The reduction of oxygen on copper in neutral unbuffered 1 mol dm(-3) N
aCl has been studied using rotating ring-disc electrodes at six oxygen
concentrations equivalent to atmospheres of 2% O-2 + N-2 to 100% 0(2)
. Steady-state potentiostatic measurements show that the reaction is f
irst order with respect to [O-2] and that, following adsorption of O-2
, the first electron transfer is rate determining. In 50% O-2 + N-2 an
d 100% O-2, a cathodic oxygen reduction peak is observed in both poten
tiodynamic and potentiostatic experiments at a disc potential of -0.3
to -0.4 V/SCE. The reaction is dominated by the overall four-electron
reduction to OH-, with only small amounts of peroxide detected by the
ring electrode at disc potentials corresponding to the formation of th
e cathodic oxygen reduction peak. Tafel slopes increase with [O-2] and
vary from -0.13(5) V in 2% O-2 + N-2 to a limiting value of -0.16 V t
o -0.18 V in air, 50% O-2 + N-2 and 100% O-2. The results are explaine
d by a mechanism involving oxygen reduction on two types of surface si
te with different reactivities. The most catalytic surface is believed
to comprise Cu(0) and Cu(I) sites, where the Cu(I) species is stabili
zed as Cu(OH)(ads) and/or submonolayer Cu2O The less catalytic site co
nsists of Cu(0) only. Oxygen reduction is believed to proceed by a ser
ies pathway involving an adsorbed peroxide intermediate on both sites.
Peroxide is reduced to OH- prior to desorption at Cu(0) sites, but so
me is released before being reduced at Cu(0)/Cu(I) sites. Surface cove
rage by catalytic Cu(0)/Cu(I) species is favoured by a higher interfac
ial pH and more positive disc potentials.