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Geometric isomers of mononuclear ruthenium(II) complexes

Facile and scalable fabrication of α-Fe2O3 photoanodes using a precursor solution containing FeIII ions and 1-ethylimidazole (EIm) in methanol was demonstrated to afford a rigidly adhered α-Fe2O3 film with a controllable thickness on a fluorine-doped tin oxide (FTO) substrate. EIm ligation to FeIII ions in the precursor solution brought about high crystallinity of three-dimensionally well-interconnected nanoparticles of α-Fe2O3 upon sintering. This is responsible for the 13.6 times higher photocurrent density (at 1.23 V vs reference hydrogen electrode (RHE)) for photoelectrochemical (PEC) water oxidation on the α-Fe2O3 (w-α-Fe2O3) photoanode prepared with EIm compared with that (w/o-α-Fe2O3) prepared without EIm. The w-α-Fe2O3 photoanode provided the highest charge separation efficiency (ηsep) value of 27% among the state-of-the-art pristine α-Fe2O3 photoanodes, providing incident photon-to-current conversion efficiency (IPCE) of 13% at 420 nm and 1.23 V vs RHE. The superior ηsep for the w-α-Fe2O3 photoanode is attributed to the decreased recombination of the photogenerated charge carriers at the grain boundary between nanoparticles, in addition to the higher number of the catalytically active sites and the efficient bulk charge transport in the film, compared with w/o-α-Fe2O3.

Artificial photosynthesis technologies require transparent thin film catalysts working efficiently for water oxidation in neutral pH media for penetration of light to the inner light-harvesting lay...

A rigidly adhered nitrogen-doped BiVO4 photoanode with a narrow band gap energy (Eg = 2.35 eV) for water oxidation was developed to attain the IPCE value of 35.1% at 440 nm which is among the highe...

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...

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) ...

Oxygen-evolution-reaction (OER) and oxygen-reduction-reaction (ORR) catalysts on a gas-diffusion carbon paper (CP) electrode have been explored as a current hot topic. Low-cost and earth-abundant F...

A non-heme Fe complex with a tetradentate dicarboxamide N4 in-plane ligand works efficiently and selectively for the electrocatalytic CO2-to-CO conversion in a homogeneous DMF solution. The complex...

Nanorod architecture of a N2-intercalated WO3 photoanode has been developed by emphasizing the dual role of N2H4, which functioned simultaneously as a structure-directing agent and as a nitrogen so...

Tunable mesoporous crystalline tungsten trioxide (WO3) was synthesized by in situ surfactant-thermal-carbonization method, using the series of 2-(alkylaminomethyl)pyridine (PAL2-n, n = 8, 12, and 16) surfactant templates with varied alkyl chain length. The d-spacing, corresponding to the pore center-to-center distance, for mesoporous WO3 annealed at 550 °C decreased from 3.8 nm to 3.1 nm with a shortened chain length of PAL2-n from n = 16 to n = 12, in response to changes in the smaller size of micellar self-assemblies of PAL2-n templates. The mesoporous structure was partially collapsed for the PAL2-8 template during the crystallization of WO3 framework, when annealed at 550 °C. The longer alkyl chain of PAL2-n yielded the thicker pore walls in WO3/PAL2-n mesocomposites, affording better thermal stabilization of the organized mesoporous structure of WO3, along with the formation of higher content of carbonaceous species in mesopores through surfactant carbonization, which act as a protective support duri...

A heterogeneous catalyst incorporating an inorganic ion cofactor for electrochemical water oxidation was exploited using a CoO(OH) nanoparticle layer-deposited electrode. The significant catalytic current for water oxidation was generated in a Na2B4O7 solution at pH 9.4 when applying 0.94 V versus Ag/AgCl in contrast to no catalytic current generation in the K2SO4 solution at the same pH. HB4O7– and B4O72– ions were indicated to act as key cofactors for the induced catalytic activity of the CoO(OH) layer. The Na2B4O7 concentration dependence of the catalytic current was analyzed based on a Michaelis–Menten-type kinetics to provide an affinity constant of cofactors to the active sites, Km = 28 ± 3.6 mM, and the maximum catalytic current density, Imax = 2.3 ± 0.13 mA cm–2. The Imax value of HB4O7– and B4O72– ions was 1.4 times higher than that (1.3 mA cm–2) for the previously reported case of CO32– ions. This could be explained by the shorter-range proton transfer from the active site to the proton-acceptin...

