Tsutomu Kajino

Selective CO2 Conversion to Formate Conjugated with H2O Oxidation Utilizing Semiconductor/Complex Hybrid Photocatalysts

Photoelectrochemical reduction of CO(2) to HCOO(-) (formate) over p-type InP/Ru complex polymer hybrid photocatalyst was highly enhanced by introducing an anchoring complex into the polymer. By functionally combining the hybrid photocatalyst with TiO(2) for water oxidation, selective photoreduction of CO(2) to HCOO(-) was achieved in aqueous media, in which H(2)O was used as both an electron donor and a proton source. The so-called Z-scheme (or two-step photoexcitation) system operated with no external electrical bias. The selectivity for HCOO(-) production was >70%, and the conversion efficiency of solar energy to chemical energy was 0.03-0.04%.

Immobilized enzymes in ordered mesoporous silica materials and improvement of their stability and catalytic activity in an organic solvent

Enzyme immobilization in mesoporous materials with various pore sizes was studied. The size of the mesopores was controlled by means of the combination of the alkyl chain lengths of surfactants and a swelling agent (triisopropyl benzene). Enzymes were selectively adsorbed to FSM-16 and MCM-41 prepared with a cationic surfactant, whose pore sizes were over the molecular diameters of the enzymes, and were not adsorbed significantly to SBA-15 prepared with a non-ionic surfactant. The higher adsorption as to FSM-16 or MCM-41 rather than on SBA-15 may be due to the ionic characteristics of the mesopore, which would be consistent with the observed larger adsorption capacity as to the cationic pigment rather than the anionic pigment of these materials. Enzymes, horseradish peroxidase and subtilisin, immobilized in FSM-16 showed the best stability and peak catalytic activity in an organic solvent when the average mesopore size just exceeded the molecular diameters of the enzymes.

Fructose/dioxygen biofuel cell based on direct electron transfer-type bioelectrocatalysis

One-compartment biofuel cells without separators have been constructed, in which d-fructose dehydrogenase (FDH) from Gluconobacter sp. and laccase from Trametes sp. (TsLAC) work as catalysts of direct electron transfer (DET)-type bioelectrocatalysis in the two-electron oxidation of d-fructose and four-electron reduction of dioxygen as fuels, respectively. FDH adsorbs strongly and stably on Ketjen black (KB) particles that have been modified on carbon papers (CP) and produces the catalytic current with the maximum density of about 4 mA cm(-2) without mediators at pH 5. The catalytic wave of the d-fructose oxidation is controlled by the enzyme kinetics. The location and the shape of the catalytic waves suggest strongly that the electron is directly transferred to the KB particles from the heme c site in FDH, of which the formal potential has been determined to be 39 mV vs. Ag|AgCl|sat. KCl. Electrochemistry of three kinds of multi-copper oxidases has also been investigated and TsLAC has been selected as the best one of the DET-type bioelectrocatalyst for the four-electron reduction of dioxygen in view of the thermodynamics and kinetics at pH 5. In the DET-type bioelectrocatalysis, the electron from electrodes seems to be transferred to the type I copper site of multi-copper oxidases. TsLAC adsorbed on carbon aerogel (CG) particles with an average pore size of 22 nm, that have been modified on CP electrodes, produces the catalytic reduction current of dioxygen with a density of about 4 mA cm(-2), which is governed by the mass transfer of the dissolved dioxygen. The FDH-adsorbed KB-modified CP electrodes and the TsLAC-adsorbed CG-modified CP electrodes have been combined to construct one-compartment biofuel cells without separators. The open-circuit voltage was 790 mV. The maximum current density of 2.8 mA cm(-2) and the maximum power density of 850 microW cm(-2) have been achieved at 410 mV of the cell voltage under stirring.

