Reoxidation Kinetics Research Articles

Function of plastoquinone in heat stress reactions of plants

The effect of high temperature treatment (40 °C, 3 h, illumination at 100 μmol m − 2 s − 1 ) on the photosynthetic electron flow in barley seedlings of different age was investigated. Thermoinduced inhibition of the liner electron flow due to partial impairment of the water oxidizing complex (WOC) and the increase in the extent of Q A − reoxidation by Tyr z ox in thylakoids isolated from 4-day-old leaves was shown by measurements of oxygen evolution using benzoquinone or potassium ferricyanide as electron acceptors, as well as by following Q A − reoxidation kinetics in the absence and presence of exogenous electron acceptors, DCBQ and DMBQ. Using HPLC analysis, an increase in the oxidation of the photoactive plastoquinone pool in young leaves under heating was shown. In older, 11-day-old leaves, heat treatment limited both photosynthetic electron flow and oxygen evolution. The same effects of heat shock on oxygen evolution caused an inhibition of electron flow on the donor side of PSII only. However, a rise in the proportion of PSII with Q A − reoxidized through recombination with the S 2/S 3 state of the WOC was observed. The addition of exogenous electron acceptors (DCBQ and DMBQ) and a donor (DPC) showed that the thermoinduced decrease in the electron transport rate was caused by an impediment of electron flow from Q A − to acceptor pool. The decrease in size of the photoactive PQ-pool and a change in the proportions of oxidized and reduced PQ in older leaves under heat treatment were shown. It was suggested that a thermoinduced change of the redox state of the PQ-pool and a redistribution of plastoquinone molecules between photoactive and non-photoactive pools are the mechanisms which reflect and regulate the response of the photosynthetic apparatus under heat stress conditions.

Modeling of fast diffusion along grain boundaries in oxide ceramics

The kinetics of oxygen exchange between oxide ceramics and the surrounding gas phase is modeled by taking account of surface exchange reactions and fast grain boundary diffusion. Numerical Laplace inversion is used with respect to a spherical grain model, while numerical solutions for a square grain model are obtained by means of the finite element method. The time dependence of the total amount of exchanged oxygen (relaxation curves) is predicted within Harrison's type-A kinetics. Appropriate relations for the effective diffusion coefficient and the effective surface exchange coefficient are given. If the grain boundary diffusion coefficient exceeds the bulk diffusion coefficient by many orders of magnitude, the new oxygen activity is attained instantaneously at the surface of each grain (grain boundary) after a step-wise alteration of the oxygen partial pressure. The rate-determining step for the oxygen stoichiometry change is slow bulk diffusion from the grain boundaries into the grains. This mechanism allows the interpretation of the re-oxidation kinetics of donor doped barium titanate ceramics.

In vivo target sites of nitric oxide in photosynthetic electron transport as studied by chlorophyll fluorescence in pea leaves.

The role of nitric oxide (NO) in photosynthesis is poorly understood as indicated by a number of studies in this field with often conflicting results. As various NO donors may be the primary source of discrepancies, the aim of this study was to apply a set of NO donors and its scavengers, and examine the effect of exogenous NO on photosynthetic electron transport in vivo as determined by chlorophyll fluorescence of pea (Pisum sativum) leaves. Sodium nitroprusside-induced changes were shown to be mediated partly by cyanide, and S-nitroso-N-acetylpenicillinamine provided low yields of NO. However, the effects of S-nitrosoglutathione are inferred exclusively by NO, which made it an ideal choice for this study. Q(A)(-) reoxidation kinetics show that NO slows down electron transfer between Q(A) and Q(B), and inhibits charge recombination reactions of Q(A)(-) with the S(2) state of the water-oxidizing complex in photosystem II. Consistent with these results, chlorophyll fluorescence induction suggests that NO also inhibits steady-state photochemical and nonphotochemical quenching processes. NO also appears to modulate reaction-center-associated nonphotochemical quenching.