proximal,proximal-(p,p)-[RuII2(tpy)2LXY]n+ (tpy = 2,2′;6′,2″-terpyridine, L = 5-phenyl-2,8-di-2-pyridyl-1,9,10-anthyridine, and X and Y = other coordination sites) yields the structurally and functionally unusual RuII(μ-OH)RuII core, which is capable of catalyzing water oxidation with key water insertion to the core (Inorg. Chem. 2015, 54, 7627). Herein, we studied a sequence of bridging-ligand substitution among p,p-[Ru2(tpy)2L(μ-Cl)]3+ (Ru2(μ-Cl)), p,p-[Ru2(tpy)2L(μ-OH)]3+ (Ru2(μ-OH)), p,p-[Ru2(tpy)2L(OH)(OH2)]3+ (Ru2(OH)(OH2)), and p,p-[Ru2(tpy)2L(OH)2]2+ (Ru2(OH)2) in aqueous solution. Ru2(μ-Cl) converted slowly (10–4 s–1) to Ru2(μ-OH), and further Ru2(μ-OH) converted very slowly (10–6 s–1) to Ru2(OH)(OH2) by the insertion of water to reach equilibrium at pH 8.5–12.3. On the basis of density functional theory (DFT) calculations, Ru2(OH)(OH2) was predicted to be thermodynamically stable by 13.3 kJ mol–1 in water compared to Ru2(μ-OH) because of the specially stabilized core structure by multiple hydrog...

A new transparent iridium oxide (IrOx) film on fluorine-doped tin oxide (FTO) electrodes were achieved from a homogeneous precursor complex solution by employing a facile spin-coating technique. The composition of the nanostructure and crystallinity of the IrOx film is tunable by a simple annealing treatment of a compact complex layer, which is responsible for their significantly different electrocatalytic performances for water oxidation. Transmission electron microscopy (TEM) observations showed uniformly dispersed small IrOx nanoparticles of dimensions ca. 2–5 nm for the film annealed at 300 °C, and the nanoparticles gradually agglomerated to form relatively large particles at higher temperatures (400 and 500 °C). The IrOx films prepared at different annealing temperatures are characterized by Raman spectroscopic data to reveal intermediate IrOx(OH)y nanoparticles with two oxygen binding motifs: terminal hydroxo and bridging oxo at 300 and 350 °C annealing, via amorphous IrOx at 400 °C, transforming ul...

The current approaches to electrochemically synthesizing valve metal-derived nanochannel films with longitudinal nanospaces aligned at a right angle to planar substrates rely on highly toxic fluoride compounds and require severe reaction conditions. Herein, we report on a fluoride-free, room-temperature electrochemical synthesis of a genuine mesoporous niobia thin film from the parent metal. The electrochemical reaction is driven by only a 1 V bias with respect to a Pt counter electrode in an aqueous solution. The solution contained an inexpensive, less toxic potassium hydroxide, and the reaction produced favorable byproducts, namely, recyclable K8Nb6O19 and H2.

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.

[(OH(2))(R-terpy)Mn(μ-O)(2)Mn(R-terpy)(OH(2)) ](3+) (R-terpy = 4'-substituted 2,2':6',2″-terpyridine, R = butoxy (BuO), propoxy (PrO), ethoxy (EtO), methoxy (MeO), methyl (Me), methylthio (MeS), chloro (Cl)) have been synthesized as a functional oxygen-evolving complex (OEC) model and characterized by UV-vis and IR spectroscopic, X-ray crystallographic, magnetometric, and electrochemical techniques. The UV-vis spectra of derivatives in water were hardly influenced by the 4'-substituent variation. X-ray crystallographic data showed that Mn centers in the Mn(III)(μ-O)(2)Mn(IV) cores for derivatives with R = H, MeS, Me, EtO, and BuO are crystallographically indistinguishable, whereas the derivatives with R = MeO and PrO gave the significantly distinguishable Mn centers in the cores. The indistinguishable Mn centers could be caused by rapid electron exchange between the Mn centers to result in the delocalized Mn(μ-O)(2)Mn core. The exchange integral values (J = -196 to -178 cm(-1)) for delocalized cores were lower than that (J = -163 to -161 cm(-1)) for localized cores, though the Mn···Mn distances are nearly the same (2.707-2.750 Å). The half wave potential (E(1/2)) of a Mn(III)-Mn(IV)/Mn(IV)-Mn(IV) pair of the derivatives decreased with an increase of the electron-donating ability of the substituted groups for the delocalized core, but it deviated from the correlation for the localized cores. The catalytic activities of the derivatives on mica for heterogeneous water oxidation were remarkably changed by the substituted groups. The second order rate constant (k(2)/mol(-1) s(-1)) for O(2) evolution was indicated to be correlated to E(1/2) of a Mn(III)-Mn(IV)/Mn(IV)-Mn(IV) pair; k(2) increased by a factor of 29 as E(1/2) increased by 28 mV.

