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The critical bottleneck for water splitting, which is important for sustainable production of hydrogen, has remained in sluggish oxygen evolving reaction (OER) requiring insufficiently low overpote...

Electrocatalytic splitting of water to oxygen and hydrogen is one of the most promising approaches for sustainable production of hydrogen as a carbon-neutral fuel. To establish efficient electrocatalytic water splitting, the overall overpotential for this reaction must be minimized via developing efficient catalysts to promote oxygen and hydrogen evolution at the anode and the cathode, respectively. However, the overpotentials for oxygen evolution are insufficiently low (162–300 mV for a current density of 10 mA cm−2), and the value less than 100 mV still remains untracked. Here, we report the unprecedentedly low of 32 mV for oxygen evolution attained by the formation of a unique motif of nickel sulfide nanowires stuffed into carbon nitride scabbards (NiSx/C3N4), demonstrating electrocatalytic water splitting at the lowest overall overpotential of 72 mV using the NiSx/C3N4 anode. This motif provides a key to guided thought for the development of efficient catalysts for oxygen evolution.

Dinuclear ruthenium(II) complexes, proximal,proximal-[Ru2(Hcptpy)2L(μ-Cl)]3+ (Ru2(μ-Cl), Hcptpy = 4′-(4-carboxyphenyl)-2,2′;6′,2″-terpyridine and L = 5-phenyl-2,8-di(2-pyridyl)-1,9,10-anthyridine) ...

A new mononuclear Ru aquo complex [Ru(C8Otpy)(H2dcbpy)(OH2)]2+ with 4,4'-dicarboxy-2,2'-bipyridine (H2dcbpy) and 4'-octyloxy-2,2':6',2''-terpyridine (C8Otpy) ligands was synthesized to investigate electrocatalytic water oxidation by the complex immobilized on a mesoporous indium-doped tin oxide (meso-ITO) electrode using a multi-potential-step chronocoulospectrometric (MPSCCS) technique. UV-visible absorption spectroscopic data indicated that [Ru(C8Otpy)(dcbpy)(OH2)] (RuOH2) is deprotonated to [Ru(C8Otpy)(dcbpy)(OH)]- (RuOH) on the meso-ITO surface even at pH 5.9 of the electrolyte solution. The cyclic voltammogram (CV) of the RuOH/meso-ITO electrode showed a pH-independent redox response at E1/2 = 0.80 V vs. Ag/AgCl in the pH range of 5-12, being assigned to a non-proton-coupled 1e- redox process of RuIIOH/RuIIIOH. The MPSCCS measurement of the RuOH/meso-ITO electrode between 0.2 and 1.5 V vs. Ag/AgCl showed that RuIV species (tentatively RuIVO) exist in a steady state of the electrocatalysis in the initial stage. This suggests that the electrochemical oxidation from RuIVO to RuVO could compete with the water nucleophilic attack for O-O bond formation involved in the rate-determining step under the employed conditions. The possibility that the water nucleophilic attack on RuIVO could also compete with the electrochemical oxidation of RuIVO to RuVO was suggested by the electrocatalytic water oxidation at a low applied potential of 1.4 V prior to the formation potential of RuVO. The MPSCCS measurement at 1.4 V for 1 h showed that RuOH is gradually transformed into an alternative catalyst (most likely RuOx nanoparticles) on the electrode. The MPSCCS technique is promising to reveal the redox reactions and catalytic aspects of molecular catalysts immobilized on an electrode for water oxidation.

A ruthenium complex showing multi-stimuli-responsive isomerization was synthesized. The catalytic activity of this complex toward water oxidation showed responses to visible-light irradiation and heat due to photoisomerization and thermal back-isomerization, respectively. DFT calculations suggested that a pendant moiety in the complex was key to controlling the catalytic activity.

Polypyridyl ruthenium(II) complexes have been widely researched as promising functional molecules. We have found unique photoisomerization reactions of polypyridyl ruthenium(II) aquo complexes. Recently we have attempted to provide insight into the mechanism of the photoisomerization of the complexes and distinguish between the distal−/proximal-isomers in their physicochemical properties and functions. Moreover, polypyridyl ruthenium(II) aquo complexes have been intensively studied as active water oxidation catalysts (WOCs) which are indispensable for artificial photosynthesis. The catalytic aspect and mechanism of water oxidation by the distal-/proximal-isomers of polypyridyl ruthenium(II) aquo complexes have been investigated to provide the guided thought to develop more efficient molecular catalysts for water oxidation. The recent progress on the photoisomerization and water oxidation of polypyridyl ruthenium(II) aquo complexes in our group are reviewed to understand the properties and functions of ruthenium complexes.

The hexagonal-shaped β -Co(OH) 2 discs were electrophoretically deposited on an electrode to yield a stably adherent and uniform β -Co(OH) 2 layer. The electrocatalytic water oxidation at the β -Co(OH) 2 layer-deposited electrode was investigated to seek efficient water oxidation catalyst for artificial photosynthesis as promising energy-providing systems for the future. The β- Co(OH) 2 layer did not work as a water oxidation catalyst in a K 2 SO 4 solution at pH 7.0 when applying 1.70 V vs. reversible hydrogen electrode (RHE), although the layer was oxidized to the CoO(OH) layer. In a phosphate buffer solution (pH 7.0), the β- Co(OH) 2 layer was dissolved and oxidatively re-deposited as the Co-Pi catalyst ( Science, 321 (2008) 1072–1075.) when applying the same potential. While in a Na 2 B 4 O 7 solution at pH 9.4, the β- Co(OH) 2 layer was oxidized to the CoO(OH) layer with the hexagonal-disc-shape remained when applying 1.7 V vs. RHE, and the formed CoO(OH) layer works efficiently for electrocatalytic water oxidation in contrast to no catalytic activity in a K 2 SO 4 solution at pH 9.4. The catalytic performance of the formed CoO(OH) layer was evaluated; the overpotential (η) of 0.42 V for current generation of 100 μA cm −2 and the Tafel slope of 0.21 ∼ 0.29 V dec −1 were provided for water oxidation at pH 9.4, which are comparable with efficient cobalt-based catalyst films reported so far.

[Ru(Rtpy)(bpy)(H2O)]2+ (1R; bpy = 2,2'-bipyridine, and Rtpy = 2,2':6',2″-terpyridine derivatives) complexes with a variety of 4'-substituent groups on Rtpy were synthesized and characterized to reveal the effects of substituents on their structures, physicochemical properties, and catalytic activities for water oxidation. The geometric structures of 1R are not considerably influenced by the electron-donating ability of the 4'-substituent groups on Rtpy. Similar multistep proton-coupled electron transfer reactions were observed for 1R, and the redox potentials for each oxidation step tended to decrease with an increase in the electron-donating ability of the substituent, which is explained by the increased electron density on the Ru center by electron-donating groups, stabilizing the positive charge that builds up upon oxidation. This is consistent with the red-shift of the absorption bands around 480 nm assigned to the metal-to-ligand charge transfer transition for 1R due to the increased d orbital energy level of the Ru center. The turnover frequency (kO2) of 1R for water oxidation catalysis, however, depended greatly on the Rtpy ligands, varying from 0.05 × 10-2 to 44 × 10-2 s-1 (as the highest kO2 was observed for R = ethoxy) by a factor of 880. A critical electron-donating ability of the 4'-substituent groups with a narrow range of Hammett constants (σp = -0.27 to -0.24) found for the highest kO2 values is valuable for understanding the great difficulty in the search for efficient water oxidation catalysts. On another front, the kO2 values increased with a decrease in the redox potentials of RuIV═O/RuV═O for 1R, indicating that the potential of formation of RuV═O species for 1R is crucial for water oxidation catalysis under the employed conditions.

