Hemin, iron(III) protoporphyrin IX chloride, was found to bind to mesoporous nanocrystalline (anatase)
TiO2 thin films from DMSO solution, K = 105 M−1 at 298 K. The reactions of iron(II) protoporphyrin
IX (heme) anchored to the TiO2 thin films, 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 of 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 vs. Ag/AgCl. Reactions with CCl4 and CHCl3 were faster at pH 8 than pH 4 (k =
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 × 10−3 s
−1
suggestive
of a multielectron transfer reaction mechanism. Dechlorinated products of propachlor and CCl4
were identified by GC/MS analysis.
