Resonance Raman spectra were obtained on a spectrometer (Renishaw, inVia) equipped with a 532 nm laser. Cu-CHA samples were first pelletized and sieved to retain ~2 mm particles, which were loaded into a temperature-controlled gas-phase reaction cell (Linkam, CCR1000). Cu-CHA samples were first treated to either 723 K in He (Airgas, Ultra-zero grade; 50 cm 3 min -1 ; 0.33 K s -1 ) and held for 2 h or to 523 K in 5% CO (Airgas, 99.999%; balance He; 50 cm 3 min -1 total flow rate; 0.33 K s -1 ) for 1 h. After pretreatments in CO, the sample chamber was purged with He (50 cm 3 min - 1 ) for 0.5 h at 523 K. The sample was then heated to a desired temperature in He (0.33 K s -1 ). Once at the desired O2-activation temperature (648 -773 K), O2 (Airgas, 99.999%) diluted in He was introduced to the sample to maintain a volumetric flow rate of 50 cm 3 min -1 and H2O (17.8 MΩ ⋅ cm) or D2O (Sigma Aldrich, 99.9% D atom) was introduced using a syringe pump (KD Scientific, Legato 100) through a liquidinjection port. Simultaneously, spectra (0.1 s, 25 co-added spectra) were obtained using line-scan mode with a long 50x objective, such that the power density was approximately 0.4 mW μm -2 . All Raman spectra were referenced to the 520 cm -1 feature of Si(111). Control experiments which varied the power of the laser (0.04 -0.8 mW µm ) and time delays between laser exposure indicate that the measured spectra are not affected by localized heating induced from the laser. In our hands, the CO gas must be purified using a combination of an in-line moisture and O2 trap (Supelco), as well as Cu turnings (Sigma Aldrich, 99%) that were heated to 553 K. We observed a discoloration of the Cu-CHA surface, which indicates contamination by metal carbonyls, when these traps were not in place. Other contaminants were removed from the O2 and He streams using inline moisture/hydrocarbon and moisture/hydrocarbon/oxygen combination purifying traps (Supelco), respectively. Consequently, we assume that the partial pressure of H2O is <10 Pa (in the absence of intentionally added water vapor), based upon specifications from the gas-trap manufacturers.