Correlation between zeolitic particle size and the low-temperature NOx storage capacity of Pd/ZSM-11 passive NOx adsorber

Abstract

The effect of zeolitic particle size on the NOx adsorption and desorption was investigated by synthesizing ZSM-11 zeolites with different particle sizes (0.5∼0.8, 1∼2, and 5∼6 μm) for doping Pd as passive NOx adsorbers (PNAs). Several characteristic techniques of XRD, HAADF-STEM, H2-TPR, CO-DRIFT, XPS and NH3-TPD were applied to clarify the essential association of the PNAs performance of Pd/ZSM-11 zeolites with the support zeolite properties and Pd active species. It was found that both the efficient Pd(II) species for assisting NOx adsorption-release and the acid amount (especially the strong acid sites) for facilitating Pd dispersion have positive correlation with the decrease of zeolite particle size. Differently, in addition to the active Pd species, meso-macroporosity properties are another key factors to determine the NOx adsorption ability of PNAs. The meso-macroporosity properties of Pd/Z11-1.5 show a negative correlation with the decrease of zeolite particle size while Pd/Z11-0.7's show the positive correlation with the decrease of zeolite particle size. As a consequence, Pd/Z11-1.5 with higher Pd(II) content but weaker meso-macroporosity properties than Pd/Z11-5.0 shows comparable adsorption capacity to Pd/Z11-5.0, while Pd/Z11-0.7 with both high Pd(II) content and good meso-macroporosity properties not only exhibits the highest NOx storage capacity, but also can release the NOx at the lowest temperature of less than 300 °C, indicating to be a good candidate for purifying NOx at the actual exhaust temperature of diesel vehicles.