Controlling reduction potentials of semiconductor-supported molecular catalysts for environment remediation of organohalide pollutants

The spectroscopic and redox properties of iron(III) protoporphyrin chloride (hemin) and cobalt(Ill) 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 10(5) M-1 and limiting surface coverages of 10(-8) mol/cm(2) 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. Spectroelectrochernical data gave isosbestic, reversible spectral changes in the visible region assigned to MIII/II redox chemistry with lambda(iso) = 410, 460, 530, 545, 568, and 593 nm for CoTCP/TiO2 and lambda(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 CC1l(4) increased from 3.9 +/- 0.7 x 10(-4) to 2.0 +/- 0.1 x 10(-3) s(-1) when the pH was raised from 4 to 8.