Showing papers on "Oxygen published in 1990"

Structural anomalies, oxygen ordering and superconductivity in oxygen deficient Ba2YCu3Ox

Abstract: We report the characterization of series of oxygen deficient Ba2YCu3Ox samples for 7 ≥ x ≥ 6 prepared by Zr gettered annealing at 440°C. Measurements include complete crystal structure analysis at 5 K by powder neutron diffraction, electron microscopy study of the oxygen ordering, and magnetic measurements of the superconducting transitions, with particular attention to the transition widths. The results show for the first time that the 90 K and 60 K plateaus in Tc as a function of oxygen stoichiometry are associated with plateaus in the effective valence of the plane coppers. We also correlate the disappearance of superconductivity for x

Where metal ions bind in proteins.

Abstract: The environments of metal ions (Li+, Na+, K+, Ag+, Cs+, Mg2+, Ca2+, Mn2+, Cu2+, Zn2+) in proteins and other metal-host molecules have been examined. Regardless of the metal and its precise pattern of ligation to the protein, there is a common qualitative feature to the binding site: the metal is ligated by a shell of hydrophilic atomic groups (containing oxygen, nitrogen, or sulfur atoms) and this hydrophilic shell is embedded within a larger shell of hydrophobic atomic groups (containing carbon atoms). That is, metals bind at centers of high hydrophobicity contrast. This qualitative observation can be described analytically by the hydrophobicity contrast function, C, evaluated from the structure. This function is large and positive for a sphere of hydrophilic atomic groups (characterized by atomic solvation parameters, delta sigma, having values less than 0) at the center of a larger sphere of hydrophobic atomic groups (characterized by delta sigma greater than 0). In the 23 metal-binding molecules we have examined, the maximum values of the contrast function lie near to observed metal binding sites. This suggests that the hydrophobicity contrast function may be useful for locating, characterizing, and designing metal binding sites in proteins.

Selective reduction of nitrogen oxides with hydrocarbons over solid acid catalysts in oxygen-rich atmospheres

Abstract: Highly selective reduction of nitrogen oxides to dinitrogen occurs to a high level in oxygen-rich atmospheres by using a small amount of propane as a reducing agent over alumina, silica-alumina, titania and zirconia catalyst. Judging from the data of activity and ammonia TPD measurement on a series of silica-alumina catalysts, acidity is suggested to be one of the main factors that determine catalytic activity.

The short range structure of sodium phosphate glasses I. MAS NMR studies

Abstract: A series of x(Na2O + H2O)·(1 − x)P2O5 glasses have been characterized by 31P and 23Na magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. High-resolution 31P NMR spectra reveal the presence of Q2-(2 bridging oxygen/tetrahedron) and Q3-(3 bridging oxygen/tetrahedron) tetrahedral sites in glasses with x 0.5. Quantitative measurements of the respective NMR site populations are in excellent agreement with Van Wazer's predictions for the structures of ionic phosphates and illustrate the depolymerizing effects of residual H2O on the glass structure. The systematic change in the Q231P chemical shift is consistent with an increase in the average π-bond character of the phosphorus-nonbridging oxygen bond as x decreases.

Active‐to‐Passive Transition in the Oxidation of Silicon Carbide and Silicon Nitride in Air

Abstract: The active-to-passive transition in the oxidation of SiC and Si3N4 was determined in a flowing air environment as a function of temperature and total pressure. The experimentally observed transition temperatures ranged from a low of 1347 C to a high of 1543 C for partial pressures of oxygen of 2.5 and 123.2 Pa, respectively. The SiC and Si3N4 samples had approximately the same transition point for a given pressure. In general, the higher the flow rate, the higher the transition temperature for a given pressure. The transitions for SiC measured in this study agree with previous data for the transition of SiC measured in pure oxygen at reduced pressures and in oxygen inert gas mixtures.

