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 02 and with persulfate ions were also determined. The rates of interaction between the porphyrin radical cations and colloidal IrOx 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* colloid resulted in generation of 02 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 02 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 < EX/2< 1.02 V) effected 02 production only in alkaline solution, whereas PdTSPP (E¡,2 = 1.1 V) gave efficient 02 evolution even in neutral solution. Further increase in Ex¡2, as achieved with InTSPP {Ex¡2 = 1.16 V), lowers the threshold pH for 02 evolution, and p.ilse 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 02 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.

Mechanistic Insights into Light‐Activated Catalysis for Water Oxidation

“…However, metalloporphyrins present several drawbacks that prevent the widespread exploitation of such chromophores with respect to the ruthenium‐based counterpart. The potential of the PS + /PS couple is in general less positive than that of Ru(bpy) 3 3+/2+ thus hampering, in some cases, activation of WOCs operating at high potentials; in these cases, working under alkaline conditions may allow to reach the suitable driving force to activate the catalytic process, as in the case of ZnTSPP (E 1/2 for ZnTSPP + /ZnTSPP = +0.87 V vs. NHE, used as the PS with colloidal iridium oxides as the WOC, at pH 11) . A similar Zn‐porphyrin was also used with iridium oxide colloids WOC, when templated in genetically engineered virus scaffolds .…”

Platinum(ii)–porphyrin as a sensitizer for visible-light driven water oxidation in neutral phosphate buffer

“…Similar effects have been observed for meso-tetrakis(4-sulfonatophenyl)porphyrins. 18,63 We further compared the changes in the absorption spectra of irradiated solutions of PS and persulfate in the absence and presence of a WOC ( Figure S15). Photodegradation is notably slower in the presence of the WOC.…”

A High Yield Synthesis of Ligand-Free Iridium Oxide Nanoparticles with High Electrocatalytic Activity

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Despite the high cost and low terrestrial abundance of iridium, hydrated iridium oxide (IrO x 3 nH 2 O) has been useful for fundamental studies of the water splitting reaction because it can be made as stable nanoparticles, and because it is highly active for water oxidation over a broad range of pH. [16][17][18][19][20][21][22][23][24][25][26] The synthesis of IrO x 3 nH 2 O colloids was first reported over 100 years ago, 27 and that synthetic method (alkaline hydrolysis of [IrCl 6 ] 2-) produces blue colloids with particle sizes in the 1-2 nm range. Recently, Murray and co-workers used this method to deposit electrode films of IrO x 3 nH 2 O, which they showed are very good electrocatalysts over a broad range of pH.…”