Masayuki Yagi

Molecular catalysts for water oxidation toward artificial photosynthesis

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.

Crystallization of Tungsten Trioxide Having Small Mesopores: Highly Efficient Photoanode for Visible‐Light‐Driven Water Oxidation

(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

Charge transfer and molecular distribution of Ru(bpy)2+3 complex dispersed in a Nafion® membrane as studied by in-situ spectrocyclic voltammetry

Charge transport through a Nafion® membrane incorporating dispersed Ru(bpy)2+3 (bpy = 2,2′-bipyridine) complex has been investigated by in-situ spectrocyclic voltammetry (SCV) measurements. The ratio RCT of the electrochemically active complexes was obtained from the visible absorption spectral change; it increased with increasing the complex concentration c in the membrane. The distribution of the intermolecular distance was taken into account using a statistical calculation. The relationship between RCT and c was derived on the basis of the center-to-center distance distribution between the nearest-neighbor molecules, and the value of the charge transfer distance R0 was obtained from this relation using a charge-hopping model. The R0 value depended on the scan rate of the potential. At high scan rates, at which bounded motion of the complex is almost negligible, R0 = 0.93 nm was taken as the charge-hopping distance between adjacent redox centers. The R0 value at slow scan rates (e.g. R0 = 1.5 nm at 2 mV s−1) includes charge hopping between adjacent redox centers as well as the bounded motion of the complex.

Charge transfer distance between tris(2,2′-bipyridine) ruthenium(II) redox centers incorporated in Nafion® membrane

Abstract 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.

Catalytic activity of [(bpy)2(H2O)Ru–O–Ru(H2O)(bpy)2]4+ for four-electron water oxidation

Abstract Water oxidation catalysis by [(bpy)2(H2O)Ru–O–Ru(H2O)(bpy)2]4+ (RuIII(OH2)–O–RuIII(OH2)) complex was studied in a homogeneous aqueous solution (AS) as well as heterogeneous Nafion membrane (HM) using Ce(IV) oxidant. The initial O2 evolution rate, VO2 (mol s−1) increased linearly at low complex concentrations under the excess Ce(IV) oxidant, showing that 4-electron water oxidation is catalyzed by one molecule of the complex. The intrinsic catalytic activities, kO2 (s−1), in the AS and HM were 4.2×10−3 s−1 and 2.4×10−3 s−1, respectively. These values are much higher than those of well-known metal and metal oxide catalysts. Comparison of catalytic activity for various metal complexes and oxides is presented.

Activity analysis of a water oxidation catalyst immobilized in a polymer membrane

The activity of a water oxidation catalyst based on a trinuclear Ru complex, Ruthenium Red (Ru-red){[(NH3)5Ru—O—Ru(NH3)4—O—Ru(NH3)5]6+} has been investigated both in a homogeneous aqueous solution (AS) and in a heterogeneous Nafion membrane (HM) using CeIV oxidant. In the AS, at higher concentrations, the oxygen (O2) evolution rate Vo2(mol s–1) decreases with increasing concentrations of Ru-red, but nitrogen (N2) evolves. The N2 evolution shows a bimolecular decomposition of the catalyst at high concentrations. In the HM, the Vo2 does not decrease even when the complex concentration in the membrane is ca. 30 times as high as the AS. A pseudo-first-order rate constant for oxidation of water by the catalyst (ko2/s–1) and a second-order rate constant for deactivation (kdeact/dm3 mol–1 s–1) were obtained. The ko2 values are close for both the AS and HM systems, indicating no significant loss of the activity in the membrane. The kdeact value decreases by an order of two in the membrane, which is ascribed to the suppression of bimolecular decomposition of the catalyst. The apparent activity (kapp/s–1) in the membrane increases upon decreasing the concentration of Ru-red due to suppression of the bimolecular decomposition. The effect of intermolecular distance distribution of the catalyst in the membrane on the catalytic activity has been analysed based on statistical calculations of the distribution. The critical decomposition distance between two adjacent complexes has been determined as 1.23 nm.

Activity analysis of a water oxidation catalyst adsorbed on an inorganic particle matrix

A highly active electrocatalytic system to oxidise water to dioxygen (O2) has been established using electrodeposited platinum black with an adsorbed trinuclear ruthenium complex (Ru-red). The catalyst turnover number for O2 evolution was 1500 h–1, four orders of magnitude greater than that of neat Pt-black. This high activity was ascribed to efficient charge transport from the electrode to the catalyst. The amount of O2 evolved increased almost linearly with the amount of Ru-red for low loadings on Pt-black but decreased after passing an optimum loaded amount. This decrease was ascribed to bimolecular decomposition of the catalysts. The catalyst activity was analysed in terms of critical decomposition distance (rd/nm) by a surface adsorption model (SAM) and a void-space adsorption model (VAM); rd was estimated to be 1.37 nm for the SAM and 1.21 nm for the VAM.

Monoclinic Ag2Mo2O7 nanowire: a new Ag-Mo-O nanophotocatalyst material.

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.

Potential-step chronocoulospectrometry of a polymer membrane incorporating tris(2,2′-bipyridine) ruthenium (II) complex

Abstract 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.

Synthesis of copper nanoparticles within the interlayer space of titania nanosheet transparent films

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.