Combination catalyst for the purification of automobile exhaust from lean-burn engine

LaCu-ZSM-5/cordierite and Ir/ZSM-5/cordierite were prepared and studied as possible catalysts for the selective reduction of nitrogen oxides in excess oxygen. LaCu-ZSM-5/cordierite exhibited good activity in C3H6-SCR reaction and a maximal nitrogen oxide conversion of ca. 80% could be obtained at 623 K under reaction conditions employed. The existence of carbon monoxide in the reaction system showed positive effects on C3H6-SCR over LaCu-ZSM-5/cordierite. Ir/ZSM-5/cordierite exhibited good activity in CO-SCR reaction and a maximal NOx conversion of ca. 55% could be obtained at 523 K. The existence of C3H6 showed negative effects on CO–SCR over Ir/ZSM-5/cordierite at relatively low temperatures. Based on the catalytic results, combination catalyst LaCu-ZSM-5/cordierite and Ir/ZSM-5/cordierite were developed for the reduction of nitrogen oxides from real lean-burn automobile exhaust by utilizing hydrocarbons and carbon monoxide as reducing agents. With contributions from both C3H6-SCR and CO–SCR, the major pollutants in lean-burn automobile exhaust, e.g. nitrogen oxides, unburned hydrocarbons and carbon monoxide, could be purified simultaneously.

► CO promotes C3H6-SCR over LaCu-ZSM-5/cordierite.
► C3H6 suppresses CO-SCR over Ir/ZSM-5/cordierite.
► Major pollutants in automobile exhaust are purified over combination catalyst.