Enhancing the Low-T NH3-SCR Activity of a Commercial Fe-Zeolite Catalyst by NH4NO3 Dosing: an Experimental and Modeling Study

Abstract

The potential improvement of DeNOx efficiency enabled by injecting calibrated amounts of NH 4 NO 3 (AN) aqueous solution into lean engine exhausts prior to NH 3 -SCR converters (Enhanced-SCR) has been investigated by dynamic and steady-state catalytic activity runs over a commercial Fe-zeolite monolith catalyst. The study was performed varying GHSV (35,000-100,000 h −1 ), temperature (150-550°C), as well as AN and NO 2 feed concentrations. The results demonstrate that already at low temperature (180°C) the DeNOx activity of the investigated catalyst at Standard-SCR conditions is greatly boosted by the addition of AN: for example, the injection of 250 ppm of NH 4 NO 3 incremented the NO conversion from ∼15 to ∼85 % at GHSV = 50,000 h −1 , with feed NO = NH 3 = 500 ppm. Furthermore, the dosed NH 4 NO 3 was completely converted, with no undesired release of N 2 O. We show also that the Enhanced-SCR pathway proceeds through a first step wherein AN rapidly oxidizes NO to NO 2 , which then goes on to react with NH 3 according to the Fast-SCR reaction. Based on this sequential reaction scheme, a simple kinetic model has been developed and successfully fitted to all the collected data.

Heterogeneous Ammonia Storage Model for NH3–SCR Modeling

“…Recently, it was found that surface ammonia nitrates play a significant role in “fast” SCR reaction. Multiple reaction steps including ammonia nitrate formation, ammonia nitrate titration by NO, and ammonia nitrate decomposition are involved in the “fast” SCR mechanism. − In order to accurately describe the fast and NO 2 SCR reactions, significant effort involving modeling of NO 2 adsorption, desorption. and interaction between ammonia nitrates and NO is needed.…”

Study on the NO removal efficiency of the lignite pyrolysis coke catalyst by selective catalytic oxidation method

“…A stable carbon framework is formed when the pyrolysis final temperature is raised to 850°C continuously, and the original active sites on the surface are stable and deactivate due to the high temperature. Therefore, the decrease of the specific surface area is not obvious after the denitration [ 43 ].…”

Mechanistic insight into low temperature SCR by ceria–manganese mixed oxides incorporated into zeolites

“…2NH 3 + 2NO + NH 4 NO 3 = 3N 2 + 5H 2 OHere, NH 4 NO 3 can simply be regarded as a masked NO 2 molecule and negates the need for the kinetically relevant oxidation of NO by O 2 under standard SCR conditions. 11 Ensuing work by Tronconi et al 16 proved this by demonstrating the equivalence of NO 2 and NH 4 NO 3 through comparison of their respective SCR rates, both of which were far beyond those of standard SCR.…”

“…Table 1 summarizes possible SCR pathways that have been suggested by others. 15,16,20,21 Please note that these are known net gas phase reactions and not elementary steps occurring on a surface. The reaction network N (1) and N (2) represent standard SCR pathways; they differ in the way that nitrous acid (HNO 2 ) is formed which is regarded as the key intermediate leading to N 2 formation.…”