Collectively, these results present evidence for the involvement of the ZCuOCuZ complex as an active intermediate for PMO and a plausible series of chemical steps for O2 activation over ZCuOH sites consistent with the observed dependence of O2 activation on O2 and H2O pressure, protons, and temperature. The precise identification of binuclear Cu species responsible for a specific chemistry (e.g., NO decomposition, CH3OH synthesis), however, remains an important scientific challenge.