A Highly Efficient Mononuclear Iridium Complex Photocatalyst for CO2 Reduction under Visible Light

Development of photocatalysts for the reduction of CO2 by sunlight is increasingly becoming an important research area owing to fossil-fuel shortage and global warming. Developing a photosynthetic system that generates solar fuel from CO2, H2O, and sunlight is a promising approach. Photocatalytic systems, including transition-metal complexes such as ruthenium(II) polypyridine carbonyl complexes, cobalt(II) trisbipyridine, and cobalt(II) macrocycles combined with a photosensitizer, can reduce CO2 with a relatively high quantum yield and high product selectivity. Among them, the rhenium(I) bipyridine (bpy) complex systems are the only mononuclear systems that exhibit definite photocatalytic activity for CO2 reduction. A typical example is fac-[Re(bpy)(CO)3Cl], developed by Lehn, which reduces CO2 to CO under UV irradiation without any additional photosensitizers. Cobalt porphyrins can also act as a CO2 reduction catalyst without a photosensitizer. [3c] A fac-[Re(bpy)(CO)3{P(OEt)3}] + complex is an efficient photocatalyst for CO2 reduction in a homogeneous system that selectively produces CO with a quantum yield of 0.38 at the ultraviolet light irradiation of 365 nm. However, the compound must be modified to allow effective use of solar energy because its absorption in the visible region is limited to wavelengths less than 440 nm. Thus, activation of highly active Re complex photocatalysts toward the visible region is necessary. Furthermore, the photocatalytic activity for CO2 reduction is very low in the presence of H2O, even at a concentration of 10 %. Therefore, for CO2 reduction, the development of metal complex photocatalysts that operate under visible light irradiation, even in the presence of H2O, is desirable. In Ir complexes, the stronger spin–orbit coupling coordinates with singlet and triplet excited states, leading to efficient luminescence and visible-light absorption from the singlet–triplet transition. Therefore, Ir complexes have been used as an emitter for electroluminescence devices, a photosensitizer for photocatalytic reactions, and a light-absorber for Gr tzel solar cells. Recently, it was reported that an Ir complex acted as a water oxidation catalyst with a sacrificial electron accepter and a CO2 reduction catalyst with an electronical bias or in the presence of hydrogen. Although Ir complexes are considered suitable for photocatalysis owing to visible-light absorption from S–T transitions and a longer lifetime of the excited state, no studies on Ir complex photocatalysts for CO2 reduction have been reported. This report describes the successful development of a novel photocatalyst, mononuclear iridium(III) terpyridine (tpy) 2-phenylpyridine (ppy) complex [Ir(tpy)(ppy)Cl] ([Irppy]), which selectively reduced CO2 to CO under visible light at 480 nm without additional photosensitizers such as in the case for Re complexes. Furthermore, advantages of the Ir complexes over Re complexes include: 1) greater photocatalytic activity for CO2 reduction; 2) CO2 reduction under visible light, such as at a wavelength of 480 nm; and 3) the photocatalytic activity is maintained (including selectivity) even in a solution containing H2O. [Ir-ppy] catalyzed the reduction of CO2 molecules to CO under visible-light irradiation. Figure 1 shows the photocatalytic formation of CO over [Ir-ppy] compared with conventional [Re(bpy)(CO)3Cl] under visible light irradiation

Photoelectrochemical reduction of CO2 in water under visible-light irradiation by a p-type InP photocathode modified with an electropolymerized ruthenium complex

Photoelectrochemical reduction of CO(2) to HCOO(-) was successfully achieved by a p-type InP photocathode modified with an electropolymerized ruthenium complex in water. This technique decreased the required applied potential for CO(2) reduction by utilizing solar energy. The carbon and proton sources of HCOO(-) were identified by a tracer experiment to be CO(2) and H(2)O, respectively.

