Revealing the effect of crystal size on the high-temperature hydrothermal stability of Cu/SSZ-13 NH3-SCR catalysts

Abstract

To reveal how crystal size improves the long-term stability of Cu/SSZ-13 zeolites under high-temperature hydrothermal aging, a series of Na/SSZ-13 supports of different crystal sizes from micron to nanometer level (0.3 to 7.3 μm) were prepared by adding different contents of milled inoculating seeds, with the same amounts of copper introduced by impregnation and degreening treatment. Physical and chemical characterizations show that all fresh Cu/SSZ-13 catalysts have identical morphology, composition, pore size, acidity feature, and the selective catalytic reduction (SCR) activity, except for different particle sizes. After the same hydrothermal aging at 800 °C, Cu/SSZ-13 with different crystal sizes exhibit different hydrothermal stabilities. The smaller-sized ones show inferior SCR performance as an obvious decline in CHA structure and active copper species can take place. Notably, the ²⁷Al NMR results show that less dealumination occurs on 7.3 μm Cu/SSZ-13, which illustrates that large crystal size shows superior stability upon hydrothermal aging. After analyzing the results of NH₃-TPD and diethylamine-TPD on H/SSZ-13, the amount of terminal hydroxyl (Al–OH) determines the stability of the CHA structure as Al–OH is easily damaged upon hydrothermal aging. Since a large-sized one has a lower amount of Al–OH, it could keep the CHA structure intact, and help to preserve most of the active sites (∼70%), especially for stabilizing 2Z-Cu²⁺ species, leading to a minor decrease in SCR performance over the whole temperature region. Our study paves an easy way to control the amount of Al–OH by changing the crystal size of SSZ-13, which could effectively improve the long-term stability of Cu/SSZ-13 catalysts.