Ruthenium(ii) complexes with polypyridyl ligands have been extensively studied as promising functional molecules due to their unique photochemical and photophysical properties as well as redox properties. In this context, we report the photoisomerization of distal-[Ru(tpy)(pynp)OH2]2+ (d-1) (tpy = 2,2';6',2''-terpyridine, pynp = 2-(2-pyridyl)-1,8-naphthyridine) to proximal-[Ru(tpy)(pynp)OH2]2+ (p-1), which has not been previously characterized for polypyridyl ruthenium(ii) aquo complexes. Herein, we review recent progress made by our group on the mechanistic insights and application developments related to the photoisomerization of polypyridyl ruthenium(ii) aquo complexes. We report a new strategic synthesis of dinuclear ruthenium(ii) complexes that can act as an active water oxidation catalyst, as well as the development of unique visible-light-responsive giant vesicles, both of which were achieved based on photoisomerization.

The catalytic aspect of a mononuclear Ru complex, [Ru(C8Otpy)(dcbpy)(OH)]− (1) (C8Otpy = 4′-octyloxy-2,2′:6′,2″-terpyridine, H2dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) on a mesoporous indium tin oxide (m-ITO) electrode was investigated by multi-potential-step chronocoulo(ampero)spectrometry (MPSCC(A)S) combined with in situ O2 evolution analysis. For the repetitive MPSCC(A)S measurement between 0.2 and 1.7 V vs Ag/AgCl, 39% of 1 recovered on the first back potential-step to 0.2 V, but 1 no longer recovered after the 4th back potential-step, although the in situ measured amount of O2 increased monotonously with time. XPS and Raman spectroscopic measurements reveal that 1 on the electrode pronouncedly undergoes the oxidative transformation to form the hydrated RuO2 as the alternative catalysts under the high applied potential of 1.7 V. The RuV=O state is more active for water oxidation than the RuIV=O state but could be prone to transform to hydrated RuO2. This tells us that the low applied potential but enough for formation of the RuIV=O species is important for 1 to work stably as a molecular catalyst on the electrode.

A series of ruthenium complexes with chloro-substituted bidentate ligands, proximal-[Ru(tpy)(Cl-pyqu)L]n+ [n = 1 for L = Cl, and n = 2 for L = OH2, tpy = 2,2′;6′,2′′-terpyridine, pyqu = 2-(2′-pyridyl)quinoline] were synthesized and their reversible photoisomerizations and thermal isomerizations were investigated experimentally. The crystal structures of the complexes indicated that introduction of a chloro substituent at the 4- or 4′-position of the pyqu ligand did not change the structure around the metal center from that of the non-substituted complex, proximal-[Ru(tpy)(pyqu)L]n+. In contrast, the 6′-substituted complexes had sterically hindered environments around the metal center. The ruthenium aqua complexes showed reversible photoisomerization between the proximal and distal isomers. The quantum yield for photoisomerization of the 6′-substituted ruthenium aqua complex was almost twice as large as those of the other derivatives. This is explained by weakening of the ligand field on the ruthenium center by introduction of a chloro substituent at the 6′-position. Thermal back isomerization from the distal isomer to the proximal one was observed for the 6′-substituted complex, but such reactions were not observed for the other derivatives. The steric hindrance in the 6′-substituted aqua complex enhanced both thermal isomerization and photoisomerization.

We report the first in situ synthesis of copper nanoparticles (CuNPs) within the interlayer space of inorganic layered semiconductors (titania nanosheet films) through the following steps. A sintered titania nanosheet (s-TNS) film was synthesised, forming a transparent, layered semiconductor film (∼2 μm thick). A considerable amount of copper ions (ca. 68% relative to the cation exchange capacity of TNSs) was intercalated in the s-TNSs using the methyl viologen-containing s-TNSs as the intermediate. The resultant copper-containing s-TNS (TNS/Cu2+) film was treated with an aqueous solution of NaBH4, resulting in a colour change. Extinction spectra of NaBH4-treated films exhibited a wide extinction band at λmax (the extinction band maximum) = 683 nm. The spectral shapes and λmax were similar to those for copper nanoparticles on TiO2 surfaces. Transmission electron microscopy analysis demonstrated the wide distribution of electron dense particles on the titania sheet of NaBH4-treated TNS/Cu2+. XRD analysis and absorption/extinction analysis with different amounts of TNSs suggest that CuNPs were formed within the interlayer space rather than the surface of TNSs through NaBH4 treatment. Repeatable oxidation and reduction behaviour, i.e. colouration and decolouration cycles of the copper species within TNS films, was investigated.

distal-[Ru(Cl-tpy)(pynp)Cl]+ (d-2Cl) (Cl-tpy = 4′-chloro-2,2′;6′,2″-terpyridine, pynp = 2-(2-pyridyl)-1,8-naphthyridine), and distal- and proximal-[Ru(Cl-tpy)(pynp)OH2]2+ (d- and p-2H2O) complexes are newly synthesized and characterized to compare structures and physicochemical properties with a 2,2′;6′,2″-terpyridine (tpy) ligand derivatives of distal-[Ru(tpy)(pynp)Cl]+ (d-1Cl), distal- and proximal-[Ru(tpy)(pynp)OH2]2+ (d- and p-1H2O). The equilibrium turned out to be involved in the aquation reaction of d-2Cl to d-2H2O in contrast to observed irreversible aquation reaction of d-1Cl under the same conditions. The kinetic analysis showed that the aquation reaction of d-2Cl is slightly slower than that of d-1Cl. The stoichiometric photoisomerization of d-2H2O to p-2H2O occurs by visible light irradiation as it is for d-1H2O, and Φ (2.1%) at 520 nm for photoisomerization of d-2H2O was higher than that (1.5%) observed for d-1H2O. d-2H2O undergoes the two-step reaction involving the successive one-proton-coupled one-electron reactions of the RuII OH2/RuIII OH and RuIII OH/RuIV O redox couples, whereas p-2H2O undergoes the one-step reaction involving the two-proton-coupled two-electron reaction of the RuII OH2/RuIV O redox couple. These redox potentials of d- and p-2H2O are higher than those for d- and p-1H2O at pH 7.0 by 10 ∼ 50 mV due to the electron-withdrawing chloro substitution. The turnover frequency ( k O 2 = 6.3 × 10 − 3 s − 1 ) of d-2H2O for water oxidation was higher than that (3.9 × 10−4 s−1) of p-2H2O by a factor of 16. kO2 for d-2H2O was also 1.6 times higher than that (3.8 × 10−3 s−1) for d-1H2O, whereas kO2 for p-2H2O was 1.2 times lower than that (4.8 × 10−4 s−1) for p-1H2O.

