ELECTRON-TRANSFER KINETICS AT MODIFIED CARBON ELECTRODE SURFACES - THE ROLE OF SPECIFIC SURFACE SITES

The electron transfer (ET) kinetics of Ru(NH3)(6)(3+/2+), IrCl62-/3-, Fe(CN)(6)(3-/4-), Fe-aq(2+/3+), and V-aq(2+/3+) were examined on several modified glassy carbon surfaces. The kinetics of the aquated ions were very sensitive to the density of surface oxides, while those of the other redox systems were not. In particular, chemical derivatization of surface carbonyl groups decreased the rate of electron transfer with Fe-3+/2+ by 2-3 orders of magnitude but had little effect on Ru(NH3)(6)(3+/2+) or IrCl62-/3-. The electron tranfer rates for Fe-3+/2+ correlated with surface C=O density determined by resonance Raman spectroscopy. Neutral, cationic, and anionic nonspecific adsorbers decreased the rates of ET with the aquated ions approximately equally but had little effect on Ru(NH3)(6)(2+/3+). The redox systems studied were classified into two groups: those which are catalyzed by surface carbonyl groups and those which are not. Possible catalytic mechanisms are considered.