where 𝑘𝑘 𝑎𝑎𝑎𝑎𝑎𝑎 is the apparent rate constant for ZCuOCuZ formation and implicitly contains the dependence of O2 activation rates on 𝑃𝑃 𝑂𝑂 2 and 𝑃𝑃 𝐻𝐻 2 𝑂𝑂 . Figures and show that values of 𝑘𝑘 𝑎𝑎𝑎𝑎𝑎𝑎 over Cu-CHA-I and Cu-CHA-M samples exhibit two distinct kinetic regimes at low (648 -673 K) and high (685 -773 K) temperature. The Cu-CHA-P control sample prepared to contain predominantly Z2Cu sites was omitted from this analysis because this sample does not facilitate PMO. At low temperatures, rates of ZCuOCuZ formation do not depend on 𝑃𝑃 𝑂𝑂 2 , exhibit saturation kinetics (i.e., a first-to-zeroth order transition) in 𝑃𝑃 𝐻𝐻 2 𝑂𝑂 , and increase exponentially with temperature under either dry conditions (Ea = 176 ± 16 kJ mol -1 ) or with 0.5 kPa H2O present (Ea = 56 ± 6 kJ mol -1 ). In contrast, at high temperatures, ZCuOCuZ formation rates depend linearly on 𝑃𝑃 𝑂𝑂 2 , remain nearly constant at all 𝑃𝑃 𝐻𝐻 2 𝑂𝑂 , and present negligible temperature dependence (i.e., are nearly barrierless). Moreover, ZCuOCuZ formation rates exhibit a normal kinetic isotope effect (𝑘𝑘 𝑎𝑎𝑎𝑎𝑎𝑎,𝐻𝐻 /𝑘𝑘 𝑎𝑎𝑎𝑎𝑎𝑎,𝐷𝐷 = 2 -3.2, 21 kPa O2, 0.5 kPa H2O or D2O) at both 648 and 723 K, which in conjunction with the functional dependence of formation rates on 𝑃𝑃 𝐻𝐻 2 𝑂𝑂 , suggests that H2O participates in the activation of O2. Comparisons between rate dependences for ZCuOCuZ formation at low (< 673 K) and high (> 685 K) temperatures suggest that Cu ions are saturated with O2-derived intermediates at low temperatures, while Cu sites are not occupied by an O2-derived species at high temperatures. ZCuOCuZ formation rates depend similarly on reactant pressures, temperature, and the isotopologue of water used for both Cu-CHA-I and Cu-CHA-M samples, which strongly suggests that the kinetically competent species probed via Raman spectroscopy within these samples are the same (i.e., corresponding to ZCuOCuZ formation from ZCu + species). Scheme 1 shows a plausible series of chemical transformations that describe the formation of ZCuOCuZ species upon contact between O2 and Cu(I) ions within Cu-CHA materials. Two ZCuOH moieties react to form a bis(μ-hydroxyl)dicopper species (ZCu(OH)2CuZ; step 1), which then dehydrates to form ZCuOCuZ species (step 2). Alternatively, treatments in He (or CO, vide infra) auto-reduce some fraction of ZCuOH sites to form ZCu + sites (step 3) that subsequently oxidize by chemisorbing molecular O2 to form ZCuO2 -intermediates (step 4). Reaction between ZCuO2 -and a proximate ZCu + forms a bridging dicopper peroxide complex (ZCuO2CuZ; step 5). These ZCuO2CuZ species may be active intermediates for PMO or may undergo O-O bond cleavage in the absence (step 6a) or presence of H2O (step 6b) to form ZCuOCuZ and release an equivalent of O 2-. These ZCuOCuZ complexes appear to form irreversibly, because the Raman features attributed to this species (580 cm -1 ) persist for hours under flowing He after activation with O2, in agreement with previous reports. Considering the processes described by Scheme 1, the rapid oxidation of Cu(I) to Cu(II) observed by XAS (Figure ) likely corresponds to step 4 or step 5.