O2 Evolution Rate Research Articles

Colloidal Synthesis of Ultrathin Monoclinic BiVO4 Nanosheets for Z-Scheme Overall Water Splitting under Visible Light

Recently, ultrathin 2D photocatalysts have attracted people’s attention due to their performances in the area of solar energy conversion. However, the synthesis of ultrathin 2D photocatalysts with a nonlayered crystal structure is still full of challenges. Herein, ultrathin 2D BiVO4 nanosheets (NSs) with monoclinic crystal structure are synthesized through a convenient colloidal two-phase method. The as-prepared BiVO4 NSs possess a thickness of less than 3 nm but a diameter larger than 1.2 μm. Furthermore, the presence of HNO3 facilitates the growth of BiVO4 NSs with nearly naked surfaces, largely exposed {010} planes, and widely distributed oxygen vacancies (VO) inside the crystalline structure, which are of great benefit to their photocatalytic activity under visible light irradiation. As a result, our ultrathin 2D BiVO4 NSs exhibit an impressive photocatalytic performance for water oxidation. The O2 evolution rate is 107.4 μmol h–1, and the apparent quantum yield (AQY) is as high as 26.1% (420 nm). Fur...

Crystallographic Controls of Polycrystalline BaNbO2n Particles Using Flux Method and Their Photocatalytic Water Oxidation Ability

Nowadays, development of energy conversion from natural sources is essential issue for construction of sustainable society and photocatalytic water oxidation using sun light is one of desirable processes to achieve it. For effective use of sun light, it is necessary to utilize visible light, since the solar provides over 50 % of visible light, but only 5 % of UV light in its whole energy. (Oxy)nitrides are promising candidates for visible-light response photocatalysts. BaNbO2N is one of oxynitrides as photocatalyst, which belongs to cubic-type perovskite. BaNbO2N exhibits superior visible-light absorption up to ca. 740 nm (band gap energy: ca. 1.7 eV) compared to other (oxy)nitrides: For example, the absorption edges are 700 nm for SrNbO2N, 650 nm for LaTaO2N and BaTaO2N, and 600 nm for LaTiO2N. Previous reports about BaNbO2N indicated that the synthetic conditions affect to the photocatalytic performance, while there is still plenty of space to further improve the efficiency. The photocatalytic performance is known to depend on a variety of crystallographic characteristics. Specific surface area, crystal habits, qualities of crystallinity, and deficiencies are the representative factors. To improve the photocatalytic performance of BaNbO2N in terms of crystallographic viewpoints, a precise understanding of the contribution of each factor to the performance would be inevitable. Herein, we prepared BaNbO2N particles with different crystallographic characteristics by controlling morphology of precursor crystals as sacrificial templates grown in a molten flux. The samples’ particle characteristics and their photocatalytic water oxidation abilities were examined to discuss the contribution of crystallographic factors to the photocatalytic performances. BaNbO2N particles were prepared through the following 2-step procedure. At first, Ba5Nb4O15 crystals were grown in molten BaCl2 flux at a solute concentration of 5 mol% at designated temperatures for 10 h. In all temperature conditions, single phases of Ba5Nb4O15 were obtained as primary particles and their sizes increased from 0.2 to 50 μm, as holding temperature increased. Subsequently, they were heated in NH3 atmosphere at 950 ºC for 10 h for nitridation. The each obtained product was identified to single-phase BaNbO2N. Looking at their morphology, segregations to porous secondary particles were found, while maintaining their initial morphologies in the secondary particles. The particle characteristics were investigated in terms of specific surface area, crystallinity, Nb states on the surface, anion deficiency, and band gap energy. Finally, we examined their photocatalytic performances by measuring photo-irradiated O2 evolution from water. It was found that O2 evolution rates differs between samples up to 8 times, depending on the precursor particle size and nitriding time. After comparing crystallographic characteristics of each BaNbO2N particle, we propose competitive effects of crystallinity and anion deficiency to dominantly determine the photocatalytic performances. The detail discussion about the effects should be introduced in terms of quantitative evaluations of the crystallographic factors in the presentation.

