Comparison of voltammetric responses of toluene and xylenes at iron(III)-doped, bismuth(V)-doped, and undoped beta-lead dioxide film electrodes in 0.50 M H2SO4

Voltammetric activities are compared for the designated compounds in 0.50 M H2SO4 at the specified film electrodes configured as rotated disks. The effective number of electrons (n(eff), equiv mol(-1)) calculated from the Koutecky-Levich plot is 3.1 +/- 0.2 for toluene oxidation at the Fe-PbO2 electrode, compared to 2.0 +/- 0.05 and 4.1 +/- 0.3 at the Bi-PbO2 and PbO2 electrodes, respectively. Gas chromatography-mass spectrometry data confirm that the primary product of toluene oxidation is benzyl alcohol (2 equiv mol(-1)) at the Fe-PbO2 electrode with production of small amounts of benzaldehyde (4 equiv mol(-1))and benzoic acid ( equiv mol(-1)) X-ray photoelectron spectroscopy data indicate the ratio of Fe: Pb is ca. 1:100 (atom: atom) in the Fe-PbO2 films. By comparison, a Bi: Pb ratio of 33:100 (atom: atom) is easily attained in Bi-PbO2 films. Nevertheless, the apparent heterogeneous rate constant (k(app), cm s(-1)) for toluene oxidation is larger at the Fe-PbO2 electrode (6.5 +/- 0.06 x 10(-3)) compared to the Bi-PbO2 electrode (2.0 +/- 0.05 x 10(-3)). X-ray absorption near-edge structure data confirm speculation that iron exists in the 3+ oxidation state with octahedral coordination by O atoms in Fe-PbO2 films. The large activity of the Fe-PbO2 electrode is attributed to the benefit of adsorption of aromatic molecules at Fe(III) sites. (C) 2001 The Electrochemical Society.