Indirect electrochemical Cr(III) oxidation in KOH solutions at an Au electrode: the role of oxygen reduction reaction.
The indirect electro-oxidation of Cr(III) by in situ generated superoxide at a gold electrode has been investigated in KOH solutions using cyclic voltammetry and UV-vis spectroscopy. It is observed that the indirect Cr(III) oxidation behavior is substantially affected by the media pH and there is a pH-modulated oxygen reduction reaction (ORR) process to generate reactive oxygen species which promotes Cr(III) oxidation. The ORR in KOH solutions is attributed to a quasi-reversible diffusion-controlled reaction. In dilute KOH solution (0.2 M), 4e reduction occurs and no reactive oxygen species are generated for the indirect Cr(III) oxidation. Moreover, Cr(III) oxidation is inhibited due to competition for the electrode active sites. As the alkaline concentration increases (3.0 M), the protonation of superoxide is greatly suppressed, and thus, 1e ORR to generate superoxide is observed. This change in mechanism facilitates the indirect Cr(III) oxidation through the superoxide as a mediator to oxidize Cr(III) to Cr(IV), which is the rate-determining step of Cr(III) oxidation to Cr(VI).
National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, People's Republic of China.
Wei
W
Jin
Michael S
MS
Moats
Shili
S
Zheng
Hao
H
Du
Yi
Y
Zhang
Jan D
JD
Miller