Unraveling the boosting low-temperature performance of ordered mesoporous Cu-SSZ-13 catalyst for NO_x reduction

Zeolites with a small pore structure are considered as the most efficient catalysts to satisfy the increasingly stringent emission requirement for diesel vehicles. Since the molecular size of NO is comparable to the pore size of small-pore zeolites, the formation of ammonium and sulphates during selective catalytic reduction of NO_x with NH₃, the limits of mass transportation become a major obstacle that reduces its catalytic performance. In this study, in order to overcome the diffusion limitation of traditional microporous Cu-SSZ-13 zeolite (Cu-SSZ-13-Trad), we successfully prepared a hierarchical Cu-SSZ-13 zeolite (Cu-SSZ-13-Meso) containing both small micropores and mesopores by adding carbon black as hard template. The experimental results exhibited that Cu-SSZ-13-Meso has high crystallinity like Cu-SSZ-13-Trad, and is rich in a large number of mesopores. Meanwhile, the hierarchical Cu-SSZ-13-Meso zeolite exhibited higher low-temperature activity (NO_x conversion reached to 86% at 160 °C) than that of the traditional Cu-SSZ-13-Trad catalyst with micropores (NO_x conversion was only 51% at 160 °C), which should be attributed to its hierarchical ordered porous structure enhancing the diffusion of the reactants and products and confirmed by the mass transportation calculation. Moreover, Cu-SSZ-13-Meso catalyst with mesopores also displayed superior hydrothermal stability, as well as water and sulfur resistance. Interestingly, the in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results found that the Cu-SSZ-13-Meso with hierarchical structure was beneficial for the diffusion or decomposition of sulphate species at low reaction temperature and thereby increased its low-temperature activity even in the presence of SO₂. This study also provides a guidance to design high-performance low-temperature active zeolite-based catalysts for the removal of NO_x to solve the problem of cold-start diesel engines.