Efficient NOx abatement by passive adsorption over a Pd-SAPO-34 catalyst prepared by solid-state ion exchange

Palladium-exchanged chabazite (Pd-CHA) zeolites as passive NOx adsorbers (PNAs) enable efficient purification of nitrogen oxides (NOx) in cold-start diesel exhausts. Their commercial application, however, is limited by the lack of facile preparation method. Here, high-performance CHA-type Pd-SAPO-34 zeolite was synthesized by a modified solid-state ion exchange (SSIE) method using PdO as Pd precursor, and demonstrated superior PNA performance as compared to Pd-SAPO-34 prepared by conventional wet-chemistry strategies. Structural characterization using Raman spectroscopy and X-ray diffraction revealed that the SSIE method avoided water-induced damage to the zeolite framework during Pd loading. Mechanistic investigations on the SSIE process by in situ infrared spectroscopy and X-ray photoelectron spectroscopy disclosed that, while PdO precursor was mainly converted to Pd2+ cations coordinated to the zeolite framework by consuming the -OH groups of the zeolite, a portion of PdO could also undergo thermal decomposition to form highly dispersed Pd° clusters in the pore channels. This simplified and scalable SSIE method paves a new way for the cost-effective synthesis of defect-free high-performance Pd-SAPO-34 zeolites as PNA catalysts.