Heterogeneously catalysed partial oxidation of acrolein to acrylic acid—structure, function and dynamics of the V–Mo–W mixed oxides

The major objective of this research project was to reach a microscopic understanding of the structure, function and dynamics of V-Mo-(W) mixed oxides for the partial oxidation of acrolein to acrylic acid. Different model catalysts (from binary and ternary vanadium molybdenum oxides up to quaternary oxides with additional tungsten) were prepared via a solid state preparation route and hydrochemical preparation of precursors by spray-drying or crystallisation with subsequent calcination. The phase composition was investigated ex situ by XRD and HR-TEM. Solid state prepared samples are characterised by crystalline phases associated to suitable phase diagrams. Samples prepared from crystallised and spray-dried precursors show crystalline phases which are not part of the phase diagram. Amorphous or nanocrystalline structures are only found in tungsten doped samples. The kinetics of the partial oxidation as well as the catalysts' structure have been studied in situ by XAS, XRD, temperature programmed reaction and reduction as well as by a transient isotopic tracing technique (SSITKA). The reduction and re-oxidation kinetics of the bulk phase have been evaluated by XAS. A direct influence not only of the catalysts' composition but also of the preparation route is shown. Altogether correlations are drawn between structure, oxygen dynamics and the catalytic performance in terms of activity, selectivity and long-term stability. A model for the solid state behaviour under reaction conditions has been developed. Furthermore, isotope exchange experiments provided a closer image of the mechanism of the selective acrolein oxidation. Based on the in situ characterisation in combination with micro kinetic modelling a detailed reaction model which describes the oxygen exchange and the processes at the catalyst more precisely is discussed.

Bidirectional electron transfer in photosystem I: Replacement of the symmetry-breaking tryptophan close to the PsaB-bound phylloquinone (A 1B) with a glycine residue alters the redox properties of A 1B and blocks forward electron transfer at cryogenic temperatures

A conserved tryptophan residue located between the A 1B and F X redox centres on the PsaB side of the Photosystem I reaction centre has been mutated to a glycine in Chlamydomonas reinhardtii, thereby matching the conserved residue found in the equivalent position on the PsaA side. This mutant (PsaB:W669G) was studied using EPR spectroscopy with a view to understanding the molecular basis of the reported kinetic differences in forward electron transfer from the A 1A and the A 1B phyllo(semi)quinones. The kinetics of A 1 − reoxidation due to forward electron transfer or charge recombination were measured by electron spin echo spectroscopy at 265 K and 100 K, respectively. At 265 K, the reoxidation kinetics are considerably lengthened in the mutant in comparison to the wild-type. Under conditions in which F X is initially oxidised the kinetics of charge recombination at 100 K are found to be biphasic in the mutant while they are substantially monophasic in the wild-type. Pre-reduction of F X leads to biphasic kinetics in the wild-type, but does not alter the already biphasic kinetic properties of the PsaB:W669G mutant. Reduction of the [4Fe–4S] clusters F A and F B by illumination at 15 K is suppressed in the mutant. The results provide further support for the bi-directional model of electron transfer in Photosystem I of C. reinhardtii, and indicate that the replacement of the tryptophan residue with glycine mainly affects the redox properties of the PsaB bound phylloquinone A 1B.

Reduction and Reoxidation Kinetics of Nickel-Based SOFC Anodes

Reduction and reoxidation kinetics of Ni-based solid oxide fuel cell (SOFC) anodes were investigated over a range of temperatures between 600 and . Dense (no open porosity) two-phase (yttria-stabilized zirconia) samples, with and without small amounts of oxide additives (CaO, MgO, ), were fabricated and then reduced in a hydrogen-containing environment. The time dependence of the reduced layer thickness at various temperatures was measured. Reoxidation studies were conducted on fully reduced anodes that were subsequently reoxidized in air over a temperature range between 650 and . A simple theoretical model was developed to describe the kinetics of reduction and reoxidation based on two series kinetic steps: diffusion and interface reaction. It was observed that the reduction kinetics was linear (interface-controlled), while the reoxidation kinetics was nearly parabolic (diffusion-controlled). Also, the kinetics of reduction was thermally activated with an activation energy of . By contrast, over the temperature range investigated, the kinetics of reoxidation was essentially independent of temperature. The interface control of the reduction process implies that gas-phase diffusion through porous , formed upon reduction of NiO to Ni, is considerably faster than the kinetics of the actual reduction reaction occurring at the interface separating the pristine and the reduced regions. By contrast, diffusion control of the reoxidation process was attributed to slow, gaseous diffusion on account of the very small amount of porosity that remains when Ni reoxidizes to NiO, developed presumably due to a slight shape change of Ni particles that may occur at high temperatures. Doping the anodes with stable oxides, such as CaO and MgO, significantly reduced both the reduction and reoxidation kinetics of Ni-based anodes.