An IrO2 colloid stabilized by citrate ions was self-assembled on an indium tin oxide (ITO) electrode when it was immersed in the colloid solution at pH 3.5. The IrO2 colloid on the ITO surface was characterized using electrochemical, inductively coupled plasma mass spectroscopic, X-ray diffraction spectroscopic, scanning electron microscopic, and atomic force microscopic techniques. The self-assembly was promoted steeply at pH 3.5 to 4.1, although it hardly occurred at pH 5.3 to 9.7. It is considered to be caused by chemical interaction between carboxylic groups on the citrate stabilizer and hydroxyl groups of the ITO surface. The adsorption isotherm of the IrO2 colloid onto the ITO surface was analyzed by a Langmuir adsorption isotherm to provide the maximum coverage and an adsorption equilibrium constant Γmax = 1.1 × 10-8 mol cm-2 and Kads = 1.8 × 104 M-1 at 25 °C, respectively. The Kads value increased from 6.7 × 103 to 1.8 × 104 M-1 with a temperature increase from 5 to 25 °C. The temperature dependen...

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.

Hybridization of [(OH(2))(terpy)Mn(mu-O)(2)Mn(terpy)(OH(2))](3+) (terpy= 2,2':6',2' '-terpyridine) (1) and mica clay yielded catalytic dioxygen (O(2)) evolution from water using a CeIV oxidant. The reaction was characterized by various spectroscopic measurements and a kinetic analysis of O(2) evolution. X-ray diffraction (XRD) data indicates the interlayer separation of mica changes upon intercalation of 1. The UV-vis diffuse reflectance (RD) and Mn K-edge X-ray absorption near-edge structure (XANES) data suggest that the oxidation state of the di-mu-oxo Mn(2) core is Mn(III)-Mn(IV), but it is not intact. In aqueous solution, the reaction of 1 with a large excess Ce(IV) oxidant led to decomposition of 1 to form MnO(4-) ion without O(2) evolution, most possibly by its disproportionation. However, MnO(4-) formation is suppressed by adsorption of 1 on clay. The maximum turnover number for O(2) evolution catalyzed by 1 adsorbed on mica and kaolin was 15 and 17, respectively, under the optimum conditions. The catalysis occurs in the interlayer space of mica or on the surface of kaolin, whereas MnO(4-) formation occurs in the liquid phase, involving local adsorption equilibria of adsorbed 1 at the interface between the clay surface and the liquid phase. The analysis of O(2) evolution activity showed that the catalysis requires cooperation of two equivalents of 1 adsorbed on clay. The second-order rate constant based on the concentration (mol g(-1)) of 1 per unit weight of clay was 2.7 +/- 0.1 mol(-1) s(-1) g for mica, which is appreciably lower than that for kaolin (23.9 +/- 0.4 mol(-1) s(-1) g). This difference can be explained by the localized adsorption of 1 on the surface for kaolin. However, the apparent turnover frequency ((kO(2))app/s(-1)) of 1 on mica was 2.2 times greater than on kaolin when the same fractional loading is compared. The higher cation exchange capacity (CEC) of mica statistically affords a shorter distance between the anionic sites to which 1 is attracted electrostatically, making the cooperative interaction between adsorbed molecules of 1 easier than that on kaolin. The higher CEC is important not only for attaining a higher loading but also for the higher catalytic activity of adsorbed 1.

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.

