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

A series of IrRu/Al₂O₃ based catalysts, where Ba promoter was added in differing amounts were applied for the lean reduction of NO_x using CO (CO-SCR), and the promotive effects of Ba addition were thoroughly analyzed. Below 225 °C, IrRu/Al₂O₃ catalyst exhibits high de-NO_x activity, but at high temperature above 250 °C, suffers from drastic decline of NO_x 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 NO_x species, following its traditional role as a NO_x 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 NO_x storage, where it is proposed that a continuous regeneration of NO_x-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 NO_x reduction using CO as a reductant is anticipated to enable the facile reduction of NO_x 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.