On the Mechanism of Ammonia SCR over Cu- and Fe-Containing Zeolite Catalysts

Abstract

New insights from recent reports are integrated to generate an improved depiction of the reaction mechanisms for the selective catalytic reduction (SCR) of NOx with ammonia over Cu-containing zeolites (mainly chabazite) and Fe-containing zeolite catalysts. The reaction network over Cu-containing zeolites is consistent with the mixed oxidation state of copper cations as determined by operando X-ray spectroscopy, with a ``solvated state'' form of Cu(II) and Cu(I) at low-temperature (<225{\textdegree}C) reaction conditions; oxidation of Cu-bounded nitrite into nitrate and the subsequent formation and decomposition of ammonium nitrate explains the formation of N2O as a by-product. A reaction network for NO reduction over Fe-containing zeolites is proposed based on a synthesis from various reported mechanisms. This proposed mechanism is consistent with operando EPR and Mossbauer spectroscopy and predicts a limited production of N2O as compared to Cu-containing zeolites.