Photosystem II Heterogeneity of in hospite Zooxanthellae in Scleractinian Corals Exposed to Bleaching Conditions

Increased ocean temperatures are thought to be triggering mass coral bleaching events around the world. The intracellular symbiotic zooxanthellae (genus Symbiodinium) are expelled from the coral host, which is believed to be a response to photosynthetic damage within these symbionts. Several sites of impact have been proposed, and here we probe the functional heterogeneity of Photosystem II (PSII) in three coral species exposed to bleaching conditions. As length of exposure to bleaching conditions (32 degrees C and 350 micromol photons m(-2) s(-1)) increased, the QA- reoxidation kinetics showed a rise in the proportion of inactive PSII centers (PSIIx), where QB was unable to accept electrons. PSIIx contributed up to 20% of the total PSII centers in Pocillopora damicornis, 35% in Acropora nobilis and 14% in Cyphastrea serailia. Changes in Fv/Fm and amplitude of the J step along fast induction curves were found to be highly dependent upon the proportion of PSIIx centers within the total pool of PSII reaction centers. Determination of PSII antenna size revealed that under control conditions in the three coral species up to 60% of PSII centers were lacking peripheral light-harvesting complexes (PSIIbeta). In P. damicornis, the proportion of PSIIbeta increased under bleaching conditions and this could be a photoprotective mechanism in response to excess light. The rapid increases in PSIIx and PSIIbeta observed in these corals under bleaching conditions indicates these physiological processes are involved in the initial photochemical damage to zooxanthellae.

Kinetic modeling of the reoxidation of vanadia-based catalysts

The reoxidation kinetics of classical vanadia-based EL10V Eurocat catalysts are compared with those obtained on a new catalytic material, prepared by DC magnetron sputtering, by means of temporal analysis of products (TAP) at reduction and reoxidation temperatures from 723 to 823 K. After reduction to different degrees of reduction with multipulses of propane, the kinetics of reoxidation were obtained from single-pulse experiments. Irreversible dissociative chemisorption of di-oxygen allowed to describe the latter over the full range of investigated conditions for EL10V1 with a single pre-exponential factor, A = 4.05 × 10 2 kg Vm 3/mol 2 s, and a single activation energy, E a = 33.41 kJ/mol. In the case of the sputtered catalysts, investigated at 773 K only, a single rate coefficient also describes the reoxidation at different degrees of reduction for every V-loading. Its value increases with increasing V-loading up to a constant value of 1.09 ± 0.11 kg Vm 3/mol 2 s, but remains lower than the corresponding rate coefficient for the reoxidation for EL10V8 which amounts to 13.30 kg Vm 3/mol 2 s.

Optical in‐situ study of the oxidation and reduction kinetics of Yb‐substituted YAG epitaxial films

The optical spectra of the Yb-substituted Yb3Al5O12 (YAG) epitaxial films and the Yb ions recharging kinetics have been investigated by means of in-situ optical spectroscopy in the temperature range between 900 and 1100 °C at oxidation and reduction condition. This work continues the studies of the spectral properties of Yb:YAG epitaxial films represented in [1] and optical in-situ study of the re-oxidation kinetics of Yb3Al5O12 (YbAG) crystal represented in [2] where the model of internal oxidation of ytterbium ions Yb2+ → Yb3+ in the bulk crystal was developed and proofed. The changes of the absorption spectra of Yb:YAG film during oxidation and reduction were attributed to the recharging process Yb3+ ↔ Yb2+. It was found that the oxidation kinetics in epitaxial films do not agree to the model [2] and do not follow a parabolic rate law in contrary to bulk YbAG single crystal. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spectroscopic studies of high-temperature processes in solids

