Activation of Pd/SSZ-13 catalyst by hydrothermal aging treatment in passive NO adsorption performance at low temperature for cold start application

Pd/SSZ-13 catalysts after hydrothermal aging (HTA) treatment at 750 °C for 25 h showed enhanced passive NO adsorption performance at temperature below 120 °C, which will be suitable for cold start application, although those calcined at 500 °C did not possess good NO adsorption capacity irrespective of the preparation methods such as incipient wetness impregnation, wet impregnation, ion-exchange, and solid state ion exchange. NO adsorption ability of HTA treated Pd/SSZ-13 samples increases gradually as a function of the metal content, up to 2 wt% Pd loading, and then decreased implying that there is an optimum chemistry between the metal and zeolite support. Combined H₂-TPR and Pd XAFS results clearly demonstrate that PdO mainly existed over the fresh samples, whereas HTA treated samples contained ionic Pd²⁺ species, indicating the redistribution of PdO into highly dispersed Pd²⁺ species within the SSZ-13 structure arising from HTA treatment. Such phenomenon was visually confirmed by comparative STEM-EDS analysis of fresh and HTA Pd/SSZ-13 samples. DRIFT results display the formation of two nitrosyl complexes adsorbed on Pd²⁺ ions, which are directly related to two desorption peaks of NO_x at 250 and 400 °C. All combined results provide the unambiguous evidence about the generation of Pd ions in SSZ-13 zeolite induced by HTA treatment, which play as the active sites for NO adsorption at low temperature.