Revealing the Formation and Reactivity of Cage-Confined Cu Pairs in Catalytic NO x Reduction over Cu-SSZ-13 Zeolites by In Situ UV-Vis Spectroscopy and Time-Dependent DFT Calculation.

The low-temperature mechanism of chabazite-type small-pore Cu-SSZ-13 zeolite, a state-of-the-art catalyst for ammonia-assisted selective reduction (NH 3 -SCR) of toxic NO x pollutants from heavy-duty vehicles, remains a debate and needs to be clarified for further improvement of NH 3 -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 NH 3 -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 O 2 -bridged Cu dimers ( i.e. , μ-η 2 :η 2 -peroxodiamino dicopper) and the proximately paired, chemically nonbonded Cu I (NH 3 ) 2 sites, and to confirm the Cu I (NH 3 ) 2 pair as a precursor to the O 2 -bridged Cu dimer. Comparative transient experiments reveal a particularly high reactivity of the Cu I (NH 3 ) 2 pairs for NO-to-N 2 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 NH 3 -SCR catalysis over Cu-SSZ-13.