Predicting Optimal Catalyst Design for Selective Catalytic Reduction

Catalytic performance of Cu-zeolite catalyst for selective catalytic reduction (SCR) of NOx with NH₃ is adversely impacted by hydrothermal aging. The design of an optimal SCR catalyst requires control of acid site distribution, particularly the balance between BrØnsted and Lewis acid sites. This simulation study analyzes the evolution of BrØnsted (ZH) and Lewis acid (Z₂Cu, ZCuOH) site densities as a function of Cu weight loading (1.5-4 wt.%) and Cu-to-framework aluminum ratio (Cu/Al_f =0.1-1) at 550 °C and 650 °C, and correlates these changes with catalytic functionality. Effect of hydrothermal aging is quantified by the change in ZH sites that transform to Z₂Cu sites on reaction with ZCuOH sites. High initial ZH, ZCuOH and temperature accelerates the loss of Brønsted site loss via dehydroxylation. Further, key SCR performance descriptors such as steady-state NOx conversion, NH₃ storage, NH₃ oxidation, N₂O selectivity, and overall SCR activity are evaluated across discrete temperature points in the range of 150-550 °C. The results indicate that moderate Cu loadings (∼2.5-3.0 wt.%) consistently provide the best trade-off between high NOx conversion and controlled NH₃ oxidation. Lower Cu loadings (1.5 wt.%) show limited active site density, resulting in reduced NOx conversion despite favorable NH₃ selectivity. Conversely, higher Cu loadings (≥3.5 wt.%) enhance NH₃ oxidation rates, leading to increased N₂O formation and diminished SCR selectivity to N₂. Overall, understanding the response of active sites to different aging conditions and the consequences on SCR performance indicators will enable robust catalyst design for real-world applications.