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 NH4NO3 (AN) aqueous solution into lean engine exhausts prior to NH3-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 NO2 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 NH4NO3 incremented the NO conversion from ∼15 to ∼85 % at GHSV = 50,000 h−1, with feed NO = NH3 = 500 ppm. Furthermore, the dosed NH4NO3 was completely converted, with no undesired release of N2O. We show also that the Enhanced-SCR pathway proceeds through a first step wherein AN rapidly oxidizes NO to NO2, which then goes on to react with NH3 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.