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

Abstract

Broadband (l 4 320 nm) irradiation of chloroform solutions of either [Ru(bpy) 2 Cl 2 ] or [Ru(bpy) 2 Cl 2 ]Cl exposed to air led to a photostationary state, in which [Ru(bpy) 2 Cl 2 ] þ predominated, and to the continuous decomposition of CHCl 3 , as evidenced by the accumulation of HCl, hydroperoxides (CCl 3 OOH and CHCl 2 OOH), and tetra-, penta-, and hexachloroethane. The addition of Cl À increased the rate of photodecomposition, while the replacement of Cl À by F À greatly decreased the rate. The observations are consistent with a photocatalytic cycle in which [Ru(bpy) 2 Cl 2 ] þ is photochemically reduced to [Ru(bpy) 2 Cl 2 ], which is thermally reoxidized by CCl 3 OO or CCl 3 OOH. 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) 2 Cl 2 ] þ is photoreduced, as in aerated solutions, while [Ru(bpy) 2 Cl 2 ] is photooxidized with chloroform as the substrate.

Photocatalysis of Chloroform Decomposition by the Hexachlororuthenate(IV) Ion

“…The induction period, the inverse dependence on the ruthenium concentration in more concentrated solutions, and the retardation by Cl À were proposed to result from the photochemical formation of a CHCl 3 complex together with a thermal backreaction as in Eq. (17). The rate of chloroform decomposition would then increase as the concentration of RuCl 5 (CHCl 3 ) À , the photocatalytically active species, increases.…”

“…This is more readily accomplished when the counterion is negatively charged; thus, chlorometallates would not normally be candidates for this mechanism. A complex that does appear to catalyze chloroform decomposition in this way is [Ru(bpy) 2 Cl 2 ] + , which, when excited, apparently oxidizes Cl− to Cl • .…”

“…This is more readily accomplished when the counterion is negatively charged; thus, chlorometallates would not normally be candidates for this mechanism. A complex that does appear to catalyze chloroform decomposition in this way is [Ru(bpy) 2 Cl 2 ] + , which, when excited, apparently oxidizes ClÀ to Cl (17). *Corresponding author email: phoggard@scu.edu (Patrick E. Hoggard) While these and other (1,4) mechanisms lead to photocatalyzed decomposition, the situation is often complicated by a transformation of the original chlorometallate complex to another species under irradiation (2)(3)(4)18), which can greatly obscure mechanistic conclusions.…”

Photocatalysis of Chloroform Decomposition by Hexachloroosmate(IV)

“…Given the relative difficulty of oxidizing chloroform by electron transfer, chloride ion (from the reduction of CHCl 3 , ) is the most probable substrate. One possibility, observed in some other photocatalytic systems (16,18), is for chloride ion to be oxidized to chlorine radicals. This would lead to some HCl formation through hydrogen abstraction by chlorine atoms, which was not observed.…”

“…In either case, radicals are created that react with oxygen to form peroxy radicals and hydroperoxides, leading to further decomposition through a chain process (17). Yet another means to effect photodecomposition is for the catalyst to undergo photoreduction by oxidizing a counterion or another substrate to create radicals, following which the catalyst is restored by a thermal process that may generate additional radicals (18). To function in aqueous environments, coordinated chlorides, at least some of them, should resist hydrolysis.…”