Correlation between Zeolitic SiO[sub.2]/Al[sub.2]O[sub.3] Ratio and the Low-Temperature NOx Adsorption Capacity of Pd/SSZ-13 under Realistic Exhaust Conditions

Passive NOx adsorber (PNA) is one of the important means to effectively reduce NOx emission control in the cold start of a diesel engine. A series of Pd/SSZ-13 catalysts with different SiO[sub.2]/Al[sub.2]O[sub.3] ratios (11, 17, and 25) were prepared using the impregnation method. Furthermore, the effect of the SiO[sub.2]/Al[sub.2]O[sub.3] ratio on the adsorption performance of Pd/SSZ-13 at low-temperature NOx under realistic exhaust conditions was studied. The results show that the Pd/SSZ-13 catalyst with a low SiO[sub.2]/Al[sub.2]O[sub.3] ratio after loading Pd has a higher specific surface area and palladium ion content. There is a negative correlation between NOx adsorption performance and the SiO[sub.2]/Al[sub.2]O[sub.3] ratio. After high-temperature heat treatment, the acid sites closely related to palladium species increase and palladium species will redisperse, producing more palladium ions. The palladium ions coexist in Pd/SSZ-13 in the form of Pd[sup.2+] and Pd[sup.+], among which Pd[sup.2+] is divided into two types: Z[sup.−]Pd[sup.2+]Z[sup.−] and Z[sup.−][Pd(II)OH][sup.+]. The NOx adsorption performance of the Pd/SSZ-13 catalyst was significantly improved, and the higher the SiO[sub.2]/Al[sub.2]O[sub.3] ratio, the more obviously the advantage of NOx adsorption performance increased after heat treatment. The NOx adsorption kinetic model of the Pd/SSZ-13 catalyst under realistic exhaust conditions was most suitably described by the pseudo-first-order model.