Session: Green Chemistry & the Environment:

Pd-Beta has significant NOx storage capacities and simultaneously traps hydrocarbons as passive NOx adsorber. However, an irreversible loss of Pd²⁺ ions was found during CO exposure as reported. In this study, the improvement in storage stability and release profile on Pd-Beta as a function of Si/Al ratio was clearly investigated. Loop tests exposed for CO and C₃H₆ suggested a significant decline in the loss ratio of NOx storage capacities and an increased desorbed proportion of NOx around 200~300 ^oC with Si/Al ratio. NOx-FTIR spectra confirmed that part of palladium sites was responsible for loss of NOx storage and meanwhile was responsible for lower-temperature NOx desorption. This part of sites was attributed to weak palladium ion, identified by H₂-TPR. The ratio of weak/strong ions reduces with Si/Al ratio. ²⁷Al NMR and DFT calculations showed two types of bridged Brønsted acid, Al(IV)_b and Al(IV)_a, and their ratio also evidently reduced with Si/Al ratio. The intense linear correlation between ratio of Al(IV)_b/Al(IV)_a and ratio of weak/strong sites suggested weak site on Al(IV)_b and strong site on Al(IV)_a. In summary, higher Si/Al ratio improves the NOx storage stability of Pd-Beta exposed for reductive exhaust and enhances the NOx desorbed profile during 200~300 ^oC. And reducing Al(IV)_b species is an only effective strategy for improvement of Pd-Beta. This would accelerate the application of Pd-Beta as passive NOx adsorber.

Yi Zhu

Stabilization of Pd-beta by increasing Si/Al ratio as passive NOx adsorbers

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