Showing papers on "Oxygen published in 2006"

Production and scavenging of reactive oxygen species in chloroplasts and their functions.

Abstract: The reaction centers of PSI and PSII in chloroplast thylakoids are the major generation site of reactive oxygen species (ROS). Photoreduction of oxygen to hydrogen peroxide (H2O2) in PSI was discovered over 50 years ago by [Mehler (1951)][1]. Subsequently, the primary reduced product was identified

Reactive Oxygen Species Signaling in Response to Pathogens

Abstract: The production of reactive oxygen species (ROS), via consumption of oxygen in a so-called oxidative burst, is one of the earliest cellular responses following successful pathogen recognition. Apoplastic generation of superoxide (O2−), or its dismutation product hydrogen peroxide (H2O2), has been

The carbon cycle and associated redox processes through time

Abstract: Earth's biogeochemical cycle of carbon delivers both limestones and organic materials to the crust. In numerous, biologically catalysed redox reactions, hydrogen, sulphur, iron, and oxygen serve pr...

Interplay between O2 and SnO2: Oxygen Ionosorption and Spectroscopic Evidence for Adsorbed Oxygen

Abstract: Tin dioxide is the most commonly used material in commercial gas sensors based on semiconducting metal oxides. Despite intensive efforts, the mechanism responsible for gas-sensing effects on SnO(2) is not fully understood. The key step is the understanding of the electronic response of SnO(2) in the presence of background oxygen. For a long time, oxygen interaction with SnO(2) has been treated within the framework of the "ionosorption theory". The adsorbed oxygen species have been regarded as free oxygen ions electrostatically stabilized on the surface (with no local chemical bond formation). A contradiction, however, arises when connecting this scenario to spectroscopic findings. Despite trying for a long time, there has not been any convincing spectroscopic evidence for "ionosorbed" oxygen species. Neither superoxide ions O(2)(-), nor charged atomic oxygen O,(-) nor peroxide ions O(2)(2-) have been observed on SnO(2) under the real working conditions of sensors. Moreover, several findings show that the superoxide ion does not undergo transformations into charged atomic oxygen at the surface, and represents a dead-end form of low-temperature oxygen adsorption on reduced metal oxide.

Concentration of Ce3+ and Oxygen Vacancies in Cerium Oxide Nanoparticles

Abstract: Temperature variations of the electron magnetic resonance (EMR) spectra and magnetization measurements are used to show that Ce3+ ions in concentration ≃18% are present in 3 nm CeO2 nanoparticles supported on silica aerogel. It is argued that the presence of Ce3+ implies the defect structure CeO2-x for ceria nanoparticles due to oxygen vacancies. This transformation of Ce4+ to Ce3+ driven by oxygen vacancies may be the key to understanding the catalytic properties of ceria.

Oxygen stoichiometry and chemical expansion of Ba0.5Sr0.5Co0.8Fe0.2O3- δ measured by in situ neutron diffraction

Abstract: The structure, oxygen stoichiometry, and chemical and thermal expansion of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) between 873 and 1173 K and oxygen partial pressures of 1 × 10-3 to 1 atm were determined by in situ neutron diffraction. BSCF has a cubic perovskite structure, space group Pm3m, across the whole T−pO2 region investigated. The material is highly oxygen deficient with a maximum oxygen stoichiometry (3 − δ) of 2.339(12) at 873 K and a pO2 of 1 atm and a minimum of 2.192(15) at 1173 K and a pO2 of 10-3 atm. Good agreement is obtained between oxygen stoichiometry data determined by neutron diffraction and thermogravimetry. In the range covered by the experiments, the thermal and chemical expansion coefficients are 19.0(5)−20.8(6) × 10-6 K-1 and 0.016(2)−0.026(4), respectively.

Physical Mechanisms of Inactivation of Bacillus subtilis Spores Using Cold Atmospheric Plasmas

Abstract: This paper presents a detailed study of the potential physical mechanisms of the microbial inactivation by cold atmospheric plasmas. With the Bacillus subtilis spores as a model microorganism and an atmospheric-plasma plume in helium flow, optical emission spectroscopy and inactivation kinetics are used to demonstrate the dominating role played by the reactive oxygen species (e.g., atomic oxygen and OH) as well as the minor contributions of the UV photons, heat, charged particles, and electric fields. To differentiate the concentrations of the reactive oxygen species, an atmospheric helium-oxygen plasma is also used for the spore inactivation. Results with the helium and the helium-oxygen plasmas are contrasted to highlight how the production of the spore-killing oxygen species may be enhanced

Chemistry and reactions of reactive oxygen species in foods.

