Laura A. Peña

Photocatalysis of Chloroform Decomposition by Hexachloroosmate(IV)

Hexachloroosmate(IV) effectively catalyzes the photodecomposition of chloroform in aerated solutions. The decomposition products are consistent with a mechanism in which excited state OsCl62− reduces chloroform, rather than one involving photodissociation of chlorine atoms. Trace amounts of ethanol or water in the chloroform lead to photosubstitution to form OsCl5(EtOH)− or OsCl5(H2O)−, neither of which is photocatalytically active.

Photocatalytic degradation of chloroform by bis(bipyridine)dichlororuthenium(III/II)

Broadband (λ > 320 nm) irradiation of chloroform solutions of either [Ru(bpy)2Cl2] or [Ru(bpy)2Cl2]Cl exposed to air led to a photostationary state, in which [Ru(bpy)2Cl2]+ predominated, and to the continuous decomposition of CHCl3, as evidenced by the accumulation of HCl, hydroperoxides (CCl3OOH and CHCl2OOH), and tetra-, penta-, and hexachloroethane. The addition of Cl− increased the rate of photodecomposition, while the replacement of Cl− by F− greatly decreased the rate. The observations are consistent with a photocatalytic cycle in which [Ru(bpy)2Cl2]+ is photochemically reduced to [Ru(bpy)2Cl2], which is thermally reoxidized by CCl3OO or CCl3OOH. In the absence of air a much slower photodecomposition reaction takes place leading to continuously increasing concentrations of chloroethanes. The data are consistent with a catalytic cycle in which [Ru(bpy)2Cl2]+ is photoreduced, as in aerated solutions, while [Ru(bpy)2Cl2] is photooxidized with chloroform as the substrate.

Photocatalysis of chloroform decomposition by the hexachlororuthenate(IV) ion

Dissolved hexachlororuthenate(IV) effectively catalyzes the photodecomposition of chloroform to hydrogen chloride and phosgene under near‐UV (λ > 345 nm) irradiation, whereby RuCl62− is not itself photocatalytically active, but is photochemically transformed into a species that is active, possibly RuCl5(CHCl3)−. Conversion to a photoactive species during irradiation is consistent with the acceleration of the decomposition rate during the early stages and with the apparent inverse dependence of the decomposition rate on the initial concentration of RuCl62−. The displacement of Cl− by CHCl3 in the coordination sphere to create the photoactive species is consistent with the retardation of photodecomposition by both Cl− and H2O. The much smaller photodecomposition rate in CDCl3 suggests that C–H bond dissociation occurs during the primary photochemical event, which is also consistent with the presence of a CHCl3 molecule in the first coordination sphere.

Photodecomposition of Chloroform Catalyzed by Unmodified MCM‐41 Mesoporous Silica

Unactivated MCM‐41 mesoporous silica catalyzes the photodecomposition of chloroform to phosgene and hydrogen chloride under near‐UV (λ > 360 nm) irradiation. The rate of photodecomposition increases toward an asymptotic limit as the O2 partial pressure is increased. Deuterochloroform does not decompose under the same experimental conditions. Low concentrations of both cyclohexane and ethanol quench the photodecomposition, whereas water, up to its solubility limit, does not. Dissolved tetraalkylammonium salts suppress photodecomposition. The data are consistent with a mechanism in which light absorption by an SiO2 defect yields an electron‐deficient oxygen atom, which then abstracts hydrogen from chloroform. The resulting CCl3 radicals react with oxygen to form a peroxy radical that decomposes, eventually yielding phosgene and hydrogen chloride.