Electrochemical water oxidation by [Ru(EtOtpy)(bpy)OH 2 ] 2+ ( 1 ) (EtOtpy = 4′-ethoxy-2,2′:6#6°6#′-terpyridine, bpy = 2,2′-bipydidine) was investigated in a homogeneous solution under weakly acidic conditions (pH 5.3). The cyclic voltammogram of a 1 aqueous solution showed that successive proton-coupled electron transfer reactions of Ru II OH 2 / Ru III OH and Ru III OH/Ru IV O redox pairs and high anodic current above 1.1 V vs SCE. Electrocatalytic water oxidation was corroborated by the bulk electrolysis at 1.5 V; the significant amount of O 2 was evolved compared with the blank during the electrolysis. Potential-step chronocoulospectrometry (PSCCS) from 0.0 V to 1.52 V vs SCE was conducted to observe the change of 1 in solution during the electrocatalysis. The in situ UV visible spectral change showed oxidation of 1 (Ru II OH 2 ) to Ru III OH and to further oxidation of Ru III OH to Ru IV O, and that the Ru IV O species mainly exists in a steady state after 200 s in the electrocatalysis. The in situ UV–vis spectral change in a reverse potential step from 1.52 V to 0.22 V vs SCE exhibited that Ru II OH 2 completely recovers by two electron re-reduction process from the steady state in the electrocatalysis. The observation of Ru IV O in a steady state suggests that a rate determining step in the catalytic cycle is oxidation of Ru IV O to Ru V O rather than the O O bonding formation by nucleophilic attack of water to Ru V O in the electrocatalysis in a homogeneous solution.

Heterogeneous water oxidation catalyses were studied as a synthetic model of oxygen evolving complex (OEC) in photosynthesis using mica adsorbing various manganese species. Distribution of manganese species formed in the oxidative dimerization reaction of [Mn(II)(terpy)2](2+) (terpy=2,2':6',2″-terpyridine) (1') with various oxidants in water was revealed. 1' was stoichiometrically oxidized to form di-μ-oxo dinuclear manganese complex, [(OH2)(terpy)Mn(III)(μ-O)2Mn(IV)(terpy)(OH2)](3+) (1) by KMnO4 as an oxidant. When Oxone and Ce(IV) oxidants were used, the further oxidation of 1 to [(OH2)(terpy)Mn(IV)(μ-O)2Mn(IV)(terpy)(OH2)](4+) (2) was observed after the oxidative dimerization reaction of 1'. The mica adsorbates with various composition of 1', 1 and 2 were prepared by adding mica suspension to the various oxidant-treated solutions followed by filtration. The heterogeneous water oxidation catalysis by the mica adsorbates was examined using a Ce(IV) oxidant. The observed catalytic activity of the mica adsorbates corresponded to a content of 1 (1ads) adsorbed on mica for KMnO4- and Oxone-treated systems, indicating that 1' (1'ads) and 2 (2ads) adsorbed on mica do not work for the catalysis. The kinetic analysis suggested that 1ads works for the catalysis through cooperation with adjacent 1ads or 2ads, meaning that 2ads assists the cooperative catalysis by 1ads though 2ads is not able to work for the catalysis alone. For the Ce(IV)-treated system, O2 evolution was hardly observed although the sufficient amount of 1ads was contained in the mica adsorbates. This was explained by the impeded penetration of Ce(IV) ions (as an oxidant for water oxidation) into mica by Ce(3+) cations (generated in oxidative dimerization of 1') co-adsorbed with 1ads.

The photoisomerization equilibrium between distal- and proximal-[Ru(tpy)(pyqu)OH2]2+ [d- and p-RuH2O, tpy = 2,2′;6′,2″-terpyridine, pyqu = 2-(2′-pyridyl)quinoline] is characterized. The kinetic analysis of the pD-dependent photoisomerization reactions (monitored by 1H NMR) of d-RuH2O and p-RuH2O shows (1) that both hydroxo isomers, distal- and proximal-[Ru(tpy)(pyqu)OH]+, are inert to photoisomerization, and (2) that the back reaction (distal to proximal) is 3.0 times faster than the forward reaction (proximal to distal). Isolation of distal- and proximal-[Ru(tpy)(pyqu)Cl]+ (d- and p-RuCl) as well as d- and p-RuH2O isomers enabled comprehensive studies on geometric structures, ligand exchange and redox reactions, and water oxidation catalysis for these isomers. The observed aquation rate constant (9.2 × 10–2 s–1 at 40 μM) of p-RuCl to form p-RuH2O is 1700 times higher than that (5.4 × 10–5 s–1 at 63 μM) of d-RuCl at 298 K owing to the steric repulsion between a chloro ligand and the 8-proton of the quinoline moiety. The turnover frequency (TOF = 1.7 × 10–3 s–1) of p-RuH2O for catalytic water oxidation is 1.7 times greater than that (1.0 × 10–3 s–1) for d-RuH2O, in contrast to the [Ru(tpy)(pynp)OH2]2+ isomer system, in which the TOF of the distal isomer is higher than that of the proximal one by one order of magnitude. The mechanisms and factors controlling the photoisomerization equilibria and water oxidation catalysis of the d- and p-RuH2O isomers are discussed on the basis of experimental and theoretical investigations.

The synthesis of silver nanoparticles (AgNPs) within the interlayer space of transparent layered titania nanosheet (TNS) films is investigated. A considerable number of silver ions (≈70% against the cation exchange capacity of the TNS) are intercalated in the TNS films using methyl‐viologen‐containing TNSs as a precursor. The silver ion (Ag+)‐containing TNS films are treated with aqueous sodium tetrahydroborate (NaBH4), resulting in a gradual color change to bright blue. Various structural analyses clearly show that crystalline AgNPs are generated within the interlayer space of the TNSs. The NaBH4‐treated films show intense and characteristic near‐infrared (NIR) extinction spectra up to 1800 nm. The stability of the AgNPs within the TNS against oxygen and moisture is also investigated, and 96% and 82% of the AgNPs remain after standing in air for 1 month and 1 year, respectively. The NIR extinctions of the AgNP‐containing TNS films are further extended by employing different preparation procedures, for example, using sintered TNS films as starting materials and irradiating the Ag+‐containing TNSs with ultraviolet (UV) light. The obtained AgNP‐containing TNS films exhibit photochemical activities in the production of hydrogen from ammonia borane under visible‐light irradiation and the decomposition of nitrogen monoxide under UV‐light irradiation.

