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

Palladium-exchanged chabazite(Pd-CHA) zeolites as passive NO_x adsorbers(PNAs) enable efficient purification of nitrogen oxides(NO_x) 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 wetchemistry 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 Pd^(2+) 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~0 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.

A novel material for passive NO_(x) adsorber:Ce-based BEA zeolite

Passive NO_(x) adsorbers(PNAs)were proposed to address the NO_(x) emissions during the cold start phase.Here we show a novel Ce-based BEA zeolite,as a noble-metal-free passive NO_(x)adsorber.The NO_(x) adsorption capacity of Ce/BEA reaches 36μmol/g in the feed gas close to realistic exhaust conditions,and the NO_(x) desorption temperature,which is around 290℃,is ideal for diesel exhaust after-treatment systems.Ce/BEA also behaves notable stability of high temperature CO exposure conditions.Multiple characterizations were performed to explore the NO_(x) adsorption chemistry of Ce/BEA.The Ce(Ⅳ)species in the BEA zeolite serves as the active center for NO_(x) adsorption.The bidentate nitrate species is responsible for the observed NO_(x) storage capacity,and the active oxygen around Ce(Ⅳ)plays a critical role in its formation.Considering the significantly better cost efficiency of Ce compared to Pd,Ce/BEA presents an enormous potential for the PNA applications and provides a novel formulation for the noblemetal choice of PNA materials.

Recent progress of Pd/zeolite as passive NO_(x) adsorber: Adsorption chemistry, structure-performance relationships, challenges and prospects

Due to the technology limitation and inferior deNO_(x) efficiency of urea selective catalytic reduction (SCR) catalysts at low temperatures, passive NO_(x) adsorber (PNA) for decrease of NO_(x), CO and hydrocarbons (HCs) during “cold start” of vehicles was proposed to meet the further tighten NO_(x) emission regulations in future. Among them, Pd modified zeolite PNA materials have received more attention because of their excellent NO_(x) storage capacity, anti-poisoning and hydrothermal stability and since Pd/zeolite PNA was proposed, a variety of advanced characterization methods have been applied to investigate its adsorption behavior and structure-performance relationship. The comprehension of the active sites and adsorption chemistry of Pd/zeolite PNA was also significantly improved. However, there are few reviews that systematically summarize the recent progress and application challenges in atomic-level understanding of this material. In this review, we summarized the latest research progress of Pd/zeolite PNA, including active adsorption sites, adsorption mechanism, material physicochemical properties, preparation methods, storage and release performance and structure-performance relationships. In addition, the deactivation challenges faced by Pd/zeolite PNA in practical applications, such as chemical poisoning, high temperature hydrothermal aging deactivation, etc., were also discussed at the micro-level, and some possible effective countermeasures are given. Besides, some possible improvements and research hotspots were put forward, which could be helpful for designing and constructing more efficient PNA materials for meeting the ultra-low NO_(x) emission regulation in the future.