Electrospinning preparation of Sn4+-doped BiFeO3 nanofibers as efficient visible-light-driven photocatalyst for O2 evolution

Metal ion doping has been regarded as an effective strategy to improve photocatalytic activity of pristine photocatalysts for rapid charge transfer and separation effects achieved by doping induced defect states. We herein report the fabrication of uniform one-dimensional (1D) Sn4+-doped BiFeO3 (Sn-BFO) nanofibers (NFs) by a facile electrospinning technique with subsequent annealing treatment. Benefiting from metal ion doping and the unique 1D structure, the as-prepared Sn-BFO NF catalyst exhibits significantly enhanced photocatalytic performance on O2 evolution as well as improved photostability under visible-light illumination. Revealed by experimental investigation as well as density functional theory (DFT) calculation, the doped Sn atoms were incorporated into the BFO lattice and the band structure of BFO was effectively modified and the defect-related energy levels were produced, which leads to the boosted light absorption and improved charge transfer and separation efficiency. Additionally, an optimal Sn4+ doping percentage of 1.0% was discovered to achieve an average O2 evolution rate of 516.4 μmol h−1 g−1, nearly two times as much as the non-doped BFO samples.

Synthesis, Structure, and Water Oxidation Activity of Ruthenium(II) Complexes: Influence of Intramolecular Redox Process on O2 Evolution

The synthesis and characterization of two new RuII complexes, a mononuclear complex [RuII(MeMPTP)(bpp)Cl]PF6 (1) and a dinuclear complex [RuII(MeMPTP)(bpp)RuII(bpy)2Cl](PF6)3 (2), [where MeMPTP = 4′‐(4‐methylmercaptophenyl)‐2,2′:6′2′′‐terpyridine, bpp = 2,3‐bis(2‐pyridyl)pyrazine, bpy = 2,2′‐bipyridine] is reported. Single‐crystal X‐ray structural determination of the compounds unambiguously confirms the formation of the complexes 1 and 2. Catalytic investigations of the complexes for the chemical oxidation of water in the presence of Ce4+ as a sacrificial oxidant reveals a higher rate of O2 evolution by the dinuclear complex 2 over the physical mixture of complex 1 and a redox mediator, [Ru(bpy)3]2+ (med), followed by the mononuclear complex 1. The enhancement in the catalytic rate of O2 evolution by 2 is ascribed to being due to the intramolecular redox process, which facilitates the facile formation of a high‐valent ruthenium–oxo species that is required for water oxidation through a cooperative effect. The present study highlights the importance of the intramolecular redox process over the intermolecular redox process.

Correlation of carbonic anhydrase and Rubisco in the growth and photosynthesis in the diatom Phaeodactylum tricornutum

Carbonic anhydrase (CA) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) are critical enzymes involved in the CO2-concentrating mechanism (CCM) and carbon fixation in phytoplankton. These enzymes play key roles in photosynthetic carbon acquisition and fixation in diatoms. However, the potential synergistic interactions of the said enzymes remain little understood. In the present study, effects of CA inhibitors on the growth, photosynthetic rate, and Rubisco activities in the diatom Phaeodactylum tricornutum were investigated. Our results showed that the growth and photosynthetic O2 evolution rate of P. tricornutum were significantly inhibited by the intracellular CA (iCA) inhibitor ethoxzolamide (EZ), but not by extracellular CA (eCA) inhibitor acetazolamide (AZ) in the long term. This suggests that the eCA was not necessarily essential for growth and photosynthesis, but rather it was the iCA activity that was more crucial in P. tricornutum. We also found that there is a significant positive correlation between Rubisco and CA activities, suggesting that the two carbon acquisition and fixation components of photosynthesis may have undergone co-evolution to optimize fundamental trade-offs. These results further indicate that the roles of eCA and iCA may also differ among diatom species or strains, and that their functions are tightly correlated with other carbon fixation components.