Phthalocyanine zinc(II) (ZnPc) was found to be adsorbed well into a Nafion (Nf) film. The kinetic analysis suggested that the adsorption of ZnPc into the Nf film is controlled by its diffusion in the Nf film with a diffusion coefficient of D = 1.9 x 10(-6) cm(2) s(-1) that is higher than those (10(-9)-10(-12) cm(2) s(-1)) of cationic redox molecules in the Nf film by 3-6 orders of magnitude. The adsorption isothermal was analyzed by a Brunauer-Emmett-Teller (BET) equation suggesting multilayer adsorption of ZnPc into the film. The BET analysis provided the amount of ZnPc for monolayer adsorption (w(m) = 1.50 x 10(-7) mol cm(-2)), from which the effective area for the ZnPc adsorption was estimated to be larger by a factor of 1.7 x 10(3) than the Nf film area (1.0 cm(2)). The absorption spectra of a Nf film adsorbing ZnPc ((Nf/ZnPc)(ads) film) exhibited two broad absorption bands at 385 and 680-750 nm without any structural features, which is significantly different from the absorption spectra of either ZnPc solution in DMF or a (Nf/ZnPc)(mix) film prepared from a DMF solution containing Nf and ZnPc by solvent evaporation. This is ascribed to the formation of a ZnPc aggregate in the (Nf/ZnPc)(ads) film. Photoluminescence data for the (Nf/ZnPc)(mix) film suggested the presence of a ZnPc monomer and dimer at equilibrium in the film with a concentration of 0.1 M and that energy transfer occurs from the monomer to the dimer in excitation of the monomer (at lambda(ex) = 609 nm) to yield emission from the dimer. By contrast, photoluminescence data for the (Nf/ZnPc)(ads) film suggested that the excited ZnPc is self-quenched significantly by the formation of the ZnPc aggregate in the film. The lesser electroactivity of ZnPc in the (Nf/ZnPc)(ads) film compared with that in the (Nf/ZnPc)(mix) film could be ascribable to more difficult diffusion of ZnPc in the former film due to the formation of the ZnPc aggregate. The adsorption of ZnPc into the Nf film was significantly regulated by simple pretreatments of the Nf film such as immersion in solvents and storage under solvent vapors. The regulation was explained by controlled physical and chemical properties of a channel for mass and ion transport that is formed by sulfonate groups, countercations, and solvent molecules in the Nf film.

We report a template-free facile technique that allows for the first ever synthesis of a monoclinic Ag2Mo2O7 nanowire (m-Ag2Mo2O7-NW), using a commercially available MoO3 particle. The nanowire possessed high crystallinity and structural homogeneity and strongly suggested that the nanowire was grown through an oriented aggregation mechanism in contrast to the case of a typical solution-phase method. The corresponding bulky counterpart showed no photoresponse; however, a complete structural transformation toward a nanowire triggered activity for O2 evolution in the presence of Ag(+) as an electron acceptor under visible-light irradiation.

Charge transport (CT) in a Nafion film incorporating [(NH3)5Ru(μ-pz)Ru(NH3)5]4+ ([RuII−RuII]4+; pz = pyrazine) was studied using a double potential-step chronoabsorptometry (DPSCA) technique. The oxidative CT by [RuII−RuII]4+/[RuII−RuIII]5+ (in a potential step from −0.2 to 0.3 V; first step) was compared with that by [RuII−RuIII]5+/[RuIII−RuIII]6+ (in a successive potential step from 0.3 to 0.7 V; second step). The predominant CTs by a physical displacement mechanism of the complexes were suggested for both of the steps. However, the rate constant of the CT for the first step was 5.2 time higher than that for the second step at 25 °C. The results on CT kinetics and dynamics revealed that the CTs in both of the steps are entropy-controlled in the activation at room temperature. The ΔH‡ (12 kJ mol-1) for the physical displacement in the second step was lower than that (24 kJ mol-1) for the first step, showing that the physical displacement for the second step is enthalpically favorable compared with that f...

Charge transport (CT) in a Nafion film incorporating [Ru(bpz)3]2+ (bpz = 2,2‘-bipyrazine) was investigated using a potential-step chronocouloabsorptometry technique. The reductive CT by [Ru(bpz)3]2+/+ was found to take place by a combination mechanism of physical displacement of the complex and charge hopping between the molecules of the complex. This is a different mechanism from that of the oxidative CT (occurring by a charge hopping mechanism) in an earlier reported Nafion/[Ru(bpy)3]2+/3+ (bpy = 2,2‘-bipyridine) system. The combination mechanism in CT was also reported in an earlier Nafion/[(NH3)5Ru−O−Ru(NH3)4−O−Ru(NH3)5]6+ (Ru-red) system. In the present [Ru(bpz)3]2+ system, the activation energy (Ea = 78 kJ·mol-1) of physical displacement is higher than that (40 kJ·mol-1) of charge hopping. In the Ru-red system, on the contrary, the Ea (13 kJ·mol-1) of physical displacement is lower than that (55 kJ·mol-1) of charge hopping. As a result, the fraction (Φc) of the contribution of charge hopping to the ...

