The Formation Of Charge-transfer-complex Of Organics/TiO₂and Their Effects On Photocatalytic Reduction Of Cr（Ⅵ） Under Visible Light Irradiation

The photocatalytic reductions of Cr（Ⅵ） in the presence of organic alcohol,organic aldehyde, carboxylic acid, alkane, chloralkane and aromatic compounds inthe TiO₂solution under visible light irradiation and the formation ofcharge-transfer-complex of organics/TiO₂were investigated. The results showed thatalcohols and carboxylic acid with TiO₂could form the charge-transfer-complexeswhich were able to absorb visible light. The photocatalytic reductions of C（rⅥ） andthe photocatalytic oxidation reaction of organics were carried out throughligand-to-metal charge transfer （LMCT） mechanism. There was synergistic effectbetween the photocatalytic redox reactions. The photocatalytic oxidation reactionwould not carry out when photocatalytic reductions stopped. It was a necessarycondition of photocatalytic reactions that electron acceptor and electron donor wereboth in the system at the same time.The photocatalytic activities of organics/TiO₂were related to the structure oforganics. The carboxyl was better than hydroxyl for the formation ofcharge-transfer-complex. Because the hydroxyl and carboxyl could activate eachother, the photocatalytic activity of organic acid with hydroxyl and carboxyl/TiO₂complex was better than that of organic acid only with carboxyl/TiO₂complex. Theformation of the complex was hindered seriously by steric hindrance effect. Theefficiency of photocatalytic reduction was enhanced as the number of functionalgroups increased. The vicinal effect in benzene ring was of great benefit to theformation of the complex. The photocatalytic activity was negatively related to thelength of the carbon chain of soluble organics with small molecular weight.Thevisible activity of lactic acid/TiO₂complex was the best. Cr（Ⅵ）could hardly bereduced by TiO₂without organics under visible light irradiation. The Cr（Ⅵ）conversion was99.35%in lactic acid/TiO₂systems after3h. The concentration of the organics, pH value of system and the crystal structureof TiO₂all influenced on the photocatalytic reduction of Cr（Ⅵ）. The Cr（Ⅵ）conversion increased gradually with the increase of the initial concentration oforganics. When the concentration of organics was beyond the certain limit, it wouldaffect the reaction rate no longer. The pH value of organics/TiO₂system significantlyaffected the photocatalytic reduction of Cr（Ⅵ）. On the one hand, the higher theconcentration of H+in the system, the higher the oxidation-reduction potential of Cr（Ⅵ） and the easier the reduction of Cr（Ⅵ）. On the other hand, less pH value wasin favour of the adsorption on TiO₂surface of Cr（Ⅵ）, which could improve theefficiency of the photocatalytic reaction. P25, anatase TiO₂and rutile TiO₂were allable to form the charge-transfer-complex with organic. Because of mixed-phaseeffect, the photocatalytic activity of P25was higher than anatase phase or rutile phaseof TiO₂.In organics/TiO₂systems, the photocatalytic reduction of Cr（Ⅵ）and thephotocatalytic oxidation reaction of organics conformed to the first-order kineticequation.