Enhancing the Low-T NH_3-SCR Activity of a Commercial Fe-Zeolite Catalyst by NH_4NO_3 Dosing: an Experimental and Modeling Study

摘要

The potential improvement of DeNOx efficiency enabled by injecting calibrated amounts of NH_4NO_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 ℃), as well as AN and NO_2 feed concentrations. The results demonstrate that already at low temperature (180 ℃) 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_4NO_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_4NO_3 was completely converted, with no undesired release of N_2O. 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.