Results are reported of spectroscopic investigations of various high-temperature processes in oxides. The redox behaviour of the garnet Yb 3Al 5O 12 is studied by means of optical spectroscopy. The reoxidation kinetics of reduced single crystalline Yb 3Al 5O 12 is found to provide an example of an internal oxidation process which differs significantly from the behaviour observed during reduction. The observation of a well-defined colour front is discussed. Manganese-doped barium titanate, BaTiO 3, represents an important electroceramic material where the valency of the dopant plays an essential role. We have studied the charge state of manganese ions in the barium titanate by electron paramagnetic resonance (EPR) spectroscopy at 1073 K as a function of oxygen partial pressure.

Effect of herbicide clomazone on photosynthetic processes in primary barley ( Hordeum vulgare L.) leaves

The effect of pre-emergently applied herbicide clomazone on the photosynthetic apparatus of primary barley leaves ( Hordeum vulgare L.) was studied. Clomazone application caused a reduction in chlorophyll ( a+ b) and carotenoid levels that was accompanied by a decline in the content of light harvesting complexes as judged from the increasing chlorophyll a/ b ratio. The pigment reduction also resulted in changes in 77 K chlorophyll fluorescence emission spectra indicating lower chlorophyll (Chl) fluorescence reabsorption and absence of the long-wavelength emission forms of photosystem I. The maximal photochemical yield of photosystem II (PSII) and the reoxidation kinetics of the primary quinone acceptor Q A − were not significantly influenced by clomazone. A higher initial slope of Chl fluorescence rise in the Chl fluorescence induction kinetic indicated an increased delivery of excitations to PSII. Simultaneously, analysis of the Chl fluorescence quenching revealed that clomazone reduced function of the electron transport chain behind PSII. The decrease in the saturation rates of CO 2 assimilation paralleled the decrease of the Chl content and has been suggested to be caused by a suppressed number of the electron transport chains in the thylakoid membranes or by their decreased functionality. The obtained results are discussed in view of physiological similarity of the clomazone effect with changes of photosynthetic apparatus during photoadaptation.

Reduction and Re-Oxidation Kinetics of Nickel-Based Solid Oxide Fuel Cell Anodes

Reduction and re-oxidation kinetics of Ni-based anodes were investigated over a range of temperatures between 650 and 800°C. The experimental part of the investigation consisted of first fabricating NiO + YSZ two- phase composites of essentially full density. The fully dense samples were subsequently reduced in a hydrogen-containing environment. The reduced layer thickness was measured as a function of time at temperature. Some of the samples were reduced for a time long enough to ensure that they were fully reduced. Subsequently, the reduced samples were re-oxidized in air at various temperatures, ranging between 650 and 800°C. In this set of experiments, the re-oxidized layer thickness was measured as a function of time at temperature. These two types of experiments were used for determining the kinetics of reduction as well as that of re-oxidation. It was observed that the reduction kinetics was linear (interface-controlled), while the re-oxidation kinetics was parabolic (diffusion-controlled). A simple theoretical analysis was conducted to describe the two processes. The interface control of the reduction process was attributed to the rapid gaseous transport through the pores formed upon reduction of NiO to Ni. By contrast, diffusion control for the re-oxidation process was attributed to the existence of a very small amount of porosity that forms when Ni re-oxidizes to NiO, which occurs due presumably to a slight shape change of Ni particles.