Co@Co3O4 Prepared in Situ from Metallic Co as an Efficient Semiconductor Catalyst for Photocatalytic Water Oxidation

This paper reported the first attempt of using Co@Co3O4 core–shell nanoparticles obtained in situ from a metallic Co precursor as a highly active and stable catalyst for the photocatalytic water oxidation. Co nanoparticle precursor was prepared through a hydrothermal process. The components of precursor and catalyst were confirmed by multiple measurements (X-ray diffraction, field emission scanning electron microscopy, scanning transmission electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, line scanning analysis, UV–vis diffuse reflectance spectroscopy, Mott–Schottky curve). The Co@Co3O4 semiconductor catalyst exhibited excellent activity for the photocatalytic water oxidation without any addition of photosensitizer or cocatalyst, with an average O2 evolution rate of 2778 μmol h–1 g–1, and the Co@Co3O4 maintained 90% of the initial activity even after the sixth run; its oxygen evolution reaction performance under λ = 600 and 765 nm still remained 16% ...

Comparative phytotoxicity of usnic acid, salicylic acid, cinnamic acid and benzoic acid on photosynthetic apparatus of Chlamydomonas reinhardtii

The effects of four phytotoxins usnic acid (UA), salicylic acid (SA), cinnamic acid (CA) and benzoic acid (BA) on photosynthesis of Chlamydomonas reinhardtii were studied in vivo to identify and localise their initial action sites on two photosystems. Our experimental evidence shows that the four phytotoxins have multiple targets in chloroplasts, which mainly lie in photosystem II (PSII), not photosystem I (PSI). They share an original action site by blocking electron transport beyond QA (primary plastoquinone acceptor) at PSII acceptor side since a fast increase of the J-step level is the greatest change in chlorophyll a fluorescence induction kinetics OJIP in C. reinhardtii cells treated with the phytotoxins. UA decreases photosynthetic activity by reducing O2 evolution rate, interrupting PSII electron transport at both the donor and acceptor sides, inactivating the PSII reaction centers (RCs), reducing the content of chlorophylls and carotenoids, destroying the conformation of antenna pigment assemblies, and casuing the degradation of D1/D2 proteins. SA damage to photosynthetic machinery is mainly attributed to inhibition of PSII electron transport beyond QA at the acceptor side, inactivation of the PSII RCs, reduction of chlorophyll content, digestion of thylakoid ploypeptides and destabilization of thylakoid membranes. Both CA and BA affect the photosynthetic process by decreasing PSII electron transport efficiency at the acceptor side and the amount of active PSII RCs. Besides, the initial cause of BA-inhibiting photosynthesis is also assocaited with the O2 evolution rate and the disconnection of some antenna molecules from PSII RCs.

X-Shaped α-FeOOH with Enhanced Charge Separation for Visible-Light-Driven Photocatalytic Overall Water Splitting.

Photocatalytic overall water splitting (POWS) is a promising route for converting solar energy into green and sustainable energy. Herein, we report a facile hydrothermal approach for the fabrication of x-shaped α-FeOOH photocatalysts containing high-index facets for POWS. The x-shaped α-FeOOH photocatalysts exhibited enhanced visible-light-driven POWS activities in comparison with that of FeOOH without x-structures, with a maximum H2 and O2 evolution rate of 9.2 and 4.7 μmol h-1 g-1 , respectively. The morphology and particle size of the α-FeOOH could be controlled by adjusting the NH4 F concentration in the precursors. The photodeposition of Pt and RuO2 on the x-shaped α-FeOOH revealed the specially separated reduction and oxidation centers on the surface of α-FeOOH, with the oxidation-active sites selectively located on the edges of the α-FeOOH x-structures. Electrochemical experiments further affirmed the enhanced charge separation in the x-shaped α-FeOOH. The smaller particle size and unique x-shape of the α-FeOOH photocatalyst were shown to enhance the POWS performance owing to the large specific surface area, high proportion of exposed high-index facets, high electron-transfer efficiency and effective separation of the photogenerated electron-hole pairs. The current study revealed that the x-shaped α-FeOOH products could serve as cost-effective and stable photocatalysts for POWS.

