Kan Fujihara

Splitting of water by electrochemical combination of two photocatalytic reactions on TiO2 particles

Photochemical splitting of water was achieved by combining two photocatalytic reactions on suspended titanium dioxide particles, namely, the reduction of water to hydrogen using bromide ions, which were oxidized to bromine and the oxidation of water to oxygen using FeIII ions, which were reduced to FeII ions. These two reactions were carried out in separate compartments and combined via platinum electrodes and cation-exchange membranes. At the electrodes, FeII ions were oxidized by bromine, and protons were transported through the membranes to maintain the electric neutrality and pH of the solutions in the two compartments. As a result, water was continuously split into hydrogen and oxygen under photoirradiation. Reversible reactions on photocatalysts often suffer from the effects of back reactions, unless the products are removed. In the present system the problem is largely prevented, because the concentrations of the products in solution are automatically maintained at a low level.

Photocatalytic oxidation of water on TiO2-coated WO3 particles by visible light using Iron(III) ions as electron acceptor

Photocatalytic oxidation of water on TiO2-coated WO3 particles was studied using iron(III) ions as the electron acceptor with the aim of constructing a photochemical energy conversion system. Although WO3 photocatalysts can utilize part of visible light, the reaction was decelerated as the concentration of iron(II) ions in solution increased. This was a marked contrast with the reaction using TiO2 photocatalysts, whose photocatalytic activity is scarcely affected by iron(II) ions in solution. In order to modify the surface of WO3 particles, they were coated with a thin TiO2 layer. Using such photocatalysts, the harmful effect by iron(II) ions on the WO3 photocatalyst was restrained to some extent, and the efficiency of photooxidation of water by visible light was improved.

Splitting of Water by Combining Two Photocatalytic Reactions through a Quinone Compound Dissolved in an Oil Phase

Two photocatalytic reactions producing oxygen and hydrogen, respectively, were combined using a quinone compound dissolved in an oil phase. As quinone compound, 2,3dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) was utilized, which was dissolved in n-butyronitrile. When an oil phase containing the reduced form of DDQ (DDHQ) was placed on an aqueous phase containing Pt-loaded TiO, particles and bromide ions, hydrogen and bromine were produced in the aqueous phase by photoirradiation of the Ptloaded TiO, particles. The bromine then oxidized DDHQ in the oil phase. Similarly, by photoirradiation of an aqueous solution containing TiO, particles and iron(III) ions, oxygen and iron(II) ions were produced. When the reaction was carried out in the double phase system consisting of the aqueous phase and the oil phase containing DDQ, DDQ was reduced to DDHQ by the iron(II) ions. The results indicate the feasibility of water splitting by combining two photocatalytic reactions through redox reactions of a quinone compound.

Forwarding reversible photocatalytic reactions on semiconductor particles using an oil/water boundary

Abstract Photocatalytic oxidation of iodide ions and hydrogen evolution proceeded on Pt-loaded titanium dioxide particles in aqueous solutions. The reaction rate levelled off as the concentration of tri-iodide ions increased. This was due to the reduction of tri-iodide ion into iodide ion on the photocatalyst. This process is the back reaction of the energy storing photocatalytic reaction. By introducing an oil phase on the aqueous solution the back reaction was retarded, because part of the tri-iodide ions were transported to the oil phase. The back reaction was much eliminated when durohydroquinone was added to the oil phase. In this case, tri-iodide ions in aqueous solution was scavenged as the result of the redox reaction between tri-iodide ion and durohydroquinone, leading to the prevention of the back reaction. This system is comparable to the functions of photosystem I and plastoquinone in the thylakoid membrane of green plants.

Excitation Migration from Photoexcited Tris(8-hydroxyquinolino)aluminium to Quinacridone in Codeposited Thin Films

Absorption, fluorescence, and time-resolved fluorescence measurements have been carried out to demonstrate the energy transfer from photoexcited tris(8-hydroxyquinolino)aluminium (ALQ) to quinacridone (QA) in codeposited thin films. All the results indicate that the excitation energy is transferred from ALQ to QA, which distribute randomly in three dimensions in the thin films, by the Forster mechanism. From the analyses of the fluorescence decay of ALQ, the critical distance for the energy transfer between ALQ and QA molecules is determined to be 26 A. This value is in good agreement with that calculated from the fluorescence spectra of ALQ, the absorption spectra of QA, and the fluorescence quantum efficiency of ALQ. This agreement also indicates that energy migration among ALQ molecules is not an efficient process.

Relay of positive holes from photoirradiated Pt-loaded TiO2 particles in an aqueous phase to t-butylhydroquinone in an oil phase

Abstract Evolution of hydrogen and oxidation of iodide ions proceeded concurrently on photoirradiated Pt-loaded TiO 2 particles which were suspended in an aqueous solution containing potassium iodide. By this reaction part of light energy was converted into chemical energy. This reaction, however, stopped at an early stage when the concentration of tri-iodide (I − 3 ) ions reached a certain level because of preferential reduction of I − 3 ions over hydrogen evolution on the surface of the photocatalyst. In order to prevent this back process, we placed an oil phase containing t -butylhydroquinone on the aqueous phase containing Pt-loaded TiO 2 and iodide ions. In this system, t -butylhydroquinone in the oil phase was oxidized by the I − 3 ions produced photocatalytically across the water/oil boundary. Such a relay of electron transfer is effective to lessen the back process of the energy storing photochemical reactions.