Progress and future challenges in passive NO adsorption over Pd/zeolite catalysts†

Future emission standards are becoming increasingly stringent. Around 50% of targeted tailpipe emissions are emitted during the cold-start period, mainly due to the ineffectiveness of catalytic converters in the after-treatment system at low temperatures (<200 °C). Passive NO_x adsorbers (PNA) have been proposed to help address cold start NO_x emissions problems. Among various PNAs, the distinctive properties of Pd/zeolites match perfectly with the required characteristics of PNA catalysts for vehicle emission control. This review summarizes significant advances in Pd/zeolite applied to PNAs, including NO storage sites, NO_x adsorption and release chemistry, synthesis methodology, and deactivation mechanisms. Future outlooks and challenges ahead for Pd/zeolite catalyst development and fundamental research are also proposed. Collectively, this review provides a theoretical and experimental foundation for the rational fabrication and application of PNA catalysts toward NO_x emission control.