Abstract: Reactive oxygen species (ROS) are formed enzymatically, chemically, photochemically, and by irradiation of food. They are also formed by the decomposition and the inter-reactions of ROS. Hydroxy radical is the most reactive ROS, followed by singlet oxygen. Reactions of ROS with food components produce undesirable volatile compounds and carcinogens, destroy essential nutrients, and change the functionalities of proteins, lipids, and carbohydrates. Lipid oxidation by ROS produces low molecular volatile aldehydes, alcohols, and hydrocarbons. ROS causes crosslink or cleavage of proteins and produces low molecular carbonyls from carbohydrates. Vitamins are easily oxidized by ROS, especially singlet oxygen. The singlet oxygen reaction rate was the highest in β-carotene, followed by tocopherol, riboflavin, vitamin D, and ascorbic acid.

Bistability of atmospheric oxygen and the Great Oxidation

Abstract: The first significant increase in atmospheric oxygen levels on Earth (the 'Great Oxidation') is thought to have occurred at least 300 million years after the evolution of oxygenic photosynthesis, but the reason for this time lag is not clear. Using a new conceptual model of the global redox system, Goldblatt et al. show that atmospheric oxygen levels could have remained either at a low or a high steady state after oxygenic photosynthesis evolved. The Great Oxidation may have been a switch between these states triggered by a relatively small environmental change. The model suggests that oxygenic photosynthesis alone is insufficient to cause an oxygen-rich atmosphere. So in the absence of additional factors, Earth might have an atmosphere containing only a few parts per million of oxygen, not the 21% which we enjoy. Equally, oxygenic photosynthesis could conceivably have evolved on planets that only have low levels of atmospheric oxygen. The history of the Earth has been characterized by a series of major transitions separated by long periods of relative stability1. The largest chemical transition was the ‘Great Oxidation’, approximately 2.4 billion years ago, when atmospheric oxygen concentrations rose from less than 10-5 of the present atmospheric level (PAL) to more than 0.01 PAL, and possibly2 to more than 0.1 PAL. This transition took place long after oxygenic photosynthesis is thought to have evolved3,4,5, but the causes of this delay and of the Great Oxidation itself remain uncertain6,7,8,9,10,11. Here we show that the origin of oxygenic photosynthesis gave rise to two simultaneously stable steady states for atmospheric oxygen. The existence of a low-oxygen (less than 10-5 PAL) steady state explains how a reducing atmosphere persisted for at least 300 million years after the onset of oxygenic photosynthesis. The Great Oxidation can be understood as a switch to the high-oxygen (more than 5 × 10-3 PAL) steady state. The bistability arises because ultraviolet shielding of the troposphere by ozone becomes effective once oxygen levels exceed 10-5 PAL, causing a nonlinear increase in the lifetime of atmospheric oxygen. Our results indicate that the existence of oxygenic photosynthesis is not a sufficient condition for either an oxygen-rich atmosphere or the presence of an ozone layer, which has implications for detecting life on other planets using atmospheric analysis12,13 and for the evolution of multicellular life.

Redox investigation of some oxides of transition state metals Ni, Cu, Fe and Mn supported on SiO2 and MgAl2O4,

Abstract: Chemical-looping combustion (CLC) and chemical-looping reforming (CLR) involve the use of a metal oxide as an oxygen carrier which transfers oxygen from combustion air to the fuel. Two interconnected fluidized beds, a fuel reactor, and an air reactor are used in both processes. In the fuel reactor, the fuel is oxidized by a metal oxide, and in the air reactor, the reduced metal is oxidized back to the original phase. In CLC, a high conversion of the fuel to CO2 and H2O is required in the fuel reactor, whereas only a partial oxidation of the fuel is desired in CLR. Oxides of Ni, Cu, Fe, and Mn supported on SiO2 and MgAl2O4 were prepared by dry impregnation and investigated under alternating reducing and oxidizing conditions in a thermogravimetric analyzer at 800−1000 °C using fuel (10% CH4, 10% H2O, and 5% CO2) and oxidizing gas (5% O2). NiO and CuO supported on both SiO2 and MgAl2O4 showed very high reactivity. However, the reactivity of NiO/SiO2 decreased as a function of the cycle number at 950 °C but w...

