During O2 activation treatments at 723 K (21 kPa O2, 80 kPa He), the formation of Raman-active CuxOy species occurs over a period of 12 ks (Figure ), while the near-complete oxidation of Cu(I) to Cu(II) approaches a steady-state within 1 ks as monitored by X-ray absorption near edge spectroscopy (XANES; Figure ). In both cases, the rate (rapp) of O2 activation (Raman) and Cu(I) oxidation (XANES) follows pseudo-first order kinetics: