O2 Evolution Rate Research Articles

Enhanced fatty acid accumulation in Isochrysis galbana by inhibition of the mitochondrial alternative oxidase pathway under nitrogen deprivation

The purpose of this study was to clarify the interrelation between the mitochondrial alternative oxidase (AOX) pathway and fatty acid accumulation in marine microalga Isochrysis galbana. Under normal conditions, the activity of the AOX pathway was maintained at a low level in I. galbana. Compared with the normal condition, nitrogen deprivation significantly increased the AOX pathway activity and fatty acid accumulation. Under nitrogen deprivation, the inhibition of the AOX pathway by salicylhydroxamic acid caused the accumulation of reducing equivalents and the over-reduction of chloroplasts in I. galbana cells, leading to a decrease in the photosynthetic O2 evolution rate. The over-production of reducing equivalents due to the inhibition of the AOX pathway under nitrogen deprivation further enhanced the accumulation of fatty acids in I. galbana cells.

Mechanism of photo-induced water oxidation with [Ce(NO3)5]−into O2

The kinetics of photo-induced water oxidation with (NH4)2Ce(NO3)6 (CAN) into O2 has been investigated under visible light (λ ≥ 400 nm). The rate of O2 evolution is the first-order law with respect to the concentration of [Ce(NO3)5]-: 4dc(O2)/dt=-dc(Ce(NO3)5-)/dt=kc(Ce(NO3)5-), where the k value is 2.0×10-3 h-1 at 25 oC. Two new peroxide species, [CeIII(NO3)3(O2)CeIII(NO3)3]2- and [CeIII(NO3)2(O2)]-, have been detected by MS/MS mass spectroscopy. The 18O isotope-labelling water oxidation experiment shows that the evolved O2 originates exclusively from water. The in situ ESR spectra demonstrates the existence of OH• and NO3• radicals. Based on these results, a more reasonable mechanism of photo-induced water oxidation with [Ce(NO3)5]- into O2 has been proposed, in which the formation of NO3• radicals is considered as the rate-determining step.

(Invited) Kinetics of Photocatalytic Multielectron Transfer By Nanoparticles of Bare or Catalyst-Loaded Titania

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 therefore it is expected that reaction rate depends on light intensity and particle size of photocatalyst, which influence the number of photons absorbed by one photocatalyst particle. However such dependences have rarely been reported and discussed so far. In this study, we report light-intensity and particle-size dependences of rate of oxygen evolution from suspensions of commercial titania samples using iodate (IO3 -) and iron(III) (Fe3+) ions as an electron acceptor under intense UV-LED irradiation (365 nm; < 300 mW). Figure 1 shows the light-intensity dependence of rate of O2 evolution from a suspension of rutile (Tayca MT-150A; 13 nm) with Fe3+. A higher-order non-linear dependence was observed in the relatively lower (< 190 mW = threshold intensity, I thr), while the rate was almost proportional to the intensity (with slope, a) in the higher intensity range, and those dependences could be reproduced by second and first-order equations, respectively. Similar bimodal dependences, with different a and I thr, were also observed when the other titania samples were used in the presence of IO3 - or Fe3+. Assuming a kinetic model (Scheme 1) in which second-photon absorption by a one photon-absorbing particle (TiO2(h+)) within its lifetime leads to oxygen evolution, a rate equation (eq. 1) was derived using parameters of lifetime of TiO2(h+), secondary photon-absorption efficiency and rate constant of oxygen evolution by a two-photon absorbing particle(TiO2*), and further derivation for lower and higher intensity limits to eqs. 2 and 3, respectively, reproduced the bimodal light-intensity dependences. Threshold intensity (I thr) at which order of light-intensity dependence formally changes from second to first is defined as intensity for the equal rates of eq. 1 and 2 are equal. Differences in I thr and a depending on the kind of electron acceptors, crystalline structure (anatase or rutile) and particle size are discussed. Figure 1

Towards clarifying what distinguishes cyanobacteria able to resurrect after desiccation from those that cannot: The photosynthetic aspect