cis-Tetraamminedichlororuthenium(III) (cis-[Ru(NH3)4Cl2]+) complex was found to work as a very active water oxidation catalyst to evolve dioxygen (O2) in an aqueous solution as well as in a Nafion membrane. The initial O2 evolution rate (VO2 (mol s-1)) was first-order with respect to the complex concentration in both the solution and membrane. The catalyst undergoes a bimolecular decomposition to deactivate at high complex concentrations. The first-order rate constant (kO2 (s-1)) for O2 evolution and the second-order rate constant (kdeact (dm3 mol-1 s-1)) for bimolecular decomposition were obtained from the kinetic analysis based on the competition between both processes. The kO2 (1.4 × 10-2 s-1) in the Nafion membrane is much higher than that (2.3 × 10-3 s-1) in the solution. The kdeact (5.1 × 10-1 dm3 mol-1 s-1) in the membrane was slightly higher than that (2.0 × 10-1 dm3 mol-1 s-1) in the solution. The catalytic activity in the Nafion membrane was analyzed in terms of the intrinsic activity (kO2 (s-1)...

Activity of hexaammineruthenium(III) complex ([Ru(NH3)6]3+) as a water oxidation catalyst was investigated in a homogeneous aqueous solution (AS) as well as in a heterogeneous Nafion membrane (HM). In the AS, the apparent catalytic activity (kapp/s-1) decreased monotonically with the concentration, which was ascribed to a bimolecular decomposition of the complex at high concentrations. The bimolecular decomposition was suppressed remarkably by incorporating the complex into the membrane. It was shown that 4-electron water oxidation is cooperatively catalyzed by two molecules of the complex in the membrane. The kapp in the HM was much higher than that in the AS at the whole concentration range, which was ascribed both to a favorable interaction for the cooperative catalysis and to the suppression of the bimolecular decomposition by immobilization of the complex. The kapp was analyzed in terms of an intrinsic activity (kO2/s-1) of the complex, cooperative catalysis distance (rco/nm) and critical decompositi...

Activity of a pentaamminechlororuthenium(III) ([Ru(NH3)5Cl]2+) complex as a water oxidation catalyst was investigated in a homogeneous aqueous solution (AS) as well as in a heterogeneous Nafion membrane (HM). In the AS, the apparent catalytic activity (kapp/s-1) decreased monotonically with the concentration. In competition with water oxidation catalysis, dinitrogen (N2) evolution by oxidation of its ammine ligands was also observed, whose rate is second order with respect to its concentration, showing that a bimolecular decomposition of the complexes takes place at high concentrations. The bimolecular decomposition was suppressed remarkably by incorporating the complex into a HM. In the HM, kapp increased with the catalyst concentration at low concentrations below 1.5 × 10-2 M, indicating that four-electron water oxidation is cooperatively catalyzed by two complex molecules. The kapp was strikingly higher for HM than for AS in the whole concentration range, which was ascribed to both a favorable interact...

Active catalysts for water oxidation to evolve O(2) are required for the construction of artificial photosynthetic devices that are expected to be promising energy-providing systems in the future. The citrate-stabilized IrO(2) colloid was self-assembled onto an indium tin oxide (ITO) electrode to form a monolayer of the colloidal IrO(2) particles when it was dipped in the colloid solution. The self-assembly could be achieved by a chemical interaction between carboxylate groups on the citrate stabilizer and hydroxyl groups on the ITO surface to form ester bonds. Efficient electrocatalysis for water oxidation was demonstrated using the electrode modified by the self-assembled IrO(2) colloid to yield the highest turnover frequency ((2.3-2.5) x 10(4) h(-1)) of IrO(2) in the hitherto-reported catalysts for electrochemical water oxidation.

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.

The activity of the electrochemical water oxidation catalyst based on Ru-red incorporated in an electrode-coated Nafion membrane was studied. The complex worked as an active catalyst, and an optimum concentration for the catalyst turnover number (TN) was exhibited. The TN increased with the concentration in low concentrations because of facilitated charge transfer between the catalysts, and the increase in the concentration brought about decreased TN due to bimolecular decomposition of the catalyst. An activity model for the electrochemical catalyst in a membrane was proposed based on intermolecular distance distribution, and the activity was analyzed in terms of a charge transfer distance (r0/nm) and a critical decomposition distance (rd/nm). Their values were obtained as 1.28 and 1.21 nm, respectively.