Complete reduction of carbon dioxide to carbon using cation-excess magnetite

Abstract: THE reduction of gaseous oxides such as CO2 and H2O is an important concern in industrial processes and pollution control. Here we report the reduction of carbon dioxide to carbon with an efficiency of nearly 100% at 290 °C using cation-excess magnetite (Fe3+δO4, δ =0.127). In this reaction, the oxygen in the CO2 is transferred, in the form of O2−, to the cation-excess magnetite, and no gas is evolved. The carbon in the CO2 is reduced to carbon (zero valence) by the addition of an electron donated from the cation-excess magnetite to maintain electrical neutrality during the transfer of the O2− to the magnetite. When we used H2O in place of CO2, hydrogen gas was evolved, indicating that the same mechanism can also reduce H2O.

On the nature of the oxygen-related defect in aluminum nitride

Abstract: The oxygen-related defect in an aluminum nitride (AIN) single crystal and in polycrystalline ceramics is investigated utilizing photoluminescence spectroscopy, thermal conductivity measurements, x-ray diffraction lattice parameter measurements, and transmission electron microscopy. The results of these measurements indicate that at oxygen concentrations near 0.75 at.%, a transition in the oxygen accommodating defect occurs. On both sides of this transition, simple structural models for the oxygen defect are proposed and shown to be in good agreement with the thermal conductivity and lattice parameter measurements, and to be consistent with the formation of various extended defects (e.g., inversion domain boundaries) at higher oxygen concentrations.

Method for reduction of nitrogen oxides with ammonia using promoted zeolite catalysts

Abstract: A method comprising passing through a zeolite catalyst, a gaseous stream containing nitrogen oxides, ammonia and oxygen to selectively catalyze the reduction of nitrogen oxides and, if excess or unreacted ammonia is present, to oxidize the excess of unreacted ammonia with oxygen to hydrogen and water The method includes the use of a zeolite catalyst composition which comprises a metal (eg, iron or copper) promoted zeolite, the zeolite being characterized by having a silica to alumina ratio of at least about 10 and a pore structure which is interconnected in all three crystallographic dimensions by pores having an average kinetic pore diameter of at least about 7 Angstroms Promoted zeolites of the above type have demonstrated high tolerance for sulfur poisoning, good activity for the selective catalytic reduction of nitrogen oxides with ammonia, good activity for the oxidation of ammonia with oxygen, and the retention of such good activities even under high temperature operations, eg, 400°C or higher, and hydrothermal conditions

Histidine oxidation in the oxygen-evolving photosystem-II enzyme

Abstract: THE evolution of oxygen as a result of light-driven water oxidation occurs in plants and is catalysed by photosystem-II (PS-II). A manganese-cluster probably acts both as the active site and as a charge-accumulating device (for a review, see ref. 1). The enzyme cycle involves five redox states which are denoted as S0–S4, depend-ing on the number of positive equivalents stored2. Oxygen is released after formation of the transient S4 state. Ca2+ is an obligatory cofactor in this process and its depletion inhibits the enzyme cycle at the step before water oxidation, that is, after formation of the S3 state3. In chelator-treated, Ca2+-depleted PS-II, a new electron paramagnetic resonance (EPR) signal arising from a formal S3 state has been reported4. It was suggested that the S3 EPR signal could originate from the oxidation of an amino acid, interacting magnetically with the manganese cluster4. There are only a few examples of amino-acid oxidation in enzyme chemistry and these are limited to tyrosine5 and tryptophan6. Here we report evidence that the S2 to S3 transition occurring in Ca2+-depleted PS-II corresponds to the oxidation of histidine.

Bacterial transformations of inorganic nitrogen in the oxygen-deficient waters of the Eastern Tropical South Pacific Ocean