Improved visible-light activity of nitrogen-doped layered niobate photocatalysts by NH3-nitridation with KCl flux

In the present study, nitrogen doping into layered perovskite niobites, KCa2Nan–3NbnO3n+1 (n = 3 or 4), was attempted to enhance their response to visible light during photocatalytic water reduction and oxidation. Although conventional nitridation, i.e., heating in NH3 stream at a high temperature, produced black-colored samples due to the undesirable reduction of Nb5+ to Nb4+ (and/or Nb3+), the use of KCl flux with NH3 stream effectively prevented the reduction of Nb5+, resulting in yellow-colored samples that had a much lower concentration of reduced Nb species. The samples prepared with KCl flux exhibited stronger absorption (at λ = 350–550 nm) when compared to those prepared without KCl flux. This was due to the higher concentration of N3– anions, as confirmed by elemental analysis and the observed peak shift in XRD patterns. On the other hand, the absorption band with λ > 550 nm, which indicated the presence of reduced species or anion vacancies, were astonishingly smaller for the samples prepared with KCl than for those without KCl. This indicated that the formation of reduced Nb species was inhibited by KCl. XPS analysis also supported this effect. The use of KCl flux also prevented the problematic decrease in the concentration of potassium cations in the interlayer space during high-temperature treatment in NH3 flow. The samples prepared with KCl flux exhibited much higher O2 and H2 evolution rates under visible-light irradiation in the presence of an electron acceptor and a donor, respectively as compared to those prepared without KCl. This clear improvement in O2 and H2 evolution was probably due to the larger amount of doped nitrogen introduced and/or the suppressed formation of reduced Nb species.

Variation in cell size of the diatom Coscinodiscus granii influences photosynthetic performance and growth.

Cell size has implications for the package effect in photon absorption as well as for metabolic scaling of metabolism. In this study, we have avoided species-related differences by using isolates of the marine planktonic diatom Coscinodiscus granii with cells of different sizes and grown at different light intensities to investigate their energy allocation strategies. To make full use of incident light, several fold variations in cellular chlorophyll a content were employed across cell size. This modulation of pigment-related light absorbance was deemed effective as similar light absorbing capacities were found in all treatments. Unexpected low values of O2 evolution rate at the highest irradiance level of 450μmol photons m-2s-1 were found in medium and large cells, regardless of more photons being absorbed under these conditions, suggesting the operation of alternative electron flows acting as electron sinks. The growth rate was generally larger at higher irradiance levels except for the large cells, in which growth slowed at 450μmol photons m-2s-1, suggesting that larger cells achieved a balance between growth and photoprotection by sacrificing growth rate when exposed to high light. Although the ratio of carbon demand to rates of uncatalysed CO2 diffusion to the cell surface reached around 20 in large cells grown under higher irradiance, the carbon fixation rate was not lowered, due to the presence of a highly effective carbon dioxide concentrating mechanism.

Liquid-Phase Measurements of Photosynthetic Oxygen Evolution.

This chapter compares two different techniques for monitoring photosynthetic O2 production: the widespread Clark-type O2 electrode and the more sophisticated membrane inlet mass spectrometry (MIMS) technique. We describe how a simple membrane inlet for MIMS can be made out of a commercial Clark-type cell, and outline the advantages and drawbacks of the two techniques to guide researchers in deciding which method to use. Protocols and examples are given for measuring O2 evolution rates and for determining the number of chlorophyll molecules per active photosystem II reaction center.

