Remarkable improvement in the NOx storage ability of Pd/SSZ-13 zeolite PNA via a facile pretreatment strategy

A facile strategy was employed to pretreat Pd/SSZ-13 zeolite obtained via the initial-impregnated method under a microwave field, aiming to greatly enhancing the NO_x storage-release ability. By investigating the effect of microwave treatment parameters (power, atmosphere and time) on the improvement of PNA performance, the optimal conditions were determined (175 W, 10 % O₂/N₂, 15 min). Under these conditions, the Pd/SSZ-13 exhibited an enhanced NO_x adsorption and desorption amount to 2.60 and 2.38 times, respectively, compared to the untreated sample after three cycles of NO-TPD. Deeper insights into the variations in Pd/SSZ-13 properties induced by the microwave radiation were clarified through a series of characterizations. The micropore surface area of Pd/SSZ-13 zeolite activated by microwave increased by 3.91 %, and the micropore volume also improved by 3.85 %, which might facilitate the adhesion of more active Pd species to the materials surface, and aid in the dispersion of PdO_x clusters into the zeolite channels. After microwave activation, the average particle size of Pd decreased from 10.09 nm to 7.08 nm. Particularly, the content of Pd(II) species such as Pd²⁺, [Pd(OH)]⁺ and PdO as active NO_x adsorption sites enhanced from 66.68 % to 68.93 %. Additionally, the microwave treatment not only increased the number of Lewis and Brønsted acid sites, but also strengthened the interaction between NO_x molecules and the zeolite skeleton. The above reasons collectively promoted the NO_x storage-release properties of Pd/SSZ-13 zeolite, showing that the microwave pretreatment is a highly efficient strategy for improving the performance of PNA materials.