Low Temperature NOx Adsorption Study on Pd-Promoted Zeolites

In this study Pd-promoted zeolites (BEA, FER, MFI and MOR) are synthesized and investigated for potential use in the low-temperature NOx adsorption. The catalysts are characterized by BET, XRD, UV-Vis and XRF, while the adsorption/desorption characteristics of the samples are investigated by NOx adsorption/TPD, with and without water in the feed. The nature of the adsorbed NOx species has been analyzed by operando FT-IR spectroscopy. Under dry conditions at 50 degrees C all the investigated zeolite frameworks are able to store significant amounts of NOx, up to 1mmol/g(cat) for the MOR sample; the presence of Pd has nota significant impact on the amounts of the stored NOx. In fact, under dry conditions at 50 degrees Cthe zeolite frameworks oxidize NO to NO2, and store NOx from NO2 over the zeolitic support. FTIR data indicates that NOx are stored in the form of nitrosonium ion NO+ and nitrate ions; nitrosyl species formed on Al3+ and Pdn+ sites have also been observed, although to a lower extent. Evidences have also been provided for the existence of gaseous/weakly interacting species (mostly NO2) contained in the pores of the zeolites. Water inhibits NO oxidation over all investigated samples, resulting in a strong decrease in the NOx storage capacity. Still, appreciable amounts of NOx could be stored at 50 degrees C(up to 60 mu mol/g(cat) for Pd/MFI) in the presence of water over the Pd-doped carriers.