(112e) Infra-Red Spectroscopic Study of NO Adsorption Mechanism on a Pd Ion-Exchanged Passive NOx Adsorber
The adsorption mechanism of NO at 50â?? on a H-ZSM-5 zeolite exchanged with Pd(NH3)42+ has been investigated utilizing DRIFTs. Two ion-exchanged zeolites with different weight percentages of Pd as well as the zeolite without ion-exchange pretreatment were compared to highlight the changes of the reaction mechanism due to the introduction of Pd. NO adsorptions after different gaseous pretreatments were examined to help identify the chemical state of Pd on the surface and the surface intermediates. The subsequent temperature programmed desorption experiments were also conducted to qualitatively determine the binding energy of the surface intermediates. After the O2 pretreatment at 500â??, Pd were dispersed into the zeolite framework and were mainly in the state of Pd ions, specifically, Pd+, Pd2+ and Pd3+.A surface reaction between NO and Pd2+ were assumed to explain the oxidation of NO and the existence of Pd+. The absorption bands of NO, NO2 adsorbed on Pd+, Pd2+ and the acid active sites of zeolite were observed and further proved. The results of three different samples were compared to highlight the benefits of Pd as an active metal in the passive NOx adsorber.