CoO and g-C3N4 complement each other for highly efficient overall water splitting under visible light

Photocatalytic hydrogen production from overall water splitting is a clean and renewable technology that can convert solar energy into chemical energy, for which developing an efficient and stable photocatalyst has been the central scientific topic. Herein, CoO/g-C3N4 heterojunction photocatalysts were fabricated through a facile solvothermal method for overall water splitting. Simultaneous evolution of H2 and O2 from pure water with the stoichiometric ratio of about 2:1 achieved with all the CoO/g-C3N4 heterojunctions as catalysts under visible light irradiation. Among of them, 30 wt.% CoO/g-C3N4 with H2 evolution rate of 2.51 μmol/h and O2 evolution rate of 1.39 μmol/h also exhibited remarkably higher photocatalytic performance and stability (over 15 cycles) than single CoO or g-C3N4. This enhanced photocatalytic activity of CoO/g-C3N4 heterojunction can be ascribed to the synergistic effect of junction and interface formed between CoO and g-C3N4. In addition, the sufficient long lifetime stability of CoO/g-C3N4 comes from the complementary advantages effect between CoO and g-C3N4, as is proved, CoO can protect g-C3N4 from H2O2 poisoning, and simultaneously the photo-induced heat from CoO during the photocatalytic process responsible for the rapid deactivation can be timely conducted to g-C3N4.

CoMn-S/CDs nanocomposite for effective long wavelength visible-light-driven photocatalytic water splitting

Photocatalytic water splitting to produce hydrogen is one of the important ways to solve energy crisis. Most transition metal sulfide semiconductors which have wide optical band gap are impractical for long wavelength visible light driven water splitting. Therefore, modulation of broad band gap has become a significant strategy and a huge challenge to achieve efficient catalyst for photocatalytic water splitting. Here, we report the synthesis of a ternary photocatalyst MnS/CoS2/CDs applied for long wavelength visible-light-driven water splitting without the requirement for any sacrificial agents. The optimal MnS/CoS2/CDs nanocomposite (CoMn-S/CDs) as an effective photocatalyst exhibits excellent photocatalytic activity with H2 evolution rate of 0.421 μmol/h and O2 evolution rate of about 0.051 μmol/h under visible light irradiation (420 nm ≤ λ ≤ 700 nm) when the concentration of CDs is 0.039 gCDs/gcatalyst. In particular, the H2 evolution rate is 0.076 μmol/h and O2 evolution rate is about 0.010 μmol/h at λ = 535 nm while the H2 evolution rate is 0.024 μmol/h and O2 evolution rate is about 0.004 μmol/h at λ = 630 nm. The efficient photocatalytic activity may attribute to the synergistic effect of the ternary sturctures, in which MnS is the major component for photocatalysis, addition of CoS2 component enables the photocatalyst to capture light of wider range and CDs provide reduction sites.

Suitability of Contact-Free Oxygen Optical Microsensors for Measuring Respiration and Photosynthesis in Green Algae

Oxygen optical microsensors are a sensitive method to monitor oxygen production and consumption in soils, sediments and aquatic systems. They have been used widely to analyze the activity and metabolism of aerobic organisms, also in ecotoxicological tests. We aimed to assess the suitability of a contact-free device to measure cell respiration and photosynthesis for future applications in ecotoxicological tests. One of the most important advantages of this modified method is that respiration and photosynthesis of test organisms which are contaminated with test chemicals can be measured without contact between sensor and test medium. Therefore, it avoids sensor contamination. In an array of calibration tests with Chlorella vulgaris in green algae medium, algae cell activity was well correlated with cell counts. Results clearly showed that, compared to O2 evolution rate, O2 assimilation rate measured by oxygen optical microsensors in a contact-free manner could better predict the algae cell counts. In a second test series we measured O2 assimilation rate in soil from a field experiment inoculated with different communities of terrestrial algae. No significant difference was observed when comparing measurements of their activity with microsensors to results obtained with the Warburg respiratory manometer. However, optical microsensors are much faster and more easily applied than the traditional manometer. Therefore, the developed method appears promising for application in ecotoxicological tests in the future.

