Investigation of crystal size effect on the NOx storage performance of Pd/ SSZ-13 passive NOx adsorbers

The crystal size effect on the NOx adsorption and desorption over Pd/SSZ-13 passive NOx adsorbers (PNA) was investigated. SSZ-13 zeolites with various crystal size (0.4, 0.8, and 2.3 mu m) presented highly textural uniformity. 0.4 mu m Pd/SSZ-13 showed the highest NOx storage capacity and robust hydrothermal stability. According to the combined XRD, HAADF-STEM, H2-TPR, and CO-TPR results, ionic Pdn+ ions were dispersed better on Pd/SSZ-13 with a smaller crystal size. The nature of Pdn+ ion distribution was investigated by bulky and external surface acidic characterization. It was found that more external surface acid sites were formed on as-prepared smallersized SSZ-13 zeolites, which contributed to easier Pdn+ ion diffusion. The kinetic study showed that the NOx adsorption on Pd/SSZ-13 obeyed the pseudo-second-order model and the NOx adsorption activation energy for Pd/SSZ-13 was 25-38 kJ/mol, independent of the crystal size. This work might guide the design of state-of-theart zeolite supported PNAs.