Metalloporphyrin-sensitized photooxidation of water to oxygen on the surface of colloidal iridium oxides: photochemical and pulse radiolytic studies

Abstract

Derivatives of TSPP (tetrakis(4-sulfonatophenyl)porphyrin) were prepared and tested as photosensitizers for oxidation of water to oxygen on the surface of colloidal iridium oxide. Triplet quantum yields, energies, and lifetimes were measured by laser flash photolysis. Rate constants for quenching the porphyrin triplet state with O{sub 2} and with persulfate ions were also determined. The rates of interaction between the porphyrin radical cations and colloidal IrO{sub x} particles were measured by pulse radiolysis for several of the compounds. The one-electron oxidation potentials of the porphyrins, measured by cyclic voltammetry, were varied between 0.7 and 1.4 V vs. NHE by using different metal centers and by substitution on the phenyl rings. Illumination of a porphyrin in the presence of sodium persulfate and an IrO{sub x} colloid resulted in generation of O{sub 2} in a process that was strongly dependent upon pH and upon the nature of the photosensitizer in the same manner as the kinetics determined by pulse radiolysis. The rate of O{sub 2} production under any conditions could be explained on the basis of thermodynamic criteria relating to either of the individual quenching or water oxidation steps. Zn porphyrins (0.9 < E{sub {1/2}} < 1.02 V) effected O{sub 2} production onlymore » in alkaline solution, whereas PdTSPP (E{sub {1/2}} = 1.1 V) gave efficient O{sub 2} evolution even in neutral solution. Further increase in E{sub 1/2}, as achieved with InTSPP (E{sub {1/2}} = 1.16 V), lowers the threshold pH for O{sub 2} evolution, and pulse radiolysis experiments confirmed that the radical cation of this porphyrin is the most reactive toward the catalyst, reacting at diffusion-controlled rates even in acidic solutions. However, the rate of O{sub 2} formation with InTSPP was very low due to the inefficient photochemistry arising from the incomplete quenching of this less-reducing porphyrin triplet state by persulfate ions.« less