Oxygen adsorption and stability of surface oxides on Cu ( 111 ) : A first-principles investigation

Abstract: As a first step towards gaining microscopic understanding of copper-based catalysts, e.g., for the low-temperature water-gas shift reaction and methanol oxidation reactions, we present density-functional theory calculations investigating the chemisorption of oxygen, and the stability of surface oxides on $\mathrm{Cu}(111)$. We report atomic geometries, binding energies, and electronic properties for a wide range of oxygen coverages, in addition to the properties of bulk copper oxide. Through calculation of the Gibbs free energy, taking into account the temperature and pressure via the oxygen chemical potential, we obtain the $(p,T)$ phase diagram of $\mathrm{O}∕\mathrm{Cu}(111)$. Our results show that for the conditions typical of technical catalysis the bulk oxide is thermodynamically most stable. If, however, formation of this fully oxidized surface is prevented due to a kinetic hindering, a thin surface-oxide structure is found to be energetically preferred compared to chemisorbed oxygen on the surface, even at very low coverage. Similarly to the late $4d$ transition metals (Ru, Rh, Pd, Ag), sub-surface oxygen is found to be energetically unfavorable.

Ether-Based Electrolytes for the Lithium/Oxygen Organic Electrolyte Battery

Abstract: The practical operation of a lithium/oxygen organic electrolyte battery depends on a significant amount of dissolved oxygen transporting through the organic electrolyte permeating the carbon black cathode before its reduction occurs. The rate of oxygen transport directly influences rate capability and discharge capacity. The organic electrolyte can be tailored to maximize the transport of oxygen while still retaining the ability to form a stable solid electrolyte interface with the lithium anode, chemical stability towards the discharge products Li 2 O 2 and Li 2 O, and oxidative stability to over 3 V. We investigated an ether-based electrolyte containing four different electrolyte salts to determine how electrolyte properties such as oxygen solubility, dynamic viscosity, and conductivity change with each electrolyte salt, and how this directly affects rate capability and discharge capacity. The results indicate that discharge capacity at 0.5 mA/cm 2 is determined by dynamic viscosity alone for these electrolytes, while discharge capacity at 0.2 and 0.05 mA/cm 2 shows no correlation with either oxygen solubility, dynamic viscosity, or conductivity. Our results demonstrate that a substantial improvement in rate capability can be achieved by optimizing electrolyte viscosity.

Effect of Pressure on the Behavior of Copper-, Iron-, and Nickel-Based Oxygen Carriers for Chemical-Looping Combustion

Abstract: The combustion process integrated by coal gasification and chemical-looping combustion (CLC) could be used in power plants with a low energy penalty for CO2 capture. This work analyzes the main characteristics related to the CLC process necessary to use the syngas obtained in an integrated gasification combined cycle (IGCC) power plant. The kinetics of reduction with H2 and CO and oxidation with O2 of three high-reactivity oxygen carriers used in the CLC system have been determined in a thermogravimetric analyzer at atmospheric pressure. The iron- and nickel-based oxygen carriers were prepared by freeze-granulation, and the copper-based oxygen carrier was prepared by impregnation. The changing grain size model (CGSM) was used for the kinetic determination, assuming spherical grains for the freeze-granulated particles containing iron and nickel and a platelike geometry for the reacting surface of the copper-based impregnated particles. The dependence of the reaction rates on temperature was low, with the a...

Temperature-Sensitive Europium(III) Probes and Their Use for Simultaneous Luminescent Sensing of Temperature and Oxygen

Abstract: Highly photostable and strongly luminescent europium(III) beta-diketonate complexes are presented that can act as new probes for optical sensing of temperature. They can be excited with the light of a 405-nm LED and possess strong brightnesses. The decay times of the probes contained in a poly(vinyl methyl ketone) film and in poly(tert-butyl styrene) microparticles are highly temperature-dependent between 0 and 70 degrees C. The temperature-sensitive microparticles were dispersed, along with oxygen-sensitive microbeads consisting of a palladium porphyrin oxygen indicator in poly(styrene-co-acrylonitrile), in a thin layer of a hydrogel to give a dually sensing material which is excitable by a single light source. The two emissions can be separated by appropriate optical filters. The response to oxygen and temperature is described by 3D plots, and unbiased values can be obtained for temperature and oxygen, respectively, from the two luminescence signals if refined in an iteration step. The sensing scheme is intended for use in temperature-compensated sensing of oxygen, in contactless sensing of oxygen and temperature in (micro)biological and medical applications, in high-resolution oxygen profiling, and for simultaneous imaging of air pressure and temperature in wind tunnels.