A Carbon Dioxide Limitation-Inducible Protein, ColA, Supports the Growth of Synechococcus sp. PCC 7002.

A limitation in carbon dioxide (CO2), which occurs as a result of natural environmental variation, suppresses photosynthesis and has the potential to cause photo-oxidative damage to photosynthetic cells. Oxygenic phototrophs have strategies to alleviate photo-oxidative damage to allow life in present atmospheric CO2 conditions. However, the mechanisms for CO2 limitation acclimation are diverse among the various oxygenic phototrophs, and many mechanisms remain to be discovered. In this study, we found that the gene encoding a CO2 limitation-inducible protein, ColA, is required for the cyanobacterium Synechococcus sp. PCC 7002 (S. 7002) to acclimate to limited CO2 conditions. An S. 7002 mutant deficient in ColA (ΔcolA) showed lower chlorophyll content, based on the amount of nitrogen, than that in S. 7002 wild-type (WT) under ambient air but not high CO2 conditions. Both thermoluminescence and protein carbonylation detected in the ambient air grown cells indicated that the lack of ColA promotes oxidative stress in S. 7002. Alterations in the photosynthetic O2 evolution rate and relative electron transport rate in the short-term response, within an hour, to CO2 limitation were the same between the WT and ΔcolA. Conversely, these photosynthetic parameters were mostly lower in the long-term response of a few days in ΔcolA than in the WT. These data suggest that ColA is required to sustain photosynthetic activity for living under ambient air in S. 7002. The unique phylogeny of ColA revealed diverse strategies to acclimate to CO2 limitation among cyanobacteria.

Quaternary Wurtzitic Nitrides in the System ZnGeN2–GaN: Powder Synthesis, Characterization, and Potentiality as a Photocatalyst

We developed a new quaternary wurtzitic nitride system by formation of the solid solution between ZnGeN2 and GaN. Near stoichiometric and monophasic powder samples in the composition Zn1–xGe1–xGa2xN2 (x ≤ 0.50) were obtained by the reduction–nitridation synthesis conducted at 900 °C. The results of crystal structure refinement clearly revealed that the cation ordering in the structure of ZnGeN2 (Pna21) tends to disappear by introducing Ga into the lattice, and the structure transforms to a simple wurtzite phase (P63mc) with the composition of x ≥ 0.33. The observed structural evolution was further confirmed by the results of 71Ga solid-state nuclear magnetic resonance (NMR) spectroscopy, showing an unsplit single peak observed for x ≥ 0.33. The dissolution of GaN into ZnGeN2 also resulted in a marked narrowing of the band gap, from the ultraviolet region of 3.42 eV to the visible-light range of 3.02–3.05 eV, depending scarcely on the value of x. The results of photocatalytic test reactions for water splitting showed that the synthesized Zn1–xGe1–xGa2xN2 solid solution possessed the H2 evolution rate of 2.8–3.6 μmol/h and the relatively high O2 evolution rate of 100.4–126.6 μmol/h, as well as the capability for overall water splitting under the visible-light irradiation of λ > 400 nm.

Creation of a 3Mn/1Fe cluster in the oxygen-evolving complex of photosystem II and investigation of its functional activity

