title: Controlling reduction potentials of semiconductor-supported molecular catalysts for environmental remediation of organohalide pollutants.
abstract: The spectroscopic and redox properties of iron(lll) protoporphyrin chloride (hemin) and cobalt(lll) 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/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(llI/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 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 CCl4 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.
authors.affiliation: Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, USA.
authors.forename: Sherine O
authors.initials: SO
authors.lastname: Obare
authors.forename: Tamae
authors.initials: T
authors.lastname: Ito
authors.forename: Gerald J
authors.initials: GJ
authors.lastname: Meyer
mesh_terms: D002384:Catalysis; D003460:Crystallization; D006427:Hemin / Q000737:chemistry*; D008652:Mesoporphyrins / Q000737:chemistry; D049329:Nanostructures; D009930:Organic Chemicals / Q000302:isolation & purification; D010084:Oxidation-Reduction; D012666:Semiconductors; D012989:Soil Pollutants / Q000302:isolation & purification*; D013057:Spectrum Analysis; D014873:Water Pollutants / Q000302:isolation & purification*
chemical_list: D008652:Mesoporphyrins; D009930:Organic Chemicals; D012989:Soil Pollutants; D014873:Water Pollutants; C506122:cobalt(lll) meso-tetra-(4-carboxyphenyl)porphyrin chloride; D006427:Hemin