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Controlling Reduction Potentials of Semiconductor-Supported Molecular Catalysts for Environmental Remediation of Organohalide Pollutants

Sherine O. Obare, Tamae Ito, and Gerald J. Meyer*

Departments of Chemistry and Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland 21218

Environ. Sci. Technol., 2005, 39 (16), pp 6266–6272

DOI: 10.1021/es048058r

Publication Date (Web): July 8, 2005

Corresponding author phone: (410)516-7319; e-mail: meyer@jhu.edu.

Abstract

The spectroscopic and redox properties of iron(III) protoporphyrin chloride (hemin) and cobalt(III) meso-tetra(4-carboxyphenyl) porphyrin chloride (CoTCP) were quantified in fluid solution and when anchored to mesoporous nanocrystalline TiO₂ thin films. Surface binding was well-described by the Langmuir adsorption isotherm model from which adduct formation constants of 10⁵ M^-1 and limiting surface coverages of 10^-8 mol/cm² were abstracted. In acetonitrile and dimethyl sulfoxide electrolytes, TiO₂ binding was found to induce a substantial negative shift in the M^III/II formal reduction potentials. In DMSO electrolyte, the Co^III/II and Fe^III/II potentials were −559 and −727 mV versus ferrocenium/ferrocene (Fc⁺/Fc) and shifted to −782 and −1063 mV, respectively, after surface binding. The Bronsted acidity of the TiO₂ surface was found to correlate with the measured reduction potentials. For TiO₂ pretreated with aqueous solutions from pH 4−9, the Co^III/II potential showed a −66 mV/pH unit change, while the Fe^III/II potential of hemin changed by −40 mV/pH from pH 1 to 14. Spectroelectrochemical data gave isosbestic, reversible spectral changes in the visible region assigned to M^III/II redox chemistry with λ_iso = 410, 460, 530, 545, 568, and 593 nm for CoTCP/TiO₂ and λ_iso = 408, 441, 500, 576, and 643 nm for hemin/TiO₂. In aqueous solution, the CoTCP reduction potentials were also found to be pH dependent upon surface binding, with CoTCP = −583 mV and CoTCP/TiO₂ = −685 mV versus Fc⁺/Fc at pH 6. For CoTCP/TiO₂, the aqueous pH dependence of the potentials was −52 mV/pH. The rate constant for heme/TiO₂ reduction of CCl₄ increased from 3.9 ± 0.7 × 10^-4 to 2.0 ± 0.1 × 10^-3 s^-1 when the pH was raised from 4 to 8.

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History

Published In Issue August 15, 2005
Received for review December 8, 2004
Revised manuscript received May 28, 2005
Accepted June 1, 2005

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Article

Controlling Reduction Potentials of Semiconductor-Supported Molecular Catalysts for Environmental Remediation of Organohalide Pollutants