Organisms inhabiting biological soil crusts (BSCs) are able to cope with extreme environmental conditions including daily hydration/dehydration cycles, high irradiance and extreme temperatures. The photosynthetic machinery, potentially the main source of damaging reactive oxygen species during cessation of CO2 fixation in desiccating cells, must be protected to avoid sustained photodamage. We compared certain photosynthetic parameters and the response to excess light of BCS-inhabiting, desiccation-tolerant cyanobacteria Leptolyngbya ohadii and Nostoc reinholdii with those observed in the “model” organisms Nostoc sp. PCC 7120, able to resurrect after mild desiccation, and Synechococcus elongatus PCC 7942 and Synechocystis sp. PCC 6803 that are unable to recover from dehydration. Desiccation-tolerant strains exhibited a transient decline in the photosynthetic rate at light intensities corresponding to the inflection point in the PI curve relating the O2 evolution rate to light intensity. They also exhibited a faster and larger loss of variable fluorescence and profoundly faster QA− re-oxidation rates after exposure to high illumination. Finally, a smaller difference was found in the temperature of maximal thermoluminescence signal in the absence or presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) than observed in “model” cyanobacteria. These parameters indicate specific functional differences of photosystem II (PSII) between desiccation tolerant and sensitive cyanobacteria. We propose that exposure to excess irradiation activates a non-radiative electron recombination route inside PSII that minimizes formation of damaging singlet oxygen in the desiccation-tolerant cyanobacteria and thereby reduces photodamage.

Catalytic Function of IrOx in the Two-Step Thermochemical CO2-Splitting Reaction at High Temperatures

Splitting CO2 into CO and O2 via a two-step thermochemical cycle by utilizing concentrated solar energy is a promising option for CO2 reduction. Herein, to enhance the solar to fuel energy conversion efficiency, IrOx as a catalyst is used to increase the fuel production rate of the LaFeO3-based two-step thermochemical CO2-splitting reaction. Compared with LaFeO3, 0.6 atom % IrOx as a catalyst that loaded on LaFeO3 can almost double the maximal CO release rate and increase the maximal O2 evolution rate 0.5 times; when a small amount of IrOx (2.5 atom %) is doped into the structure of LaFeO3 through a solution combustion method, the initial CO generation rate can be increased by 5-fold. This work demonstrates that the catalytic function is still necessary in the two-step thermochemical CO2-splitting reaction even at high temperatures.

Double-diffusion-based synthesis of BiVO4 mesoporous single crystals with enhanced photocatalytic activity for oxygen evolution.

BiVO4 mesoporous single crystals (MSCs) were successfully prepared, for the first time, by a one-step hydrothermal method using the acidified BiVO4 precursor solution pre-impregnated silica as the template. It was revealed that the BiVO4 MSCs were formed by a double-diffusion mechanism. The O2 evolution rate over BiVO4 MSCs was improved nearly 10 times than that over BiVO4 bulk single crystals.

Factors affecting oxygen evolution through water oxidation on polycrystalline titanium dioxide

A linear correlation between crystallite size and O2evolution rate was obtained, due to the efficient spatial separation of photogenerated carriers.

Ca-assisted nitrogen-rich molecular approach for the synthesis of monodisperse tantalum oxide and (oxy)nitride nanocrystals for visible light&amp;ndash;induced water oxidation

The visible-light-induced photocatalytic water oxidation activity without cocatalyst has been demonstrated for the first time on tantalum (oxy)nitride nanocrystals synthesized by a Ca-assisted nitrogen-rich molecular approach. Monodisperse TaON and Ta3N5 nanocrystals were synthesized by using suitable urea /Ta ratios in the presence of Ca2+ions. The as-synthesized (oxy)nitrides are in the form of spherically-shaped and well-defined nanocrystals with an average diameter of ca. 7 nm. Both nanocrystals showed absorption edges in the visible region. The band gap energies of TaON and Ta3N5 nanocrystals were determined from the Tauc’s plots of the UV ­ Vis diffuse reflectance spectra to be 2.27 and 2.08 eV for indirect allowed transitions,respectively. The O2 evolution rate was increased and reached the maximum in the following order: 2.01, 3.32, and 4.66 mol·h¹1 for the samples synthesized with Ca2+/urea molar ratio of 0.10, 0.50, and 0.25, respectively, due to the decrease in the optical band gap. ©2016 The Ceramic Society of Japan. All rights reserved.