The influencing factors on heterogeneous water oxidation catalysis (WOC) were investigated in a synthetic photosystem II model developed by adsorbing [(OH2)(terpy)MnIII(μ-O)2MnIV(terpy)(OH2)]3+ (terpy = 2,2′:6′,2″-terpyridine) (1) as an oxygen evolving center onto mica. For chemical WOC using a Ce4+ oxidant, the catalytic activity of 1 on mica increased by a factor of 2.3 or 1.4 by co-adsorption (0.015 mmol g−1) of redox-inactive trications of Al3+ or Ce3+ with 1 (0.15 mmol g−1), respectively, whereas it decreased by co-adsorption (0.25 mmol g−1) of excess Al3+ or Ce3+. The cooperative catalysis by two equivalents of the adsorbed 1 for water oxidation could be facilitated by enrichment of 1 by trications at their low co-adsorption conditions. The decreased catalytic activity at high trications co-adsorption conditions could be explained by impeded penetration of Ce4+ oxidant ions into a mica interlayer. For photochemical WOC containing a [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) photoexcitation center in mica, the drying treatment at 65 °C under the vacuum after 1 adsorption was required in adsorbate preparation, possibly to maintain favorable arrangement of 1 and [Ru(bpy)3]2+ in a mica interlayer. The drying treatment at 65 °C under the vacuum after [Ru(bpy)3]2+ adsorption inactivated the photochemical WOC. The proton-coupled electron transport from interior [Ru(bpy)3]2+ centers to ones near the surface in mica is considered to be suppressed by the drying treatment, which could be responsible for the inactivated photochemical WOC.

Artificial photosynthesis is one of the promising clean-energy-providing systems of the future. Development of an efficient catalyst for water oxidation to evolve O2 is indispensable for the construction of artificial photosynthetic devices. The photosynthetic oxygen evolving complex (OEC) composed of a tetranuclear manganese cluster serves as an excellent example of an efficient catalyst for water oxidation. Recently, significant progress has been reported in the development of manganese complexes as OEC models. The molecular aspects and activities of these manganese complexes toward water oxidation catalysis were reviewed in terms of homogeneous and heterogeneous catalysis systems. These could provide hints to design an efficient catalyst, as well as a key model reaction to understand the O2 evolution mechanism in photosynthesis.

A ligation of Ru(tpy)Cl3 (tpy = 2,2':6',2"-terpyridine) with 2-(2-pyridyl)-1,8-naphthyridine) (pynp) in the presence of LiCl gave distal-[Ru(tpy)(pynp)Cl](+) (d-1Cl) selectively, whereas the ligation gave proximal-[Ru(tpy)(pynp)OH2](2+) (p-1H2O) selectively in the absence of halide ions. (The proximal/distal isomers were defined by the structural configuration between the 1,8-naphthyridine moiety and the aquo or chloro ligand.) An aquation reaction of d-1Cl quantitatively afforded distal-[Ru(tpy)(pynp)OH2](2+) (d-1H2O) in water, and d-1H2O is quantitatively photoisomerized to p-1H2O. The mechanism of the photoisomerization was investigated by transient absorption spectroscopy and quantum chemical calculations. The temperature dependence of the transient absorption spectral change suggests existence of the thermally activated process from the (3)MLCT state with the activation energy (ΔE = 49 kJ mol(-1)), which is close to that (41.7 kJ mol(-1)) of the overall photoisomerization reaction. However, quantum chemical calculations suggest another activation process involving the conformational change of the pentacoordinated distal structure to the proximal structure. Quantum chemical calculations provide redox potentials and pK(a) values for proton-coupled electron transfer reactions from Ru(II)-OH2 to Ru(IV)═O in good agreement with experiments and provide an explanation for mechanistic differences between d-1H2O and p-1H2O with respect to water oxidation. The calculations show that water nucleophilic attack (WNA) on d-[Ru(V)-O](3+) (the ruthenyl oxo species derived from d-1H2O, calculated ΔG(‡) of 87.9 kJ/mol) is favored over p-[Ru(V)-O](3+) (calculated ΔG(‡) of 104.6 kJ/mol) for O-O bond formation. Examination of the lowest unoccupied molecular orbitals in d- and p-[Ru(V)-O](3+) indicates that more orbital amplitude is concentrated on the [Ru-O] unit in the case of d-[Ru(V)-O](3+) than in the case of p-[Ru(V)-O](3+), where some of the amplitude is instead delocalized over the pynp ligand, making this isomer less electrophilic.

A novel type of energy donor–acceptor system on a clay surface has been prepared. The energy transfer between an energy-donating cationic pyrene derivative (An-Py2+) and an energy-accepting tris(bipyridine)ruthenium complex (Ru2+) on the clay surface was investigated using absorption, emission, and lifetime measurements. An obvious energy transfer was observed, and one Ru2+ molecule quenched the emission from five molecules of An-Py2+ with an emission quenching efficiency of 85% on the clay surface. This suggests that the light energies absorbed by five of the An-Py2+ molecules were accumulated in the one Ru2+ molecule. Near-quantitative emission quenching was observed for stoichiometric amounts of An-Py2+ and Ru2+. The apparent quenching rate constant is approximately 1017 L mol–1 s–1, and thus the quenching rate constant is 107–108 times higher than the diffusion rate constant in a homogeneous solution.

Combinations of metal oxide semiconductors and gold nanoparticles (AuNPs) have been investigated as new types of materials. The in situ synthesis of AuNPs within the interlayer space of semiconducting layered titania nanosheet (TNS) films was investigated here. Two types of intermediate films (i.e., TNS films containing methyl viologen (TNS/MV2+) and 2-ammoniumethanethiol (TNS/2-AET+)) were prepared. The two intermediate films were soaked in an aqueous tetrachloroauric(III) acid (HAuCl4) solution, whereby considerable amounts of Au(III) species were accommodated within the interlayer spaces of the TNS films. The two types of obtained films were then soaked in an aqueous sodium tetrahydroborate (NaBH4) solution, whereupon the color of the films immediately changed from colorless to purple, suggesting the formation of AuNPs within the TNS interlayer. When only TNS/MV2+ was used as the intermediate film, the color of the film gradually changed from metallic purple to dusty purple within 30 min, suggesting that aggregation of AuNPs had occurred. In contrast, this color change was suppressed by using the TNS/2-AET+ intermediate film, and the AuNPs were stabilized for over 4 months, as evidenced by the characteristic extinction (absorption and scattering) band from the AuNPs.

Synthetic models of oxygen evolving complex (OEC) are used not only to gain better understanding of the mechanism and the roles of cofactors for water oxidation in photosynthesis, but also as water oxidation catalysts to realize artificial photosynthesis, which is anticipated as a promising solar fuel production system. However, although much attention has been paid to the composition and structure of active sites for development of heterogeneous OEC models, the cofactors, which are essential for water oxidation by the photosynthetic OEC, remain little studied. The high activity of CoO(OH) nanoparticles for electrocatalytic water oxidation is shown to be induced by a CO32- cofactor. The possibility of CO32- ions acting as proton acceptors for O-O bond formation based on the proton-concerted oxygen atom transfer mechanism is proposed. The O-O bond formation is supposed to be accelerated due to effective proton acceptance by adjacent CO32- ions coordinated on the CoIV center in the intermediate, which is consistent with Michaelis-Menten-type kinetics and the significant H/D isotope effect observed in electrocatalysis.

