Structure Sensitivity of NO Reduction over Iridium Catalysts in HC–SCR

Ir black with different crystallite sizes and Ir–H–ZSM-5 prepared using two different precursors were investigated for their behavior in selective catalytic reduction using hydrocarbons as reducing agents (HC–SCR). Emphasis was given to the study of the influence of time onstream on N2 yield, the change in Ir crystallite size, and the change in the ratio of Ir : IrO2. Under reaction conditions at 450°C Ir–H–ZSM-5 did not reach steady-state catalytic behavior within 32 h. In contrast, unsupported Ir black showed higher initial yields of nitrogen and approached steady-state considerably faster. Ir black with the largest crystallite size (45 nm) required the shortest time for reaching steady-state behavior. The influence of crystallite size on the reaction of Ir with O2 and NO was addressed and related to the increase in N2 yields with increasing Ir crystallite size in the reduction of NO using propene as a reducing agent. Comparative pulse thermoanalysis studies of NO and O2 adsorption on Ir black with different crystallite sizes (5–45 nm) revealed that the relative uptake of NO (mNO/mO2) increases strongly with increasing crystallite size. This behavior is due to a strong structure sensitivity of NO adsorption, whereas O2 adsorption is relatively insensitive to crystallite size. The improved yield of N2 with increasing crystallite size under HC–SCR conditions is traced to the higher surface concentration of NO relative to O2 with increasing crystallite size.