REACTIONS OF HEME CATALYSTS AT NANOCRYSTALLINE TiO2 THIN FILM INTERFACES WITH ORGANOHALIDE POLLUTANTS
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Organohalide compounds are one of major pollutants on Environmental Protection Agency’s (EPA) contaminants candidate list (www.epa.gov.safewater). Chapter 1 represents the overview of environmental detoxification of groundwater contaminants, electron transfer mechanisms, and the advantages of surface modified nanocrystalline TiO2 thin films. Chapter 2 describes the enhanced reactivity of heme/TiO2 compared to heme in fluid solutions. The photoreduction of organohalides, CCl4, CBr4, and CHCl3 and chloroacetanilides alachlor (2-chloro-2',6'-diethyl-N-(methoxymethyl)acetanilide) and propachlor (2-chloro-N-isopropylacetanilide) by iron(II) protoporphyrin IX chloride (heme) in fluid solution and anchored to a mesoporous nanocrystalline (anatase) TiO2 thin film immersed in solution is reported. The hemes were reacted with organic halides in the dark. Second-order kinetic rate constants of heme/TiO2 were quantified and were found to be larger than the corresponding rate constants for heme in fluid solution. Chapter 3 explains that the synergy effect of heme/TiO2 is partially due to the negative shifts in the formal reduction potentials of the catalysts upon surface binding. 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. In acetonitrile and dimethyl sulfoxide electrolytes, TiO2 binding was found to induce a substantial negative shift in the MIII/II formal reduction potentials. In DMSO electrolyte, the CoIII/II and FeIII/II potentials were -559 and -727 mV versus ferrocenium/ferrocene (Fc+/Fc) and shifted to -782 and -1063 mV, respectively, after surface binding. For TiO2 iii pretreated with aqueous solutions from pH 4-9, the CoIII/II potential showed a -66 mV/pH unit change, while the FeIII/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 MIII/II redox chemistry with iso = 410, 460, 530, 545, 568, and 593 nm for CoTCP/TiO2 and 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. In Chapter 4, photodriven multi-electron transfer (MET) processes are described. Hemin (iron protoporphyrin IX) has been anchored to ~15 nm TiO2 nanocrystallites (anatase) in ~8 μm thick mesoporous thin films. Band gap excitation of these materials in methanol or aqueous (pH 4 or 8) solutions leads to the reduction of hemin to heme (FeIII FeII) and the production of TiO2(e-), heme/TiO2(e-). The mechanisms and second-order rate constants for the reduction of bromobenzene, chlorobenzene, dichlorobenzene, and trichloroethylene were quantified. In all cases, the concentration of TiO2(e-) was found to decrease to near zero before the hemes were oxidized to hemin. Comparative studies with TiO2(e-) that were not functionalized with hemes indicate that organohalide reduction is mediated by the hemes. Reactions of 6-bromo-1-hexene with heme/TiO2(e-) demonstrate multi-electron transfer reactivity and show that heme/TiO2(e-) nanocrystallites deliver two electrons to RX within 4.5 μs. In Chapter 5, the reactions of heme/TiO2 catalysts in aqueous solution, including reaction orders, MET processes, heme mediated mechanisms, and reaction products, were examined. Hemin was found to bind to mesoporous nanocrystalline (anatase) TiO2 thin films from DMSO solution, Keq = 105 iv M-1 at 298 K. The reactions of heme/TiO2 with CCl4, CHCl3, propachlor, and trichloroethylene were investigated in methanol, and pH 4 and 8 aqueous solution. The reactions were found to be first-order in heme/TiO2 and in organohalide. Second-order rate constants measured in aqueous solution increased in the order CCl4 > propachlor > trichloroethylene > CHCl3. The FeIII/II formal reduction potentials of heme/TiO2 were estimated by cyclic voltammetry and found to be –340 mV at pH 4 and –600 mV at pH 8 versus Ag/AgCl. Reactions with CCl4 and CHCl3 were faster at pH 8 than pH 4 (kobs = 14.1 ± 0.7 and 0.63 ± 0.03 M-1 s-1 at pH 4 vs. k = 69 ± 3 and 1.33 ± 0.07 M-1 s-1 at pH 8, respectively), which was attributed to the more negative formal FeIII/II reduction potential at higher pH. The rate constants for propachlor reduction were similar at both pHs. Experiments were also performed with excess electrons trapped in TiO2, TiO2(e-), and heme/TiO2(e-). The reaction of CCl4 was 20 times faster with heme/TiO2(e-) compared to TiO2(e-) alone. In the case of trichloroethylene, no reaction was observed for TiO2(e-), but rapid reactivity was observed for heme/TiO2(e-), kobs = 1.0 x 10-3 s-1 suggestive of a multi-electron transfer reaction mechanism. Dechlorinated products of propachlor and CCl4 were identified by GC/MS analysis. Chapter 6 describes the effects of axial ligand on the reactivity of heme catalysts. The reactivity of heme complexes anchored to nanocrystalline mesoporous TiO2 thin films, FeII/TiO2 and bis(pyr)FeII/TiO2, toward organohalides pollutants was investigated in pH 4 aqueous solution. Pyridine were found to bind to iron center of the porphyrins with binding constants of 9.3 ± 0.4 x 10-4 M-2. Spectroscopic data gave isosbestic points at λiso = 392, 456, and 570 nm upon addition of pyridine to FeII/TiO2. Pyridine axial ligation affected the redox properties of iron porphyrin, the estimated FeIII/II/TiO2 and (pyr)FeIII/II/TiO2 formal reduction potentials v were E° = -900 mV and -160 mV versus ferrocene/ferrocenium (Fc+/Fc) in pH 4. The reaction rates of FeII/TiO2 with halomethanes were consistently faster than bis(pyr)FeII/TiO2 complexes, partly due to the more negative reduction potentials of FeIII/II/TiO2. Multi electron transfer (MET) reactions of bis(pyr)FeII/TiO2(e-) with CCl4 in MeOH formed no carbene adducts, suggestive of the outer sphere electron transfer mechanism when two pyridine axial ligands were present. The detection of tetrachloroethylene as a product of multi-electron transfer supported two-electron reduction of CCl4. The linear free energy relationships for the second-order rate constants, k, versus thermodynamics of the reactions, ΔG, indicated the correlation between the reaction rates and the reduction potentials of FeIII/II and organohalides.