Gad S. Nahor

Iridium oxide hydrosols as catalysts for the decay of zinc porphyrin radical cations in water

Iridium oxide hydrosols, containing both IrIII and IrIV sites within a complex oxyhydroxide structure, have been prepared and their interaction with water-soluble zinc porphyrin radical cations has been studied. The colloids possess a negative surface charge so that the positively charged zinc porphyrins are bound to the colloid by electrostatic attraction whilst negatively charged porphyrins are repelled from the surface. Oxidation of IrIII sites can be accomplished with fairly mild oxidants and, at most pH values, the zinc porphyrin radical cations are capable of driving this process. The resultant IrIV sites can undergo further redox reactions, the outlet for any stored oxidizing equivalents being dependent upon the type of porphyrin used. With positively charged porphyrins, discharge of the colloid involves oxidation of surface-bound porphyrin. Using negatively charged porphyrins, the colloid catalyses the oxidation of water to O2 with remarkably high efficiency. Evolution of O2 is restricted, however, to pH > 7 owing to thermodynamic factors associated with the porphyrin. The photo-oxidation of water to O2, under sacrificial conditions, is found to proceed with an initial quantum efficiency of 72% for an optimized system.

Radiolytic reduction of tetrapyridylporphyrins. Formation of a stable radical from tetrakis(N-methyl-2-pyridyl)porphyrin

One-electron and multi-electron reduction of the three isomeric free-base porphyrins derived from TMPyP [tetrakis(N-methylpyridyl)porphyrin] in aqueous solutions under various conditions have been studied by spectrophotometric pulse radiolysis and γ-radiolysis. Similar experiments were carried out with TPyP (tetrapyridylporphyrins) in acidic solutions. Significant differences were found in the behaviour of the three isomers, in the stability of their one-electron and two-electron reduction products and in the effect of pH. An exceptionally stable radical was formed upon one-electron reduction of the ortho isomer, TM2PyP [tetrakis(N-methyl-2-pyridyl)porphyrin], at high pH. The stability of this radical is due to inhibition of protonation, a necessary step in the decay of the radical to final reduction products. Protonation is probably inhibited by the positive charges on the 2-pyridyl groups, which are very close to the meso positions of the porphyrin, and by steric effects of the ortho N-methyl groups, which inhibit deformation of the porphyrin ring and thus retard protonation. Somewhat stable radicals were also found in strongly acidic solutions of TM2PyP and T2PyP.

Reduction of dinitrogen to ammonia in aqueous solution mediated by colloidal metals

Reduction of N2 to NH3 has been investigated by radiolysis and photolysis in N2-saturated aqueous solutions containing colloidal dispersions of Ru, Pt or Rh stabilized either by neutral [poly(vinyl alcohol), PVA] or charged (polybrene, PB, or polystyrene sulphonate, PSS) polymers. Reducing radicals formed from propan-2-ol, methyl viologen, or 4-sulphonatomethylbenzophenone (SBP) were produced radiolytically or photochemically and used to charge the colloidal particles. The yield of NH3 in γ-radiolysis experiments was negligible with Ru/PSS, small with Rh/PB and Pt/PVA, and high with Ru/PB and Ru/PVA. The yield depended on the irradiation time, colloid concentration, type of reducing radical, and pH. Detailed studies with Ru/PVA gave radiation efficiencies close to unity at short irradiations (dose ⩽ 1 kGy). At longer irradiations the concentration of NH3 increased at a much slower rate until the reaction stopped. The results suggest fast and efficient reduction of any N2 that was preadsorbed at highly reactive sites on the surface of the colloid. As this N2 was consumed the active sites were destroyed by hydrogen produced from water reduction. Upon photolysis, H2 and NH3 were produced in competition; longer irradiation times or lower pH favouring water reduction. In all cases, the overall yields of NH3 remained very low. Experiments with colloidal oxides of iron or ruthenium gave very low yields of NH3.

Pulse radiolysis of sulfonato-propyl-viologen (SPV) in aqueous solution containing colloidal platinum

Abstract The pulse radiolysis of the zwitterion sulfonato-propyl-viologen (SPV) was studied in aqueous solutions containing 2-propanol. SPV reacts with the hydrated electron and the 2-propanol radical yielding the anion radical, SPV − . The absorbance changes due to the formation of SPV − were followed at 578 nm ( ϵ 578 = 1.0 × 10 4 M −1 cm −1 ). In the absence of colloidal platinum (Ptc) or air, SPV − is stable for at least several hours. In the presence of Ptc a single decay process was observed. The results are compared with the nitrobenzene-Ptc and methylviologen-Ptc systems.