An optical in-situ study of the re-oxidation kinetics of mixed valent Yb3Al5O12

The optical spectra of blue Yb-garnet single crystals have been studied in the range of 200 to 1200 nm. The spectra exhibit absorption bands due to Yb2+ and Yb3+ ions including 4f–4f transitions on Yb3+ as well as 4f–5d transitions on Yb2+ ions, and a band due to Yb2+–Yb3+ intervalence charge-transfer (IVCT) responsible for the blue colour of the crystal. At high temperatures, the blue colour is found to bleach in an oxidising atmosphere. The oxidation reaction was studied by means of optical spectroscopy at in-situ conditions in the temperature range between 861 and 1065°C. Basically, the oxidation kinetics were found to follow a parabolic rate law. The kinetics have been analysed in the framework of a refined model of internal oxidation accounting for both the transport of the oxidising defects and the diffusion of the reduced component Yb2+.

Photosynthetic energy conversion under extreme conditions—I: important role of lipids as structural modulators and energy sink under N-limited growth in Antarctic sea ice diatoms

The availability of dissolved nutrients such as nitrate under extreme low temperatures is a strong determinant in the development and growth of ice diatoms. Consequently we investigated regulation of photosynthesis in a mixed culture of three diatom species, which grew in chemostats at −1 °C, 15 μmol photons m −2 s −1 under N-limitation. When nitrogen is limiting, pigment–protein complexes are one of the most affected structures under low-light conditions. The loss of integral polar thylakoid components destabilized the bilayer structure of the membrane with consequences for lipid composition and the degree of fatty acid desaturation. N-Limitation caused a decrease in monogalactosydiacylglycerol (MGDG) and a simultaneous increase in bilayer forming digalactosyldiacylglycerol (DGDG). Their ratio MGDG:DGDG decreased from 3.4±0.1 to 1.1±0.4, while 20:5 n-3 fatty acids of chloroplast related phospholipid classes such as phosphatidylglycerol (PG) increased under N-limitation. These data reveal that lipids are important components, required to sustain membrane structure under a deficiency of integral membrane bound proteins and pigments. Nonetheless, energy conversion at photosystem II is still affected by N-limitation despite this structural regulation. Photosynthetic quantum yield ( F v/ F m) and electron transport rates decreased under N-limitation caused by an increasing amount of electron acceptors (second stable electron acceptor=Q B) which had slower reoxidation kinetics. The energy surplus under these conditions is stored in triacylglycerols, the main energy sink in Antarctic sea ice diatoms under N-limitation.

Re-oxidation kinetics of a VPO catalyst

Oxidation of vanadium pyrophosphate (VPO) to the orthophosphate was investigated by temperature programmed oxidation (TPO) and by weight gain measurements in a TGA unit as a model for the re-oxidation of a catalyst in a two-bed process in the partial oxidation of butane to maleic anhydride (MA). It was demonstrated by FTIR observations that the weight gain corresponds to the VPO oxidation. The results of separate measurements of a catalyst alternatively exposed to oxygen and to a stream containing butane should also be applicable to a VPO catalyst employed in a two-bed process. The VPO oxidation measurements demonstrate that re-oxidation of a VPO catalyst in a two-bed process should be carried out at a higher temperature than the butane partial oxidation. When this is carried out, significant performance improvements in the two-bed process are observed.

Determination of the oxygen ion diffusion coefficients in mixed conductors by the volumetric measurement of the reoxidation kinetics

Abstract Mixed conducting ceramics (ionic and electronic conducting oxides) have a high application potential for the oxygen separation from oxygen containing gas mixtures (mostly air). Because the ion diffusion controls frequently the rate of the total permeation flux, a simple and fast determination of the oxygen diffusion coefficient is the prerequisite for an efficient material screening. In this work, the volumetric measurement of the reoxidation of chemically reduced mixed conductors was used for the determination of oxygen ion diffusion coefficients. The described method has advantages compared to the frequently used mass relaxation technique, e.g. a higher measuring effect, the suitability for fast processes (by use of large samples) and a high resistance to abrupt pressure changes. The checkup of the measurement method on the reference materials SrFe 1− x Co x O z ( x =0.6) and La 0.79 Sr 0.2 MnO 3+ z indicates a sufficient agreement with previous published values. The experimental results show a high internal consistency, as it is essential for a screening method. Diffusion coefficients are independently on the oxygen concentration over a wide range of the oxygen stoichiometry for all tested materials. Low activation energies in the range 0.10–0.5 eV were found. These latter results are opposed to the majority of publications on mixed conducting oxides. Additionally, new promising perovskite compositions in the system Sr 1− x Ca x Mn 1− y Fe y O 3− z were presented.