Artificial photosynthesis is expected as one of promising clean energy-providing systems in future. Development of an efficient catalyst for water oxidation to evolve O2 is a key task to yield a breakthrough for construction of an artificial photosynthetic device. Recently, significant progress has been reported in development of the molecular catalysts for water oxidation based on manganese, ruthenium and iridium complexes. The molecular aspects of the catalysts in chemical, electrochemical, and photo- or photoelectro-driven water oxidation were reviewed to provide hints to design an efficient catalyst. This review will mainly cover the related progress in the last 5 or 6 years.

Artificial photosynthesis is anticipated as one of the promising clean energy-providing systems for the future. The development of an efficient catalyst for water oxidation to evolve O2 is a key task to yield a breakthrough for construction of artificial photosynthetic devices. Recently, significant progress has been reported in the development of the molecular catalysts for water oxidation based on manganese, ruthenium and iridium. The molecular aspects of the catalysts reported in the last decade were reviewed to provide hints to design an efficient catalyst, as well as to gain clues to reveal the mechanism of O2 evolution at photosynthetic oxygen evolving complex in nature.

An anodic alumina supported 17.9 wt% Ni catalyst shows an excellent catalytic performance during steam reforming of methane at 700°C under W/F = 6.33 × 10–3 g-cat·h/L, even when compared with some commercial Ni and Ru reforming catalysts. This is considered to be the result of the existence of an interfacial NiAl2O4 layer, which could anchor the top metallic nickel particles to prevent them from reacting with the alumina support and sintering, thereby, effectively suppressing carbon deposition. Nevertheless, for the industrialization of anodic nickel catalysts, some effort should be made to alleviate the deterioration of anodic supports, because when subjected to a higher W/F as 1.27 × 10–3 g-cat·h/L, the catalyst shows irreversible deactivation, and little improvement is seen by increasing nickel spinel content and metal loading or modifying the impregnation solution pH.

A 17.9 wt.% Ni/Al 2 O 3 /alloy catalyst with porous anodic alumina support was subjected to steam reforming of methane after 700 °C steam purging and no reactivity was shown because of surface oxidation of sintered Ni particles. When 0.05 wt.% Ru doped on this catalyst, it exhibited self-activation and self-regenerative activity, resulted from the hydrogen spillover over Ru and/or Ru–Ni alloy. Moreover, a 97% CH 4 conversion was kept for 20 h over the catalyst under electrical heating pattern.

A [Ru(bpy) 3 ] 2+ (bpy = 2,2′-bipyridine)/WO 3 hybrid (denoted as Ru-WO 3 ) film was prepared as a base layer on an indium tin oxide electrode by electrodeposition from a colloidal solution containing peroxotungstic acid, [Ru(bpy) 3 ] 2+ and poly(sodium 4-styrenesulfonate). A ruthenium purple (RP, Fe III 4 [Ru II (CN) 6 ] 3 , denoted as Fe III -Ru II ) layer was electrodeposited on a neat WO 3 film or a Ru-WO 3 film from an aqueous RP colloid solution to yield a WO 3 /RP bilayer film or a Ru-WO 3 /RP bilayer film, respectively. The spectrocyclic voltammetry measurement reveals that Fe II -Ru II is oxidized to Fe III -Ru II by a geared reaction of [Ru(bpy) 3 ] 2+/3+ and Fe III -Ru II is reduced by a geared reaction of H x WO 3 /WO 3 in the Ru-WO 3 /RP film. These geared reactions produced electrochromic hysteresis of the RP layer. However, the absorbance change in the hysteresis was smaller than that for the Ru-WO 3 /Prussian blue bilayer film reported previously, resulting from the lower electroactivities of any redox component for the Ru-WO 3 /RP film. The lower electroactivities could be explained by the specific interface between the Ru-WO 3 and RP layers. It might contribute to either an increase of the interfacial resistance between the Ru-WO 3 and RP layers, or formation of the physically precise interface between the layers to make it difficult for counter ions to be transported in the interfacial liquid phase involved in the redox reactions in the film. The specific interface at the Ru-WO 3 and RP layers could be formed possibly by the electrostatic interaction between [Ru(bpy) 3 ] 2+ and terminal [Ru(CN) 6 ] 4− moieties of RP. It could be suggested by the decreased redox potential of [Ru(bpy) 3 ] 2+ in the Ru-WO 3 layer from 1.03 to 0.61 V by formation of the RP layer.

A series of plate-type metal-monolithic anodic alumina supported nickel catalysts were employed to investigate their reactivity in the steam reforming of methane reactions. After H2 reduction, a fresh 4.7 wt% Ni/γ-Al2O3/alloy catalyst provided only a short-term activity, and then deactivated quickly. By the temperature programmed reaction technologies, the oxidation of surface sintered metallic Ni particles in the SRM test was suggested to be the main reason for the catalyst deactivation. As a result, a 17.9 wt% Ni/NiAl2O4/γ-Al2O3/alloy catalyst with an interfacial NiAl2O4 layer was synthesized, which showed excellent SRM reactivity at 700 °C and this was believed to be resulted from the existence of interfacial NiAl2O4 layer which anchored the top metallic nickel particles and effectively suppressed catalyst sintering. Moreover, no unfavorable effect was evidenced over the SRM reactivity of anodic Ni catalysts when applying the electrically heated pattern. Nevertheless, for the industrialization of anodic nickel catalysts, some efforts should be made to alleviate the deterioration of anodic supports and the catalysts sintering under the SRM reactions.

A nanocomposite film of tungsten trioxide (WO 3 ) and [Ru(bpy) 3 ] 2+ (bpy = 2,2'-bipyridine) (denoted as Ru-WO 3 ) was prepared from an aqueous colloidal triad solution containing peroxotungstic acid, [Ru(bpy) 3 ] 2+ , and poly(sodium 4-styrenesulfonate) by an electrodeposition technique. The electrochemical features of the Ru-WO 3 film were investigated using cyclic voltammetry (CV) and potential-step chronoamperospectrometry (PSCAS) techniques, compared with those in a [Ru(bpy) 3 ] 2+ /Nafion (Ru-Nf) film. PSCAS data spectrophotometrically showed that Ru II is completely oxidized for 0.3 s in the Ru-WO 3 film, in contrast to the corresponding reaction completed for 30 s in the Ru-Nf film. The apparent diffusion coefficient (D app = (0.1-1.1) x 10 -7 cm 2 s -1 ) for charge transport (CT) by a Ru II /Ru III redox in the Ru-WO 3 film was higher than that (2.4 x 10 -10 cm 2 s -1 ) in the Ru-Nf film by 2 or 3 orders of magnitude. Activation energy (E a ) for CT (14.1 kJ mol -1 ) for the Ru-WO 3 film was 3.5 times lower than that (E a = 49.8 kJ mol -1 ) for the Ru-Nf film. The lower E a could be responsible for the faster CT in the Ru-WO 3 film than the Ru-Nf film. D app in the Ru-WO 3 film increased linearly with an increase of the final applied potential (E f ) for PSCAS from 1.2 to 1.5 V vs SCE and saturated above E f = 1.5 V. The mechanism for CT in the Ru-WO 3 film is proposed, in which electrons are injected from [Ru(bpy) 3 ] 2+ into the conduction band (CB) of WO 3 and go through there to a collector electrode.

