Understanding the chemistry during the preparation of Pd/SSZ-13 for the low-temperature NO adsorption: The role of NH4-SSZ-13 support

The chemistry during the preparation of Pd/SSZ-13, including impregnation and calcination process, was investigated, using Pd(NO3)2 as precursor and NH4-SSZ-13 as the support. Special attention was paid on analyzing the improvement effect of NH4-SSZ-13 support on Pd2+ ions dispersion. The Pd(NO3)2 precursor remained intact after impregnation and transformed to Pd(NH3)x2+ during calcination at 200−290 °C, and then converted to Pd2+ ions occupying the ion-exchange sites by oxidizing the NH3 ligands to N2 at 290−450 °C. The formation of Pd(NH3)x2+ intermediates is a critical factor for achieving high Pd2+ dispersion, probably due to the fact that NH3 ligands give the intermediates high mobility, facilitating their movement to the ion-exchange sites. The PdO formation might be related to the excessive reduction of Pd2+ sites to metallic Pd, when catalyzing the oxidation of NH3 ligands and the NH4+ on the Brønsted acid sites.