance spectroscopy and DRIFT spectroscopy, we observed directly the formation of ammonium ion (NH 4 + ) intermediates resulting from the interaction of NO and NH 3 on Fe-ZSM-5 catalysts for selective catalytic reduction by NH 3 (NH 3 −SCR). The formed NH 4 + intermediates, indicating the activation of NO in the presence of adsorbed NH 3 , were found to be strongly related to the NH 3 −SCR activity of Fe-ZSM-5 catalysts at low temperatures. These findings, which are not easily achievable by conventional methods, provide new and important perspectives to understand mechanistically the NH 3 −SCR reaction over Fe-zeolite catalysts.

Fe‐Beta@CeO2 core‐shell catalyst with tunable shell thickness for selective catalytic reduction of NOx with NH3

“…Fe-based zeolite have been extensively studied in the NH 3 -SCR reaction over the past decades. [6][7][8][9] However, the low-temperatures catalytic activity of Fe-based catalysts is still to be improved. 10,11 Moreover, the hydrothermal stability of this kind of catalysts needs to be improved.…”

In Situ Spectroscopic Studies of Proton Transport in Zeolite Catalysts for NH3-SCR

“…Nevertheless, more elementary processes associated with the NH 3 storage and conversion, in particular, the proton transport, are not fully understood. It is known that proton transport, which can take place either from the bridging hydroxyl groups (Brønsted acid sites) to reactant molecules or between the reaction intermediates, plays an important role in a series of catalytic reactions such as methylation [14], cracking and methanol-to-olefin [15,16], as well as abatement of NO x emissions [3,4,17]. Comprehensive experimental and theoretical investigations had been performed over zeolites with different framework types (FAU, BEA, MFI, FER, CHA, etc.…”

“…In the last years, we applied electrical impedance spectroscopy (IS) to study the proton transport in various zeolites (such as H-ZSM-5, Fe-ZSM-5, Cu-ZSM-5, Cu-SSZ-13, Cu-SAPO-34, etc.) as NH 3 -SCR catalysts under in situ or operando conditions [17,[25][26][27][28][29][30][31]. In NH 3 -SCR over zeolite catalysts, the adsorption of NH 3 molecules on Brønsted acid sites leads to the formation of ammonium ions (NH 4 + ), which interact further with NH 3 molecules forming NH 4 + ·(NH 3 ) n complexes at low temperatures [2].…”

“…The formed NH 4 + and NH 4 + ·(NH 3 ) n complexes, which can provide additional paths or carriers for proton transport [26,[32][33][34], lead to increased proton conductivities which can be monitored by IS in a broad frequency range (mHz-GHz) [27,[32][33][34][35][36]. The consumption of adsorbed NH 3 , either by desorption or SCR conversion, leads to decreased conductivity due to a loss of proton carriers [17,26]. A further combination of IS and diffuse reflection infrared Fourier transformation spectroscopy (in situ IS-DRIFTS) allowed us to achieve a molecular understanding of the proton transport processes and their impact in NH 3 -SCR catalysis [17,[27][28][29].…”

“…The consumption of adsorbed NH 3 , either by desorption or SCR conversion, leads to decreased conductivity due to a loss of proton carriers [17,26]. A further combination of IS and diffuse reflection infrared Fourier transformation spectroscopy (in situ IS-DRIFTS) allowed us to achieve a molecular understanding of the proton transport processes and their impact in NH 3 -SCR catalysis [17,[27][28][29].…”