Solvent and electrode kinetic effects on the anodic oxidation of Br^- in water and some dipolar aprotic solvents on bare and platinized Pt and C surfaces ^†

Abstract

The solvent effects on the exchange current densities (i_o)for anodic oxidation of Br^- on platinum (Pt),-graphite (C) and their platinized surfaces, (Pt)Pt or (C)Pt at 15, 25 and 35°C have been determined in water as well as in some pure non-aqueous solvents like acetonitrile (ACN), N,N-dimethylformamide (DMF) and propylene carbonate (PC) by potentiostatic polarization technique under near equilibrium condition. For all the electrodes except C, the observed order of io values in different organic solvents followed the sequence: ACN> DMF> PC and that on C electrode: ACN > PC > DMF. On the other hand, for any solvent except water, the observed io values on different electrodes are in the order: (C)Pt≥(Pt)Pt>Pt>C and that in water: (Pt)Pt>Pt≥(C)Pt>C.. The kinetic solvent effect (KSE) data at 25°C relative to that in water have been analyzed in the light of relative stabilization of the initial state (Br^-] and the transition state (TS), as well as the extent of free surface (1 - <i><i><img src='/image/spc_char/theta.gif' border=0></i></i>_s,), the coverage, <img src='/image/spc_char/theta.gif' border=0>_sbeing guided by image interactions' inducted by dipolar and dispersion interactions of the cosolvents with the surfaces. The observed virtual heats of activation at the reversible potentia. (<img src='/image/spc_char/delta.gif' border=0><i>H</i>^{0 <i>#</i>}) which ref1ectintrinsic electrocatalytic activities of the electrodes, being of the order C≥ Pt > (Pt)Pt > > (C)Pt, suggest that graphite (C) electrode is intrinsically a weaker electrocatalyst than both the (C) Pt and (Pt)pt or even bare Pt electrode and that the superiority of (C)Pt over (Pt)Pt is seemingly due to the porous nature of C substrate.