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 O(2) 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 SiO(2) defect yields an electron-deficient oxygen atom, which then abstracts hydrogen from chloroform. The resulting CCl(3) radicals react with oxygen to form a peroxy radical that decomposes, eventually yielding phosgene and hydrogen chloride.

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 RuCl6(2-) 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 RuCl6(2-) . 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 H2 O. 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.

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 OsCl(6)(2-) reduces chloroform, rather than one involving photodissociation of chlorine atoms. Trace amounts of ethanol or water in the chloroform lead to photosubstitution to form OsCl(5)(EtOH)(-) or OsCl(5)(H(2)O)(-), neither of which is photocatalytically active.

Chlorochromate ion as a catalyst for the photodegradation of chloroform by visible light.

Exposure of solutions of tetrabutylammonium chlorochromate in chloroform to UV or blue light causes decomposition of the chloroform and the buildup of HCl and peroxides in solution. The CrO(3)Cl(-) is converted during irradiation to CrO(2)Cl(2), which forms a suspension in the chloroform, and then to CrOCl(4)(-). CrO(2)Cl(2) does not by itself catalyze photodecomposition. The initial rate of HCl formation shows an apparently linear dependence on the incident light intensity and on the fraction of light absorbed by chlorochromate, but different values for the apparent quantum yield at 435 nm with high and low concentrations imply a nonlinear contribution to the rate. It is proposed that, at least initially, a cycle involving photoreduction of a Cr(vi) species and thermal reoxidation of Cr(v) by CCl(3)OOH produces radicals that initiate further decomposition.