Obare Sherine O, Ito Tamae, Meyer Gerald J
Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, USA.
Environ Sci Technol. 2005 Aug 15;39(16):6266-72. doi: 10.1021/es048058r.
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.
在流体溶液中以及固定在介孔纳米晶TiO₂薄膜上时，对氯化铁（III）原卟啉（血红素）和氯化钴（III）中-四-（4-羧基苯基）卟啉（CoTCP）的光谱和氧化还原性质进行了定量分析。表面结合情况可以很好地用朗缪尔吸附等温线模型来描述，从中提取出加合物形成常数为10⁵ M⁻¹，极限表面覆盖度为10⁻⁸ mol/cm²。在乙腈和二甲基亚砜电解质中，发现TiO₂结合会导致M（III/II）形式还原电位出现显著负移。在DMSO电解质中，相对于二茂铁鎓/二茂铁（Fc⁺/Fc），Co（III/II）和Fe（III/II）的电位分别为-559和-727 mV，表面结合后分别移至-782和-1063 mV。发现TiO₂表面的布朗斯特酸度与测得的还原电位相关。对于用pH 4 - 9的水溶液预处理的TiO₂，Co（III/II）电位显示出-66 mV/pH单位的变化，而血红素的Fe（III/II）电位在pH 1至14范围内变化为-40 mV/pH。光谱电化学数据给出了在可见光区域内等吸收、可逆的光谱变化，这些变化归因于M（III/II）氧化还原化学，对于CoTCP/TiO₂，等吸收波长λ(iso) = 410、460、530、545、568和593 nm，对于血红素/TiO₂，λ(iso) = 408、441、500、576和643 nm。在水溶液中，还发现CoTCP的还原电位在表面结合后也与pH有关，在pH 6时，相对于Fc⁺/Fc，CoTCP = -583 mV，CoTCP/TiO₂ = -685 mV。对于CoTCP/TiO₂，电位的水溶液pH依赖性为-52 mV/pH。当pH从4升高到8时，CCl₄的血红素/TiO₂还原速率常数从3.9 ± 0.7×10⁻⁴增加到2.0 ± 0.1×10⁻³ s⁻¹。
