Deactivation and regeneration studies on Pd-containing medium pore zeolites as passive NOx adsorbers (PNAs) in cold-start applications

Two Pd-containing medium pore zeolite frameworks, MFI and MWW, have been evaluated as passive NOx adsorbers (PNAs) in automotive applications. The NOx adsorption/desorption behavior of Pd-containing standard ZSM-5 and MCM-22 zeolites with analogous physico-chemical properties (Si/Al~10–12, 1%wt Pd, crystal size of ~200–400 nm), has been first studied. Pd/ZSM-5 shows better low-temperature NOx capacity than Pd/MCM-22 (0.83 and 0.55 μmol NOx/μmol Pd), but, in contrast, Pd/MCM-22 is able to desorb NOx at remarkable lower temperatures (~50 °C fewer). In order to evaluate the influence of the textural properties of the MWW-type materials on the NOx adsorption/desorption behavior, a delaminated DS-ITQ-2 and nano-MCM-22, have also been prepared. The deactivation of the different Pd-containing medium pore zeolites and their posterior regeneration have been systematically studied by subjecting the samples to CO-ageing treatments at different temperatures (from 150 to 650 °C) and hydrothermal treatments at 750 °C, respectively. Pd/ZSM-5 is able to almost fully recovery the former NOx adsorption capacity, whereas Pd/MWW materials only recuperate half of their initial NOx adsorption capacity, fact that can be explained by the larger critical sizes of part of the agglomerated metal particles on the external surface of these materials.