Investigation of a Ba0.5Sr0.5Co0.8Fe0.2O3−δ based cathode IT-SOFC: I. The effect of CO2 on the cell performance

Abstract: A Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) based cathode intermediate temperature solid oxide fuel cell (IT-SOFC) was fabricated and tested. The effect of carbon dioxide on the performance of BSCF cathode was evaluated at temperatures ranging from 450 to 750 °C. The current density was recorded at a constant discharge of voltage value and the electrochemical impedance spectra (EIS) measurements were carried out in the absence and in the presence of CO2 in the oxidant gas line (cathode). It was found that the presence even of relatively small quantities (0.28–3.07%) of CO2 negatively affects the H2-IT-SOFC performance. It was shown that as the CO2 content in the cathode side increases and as the operation temperature decreases, the fuel cell performance is seriously aggravated up to 550 °C in a reversible way. A further decrease of the operation temperature deteriorates the SOFC performance irreversibly. However, the cell performance can be recovered after treatment at 800 °C in pure oxygen. It was also shown that as the CO2 content increases, the rate of oxygen electrochemical reduction decreases and the corresponding apparent activation energy increases linearly. The EIS results show that the interface resistance increases dramatically after carbon dioxide is added into the oxidant gas line. It is believed that carbon dioxide and temperature, acting in a synergetic way, decrease at least the cathode activity for oxygen reduction. This behaviour could be attributed to the strong carbon dioxide adsorption on the BSCF surface and to the formation of carbonates at temperatures as low as 500 and 450 °C.

Imaging Adsorbate O−H Bond Cleavage: Methanol on TiO2(110)

Abstract: We investigated methanol adsorption and dissociation on bridge-bonded oxygen vacancies of the TiO2(110)-(1×1) surface using in situ scanning tunneling microscopy. We provide the first direct evidence that methanol dissociates on oxygen vacancies via O−H bond scission rather than C−O scission. For CH3OH coverages lower than the oxygen vacancy concentration, stationary methoxy−hydroxyl pairs form. At CH3OH coverages close to the oxygen vacancy concentration undissociated mobile CH3OH interacts with methoxy−hydroxyl pairs and facilitates the movement of hydroxyl away from the methoxy group.

Control of Si nanowire growth by oxygen.

Abstract: Semiconductor nanowires formed using the vapor-liquid-solid mechanism are routinely grown in many laboratories, but a comprehensive understanding of the key factors affecting wire growth is still lacking. In this paper we show that, under conditions of low disilane pressure and higher temperature, long, untapered Si wires cannot be grown, using Au catalyst, without the presence of oxygen. Exposure to oxygen, even at low levels, reduces the diffusion of Au away from the catalyst droplets. This allows the droplet volumes to remain constant for longer times and therefore permits the growth of untapered wires. This effect is observed for both gas-phase and surface-bound oxygen, so the source of oxygen is unimportant. The control of oxygen exposure during growth provides a new tool for the fabrication of long, uniform-diameter structures, as required for many applications of nanowires.

Impact of Oxygen on Photopolymerization Kinetics and Polymer Structure

Abstract: The effects of varying polymerization conditions on the extent of oxygen inhibition of free-radical photopolymerization were investigated using both experimental and modeling efforts. Specifically, sample thickness, initiation rate, and oxygen concentration were varied, and the resulting photopolymerization kinetics were studied using real-time FTIR and a comprehensive photopolymerization model. As the sample thickness was increased, the overall average double-bond conversion increased due to the greater conversion in the lower depths of the films. Because of the dramatic inhibition at the top of the film, this led to heterogeneity in the film's conversion as shown with the comprehensive model. Increasing the initiation rate decreased the inhibition time and increased the overall average double-bond conversion due to an increase in the overall radical production, allowing better competition of the propagation reaction with the inhibition reaction. The oxygen concentration was varied, and as the oxygen con...