Nanowire of nickel molybdate hydrate, being recognized as an emerging supercapacitor material, was synthesized from the intercrystal self-assembly process (commonly referred to as oriented aggregation or attachment). The detailed lattice image of a NiMoO4·0.75H2O nanowire and the intermediate nanostructure before reaching the interplanar binding were successfully captured by means of high-resolution transmission and scanning electron microscopies. NiMoO4·0.75H2O possessed highly crystalline surface and internal nanostructures.

A new series of proximal,proximal-[Ru2(tpy)2(L)XY](n+) (p,p-Ru2XY, tpy = 2,2':6',2″-terpyridine, L = 5-phenyl-2,8-di(2-pyridyl)-1,9,10-anthyridine, X and Y = other coordination sites) were synthesized using photoisomerization of a mononuclear complex. The p,p-Ru2XY complexes undergo unusual reversible bridge-exchange reactions to generate p,p-Ru2(μ-Cl), p,p-Ru2(μ-OH), and p,p-Ru2(OH)(OH2) with μ-Cl, μ-OH, as well as hydroxo and aquo ligands at X and Y sites of p,p-Ru2XY, respectively. The geometric and electronic structures of these complexes were characterized based on UV-vis and (1)H NMR spectra, X-ray crystallography, and density functional theory (DFT) calculations. (1)H NMR data showed C2 symmetry of p,p-Ru2(OH)(OH2) with the distorted L chelate and nonequivalence of two tpy ligands, in contrast to the C2v symmetry of p,p-Ru2(μ-Cl) and p,p-Ru2(μ-OH). However, irrespective of the lower symmetry, p,p-Ru2(OH)(OH2) is predominantly formed in neutral and weakly basic conditions due to the specially stabilized core structure by multiple hydrogen-bond interactions among aquo, hydroxo, and backbone L ligands. The electrochemical data suggested that p,p-Ru2(OH)(OH2) (Ru(II)-OH:Ru(II)-OH2) is oxidized to the Ru(III)-OH:Ru(III)-OH state at 0.64 V vs saturated calomel electrode (SCE) and further to Ru(IV)═O:Ru(IV)-OH at 0.79 V by successive 1-proton-coupled 2-electron processes at pH 7.0. The cyclic voltammogram data exhibited that the p,p-Ru2(OH)(OH2) complex works more efficiently for electrocatalytic water oxidation, compared with a similar mononuclear complex distal-[Ru(tpy)(L)OH2](2+) (d-RuOH2) and p,p-Ru2(μ-Cl) and p,p-Ru2(μ-OH), showing that the p,p-Ru2 core structure with aquo and hydroxo ligands is important for efficient electrocatalytic water oxidation. Bulk electrolysis of the p,p-Ru2(OH)(OH2) solution corroborated the electrocatalytic cycle involving the Ru(III)-OH:Ru(III)-OH state species as a resting state. The mechanistic insight into O-O bond formation for O2 production was provided by the isotope effect on electrocatalytic water oxidation by p,p-Ru2(OH)(OH2) and d-RuOH2 in H2O and D2O media.

Tetraphenylporphine zinc(II) (ZnTPP) was found to be adsorbed from its CH2Cl2 solution into a Nafion (Nf) film. The characteristics of the adsorption of ZnTPP into the Nf film were studied using a visible absorption spectroscopic technique. The initial rate (v0, mol cm(-2) s(-1)) for uptake of ZnTPP was saturated with increasing ZnTPP concentration (c0, M) in the solution. This kinetic profile was analyzed in terms of a Michaelis-Menten model considering preequilibrium of ZnTPP adsorption between the solution and the outer layer of the Nf film, followed by diffusion to an inner bulk region, giving a maximum diffusion reflux of v(max) = (2.2 +/- 0.2) x 10(-13) mol cm(-2) s(-1). This is different from the kinetics for the Nf/phthalocyanine zinc(II) (ZnPc) film, which gives a linear plot of v(0) vs c(0). This can be explained by the relatively slow diffusion of ZnTPP in the film compared to that of ZnPc because of steric factors: ZnTPP contains bulky tetraphenyl moieties attached perpendicular to a porphyrin ring, whereas ZnPc has higher planarity. The isotherm for the adsorption of ZnTPP into the Nf film was analyzed using a Langmuir isotherm equation, yielding an equilibrium constant of (3.6 +/- 1.1) x 10(6) M(-1) and a saturated amount of adsorbed ZnTPP of (1.8 +/- 0.1) x 10(-9) mol cm(-2), suggesting monolayer adsorption of ZnTPP on the hydrophobic polymer network interfacial with hydrophilic transport channels without significant intermolecular overlap. This is in contrast to the multilayer adsorption mode suggested for the ZnPc adsorption. The tetraphenyl moieties could prevent the stacking of ZnTPP for multilayer adsorption.

Electrochemical reactions of Ruthenium purple, Fe 4 III [Ru II (CN) 6 ] 3 (RP; Fe III -Ru II ) were studied using a spectrocyclic voltammetry (SCV) technique. The SCV measurement for an RP film coated on an ITO electrode showed a reversible redox between RP and Ruthenium white (RW; Fe II -Ru II ) at 0.14 V vs saturated calomel reference electrode (SCE). An RP film was electrodeposited on a hybrid film of tungsten trioxide (W03)/tris(2,2'-bipyridine)ruthenium(II) ([Ru(bpy) 3 ] 2+ ; bpy=2,2'-bipyridine)/poly(sodium 4-styrenesulfonate) (PSS) (denoted as WRP film) from a colloidal solution containing 0.5 mM FeCl 3 , 0.5 mM K 4 [Ru(CN) 6 ] and 40 mM KCl using a potentiodynamic multi-sweep technique. In a cyclic voltammogram (CV) of a WRP/RP film, a redox response was observed at 0.61 V in addition to essential redox responses of WRP hybrid film (a [Ru(bpy) 3 ] 2+ /Ru(bpy) 3 ] 3+ redox at 1.03 V and a H x WO 3 / WO 3 redox below 0.09 V), but a redox response of RW/RP was not observed at 0.14 V. The SCV measurement for the WRP/RP film suggested that the redox response at 0.61 V is attributed to a redox of [Ru(bpy) 3 ] 2+ /[Ru(bpy) 3 ] 3+ interacted electrostatically with RP. It also showed that RW is oxidized to RP via [Ru(bpy)3]2+/[Ru(bpy)3]3+ redox and RP is reversibly reduced to RW via H x WO 3 /WO 3 redox. This unique geared electrochemical reaction for the WRP/RP film leads to a hysteresis property of an RW/RP redox.

