Revealing the Formation and Reactivity of Cage-Confined Cu Pairs in Catalytic NO Reduction over Cu-SSZ-13 Zeolites by...

The low-temperature mechanism of chabazite-type small-pore Cu-SSZ-13 zeolite, a state-of-the-art catalyst for ammonia-assisted selective reduction (NH₃-SCR) of toxic NO pollutants from heavy-duty vehicles, remains a debate and needs to be clarified for further improvement of NH₃-SCR performance. In this study, we established experimental protocols to follow the dynamic redox cycling (i.e., Cu^II ↔ Cu^I) of Cu sites in Cu-SSZ-13 during low-temperature NH₃-SCR catalysis by in situ ultraviolet-visible spectroscopy and in situ infrared spectroscopy. Further integrating the in situ spectroscopic observations with time-dependent density functional theory calculations allows us to identify two cage-confined transient states, namely, the O₂-bridged Cu dimers (i.e., μ-η²:η²-peroxodiamino dicopper) and the proximately paired, chemically nonbonded Cu^I(NH₃)₂ sites, and to confirm the Cu^I(NH₃)₂ pair as a precursor to the O₂-bridged Cu dimer. Comparative transient experiments reveal a particularly high reactivity of the Cu^I(NH₃)₂ pairs for NO-to-N₂ reduction at low temperatures. Our study demonstrates direct experimental evidence for the transient formation and high reactivity of proximately paired Cu^I sites under zeolite confinement and provides new insights into the monomeric-to-dimeric Cu transformation for completing the Cu redox cycle in low-temperature NH₃-SCR catalysis over Cu-SSZ-13.