Pd-based zeolites are versatile catalysts in automotive exhaust aftertreatment as passive nitrogen oxide (NOx) adsorbers (PNAs) and methane (CH4) oxidation catalysts (MOCs). The versatility is derived from the Pd speciation facilitated by the zeolite framework in the forms of ion-exchanged Pd and PdO nanoparticles. Ion-exchanged Pd species are identified to be the adsorption sites for PNAs, whereas PdO nanoparticles are the active sites for CH4 oxidation. Maintaining the desired Pd species is essential for ensuring stable long-term performance in both PNAs and MOCs; however, Pd speciation is dynamic under long term exposure to ambient conditions. Herein, 0.5 wt.% Pd/zeolite catalysts were synthesized as Pd(0.5)/BEA (Si/Al = 12.5), Pd(0.5)/ZSM-5 (15), and Pd(0.5)/SSZ-13 (12.5) and aged under ambient conditions. Evaluation of the PNA and CH4 oxidation performance of the fresh and aged catalysts was used to probe ion-exchanged Pd and PdO nanoparticles, respectively. The NOx adsorption capacity of fresh Pd(0.5)/BEA (NOx/Pd molar ratio = 0.78) decreased after aging (NOx/Pd = 0.25), while the CH4 conversion at 600 oC improved from 10 to 60%. The probe reactions indicate a decrease in ion-exchanged Pd and an increase in PdO nanoparticles of the fresh and aged Pd(0.5)/BEA, respectively. Pd speciation was also monitored using H2-TPR for ion-exchanged Pd and O2-TPD for PdO. CO-diffuse reflectance FT-IR spectroscopy (DRIFTS) of the fresh and aged Pd(0.5)/BEA catalyst (Fig. 1) showed a decrease in ion-exchanged Pd at 2134 cm-1 and an increase in pore confined Pd ions at 2213 and 2192 cm-1. The presence of the pore confined Pd ions implies solvation by H2O of the ion-exchanged Pd and detachment from the zeolite framework to become mobile. The mobility of Pd as promoted by H2O is a possible mechanism for the increased formation of PdO nanoparticles suggesting that ambient moisture plays a role in Pd speciation.

Effect of Pd speciation on CH4 oxidation and passive NOx adsorption/desorption performance over zeolite-based catalysts

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