Promotive effects of Ba addition on lean NOx reduction by CO over IrRu/Al2O3 catalyst

Abstract

A series of IrRu/Al2O3 based catalysts, where Ba promoter was added in differing amounts were applied for the lean reduction of NOx using CO (CO-SCR), and the promotive effects of Ba addition were thoroughly analyzed. Below 225 degrees C, IrRu/Al2O3 catalyst exhibits high de-NOx activity, but at high temperature above 250 degrees C, suffers from drastic decline of NOx conversion. Promotion of Ba enhances the catalytic activity by suppressing NO2 production in the high temperature region, which expands the operation window. Ba species is capable of trapping the NOx species, following its traditional role as a NOx storage compartment. However, when compared to the Ba/IrRu/Al2O3 catalyst, physically mixing Ba compartment with IrRu/Al2O3 catalyst did not lead to substantial promotion. Thus, participation of Ba species during the reaction is not limited to NOx storage, where it is proposed that a continuous regeneration of NOx-trapping Ba sites takes place by direct reaction with the CO reductant catalyzed by IrRu species. It is also revealed that promotion of Ba led to fundamental alterations in IrRu species, by maintaining their active metallic states under oxidizing reaction conditions and modifying their intrinsic activity towards NO bond cleavage, thereby leading to enhanced activity at elevated temperatures. The utilization of Ba/IrRu/Al2O3 catalysts in lean NOx reduction using CO as a reductant is anticipated to enable the facile reduction of NOx over a wide temperature range with high efficiency and stability, while also eliminating the need to employ conventional urea-based reductant, even under oxidizing conditions.