Low Temperature NO_x 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 NO_x adsorption. The catalysts are characterized by BET, XRD, UV–Vis and XRF, while the adsorption/desorption characteristics of the samples are investigated by NO_x adsorption/TPD, with and without water in the feed. The nature of the adsorbed NO_x species has been analyzed by operando FT-IR spectroscopy. Under dry conditions at 50 °C all the investigated zeolite frameworks are able to store significant amounts of NO_x, up to 1 mmol/g_cat for the MOR sample; the presence of Pd has not a significant impact on the amounts of the stored NO_x. In fact, under dry conditions at 50 °C the zeolite frameworks oxidize NO to NO₂, and store NO_x from NO₂ over the zeolitic support. FTIR data indicates that NO_x 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 NO_x storage capacity. Still, appreciable amounts of NO_x could be stored at 50 °C (up to 60 µmol/g_cat for Pd/MFI) in the presence of water over the Pd-doped carriers.