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Copper-ion-exchanged SSZ-13 has been demonstrated to be an effective catalyst for the ammonia selective catalytic reduction of NOx. However, the Cu-SSZ-13 is still susceptible to hydrothermal degradation, which reduces the catalyst lifetime. We herein report that the hydrothermal stability of Cu-SSZ-13 can be remarkably enhanced through loading with a small amount of cerium. To clearly reveal the enhancement effect of cerium, we focused on an aluminum-rich Cu-SSZ-13 zeolite (Si/Al ratio: 6.5) with copper loadings ranging from ca. 3.4% to 4.1%, which was found to give high hydrothermal stability while still suffering from considerable degradation upon severe hydrothermal aging. Different cerium loadings were investigated, and a cerium loading with a limited range of ca. 0.2–0.4 wt % resulted in the best enhancement. The introduction of only a small amount of cerium was beneficial because it enabled a high copper loading and abundant Brønsted acidity sites that are needed for high SCR activities over a broad temperature range. Various characterization methods, including Rietveld refinement, transmission electron microscopy with energy-dispersive X-ray spectroscopy, scanning electron microscopy, and electron-spin resonance spectroscopy revealed that the small amount of cerium ions loaded into the Cu-SSZ-13 occupied the ion-exchange sites. The appropriate amount of cerium cations at the ion-exchange sites better stabilized the framework of SSZ-13 and thus led to the enhancement effect. By contrast, excessive ion-exchange with cerium adversely affected the hydrothermal stability of the Cu-SSZ-13, resulting in degraded SCR performance.
Usui et al. (Wed,) studied this question.