Multi-electron transfer from heme-functionalized nanocrystalline TiO2 to organohalide pollutants

Hemin (iron protoporphyrin IX) has been anchored to approximately 15 nm TiO2 nanocrystallites (anatase) in approximately 8 mum 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 mus.