Extraction of Mn cations from the oxygen-evolving complex (OEC) of Ca-depleted PSII membranes (PSII[-Ca,4Mn]) by reductants like hydroquinone (H2Q) occurs with lower efficiency at acidic pH (2Mn/reaction center [RC] are extracted at pH5.7) than at neutral pH (3Mn/RC are extracted at pH6.5) [Semin et al. Photosynth. Res. 125 (2015) 95]. Fe(II) also extracts Mn cations from PSII(-Ca,4Mn), but only 2Mn/RC at pH6.5, forming a heteronuclear 2Mn/2Fe cluster [Semin and Seibert, J. Bioenerg. Biomembr. 48 (2016) 227]. Here we investigated the efficiency of Mn extraction by Fe(II) at acidic pH and found that Fe(II) cations can extract only 1Mn/RC from PSII(-Ca,4Mn) membranes at pH 5.7, forming a 3Mn/1Fe cluster. Also we found that the presence of Fe cations in a heteronuclear cluster (2Mn/2Fe) increases the resistance of the remaining Mn cations to H2Q action, since H2Q can extract Mn cations from homonuclear Mn clusters of PSII(-Ca,4Mn) and PSII(-Ca,2Mn) membranes but not from the heteronuclear cluster in PSII(-Ca,2Mn,2Fe) membranes. H2Q also cannot extract Mn from PSII membranes obtained by incubation of PSII(-Ca,4Mn) membranes with Fe(II) cations at pH5.7, which suggests the formation of a heteronuclear 3Mn/1Fe cluster in the OEC. Functional activity of PSII with a 3Mn/1Fe cluster was investigated. PSII preparations with a 3Mn/1Fe cluster in the OEC are able to photoreduce the exogenous electron acceptor 2,6-dichlorophenolindophenol, possibly due to incomplete oxidation of water molecules as is the case with PSII(-Ca,2Mn,2Fe) samples. However, in the contrast to PSII(-Ca,2Mn,2Fe) samples PSII(-Ca,3Mn,1Fe) membranes can evolve O2 at a low rate in the presence of exogenous Ca2+ (at about 27% of the rate of O2 evolution in native PSII membranes). The explanation for this phenomenon (either water splitting and production of molecular O2 by the 3Mn/1Fe cluster or apparent O2 evolution due to minor contamination of PSII(3Mn,1Fe) samples with PSII(-Ca,4Mn) membranes) is discussed.

Characterization of a heat-tolerant Chlorella sp. GD mutant with enhanced photosynthetic CO2 fixation efficiency and its implication as lactic acid fermentation feedstock

BackgroundFermentative production of lactic acid from algae-based carbohydrates devoid of lignin has attracted great attention for its potential as a suitable alternative substrate compared to lignocellulosic biomass.ResultsA Chlorella sp. GD mutant with enhanced thermo-tolerance was obtained by mutagenesis using N-methyl-N′-nitro-N-nitrosoguanidine to overcome outdoor high-temperature inhibition and it was used as a feedstock for fermentative lactic acid production. The indoor experiments showed that biomass, reducing sugar content, photosynthetic O2 evolution rate, photosystem II activity (Fv/Fm and Fv′/Fm′), and chlorophyll content increased as temperature, light intensity, and CO2 concentration increased. The mutant showed similar DIC affinity and initial slope of photosynthetic light response curve (α) as that of the wild type but had higher dissolved inorganic carbon (DIC) utilization capacity and maximum photosynthesis rate (Pmax). Moreover, the PSII activity (Fv′/Fm′) in the mutant remained normal without acclimation process after being transferred to photobioreactor. This suggests that efficient utilization of incident high light and enhanced carbon fixation with its subsequent flux to carbohydrates accumulation in the mutant contributes to higher sugar and biomass productivity under enriched CO2 condition. The mutant was cultured outdoors in a photobioreactor with 6% CO2 aeration in hot summer season in southern Taiwan. The harvested biomass was subjected to separate hydrolysis and fermentation (SHF) for lactic acid production with carbohydrate concentration equivalent to 20 g/L glucose using the lactic acid-producing bacterium Lactobacillus plantarum 23. The conversion rate and yield of lactic acid were 80% and 0.43 g/g Chlorella biomass, respectively.ConclusionsThese results demonstrated that the thermo-tolerant Chlorella mutant with high photosynthetic efficiency and biomass productivity under hot outdoor condition is an efficient fermentative feedstock for large-scale lactic acid production.

