Insights into the mechanisms of NH₃ inhibition on Cu-CHA SCR catalysts

Ammonia (NH₃) inhibition during the selective catalytic reduction (SCR) of nitrogen oxides (NO_x) over Cu-exchanged chabazite (Cu-CHA) catalysts limits low-temperature performance, yet its molecular origin remains unclear. Here we show that excess NH₃ selectively suppresses the oxidation half-cycle of the SCR reaction while leaving the reduction half-cycle largely unaffected. By combining kinetic measurements, operando electron paramagnetic resonance spectroscopy, and density functional theory calculations, we identify hindered mobility of Cu⁺ ions as the key factor. Specifically, NH₃ coordination increases the energy barrier for Cu⁺ diffusion, preventing the formation of reactive Cu²⁺-oxo dimer intermediates required for efficient oxidation. Spectroscopic measurements further reveal that the extent of inhibition depends strongly on temperature and copper loading. These insights provide a mechanistic basis for mitigating NH₃ inhibition, suggesting that improved catalyst design and optimized operating conditions can enhance low-temperature SCR performance in practical emission control systems.