Photoreductive Dehalogenation of Bromoform with Titanium Dioxide-Cobalt Macrocycle Hybrid Catalysts

The hybrid semiconductor-macrocycle catalyst, TiO₂-Cotetrasulfophthalocyanine, (TiO₂—CoTSP) effectively enhances the solar-promoted reductive dehalogenation of bromoform (CHBr₃) under anaerobic conditions. Reaction rates are 4–10 times faster than those obtained using silanized TiO₂, unmodified TiO₂, or direct (uncatalyzed) solar photolysis under the same conditions. CHBr₃ is reduced to its lower homologues, dibromomethane (CH₂—Br₂), and bromomethane (CH₃Br). HBr is also produced. No other major dehalogenation products are observed, although methane is found in trace amounts after prolonged photolysis. 2-Propanol, the sacrificial electron donor, is oxidized stoichiometrically to acetone. The catalyst is stable in sunlight for at least 30 h without loss of activity. The reaction mechanism is postulated to involve nucleophilic attack of Co(I)TSP, generated by the semiconductor, on CHBr₃. The organometallic complex that is formed, TiO₂—CoTSP—CHBr₂, is postulated to subsequently undergo photolysis via homolytic Co—C bond cleavage to regenerate the catalyst. 2-Propanol is oxidized by valence band holes.