Hydrogenated CeO(2-x)S(x) mesoporous hollow spheres for enhanced solar driven water oxidation.

A facile route for the fabrication of hydrogenated sulfur-doped CeO2 (H-CeO(2-x)S(x)) mesoporous hollow spheres is reported. The spheres exhibited excellent photocatalytic activity due to the synergistic effect of the higher sulfur doping level and hydrogen post-treatment.

Facile one-pot solvothermal preparation of Mo-doped Bi2WO6biscuit-like microstructures for visible-light-driven photocatalytic water oxidation

The adsorption behavior and the separation efficiency of photogenerated electron–hole pairs are two important elements in estimating the photocatalytic activity of a photocatalyst. In this work, we have developed a facile one-pot solvothermal method for the preparation of Mo-doped Bi2WO6 with uniform three-dimensional (3D) hierarchical porous biscuit-like microstructures (PBMs). Mo doping is found to have two important roles in the synthesis of Bi2WO6 particles, leading to porous microstructures and adjusting band gaps of the Bi2MoxW1−xO6 particles. The band structure of the as-prepared porous Bi2MoxW1−xO6 products is characterized by UV-vis diffuse reflectance spectroscopy and valence-band X-ray photoelectron spectroscopy. Density functional theory (DFT) calculations give further insights into the band structure of the Bi2MoxW1−xO6 products. In all the samples, Bi2Mo0.21W0.79O6 PBMs exhibit a very efficient catalytic performance in oxidizing water under visible light irradiation (λ > 420 nm), with an average O2 evolution rate of up to 147.2 μmol h−1 g−1 and an apparent quantum efficiency (QE) of 3.1% at 420 nm, representing a 2 times more enhancement compared with the non-doped Bi2WO6 sample. This study provides a simple method for designing metal-doped semiconductors with porous structures for different applications.

XPS Study of the Interface Evolution of Carbonaceous Electrodes for Li-O2 Batteries during the 1st Cycle

The surface chemistry of the commonly employed positive electrode substrate Super C65 carbon was investigated during the 1st cycle of a Li-O2 battery with a typical ether electrolyte (0.2 M LiTFSI in Diglyme) by performing in situ online electrochemical mass spectrometry (OEMS), ex situ scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). During discharge, a nanometer-thin (< 5 nm) layer of Li2O2 forms homogeneously throughout the electrode before it is passivated at a final specific charge of ∼365 mAh/g. Higher discharge charge rates lead to thinner and more densely packed layers of Li2O2. Electrolyte decomposition also occurs in parallel, as evidenced by the continuous increase of LiF and sulfur containing species of the degraded LiTFSI salt. On charge, O2 evolves initially according to a 2e−/O2 rate, but as the cell over-potentials increase, O2 evolution rate approaches 4e−/O2, thus demonstrating a significant extent of parasitic side-reactions. Unlike the discharge, the charge leads to inhomogeneous electrode reactions causing a continuous removal of Li2O2 with no indication of LiO2 or Li2O intermediates. The decomposition side-products formed during discharge are also removed and the spectra of the pristine electrode are nearly retained at a full charge. Our results show that the analysis of Li-O2 battery chemistry requires broad approach based on the combination of various electrode surface sensitive techniques, such as XPS and OEMS, in order to provide further fundamental understanding of the mechanisms behind the complex electrochemistry involving the Li and O2 as well as several other predominant degradation products of the electrode and electrolyte.

