Ammonium Nitrate Temperature‐Programmed Decomposition on Fe–Zeolite Catalysts: Effect of Deposition Method

Abstract

The N2O formation during NOx selective catalytic reduction by NH3 is often attributed to NH4NO3 intermediate decomposition. Thermal decomposition of NH4NO3 has been extensively studied owing to the combination of its wide use as fertilizer and its dangerous properties. However, the decomposition of NH4NO3 in the presence of a catalyst has generated less attention. The decomposition of NH4NO3 may yield NxOy‐type products such as NO, NO2, N2O, N2 as well as HNO3 and H2O. This work focuses on the emissions of N2O. The deposition of NH4NO3 onto a Fe–zeolite catalyst was carried out by dry impregnation, by using a dilute solution of NH4NO3, and by a mechanical mixture. In the absence of catalyst, the release of N2O strongly depends on the conditions of the experiment (static or dynamic). The presence of a catalyst and the method of mixing ammonium nitrate into it affects its decomposition. Nitrogen formation is much more significant for the impregnated sample. For the mechanically mixed sample, the ammonium nitrate is mainly decomposed into N2O and NO2. N2O from ammonia oxidation was also detected in the presence of a catalyst.

Formation and Reactions of NH4NO3 during Transient and Steady-State NH3-SCR of NOx over H-AFX Zeolites: Spectroscopic and Theoretical Studies

“…To provide direct evidence for reaction (6), we carried out in situ IR analyses of NH4NO3-loaded H-AFX under a flow of NO at elevated temperatures. Based on a literature method, 23,44,45 NH4NO3-loaded H-AFX (NH4NO3 loading of 1 mmol g -1 ) was prepared by the impregnation method, via adding 200 μL of a 0.2 M NH4NO3 solution dropwise to 40 mg of H-AFX, followed by drying in an oven at 80 °C for 5 h. After obtaining the background IR spectrum of the NH4NO3-impregnated H-AFX at 30 °C under He, the IR cell was heated (10 °C min -1 ) under a flow of 500 ppm NO/He. Figure 7 shows the IR difference spectra of the NH4NO3-impregnated H-AFX at various temperatures.…”

Mitigated ammonium nitrate inhibition in SCR over Cu-SSZ-13 + Ce/Mn-oxide composite catalysts: insights from temperature-programmed desorption analysis

“…From the literature, where this reaction is catalyzed by Brønsted-acid sites (BAS), it occurs via the following steps: 37–39 NH 4 NO 3 ⇌ NH 3 + HNO 3 HNO 3 + H + → NO 2 + + H 2 ONH 3 + NO 2 + → NH 3 NO 2 + → N 2 O + H 2 O + H + NO 2 evolution is also observed at similar temperature but much lower intensity versus N 2 O, and thus likely similarly originating from NH 4 NO 3 decomposition. We believe that NO 2 evolves from NH 4 NO 3 dissociation to NH 3 and HNO 3 (eqn (3)), followed by HNO 3 dissociation to NO 2 : 16,40 2HNO 3 → 2NO 2 → 0.5O 2 + H 2 OTo confirm the little influence of adsorbed NH 3 on the N 2 O and NO 2 evolution from HSZ during fast-SCR-TPD, NH 3 evolution from Fig. 4B is compared with NH 3 evolution from NH 3 -TPD analysis on HSZ and the results are shown in Fig.…”

“…The low temperature of NH 4 NO 3 decomposition at 206 °C also confirms the results from Pereira et al who hypothesized the confinement effect that zeolite has on NH 4 NO 3 , in that NH 4 NO 3 is not easily vaporized when “trapped” in the zeolite cage and thus disfavoring dissociation to gaseous NH 3 and HNO 3 . 16 Aside from NH 4 NO 3 dissociation, noncatalytic NH 4 NO 3 decomposition is also accompanied by NH 4 NO 3 sublimation (eqn (2)) that was supported by post-analysis of white solid deposits that were formed at the unheated part of the downstream reactor outlet.…”

“…4–12 However, carrying out the fast SCR reaction at low temperatures is challenged by temporary deactivation by solid NH 4 NO 3 deposits formed via a reaction between NO 2 and NH 3 : 12–14 2NO 2 + 2NH 3 → NH 4 NO 3 + N 2 + H 2 OThe adverse impact of NH 4 NO 3 deactivation does not subside until the exhaust temperature surpasses the NH 4 NO 3 melting point (170–180 °C). Above this temperature, solid NH 4 NO 3 undergoes: (1) sublimation to gas-phase NH 4 NO 3 (eqn (2)), 15 (2) dissociation to NH 3 and HNO 3 (eqn (3)), 13,15,16 (3) thermal decomposition to NO x (eqn (4)–(6)), 16–18 and/or catalytic reduction by NO (eqn (7)). 19,20 In current aftertreatment systems where the average exhaust temperature is higher than the NH 4 NO 3 melting point, the NH 4 NO 3 -induced deactivation is temporary and occurs only during “cold-start”.…”