Charge transport (CT) in a Nafion membrane containing μ-oxobis[aquabis(2,2′-bipyridine)ruthenium(III)] complex, [(bpy)2(H2O)RuORu(H2O)(bpy)2]4+ (bpy=2,2′-bipyridine, abbreviated to RuIIIORuIII) was investigated by potential-step chronocoulospectrometry (PSCCS). Electrochemical reduction of RuIIIORuIII in the membrane occurred irreversibly to form [Ru(bpy)2(OH2)2]2+ monomer. The CT by reduction of RuIIIORuIII in the membrane was suggested to take place by physical displacement of the complex, which is quite different from the mechanism in the CT by oxidation of RuIIIORuIII in the same membrane in which charge is transported by charge hopping based on reversible redox reaction between RuIIIORuIII and RuIIIORuIV. The fractions of the electrochemically reacted complex in the membrane for the oxidative CT was dependent on the complex concentration, and the yield was low (maximum fraction=0.42 at 0.87 M) relative to the reductive CT. By contrast, the fraction for the reductive CT was independent of the concentration over 0.12 M and close to unity. The different concentration dependence of the fraction was discussed related to the difference in the CT mechanism.

Tetra-cationic pyrene derivative (Py(4+)) and tris(bipyridine)ruthenium(II) (Ru(2+)) were hybridized onto the surface of a synthesized clay. We observed the remarkable stimulation of excited Py(4+) emission quenching on the clay surface, with a very large apparent quenching rate constant (kq = 7.4 ± 0.7 × 10(15) L mol(-1) s(-1)).

Cores for thought! A new [Mn4 O4 ]6+ "cubane" core complex (L6 Mn4 O4 ) with six facially bridging phosphinate chelate ligands (L- =(MePh)2 PO2- ) was synthesized. Photo-excitation releases molecular O2 by intramolecular coupling of two core oxygen atoms and selective rearrangement to a [Mn4 O2 ]6+ "butterfly" core ([L5 Mn4 O2 ]+ ; see scheme). Thus the Mn4 O4 cubane core exhibits unique reactivity in O2 evolution which may account for its presence in the photosynthetic enzyme.

The activity of tris(ethylenediamine)ruthenium (III) complex, [Ru(en)3]3+, as a water oxidation catalyst was studied in a homogeneous aqueous solution and a heterogeneous Nafion (Nf) membrane. In the aqueous solution, the apparent catalytic activity (kapp (s−1)) decreased monotonously with the concentration due to a bimolecular decomposition of the complex. The bimolecular decomposition of the complex was remarkably suppressed by incorporating it into a Nf membrane. An optimum complex concentration for kapp in the Nf membrane was exhibited, which was explained both by a cooperative catalysis and a bimolecular decomposition of the complex. The kapp in the Nf membrane was analyzed in terms of an intrinsic catalytic activity (kO2 (s−1)) of the complex, a cooperative catalysis distance (rco (nm)) and a critical decomposition distance (rd (nm)) between them based on intermolecular distance distribution to obtain the kO2=8.5×10−5 s−1, rco=1.44 nm and rd=1.07 nm. The results in the [Ru(en)3]3+ system were compared with those obtained in the [Ru(NH3)6]3+ system.

Electrocatalytic water oxidation was carried out using a basal-plane pyrolytic graphite (BPG) electrode coated with electrodeposited Platinum black (Pt-black) adsorbing [Ru(NH 3 ) 5 Cl] 2+ . The amount of O 2 evolved ( V O 2 , mol h −1 ) remarkably increased by adsorbing the complex onto Pt-black. The plots of V O 2 vs. the complex amount on the Pt-black gave a sigmoidal curve, which was explained by both the cooperative catalysis by two molecules of the complex and their bimolecular decomposition. The electrocatalytic activity of the complex was analyzed in terms of its intrinsic activity, k (h −1 ), cooperative catalysis distance, r co (nm) and critical decomposition distance, r d (nm) for both the void space adsorption model (VAM) and surface adsorption model (SAM) based on intermolecular distance distribution. k =940 h −1 , r co =1.21 nm and r d =0.97 nm for VAM and 950 h −1 r co =0.82 nm and r d =0.78 nm for SAM were obtained, respectively. Remarkable high intrinsic activities are ascribed to the efficient charge transfer from the electrode to the complex attached to the Pt-black.

The activity of an electrochemical water oxidation catalyst based on pentaamminechlororuthenium(III) ([Ru(NH 3 ) 5 Cl] 2+ ) complex incorporated in an electrode-coated Nafion membrane was investigated. The complex worked as an active electrochemical catalyst, and an optimum concentration for the turnover number (TN) of the catalyst in the O 2 evolution was exhibited. The TN increased with the concentration at low concentrations, which was ascribed either to charge transfer between the catalysts or to cooperative catalysis by the complex. The TN decrease at high concentrations was ascribed to bimolecular decomposition of the catalyst. The catalyst activity (TN) was analyzed in terms of a charge transfer distance between the catalysts ( r 0 /nm), a cooperative catalysis distance by the complex ( r co /nm), and a critical decomposition distance of the catalyst ( r d /nm) based on intermolecular distance distribution. It was suggested that the activity was controlled mainly by r co and r d , and the values r co =1.26 nm and r d =0.82 nm were obtained.

Electrocatalytic water oxidation was carried out using a basal-plane pyrolytic graphite (BPG) electrode coated with electrodeposited Au particles adsorbing Ru-red ([(NH3)5Ru-O-Ru(NH3)4-O-Ru(NH3)5]6+). The amount of O2 evolved, VO2 (mol h−1), increased remarkably by adsorbing the complex onto Au particles. The plots of VO2 versus the complex amount on Au particles gave an almost straight line at its low surface coverage, but deviated downward with an increase in its amount, which is ascribed to the bimolecular decomposition. The intrinsic turnover number (TN) of the complex is larger than that of the Au particle by three orders of magnitude. The catalytic activity of the complex was analyzed in terms of its intrinsic TN, k (h−1) and critical decomposition distance, rd (nm) based on intermolecular distance distribution to obtain k=21.4 h−1 and rd=1.11 nm.