Synthesis of Ordered Mesoporous CuO/CeO₂ Composite Frameworks as Anode Catalysts for Water Oxidation.

Cerium-rich metal oxide materials have recently emerged as promising candidates for the photocatalytic oxygen evolution reaction (OER). In this article, we report the synthesis of ordered mesoporous CuO/CeO2 composite frameworks with different contents of copper(II) oxide and demonstrate their activity for photocatalytic O2 production via UV-Vis light-driven oxidation of water. Mesoporous CuO/CeO2 materials have been successfully prepared by a nanocasting route, using mesoporous silica as a rigid template. X-ray diffraction, electron transmission microscopy and N2 porosimetry characterization of the as-prepared products reveal a mesoporous structure composed of parallel arranged nanorods, with a large surface area and a narrow pore size distribution. The molecular structure and optical properties of the composite materials were investigated with Raman and UV-Vis/NIR diffuse reflectance spectroscopy. Catalytic results indicated that incorporation of CuO clusters in the CeO2 lattice improved the photochemical properties. As a result, the CuO/CeO2 composite catalyst containing ~38 wt % CuO reaches a high O2 evolution rate of ~19.6 µmol·h−1 (or 392 µmol·h−1·g−1) with an apparent quantum efficiency of 17.6% at λ = 365 ± 10 nm. This OER activity compares favorably with that obtained from the non-porous CuO/CeO2 counterpart (~1.3 µmol·h−1) and pure mesoporous CeO2 (~1 µmol·h−1).

Ni-doped InN/GaZnON composite catalyst for overall water splitting under visible light irradiation

A (oxy)nitride composite- InN/GaZnON- is synthesized and utilized as the photocatalyst for visible-light-driven overall water splitting. The effect of Ni doping on the performance of the composite photocatalyst is investigated. The X-ray diffraction pattern indicates that the catalyst is a composite material consisting of InN and GaZnON phases. Ni is doped into the crystal lattices of the two phases. Ni doped InN/GaZnON composite achieves overall water splitting even without the loading of an additional cocatalyst. The doping of Ni greatly improves the visible light absorption and the water splitting activity. With the optimum elemental mole ratio, In/Ga/Zn/Ni = 1/1/1/0.05, H2 and O2 evolution rates achieve 2.23 and 1.15 μmol h−1, respectively.

Post-illumination transient O2 -uptake is driven by photorespiration in tobacco leaves.

This study aims to elucidate the molecular mechanism for the transient increase in the O2 -uptake rate in tobacco (Nicotiana tabacum cv Xanthi) leaves after turning off actinic lights (ALs). The photosynthetic O2 evolution rate reaches a maximum shortly after the onset of illumination with ALs and then decreases to zero in atmospheric CO2 /O2 conditions. After turning off the ALs, tobacco leaves show a transient increase in the O2 -uptake rate, the post-illumination transient O2 -uptake, and thereafter, the O2 -uptake rate decreases to the level of the dark-respiration rate. Photosynthetic linear electron flow, evaluated as the quantum yield of photosystem II [Y(II)], maintained a steady-state value distinct from the photosynthetic O2 -evolution rate. In high-[CO2 ] conditions, the photosynthetic O2 -evolution rate and Y(II) showed a parallel behavior, and the post-illumination transient O2 -uptake was suppressed. On the other hand, in maize leaves (a C4 plant), even in atmospheric CO2 /O2 conditions, Y(II) paralleled the photosynthetic O2 -evolution rate and the post-illumination transient O2 -uptake was suppressed. Hypothesizing that the post-illumination transient O2 -uptake is driven by C3 plant photorespiration in tobacco leaves, we calculated both the ribulose 1,5-bisphosphate carboxylase- and oxygenase-rates (Vc and Vo) from photosynthetic O2 -evolution and the post-illumination transient O2 -uptake rates. These values corresponded to those estimated from simultaneous chlorophyll fluorescence/O2 -exchange analysis. Furthermore, the H+ -consumption rate for ATP synthesis in both photosynthesis and photorespiration, calculated from both Vc and Vo that were estimated from chlorophyll fluorescence/CO2 -exchange analysis, showed a positive linear relationship with the dissipation rate of the electrochromic shift signal. Thus, these findings support our hypothesis.