A nuclear mutant of Arabidopsis thaliana selected for enhanced sensitivity to light‐chill stress is altered in PSII electron transport activity

Chilling in the light imposes a considerable level of stress on the photosynthetic apparatus, resulting in a decrease of photosystem II activity and the quenching of maximum and variable fluorescence. We selected in a fah‐1 mutagenized population of Arabidopsis thaliana, which permits a direct visible evaluation of the intensity of chlorophyll (Chl) fluorescence, a monogenic recessive nuclear mutant hypersensitive to photoinhibition induced by light and cold. The major phenotypic trait of the mutant is the appearance of chlorotic areas on developed leaves. Photochemical analyses indicate that the mutant is hypersensitive to photoinhibition in excess light in the cold but also at room temperature. The susceptibility to photoinhibition is a consequence of perturbations in photochemistry already present in unstressed plants. Such perturbations result in a greater fraction of the primary acceptor QA remaining in the reduced state even at low light fluxes. From estimates of the number of total and functional PSII units and measurements of PSII quantum yield and QA− reoxidation kinetics, the basic lesion of the mutant seems restricted to PSII photochemistry likely affecting the rate of electron transport from QA− to QB.

Measurements of Phytoplankton of Sub-Nanomolar Chlorophyll Concentrations by a Modified Double-Modulation Fluorometer

A modification of the double-modulation fluorometer is described that allows measuring very dilute phytoplankton samples. The high sensitivity is achieved by increasing the sample volume and by collecting the fluorescence from the large volume by an integrating sphere. The sensitivity of the instrument increased approximately proportionally to the volume of the sample. A further improvement of the sensitivity was achieved by replacing the PIN photodiode of the earlier versions by a photomultiplier. The instrument was used to measure fluorescence induction, F0 and Fm parameters, and QA -reoxidation kinetics at concentrations at and below 100 pM chlorophyll.

Niobium based tetragonal tungsten bronzes as potential anodes for solid oxide fuel cells: synthesis and electrical characterisation

In this paper we report studies on a range of niobate based tungsten bronzes, with a view to analysing their potential as anode materials in SOFCs. Six systems were studied, (Sr1−xBax)0.6Ti0.2Nb0.8O3, Sr0.6−xLaxTi0.2+xNb0.8−xO3, (Sr0.4−xBax)Na0.2NbO3, (Ba1−xCax)0.6Ti0.2Nb0.8O3, Ba0.5−xAxNbO3 (A=Ca, Sr), and Ba0.3NbO2.8, and the electrical conductivities were examined over a range of oxygen partial pressures (10−20–1 bar). All the systems showed good conductivity in low oxygen partial pressures, with values as high as 8 S cm−1 at 930°C (P(O2)=10−20 bar). As the oxygen partial pressure was raised the conductivity dropped showing in most cases an approximate [P(O2)]−1/4 dependence and good re-oxidation kinetics. Of all the samples studied the (Sr1−xBax)0.6Ti0.2Nb0.8O3 and (Ba1−xCax)0.6Ti0.2Nb0.8O3 systems appear most promising for potential use as anode materials in SOFCs.

Two distinct states of the thylakoid bf complex

Under normal physiological conditions the state of the cyt bf complex is characterized by rapid reoxidation kinetics of cyt b-563 following flash-illumination. It is known that these kinetics are dramatically slowed down under oxidizing conditions. Here we show that this slow-down of cyt b-563 oxidation is the consequence of a relatively slow (half-time of several minutes) transformation of the cyt bf complex into a distinctly different state (termed state-s). Reversal to the normal state requires strong reductive treatment or light-induced electron transport. The results are in line with a recent model of functional cyt bf dimers [Cramer et al., Annu. Rev. Plant Physiol. Plant Mol. Biol. 47 (1996), 477–508], if it is assumed that state-s reflects the monomeric state of the bf complex.