An Assessment of Zeolite Framework Effect for Low-Temperature NOX Adsorbers

Abstract

Pd-promoted zeolites (Y, ZSM-5, FER, SSZ-13) were prepared and characterized to analyze their properties as low-temperature NOx adsorbers. The samples were investigated by BET and XRD and by in situ FT-IR spectroscopy of CO and NO adsorption to probe the Pd sites and the nature of the adsorbed NOx species. The NOx adsorption/desorption performances at low temperatures were examined by microreactor measurements upon NO/O2 adsorption followed by TPD in the presence of water and carbon dioxide. It was enlightened that: (i) the zeolite framework influences the Pd dispersion: the smaller the zeolite cage, the higher the Pd dispersion, irrespective of the Si/Al ratio. Accordingly, the following Pd dispersion order has been observed, inversely to the zeolite cage size: Pd/SSZ-13 > Pd/ZSM-5 ~ Pd/FER >> Pd/Y; (ii) Pd is present as isolated Pdn+ species and in PdOx particles; (iii) the Pd dispersion governs the NOx storage capacity: the smaller the zeolite cage, the higher the Pd dispersion and the storage capacity; (iv) NO adsorbs mainly in the form of Pd nitrosyls and nitrates; (v) NO desorption occurs both at a temperature below 200 °C and in a high-temperature range (near 350 °C).