Effect of low irradiance on the photosynthetic performance and spiking of Phalaenopsis

Lowering irradiance can delay the flower stalk, i.e., spike development, in order to schedule flowering time of Phalaenopsis; however, the effect on photosynthetic performance and spiking inhibition remains poorly understood. We compared light and shade treatments of Phalaenopsis aphrodite subsp. formosana in order to determine how limiting light affects day-night changes in the photosynthetic capacity of leaves and the carbon pool of leaves and stems resulting in delayed spiking. The low irradiance treatment [20 μmol(photon) m−2 s−1] for six weeks did not affect potential functions of photosynthetic apparatus estimated by chlorophyll a fluorescence analysis, but it significantly reduced the net CO2 uptake and O2 evolution rates, carbohydrate and organic acid concentrations, and amplitudes of CAM activity in new and fully expanded leaves of Phalaenopsis and delayed the spiking compared with the control kept at 150 μmol(photon) m−2 s−1. The shortened stem contained a remarkably high sucrose concentration, accounting for more than 80% of total soluble sugars for both treatments throughout the day. Moreover, the sucrose concentration was unaffected by the lowering of irradiance. The relationship between the sucrose content and spiking seemed to be loose; the major factor(s) for spiking in Phalaenopsis remained to be ascertained as the flower stalk bud is attached to the shortened stem.

KCl flux-induced growth of isometric crystals of cadmium-containing early transition-metal (Ti4+, Nb5+, and Ta5+) oxides and nitridability to form their (oxy)nitride derivatives under an NH3 atmosphere for water splitting application

In this study, we have attempted to experimentally validate the results of previous theoretical calculations predicting the possible formation of the CdTiO3−xNy, CdNbO2N, and CdTaO2N phases by applying conventional one- and two-step fabrication methods under an NH3 flow. For the two-step method, CdTiO3, Cd2Nb2O7, and Cd2Ta2O7 crystals were first grown by a KCl flux method, and the effects of solute concentration and cooling rate on the crystal growth were studied. The formability of their (oxy)nitride derivatives was investigated by changing the nitridation temperature (750–950°C) and time (1–10h) of oxide precursors. It was found that the CdTiO3−xNy, CdNbO2N, and CdTaO2N phases cannot be formed by the applied conventional methods due to the low volatilization temperature of cadmium and the susceptibility of titanium and niobium to reduction under an NH3 atmosphere. Under high-temperature NH3 atmosphere, only Cd2Ta2O7 was fully converted to single-phase Ta3N5. The results from the photocatalytic O2 evolution test of bare and CoOx-loaded Ta3N5 crystalline structures, converted from Cd2Ta2O7 (Cd–Ta3N5) and Na2CO3-treated Ta2O5 (Na–Ta3N5) and Cd2Ta2O7 (Na–Cd–Ta3N5) crystals by nitridation at 850°C for 20h under an NH3 flow, revealed that the CoOx-loaded Ta3N5 showed more than two times higher O2 evolution rate (655μmol), whereas the CoOx-loaded Cd–Ta3N5 and Na–Cd–Ta3N5 exhibited nearly four (501μmol) and three (422μmol) times higher O2 evolution rates at 5h compared with their bare counterparts. An improved photocatalytic activity for O2 evolution is related to the higher density of nucleation centers of CoOx nanoparticles in the form of dangling bonds in porous Ta3N5 structures and long-lived photogenerated holes, as attested by time-resolved absorption spectroscopy.

