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 sever al modified glassy carbon surfaces. The kinetics of the aquated ions w ere very sensitive to the density of surface oxides, while those of th e other redox systems were not. In particular, chemical derivatization of surface carbonyl groups decreased the rate of electron transfer wi th 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+ cor related with surface C=O density determined by resonance Raman spectro scopy. Neutral, cationic, and anionic nonspecific adsorbers decreased the rates of ET with the aquated ions approximately equally but had li ttle effect on Ru(NH3)(6)(2+/3+). The redox systems studied were class ified into two groups: those which are catalyzed by surface carbonyl g roups and those which are not. Possible catalytic mechanisms are consi dered.