Charge transfer in a Nafion membrane incorporating a trinuclear ruthenium complex, Ru-red ([(NH3)5Ru(μ-O)Ru(NH3)4(μ-O)Ru(NH3)5]6+), was studied using potential-step chronoamperospectrometry (PSCAS). Reversible oxidation of Ru-red (RuIII–RuIV–RuIII) to Ru-brown (RuIV–RuIII–RuIV) was observed under a potential-step from 0 to 0.5 V vs. SCE in the PSCAS measurement. The charge transport rate by the oxidation of Ru-red to Ru-brown in the membrane depended on the complex concentration and was analyzed by both the physical displacement of the complexes and charge hopping between them. It was found that the charge transport takes place in the membrane by both the physical displacement and charge hopping. The charge transfer distance was analyzed based on a intermolecular distance distribution between the redox centers, and the real charge hopping distance was estimated to be 1.1 nm.

Charge transport in a Nafion membrane incorporating a μ-oxobis[aqabis(2,2′-bipyridine)ruthenium(III)] complex, [(bpy)2(H2O)Ru–O–Ru(H2O)(bpy)2]4+ [abbreviated to RuIII(OH2)–O–RuIII(OH2)], was studied by using a potential-step chronocoulospectrometry (PSCCS). The oxidation of RuIII(OH2)–O–RuIII(OH2) to RuIII(OH2)–O–RuIV(OH) was observed on a potential-step from 0 V vs. SCE to 1.0 V in the PSCCS measurement. The charge transport rate, vCT, is remarkably depended on the complex concentration. The analysis showed that the charge propagation takes place by charge hopping, whose rate constant is dependent on the complex concentration. Charge hopping distance between the complexes in the membrane was analyzed to be 1.09 nm based on an intermolecular distance distribution between them.

Redox reaction and charge transport of trinuclear ruthenium complex, Ru-red ([(NH3)5Ru(μ-O)Ru(NH3)4(μ-O)Ru(NH3)5]6+) were investigated in a Nafion (Nf) membrane as well as in a homogeneous aqueous solution using a potential-step chronoamperospectrometry (PSCAS) in addition to a cyclic voltammetry (CV). It has been shown in CV that Ru-red is oxidized to Ru-brown and then oxidatively transforms to stable redox species in the membrane during repetitive cyclic scans. In a potential-step from 0 to 0.5 V (vs SCE) in PSCAS, a reversible oxidation of Ru-red to Ru-brown was observed in both the membrane and solution. The charge transport based on Ru-red/Ru-brown took place in the membrane by both physical displacement of the complex and charge hopping between them, while in the aqueous solution charge was transported only by physical displacement of the complex. In a potential-step from 0.5 to 1.5 V (vs SCE), an irreversible oxidative transformation of Ru-brown takes place to produce the species which has almost no absorption in the visible region. The transformed species were stable and redox active in the membrane.

Artificial photosynthesis is one of the promising clean-energy-providing systems of the future. Development of an efficient catalyst for water oxidation to evolve O2 is indispensable for the construction of artificial photosynthetic devices. The photosynthetic oxygen evolving complex (OEC) composed of a tetranuclear manganese cluster serves as an excellent example of an efficient catalyst for water oxidation. Recently, significant progress has been reported in the development of manganese complexes as OEC models. The molecular aspects and activities of these manganese complexes toward water oxidation catalysis were reviewed in terms of homogeneous and heterogeneous catalysis systems. These could provide hints to design an efficient catalyst, as well as a key model reaction to understand the O2 evolution mechanism in photosynthesis.

Redox reactions of tris(2,2′-bipyridine)ruthenium (II) (Ru(bpy) 3 2+ ) as well as charge transport in a Nafion ® membrane were investigated using potential-step chronocoulospectrometry (PSCCS). The concentration of oxidized RuIII) obtained from coulometric data was higher than that obtained from spectrophotometric changes, showing that the amount of charge passed does not correspond quantitatively to a real change of the redox center in the electrochemical reaction. Both the charge transport rates obtained from the spectrophotometric and coulometric data were analyzed by a combined process of ahysical displacement and charge hopping. It was shown that charge hopping between the complexes takes place predominantly to transport charges in the membrane; the second order rate constant (6.0 × 10 −1 M −1 s −1 ) of the charge hopping obtained from the coulometric data was twice that (2.5 × 10 −1 M −1 s −1 ) calculated from the spectrophotometric data.

(pore diameter, about 2–3 nm) can offer 1) a large internal surface area; and 2) ashorter solid-state carrier diffusion length in the nanosizedwall ( 15 nm) mesoporous systems. How-ever, particularly in case of WO

The effect of carbon dioxide on corrosion rate, polarization curve and hydrogen content has been investigated for carbon steels in acetate solutions and sodium carbonate solution with a pH range of 2 to 9 at 313 K under carbon dioxide and nitrogen atmospheres. In acid solutions less than pH 6 both general corrosion and hydrogen absorption are accelerated by the existence of carbon dioxide, but not clear in the intermeadiate pHs 5 and 6. In the solutions more than pH 6 little corrosion rate is observed regardless of carbon dioxide, whereas hydrogen content is surely detected only at the existence of carbon dioxide. The anodic and cathodic polarization curves show that their reaction overpotentials are affected by carbon dioxide, whose effect changes depending upon pH. The results obtained are qualitatively explained in terms of the formation of iron carbonate and the adsorption of carbonate ions.

Charge transfer between Ru(bpy) 2+ 3 (where bpy is 2,2′-bipyridine) complexes incorporated into a Nafion® membrane has been investigated using in situ spectrocyclic voltammetry and statistical calculation of the intermolecular distance between the complexes. The membrane was prepared either by a mixture casting method in which an alcoholic mixture solution of Nafion and the complex was cast to form a membrane, or by an adsorption method in which the complex was adsorbed from its aqueous solution into a precoated Nafion® membrane. It has been found that the electrochemical reactivity of the complex in a membrane prepared by the adsorption method is much higher than that prepared by the mixture casting method. The charge transfer distance between the complexes ( R 0 ) in the membrane prepared by the adsorption method was 1.6 nm (scan rate, 2 mV s −1 ; pH 5.5), when assuming a random dispersion of the complex, which is significantly longer than that (1.1 nm) in the membrane prepared by the mixture casting method. The emission lifetime of the photoexcited Ru(bpy) 2+ 3 in the membrane suggests that the complex is localized in the membrane when it is prepared by an adsorption method; such localization would make the calculated R 0 value greater than the real value. The fraction of the volume in the membrane where the complex is adsorbed was estimated as 20 ± 3%, by comparing the real local complex concentration in the membrane obtained from the R 0 value (1.1 nm) for the mixture casting system with the apparent average complex concentration.

A first and unique preparation is reported of a WO 3 /tris(2,2'-bipyrizine)ruthenium (II) (Ru(bpy) 3 2+) composite film by electrodeposition from a colloidal solution containing peroxotungstic acid and Ru(bpy) 3 2+ that is remarkably stabilized by poly(sodium 4-styrensulfonate). The composite film clearly demonstrated multiple electrochromism. The photoexcited Ru(bpy) 3 2+ was found to be quenched completely by WO 3 in the composite film, which is responsible for an electronic interaction of Ru(bpy) 3 2+ with WO 3 . The complete quenching led to a photocharging-discharging character and a steady-state photoanodic current induced by visible light.