The photocatalytic decomposition of chloroform by tetrachloroaurate(III)

Near-UV irradiation of solutions of (Bu4N)AuCl4 in aerated ethanol-stabilized chloroform causes the continuous decomposition of chloroform, as evidenced by the production of many equivalents of HCl and peroxides. At the outset of irradiation, most of the AuCl4- is reduced to AuCl2-, but the reduction stops and is reversed. The same experiments done in ethanol-free chloroform cause chloroform decomposition only until the irreversible reduction of the gold is complete. In deoxygenated ethanol-free chloroform, irreversible reduction to AuCl2- is accompanied by the formation of HCl and CCl4, while the main decomposition products in deoxygenated ethanol-stabilized chloroform are HCl and C2Cl6. It is proposed that, in ethanol-free chloroform, photoreduction of AuCl4- begins with the concerted elimination of HCl from an association complex of CHCl3 with AuCl4-, and that ethanol suppresses {CHCl3 center dot AuCl4-} complex formation, leaving a slower radical process to carry out the photoreduction of AuCl4- in ethanol-stabilized chloroform. In the presence of oxygen, the radical process causes a build-up of CCl3OOH, which reoxidizes AuCl2- to AuCl4- and allows the photodecomposition of CHCl3 to continue indefinitely.