Figure . In situ steady-state Raman spectra (λex = 532 nm) of Cu-CHA samples containing predominantly Z2Cu sites (Cu-CHA-P), ZCuOH sites (Cu-CHA-I), or mixtures thereof (Cu-CHA-M) after O2 activation (21 kPa O2, 80 kPa He, 723 K). Samples were pretreated in He at 723 K for 2 h prior to the introduction of O2. All spectra have been normalized to the most-intense feature (~450 cm -1 ) and are vertically offset for clarity.