Nature of surface oxygen intermediates on TiO2 during photocatalytic splitting of water

Titanium dioxide (TiO2) is the most widely studied solid photocatalyst, but when applied to photocatalytic splitting of water to H2 and O2, the evolution rate of H2 is low and decreases with reaction time. The origin of the decreasing evolution rate for the photocatalytic splitting of water was investigated for the first time by directly monitoring the surface species on TiO2 during water photocatalysis with in situ attenuated total reflectance (ATR)-Fourier transform infrared (FTIR) spectroscopy. The in situ ATR-FTIR spectroscopic analysis during UV illumination of TiO2 immersed in water reveals that surface dioxygen and hydroxyl species are formed on TiO2: charged TiOOH−, peroxo Ti(O2)2−, and bridging Ti(OH+)Ti groups. The accumulation of these surface oxygenated species on the TiO2 photocatalyst blocks the activation of H2O on the surface titania sites and is responsible for the decreasing H2 evolution rate and absence of O2 evolution.

(Invited) Photon-Density Dependence of Photocatalysis: Can Standard Electrode Potential be Applied to Particulate Photocatalysis?

For heterogeneous photocatalysis, it is well known that photocatalytic oxidation of water into oxygen (O2) is induced by particulate photocatalysts in the presence of electron acceptors. This reaction has been presumed to proceed through a four-electron (hole) process, and it is thereby expected that reaction rate and efficiency depend on irradiation-light intensity and particle size of a photocatalyst, which influence the number of photons absorbed by one photocatalyst particle. In general, for the improvement of reaction efficiency, the reaction of electron-hole pairs such as enhancing charge separation and decreasing recombination has been focused, but photoabsorption, the first step of photocatalytic reaction, has been negligibly discussed so far. In this study, we analyzed light-intensity and particle–size dependences of rate of photocatalytic O2 evolution using commercial titania powders in the presence of iodate (IO3 - ) or iron(III) (Fe3+) ion as an electron acceptor for clarification of correlation of reaction rate with photoabsorption. Photoirradiation on titania suspensions (30 mg in 3.0-mL deaerated aqueous electron-acceptor solutions in a quartz cell) was performed using high light-intensity UV-LEDs (365 nm; NS Lighting ULEDN-101 and Hamamatsu Photonics L11921-400) and an aperture (1.0 cm square) with overall intensity up to ca. 340 mW. The observed light-intensity dependence of rate with small-size (4.4 nm) anatase titania as a photocatalyst using IO3 - as an electron acceptor, in the relatively lower intensity region, was second (< 100 mW) and first-order (< 280 mW) as was also seen in the reactions with small-size (13 nm) rutile titania even in the cases in which Fe3+ or IO3 - was used as an electron acceptor. This dependence is attributable to two-electron process (to yield hydrogen peroxide) and the linear (first-order) dependence at the middle-intensity region might be due to the sufficiently high intensity to guarantee the two-photon absorption by one photocatalyst particle. At the further higher intensity (> ca. 280 mW), on the other hand, fourth-order dependence appeared. Since the electrode potential of four-electron reaction (+1.23 V) is lower than two-electron reaction (+1.77 V), it was presumed that the probability of four positive-hole accumulation in one particle under high-intensity irradiation became appreciable to induce four-electron transfer to water. On the other hand, large-size (ca. 350 nm) rutile titania showed only first-order light-intensity dependence only in the whole range of light intensity, suggesting sufficient accumulation of positive holes in the particles. These results suggest that multi-electron reaction mechanism depends on the density of electron (hole) in one photocatalyst particle, i.e., the rate is digitally controlled when size of photocatalyst particles is small [1]. It is, therefore, expected that the efficiency of photocatalytic multielectron-transfer reactions can be improved so much by only focusing the irradiation beam without changing the overall intensity. At the same time, it is suggested that for heterogenous photocatalysis using particulate semiconductors it is not straightforword to predict the possiblity of reaction on the basis of standard electrode potentials and band position assuming the guaranteed multielectron transfer. [1] S. Takeuchi, M. Takashima, M. Takase and B. Ohtani, in preparation.