New insight into calcium tantalate nanocomposite photocatalysts for overall water splitting and reforming of alcohols and biomass derivatives

The photocatalytic properties of different calcium tantalate nanocomposite photocatalysts with optimized phase composition were studied without the addition of any co-catalysts in the photoreforming of different alcohols including the biomass conversion by-product glycerol, as well as after modification with double-layered NiOx (Ni/NiO) co-catalyst in overall water splitting (OWS). Nanocomposite photocatalyst consisting of cubic α-CaTa2O6/orthorhombic β-CaTa2O6 coexisting phases always possesses the highest photocatalytic performance. For overall water splitting, a loading of 0.5 wt. % NiOx exhibits the best activities with stable stoichiometric H2 and O2 evolution rates.

Heat transfer and fluid flow analysis of a 4 kW solar thermochemical reactor for ceria redox cycling

A solar reactor consisting of a cavity-receiver containing a reticulated porous ceramic (RPC) foam made of CeO2 is considered for effecting the splitting of H2O and CO2 via a thermochemical redox cycle. A transient 3D heat and mass transfer model of the reduction step is formulated and solved using Monte-Carlo ray-tracing coupled to computational fluid dynamics. Experimental validation is accomplished in terms of measured temperatures and O2 evolution rates obtained with a solar reactor prototype tested under high-flux radiative power inputs in the range 2.8–3.8kW and mean solar concentration ratios up to 3024 suns. Critical temperatures of up to 2250K induced CeO2 sublimation, which in turn affected detrimentally the solar reactor performance. The model is applied to analyze an improved geometrical design with alternative flow configuration, enabling more uniform radiative absorption and temperature distributions, and resulting in a higher solar-to-fuel energy conversion efficiency.

Effects of heat stress on photosynthetic electron transport in a marine cyanobacterium Arthrospira sp.

Arthrospira (Spirulina) is widely used as human health food and animal feed. In cultures grown outdoors in open ponds, Arthrospira cells are subjected to various environmental stresses, such as high temperature. A better understanding of the effects of high temperature on photosynthesis may help optimize the productivity of Arthrospira cultures. In this study, the effects of heat stress on photosynthetic rate, chlorophyll a fluorescence transients, and photosystem (PS) II, PSI activities in a marine cyanobacterium Arthrospira sp. were examined. Arthrospira cells grown at 25 °C were treated for 30 min at 25 (control), 30, 34, 37, or 40 °C in the dark. Heat stress (30–37 °C) enhanced net photosynthetic O2 evolution rate. Heat stress caused over-reduction PSII acceptor side, damage of donor side of PSII, decrease in the energetic connectivity of PSII units, and decrease in the performance of PSII. When the temperature changed from 25 to 37 °C, PSII activity decreased, while PSI activity increased, the enhancement of photosynthetic O2 evolution was synchronized with the increase in PSI activity. When temperature was further increased to 40 °C, it induced a decrease in photosynthetic O2 evolution rate and a more severe decrease in PSII activity, but an increase in PSI activity. These results suggest that PSI activity was the decisive factor determining the change of photosynthetic O2 evolution when Arthrospira was exposed to a temperature from 25 to 37 °C, but then, PSII activity became the decisive factor adjusting the change of photosynthetic O2 evolution when the temperature was increased to 40 °C.

Hematite Films Decorated with Nanostructured Ferric Oxyhydroxide as Photoanodes for Efficient and Stable Photoelectrochemical Water Splitting

Hematite photoanodes are decorated with nanostructured FeOOH by photoelectrodeposition. An obvious cathodic shift in the photocurrent onset potential is observed, while four‐times enhancement of photocurrent density enhancement is acheived with FeOOH present. This can be ascribed to the high reaction area for the structure and high electrocatalytic activity of nanostructured FeOOH, which increases the amount of photogenerated holes involved in the water oxidation reaction and accelerates the kinetics of water oxidation. Furthermore, the obtained Fe2O3/FeOOH photoanode achieves considerable O2 evolution rate (10.1 μmol h−1 cm−2) under AM 1.5 G illumination and is maintained for as long as 70 h. The Fe2O3/FeOOH films show visible light response, high photocurrent density, and long‐term stability, and they are well qualified photoanode materials and a promising candidate for photoelectrochemical water splitting.