Abstract: Rates of transformations of inorganic nitrogen were measured in the low oxygen, subsurface waters (50–450 m) of the Eastern Tropical South Pacific during February 1985, using 15N tracer techniques. Oxygen concentrations over the entire region were in a range (O2 Measured rates of nitrate reduction and estimated rates of denitrification were sufficient to respire nearly all of the surface primary production that might be transported into the oxygen deficient zone. These results imply that the supply of labile organic material, especially from the surface, was more important than oxygen concentration in modulating the rates of nitrogen transformations within the low oxygen water mass of the Eastern Tropical South Pacific. The pattern of nitrite oxidation and nitrite reduction activities in the oxygen minimum zone supports the hypothesis ( Anderson et al., 1982, Deep-Sea Research, 29, 1113–1140) that nitrite, produced from nitrate reduction, can be recycled by oxidation at the interface between low and high oxygen waters. Rates for denitrification, estimated from nitrate reduction rates, were in harmony with previous estimates based on electron transport system (ETS) measurements and analysis of the nitrate deficit and water residence times. Assimilation rates of NH4+ were substantial, providing evidence for heterotrophic bacterial growth in low oxygen waters. Ambient concentrations of ammonium were maintained at low values primarily by assimilation; ammonium oxidation was an important mechanism at the surface boundary of the low oxygen zone.

Mechanism of enhanced oxygen transfer in fermentation using emulsified oxygen-vectors.

Abstract: Limitations of oxygen transfer in fermentation can be solved using auxiliary liquids immiscible in the aqueous phase The liquids (called oxygen-vectors) used in this study were hydrocarbon (n-dodecane) and perfluorocarbon (forane F66E) in which oxygen is highly soluble (549 mg/L in n-dodecane and 118 mg/L in forane F66E at 35 degrees C in contact with air at atmospheric pressure) It has been demonstrated that the use of n-dodecane emulsion in a culture of Aerobacter aerogenes enabled a 3 5-fold increase of the volumetric oxygen transfer coefficient(k(L)a) calculated on a per-liter aqueous phase basis The droplet size of the vector played a crucial role in the phenomena When a static contact between gas bubble and vector droplet was established in water, the vector covered the bubble, in agreement with positive values of the spreading coefficient for these fluids The determination of the oxygen transfer coefficients (k(L)) in a reactor with a definite interfacial area enabled the main resistance to be located in the boundary layer of the waterside either for a gas-water or a vector-water interface Because oxygen consumption by weakly hydrophobic cells can only occur in the aqueous phase, the oxygen transfer is achieved according to the following pathway: gas-vector-water-cell Finally, a mechanism for oxygen transfer within this four-phased system is proposed

Oxygen optrode for use in a fiber-optic glucose biosensor.

Abstract: An optical fiber oxygen sensor, based on the dynamic quenching of the luminescence of tris(1,10-phenanthroline)-ruthenium(II) cation by molecular oxygen, is presented. The complex is adsorbed onto silica gel, incorporated in a silicone matrix possessing a high oxygen permeability, and placed at the tip of the optical fiber. Oxygen has been monitored continuously in the 0-750 Torr range, with the detection limit being as low as 0.7 Torr. The device has been applied to the development of a fast responding and highly sensitive fiber-optic glucose biosensor based on this highly sensitive oxygen transducer. The sensor relates oxygen consumption (as a result of enzymatic oxidation) to glucose concentration. The enzyme is immobilized on the surface of the oxygen optrode; carbon black is used as an optical isolation in order to prevent ambient light and sample fluorescence to interfere. Measurements have been performed in a flow-through cell in air-equilibrated glucose standard solutions of pH 7.0. The effects of enzyme immobilization procedures (including enzyme immobilization on carbon black) as to response times (around 6 min), analytical ranges (0.06-1 mM glucose), reproducibility in sensor construction, and long-term stability have been studied as well.

Analysis of the modified complete Oregonator accounting for oxygen sensitivity and photosensitivity of Belousov-Zhabotinskii systems

Abstract: The Oregonator model of the Belousov-Zhabotinsky (BZ) reaction is modified by introduction of a bromide flow corresponding to the influences of oxygen and for light on BZ systems. Due to the flow term, the Oregonator may exhibit three stationary solutions with corresponding bistability. Hopf bifurcations with emanating periodic solutions occur on two independent stationary solution branches. Bifurcations appear to be subcritical if oscillations are of relaxational type due to different time scales for each variable. With decreasing differences in these time scales, supercritical Hopf bifurcations occur emanating period solutions with small amplitudes which are sensitive to macroscopic excitations

