The effects of hydrothermal aging on NH3 oxidation over high-loading Cu-SSZ-13 catalysts: an experimental and kinetic modelling study

Abstract

This study investigates how hydrothermal aging affects NH₃ oxidation performance in high-loading Cu-SSZ-13 catalysts, aiming to reveal the intrinsic relationship between Cu species and catalytic activity. A series of catalysts, after being subjected to various hydrothermal aging conditions, were characterised using NH₃-TPD, NO + NH₃-titration, H₂-TPR, and NH₃ oxidation tests to quantify the evolution of Brønsted acid sites and key Cu species (ZCuOH, Z₂Cu, and CuO_x). The results indicate that ZCuOH serves as the dominant active site in the low-temperature region, with a distinct linear correlation between its content and the low-temperature NH₃ oxidation activity. In the high-temperature region, both ZCuOH and CuO_x contribute to the NH₃ oxidation activity, while Z₂Cu exhibits little contribution. Based on the internal selective catalytic reduction (i-SCR) mechanism, a kinetic model was established that successfully simulates the experimental trends of NH₃ conversion and NO concentration in both the low- and high-temperature regions. The results confirm that hydrothermal aging influences the two-step NH₃ oxidation process (NH₃ → NO and NO + NH₃ → N₂) by altering the distribution of Cu species. Notably, the model incorporates CuO_x sites, overcoming the limitations of previous models in simulating high Cu loading catalyst samples. This work confirms the reliability of the NH₃ oxidation kinetic model and the rationality of the i-SCR mechanism, providing valuable insights for optimising the hydrothermal stability and catalytic performance of Cu-SSZ-13 catalysts.