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

Ammonia (NH3) inhibition during the selective catalytic reduction (SCR) of nitrogen oxides (NOx) 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.