In NH₃‐SCR over Cu‐CHA catalysts in the low‐temperature range 150–300 °C, the activation of oxygen occurs via oxidation of a pair of mobile (NH₃)₂Cu^I‐complexes located in the cages of the zeolite. In this step, a reactive [(NH₃)₄Cu₂O₂]²⁺‐peroxo complex (μ‐η²,η²‐peroxo diamino dicopper(II)‐complex) is formed. The chemistry of this complex determines several catalytic properties of the Cu‐CHA catalyst. The reaction of NO with the [(NH₃)₄Cu₂O₂]²⁺‐peroxo complex governs the NH₃‐SCR activity. A reaction of the [(NH₃)₄Cu₂O₂]²⁺‐peroxo complex with ammonia hinders the reaction of NO with the complex, thus leading to an inhibition of the NH₃‐SCR reaction. Finally, the deactivation in presence of SO₂ is due to a reaction of SO₂ with the [(NH₃)₄Cu₂O₂]²⁺‐peroxo complex, leading to the formation of Cu−S compounds in the catalyst. In this review, the characterization and the reactions of the [(NH₃)₄Cu₂O₂]²⁺‐peroxo complex with NO, NH₃, and SO₂, and mean‐field kinetic models based on first principles calculations for NH₃‐SCR activity and SO₂ poisoning are discussed.

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