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Gold( 111) porphyrins are easily reduced to the corresponding radical anions, which are stable in water over a wide pH range. Further reduction results in formation of the phlorins with relatively little tendency to form chlorins. Both radical anion and phlorin will reduce water to H2 on the surface of a colloidal P t catalyst. From kinetic studies the radical anion is by far the better reducing species. Using NADH as reducing agent, a photosystem has been devised which results in the overall storage of visible light. Oxidation of the gold(rI1) porphyrins results in destruction of the porphyrin ring.Gold(m) porphyrins have a d 8 electron configuration and show a preference for forming square-planar complexes, even in solution.' This means that axial ligands and solvent molecules will be only very weakly coordinated to the central metal ion and the overall complex will be cationic. In aprotic solvents, reduction of gold( 111) tetraphenylporphyrin (AuTPP') results in formation of the radical anion (I?,/, = -0.54 V vs. SCE) in which the extra electron is delocalised over the porphyrin ring.',2 Addition of a second electron also occurs at the porphyrin ring ( E l / , = -0.98 V vs. SCE), forming a rr-dianion.' There is no experimental evidence to suggest reduction of the gold(II1) ion.' Oxidation of AuTPP+ is difficult ( El/, = 1.69 V us. SCE), but it also involves only the porphyrin ring.2 We have investigated the redox chemistry of gold( 111) porphyrins in aqueous solution as part of a series of studies concerned with the factors that influence the stability of metalloporphyrin radical ions in water.4 It is shown that the radical anion is extremely stable, reduction of the central metal ion does not occur and the radical cation is unstable.Having demonstrated the high stability of the radical anion in water, which is unusual, it is of interest to use it to reduce water to H2. Consequently, the interaction between the radical anion and a colloidal Pt catalyst has been studied. Under steady-state conditions in mildly acidic solution, this interaction results in formation of H,. Unlike earlier studies in which colloidal Pt was used to catalyse reduction of water via metalloporphyrin radical anions, '. gold( 111) porphyrins are quite resistant towards chlorin formation. The instability of the corresponding radical cation precludes its interaction with a colloidal 0,-evolving c a t a l y ~t . ~ ExperimentalSamples of gold( 111) meso-tetrakis( N-methyl-4-pyridy1)porphyrin chloride [ AuTMPyP-(4)+], gold( 111) meso-tetrakis( N-methyl-3-pyridy1)porphyrin chloride [ AuTM PyP( 3)+] and gold( 111) rneso-tetrakis(4-sulphonatopheny1)porphyrin (AuTSPP+) were prepared as follows. The corresponding free-base porphyrin (0.2 g), as received from Midcentury Chem. Co., was dissolved in water (50 cm') containing pyridine (10 cm') and LiCl (2 8). The solution was heated to 80 "C and a solution of KAuC14 (0.4 g) in water (20 cm3)

This study throws light on utilization of visible light in removal dyes from wastewater using commercial PbCrO 4 /TiO 2 (P-25) composite which was prepared by grinding TiO 2 (P-25) with PbCrO 4 to decrease the adhesion properties of PbCrO 4 . As-prepared catalyst was characterized by DRS, XRD and N 2 adsorption-desorption. The morphology was examined by SEM. The photodegradation of rhodamine B (Rh B) product in the presence of PbCrO 4 / TiO 2 (P-25) composite was dependent on the pH of the medium. In 2 \ pH \ 10, Rh B was involved in photodegradation via N-deethylation terminated at rhodamine 110 while at pH = 2 rhodamine 110 underwent chromophore destruction. Obtained data showed that HO 2• /O 2•-species were involved in degradation of dye. Commercial PbCrO 4 /TiO 2 (P-25) composite is considered as good visible light-sensitive photocatalyst for removing Rh B.