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

Abstract

Recent efforts to increase the low-temperature activity of zeolite-based selective catalytic reduction (SCR) catalysts has led to the exploration of hybrid materials comprised of a metal oxide and zeolite phase. However, the role of each component in promoting low temperature activity and their interaction with each other is not well understood. Herein, we attempt to understand the low temperature promotion by synthesizing a series of ceria–manganese mixed oxides introduced to a H-SSZ-13 zeolite via incipient wetness impregnation of the oxide precursors. Our data suggests that the mixed oxide phase provides access to surface fast SCR reaction channels via SCR generation of adsorbed nitrogen dioxide and its derivatives. At low temperature (100–170 °C), where the SCR promotion is greatest, we show that it is unfavorable for the adsorbed nitrogen dioxide (derivatives) to react with ammonia to form ammonium nitrate. This implies that fast SCR pathways remain accessible on the oxide at low temperatures and are not blocked by ammonium nitrate deposits as they are on the pure zeolite component. We hypothesize that this a contributing factor for the observed low temperature SCR promotion. Our results may benefit the current understanding of hybrid SCO–SCR catalysts and lead to further technological development in this area.