The Multi-layered Structure of Dps with a Novel Di-nuclear Ferroxidase Center

The crystallization of cellular components represents a unique survival strategy for bacterial cells under stressed conditions. A highly ordered, layered structure is often formed in such a process, which may involve one or more than one type of bio-macromolecules. The main advantage of biocrystallization has been attributed to the fact that it is a physical process and thus is independent of energy consumption. Dps is a protein that crystallizes to form a multi-layered structure in starved cells in order to protect DNA against oxidative damage and other detrimental factors. The multi-layered crystal structure of a Dps protein from Bacillus brevis has been revealed for the first time at atomic resolution in the absence of DNA. Inspection of the structure provides the first direct evidence for the existence of a di-nuclear ferroxidase center, which possesses unique features among all the di-iron proteins identified so far. It constitutes the structural basis for the ferroxidase activity of Dps in the crystalline state as well as in solution. This finding proves that the enzymatic process of detoxification of metal ions, which may cause severe oxidative damage to DNA, is the other important aspect of the defense mechanism performed by Dps. In the multi-layered structure, Dps dodecamers are organized in a highly ordered manner. They adopt the classic form of hexagonal packing in each layer of the structure. Such arrangement results in reinforced structural features that would facilitate the attraction and absorption of metal ions from the environment. The highly ordered layered structure may provide an ideal basis for the accommodation of DNA between the layers so that it can be isolated and protected from harmful factors under stress conditions.

Selective CO2 conversion to formate in water using a CZTS photocathode modified with a ruthenium complex polymer

Highly selective photoelectrochemical CO(2) reduction (>80% selectivity) in water was successfully achieved by combining Cu(2)ZnSnS(4) (CZTS) with a metal-complex electrocatalyst. CZTS, a sulfide semiconductor that possesses a narrow band gap and consists of earth-abundant elements, is demonstrated to be a candidate photoabsorber for a CO(2) reduction hybrid photocatalyst.

New Pulp Biobleaching System Involving Manganese Peroxidase Immobilized in a Silica Support with Controlled Pore Sizes

ABSTRACT Attempts have been made to use manganese peroxidase (MnP) for chlorine-free pulp biobleaching, but they have not been commercially viable because of the enzymes low stability. We developed a new pulp biobleaching method involving mesoporous material-immobilized manganese peroxidase from Phanerochaete chrysosporium. MnP immobilized in FSM-16, a folded-sheet mesoporous material whose pore size is nearly the same as the diameter of the enzyme, had the highest thermal stability and tolerance to H2O2. MnP immobilized in FSM-16 retained more than 80% of its initial activity even after 10 days of continuous reaction. We constructed a thermally discontinuous two-stage reactor system, in which the enzyme (39°C) and pulp-bleaching (70°C) reactions were performed separately. When the treatment of pulp with MnP by means of the two-stage reactor system and alkaline extraction was repeated seven times, the brightness of the pulp increased to about 88% within 7 h after completion of the last treatment.

Visible light-sensitive mesoporous N-doped Ta2O5 spheres: synthesis and photocatalytic activity for hydrogen evolution and CO2 reduction

Crystallized mesoporous tantalum pentoxide spheres (CMTS) with particle diameters of ca. 100–500 nm and composed of Ta2O5 nanocrystals were synthesized for the first time by a combination of the sol–gel process and heat-treatment with the aid of carbon reinforcement. The specific surface area of the CMTS was up to 105 m2 g−1 and the pore diameter was controllable in the range of 5.6–17 nm by changing the crystallization temperature. Visible light-sensitive p-type N-doped Ta2O5 (N-CMTS) containing 5 at% N was successfully obtained by treatment of CMTS with ammonia, while retaining the mesoporosity and morphology of CMTS. N-CMTS exhibited excellent photocatalytic activity for hydrogen evolution and CO2 reduction (with ruthenium-complex) under visible light irradiation (≥410 nm) due to their larger surface area and controlled morphology compared with previously reported N-doped Ta2O5 fine particles.

Dual functional modification by N doping of Ta2O5: p-type conduction in visible-light-activated N-doped Ta2O5

We report dual functional modulation, both p-type conduction and band gap narrowing, of Ta2O5 semiconductor induced by heavy doping of nitrogen in films sputtered in N2/Ar mixture and ammonia-treated powders. The N doping induced a redshift in the optical absorption edge from 320 to 500 nm, resulting in the absorption of visible light. Simultaneously, the N doping caused a change in the conduction from n-type to p-type. As a result, the N–Ta2O5 photoelectrode containing 7.6 or 16.1 at. % of N exhibited a distinct cathodic photocurrent (due to p-type conduction) in solutions under visible light irradiation (>410 nm).