Understanding the Mechanism of Low-Temperature NH₃-SCR Half-Cycle on Cu-CHA Using Automated Reaction Path Search

Nitrogen oxides (NO_x) emitted from diesel engines are major air pollutants. Copper-exchanged chabazite (Cu-CHA) can efficiently remove NO_x via ammonia-assisted selective catalytic reduction (NH₃-SCR), yet its molecular mechanism remains unclear. In this study, the low-temperature NH₃-SCR half-cycle on Cu-CHA was analyzed using density-functional theory (DFT) calculations combined with an automated reaction path search, and the reaction path networks consisting of many local minima and connecting paths were obtained. The results show that the side-on dimer [Cu₂^IIO₂(NH₃)₄]²⁺ reacts with two adsorbed NO molecules and exergonically cleaves into two mononuclear [Cu^II(NH₃)₂(O₂N)]⁺ complexes. Each monomer then reacts with one NO and one NH₃ to produce HONO, NH₂NO, and the reduced [Cu^I(NH₃)₂]⁺. Intracomplex N-N bond formation (ΔG^‡ = 102.2 kJ/mol) emerges as the rate-determining step of the reduction half-cycle, simultaneously reducing Cu^II to Cu^I. Orbital-level analyses indicate that Cu-mediated ligand-ligand coupling is significant in N-N bond formation.