Activation of passive NOx adsorbers by pretreatment with reaction gas mixture

Abstract

Pd/HZSM-11 prepared by the incipient wetness impregnation method was applied as a passive NOx adsorber (PNA) at low temperatures. The NO storage and release properties of Pd/HZSM-11 have been investigated by cyclic operation. It was found that the NO storage capacity of Pd/HZSM-11 at 100 °C significantly increased with repeated adsorption-release cycles. The NOx storage capacity increased by ~49% between the first and 5th adsorption-release cycle. Encouraged by these results, the fresh sample was pretreated in the feeding reaction gas for 2 h. This pretreatment resulted in highly enhanced NO storage capacity, comparable to the storage capacity of the sample after 5 adsorption-release cycles. The nature of Pd species in the sample has been characterized by XPS, NH3-TPD, N2 adsorption, and FTIR with CO titration. Pretreatment by O2 or NO + O2 at 500 °C led to highly dispersed Pd (II) species in the zeolite that were responsible for the capture of NO at low temperatures, thereby contributing to the enhanced NO storage capacity.