Oxidation of Single‐Crystal Silicon Carbide Part I . Experimental Studies

Abstract: The oxidation of single crystal in dry oxygen (10−3–1 atm and 1200°–1500°C) followed parabolic kinetics. Two different apparent activation energies were calculated for oxidation of the (0001) C faces of , approximately 120 kJ/mol below 1350°C and 260 kJ/mol above 1350°C. Two regimes were not apparent for oxidation of the (0001) Si faces, and apparent activation energies lay between 223 and 298 kJ/mol. Double oxidation experiments using and indicated that the process is dominated by the transport of molecular oxygen at lower temperatures (<1300°C) with a substantial contribution from diffusion of ionic oxygen at higher temperatures. Epitaxially grown Si 13C films on alpha‐Si 12C substrates via CVD were used to study carbon transport behavior during oxidation of . Depth profiles for carbonaceous species using SIMS showed that carbon can transport quickly through the oxide layer, which eliminates the possibility that transport of carbonaceous species is rate controlling in the oxidation of . Oxidation mechanisms of are discussed on the basis of these results.

X-ray analysis of the incommensurate modulation in the 2:2:1:2 Bi-Sr-Ca-Cu-O superconductor including the oxygen atoms

Abstract: Single-crystal x-ray diffraction data on the 2:2:1:2 Bi-Sr-Ca-Cu-O superconducting phase have been used in a four-dimensional refinement of the modulations of both the metal and oxygen atoms. An expression for an analytical temperature-factor modulation function has been derived and applied to the bismuth atoms. The large value of the temperature factor of Bi in some of the unit cells indicates a contribution from static disorder. The modulation of the oxygen atoms in the bismuth layer is best described by a function that allows the displacement to vary linearly with unit-cell position and incorporates the possibility of excess oxygen near the end of a modulation period. The results on oxygen occupancy and the behavior of the Bi temperature factor are compatible with models that attribute the origin of the modulation to the existence of extra oxygen atoms in the Bi-O layers. Refinement in the noncentrosymmetric super space group M:A2aa:1\ifmmode\bar\else\textasciimacron\fi{}11 leads to significantly better results than the centrosymmetric treatment.

Radiofrequency plasma deposition of oxygen-containing films on polystyrene and poly(ethylene terephthalate) substrates improves endothelial cell growth.

Abstract: Polystyrene and poly(ethylene terephthalate) substrates were modified by radiofrequency plasma deposition with organic vapors comprised of carbon, oxygen, and hydrogen (acetone, methanol, glutaraldehyde, formic acid, allyl alcohol, and ethylene oxide). The treatments resulted in the deposition of a film at least 100 A thick containing up to 26% atomic oxygen at the surface. A high oxygen incorporation was observed for vapors with a large oxygen-to-carbon ratio. Bovine aortic endothelial cell growth measured on acetone, methanol, and glutaraldehyde films was linearly correlated with the oxygen content of the treated surfaces. Nitrogen was incorporated in the surface by blending nitrogen gas into the organic vapors used for plasma deposition. The resulting nitrogen-containing substrates exhibited a high affinity for serum fibronectin but a moderate cell growth.

Method for monitoring glucose

Abstract: A method for monitoring the glucose level in a body fluid uses an apparatus which includes a conjugate of glucose oxidase and a fluorescent dye coated onto an optical fiber in contact with the body fluid, a source of excitation light and a fluorescence emission detector. Glucose is oxidized by oxygen in the body fluid causing a decrease in oxygen concentration at the enzyme. The fluorescent dye is sensitive to oxygen quenching so that, when the oxygen concentration decreases, fluorescence emission increases in direct proportion to the glucose concentration in the fluid.

Measurement and modelling of oxygen release from roots of phragmites australis

Abstract: Radial oxygen flux from adventitious and lateral roots of Phragmites australis in stagnant and streaming oxygen-free waters were measured polarographically. The potential for oxygen release to anaerobic sediments was predicted by extrapolating from this data and by mathematical modelling, and the oxidation of sediments demonstrated by indicator dye and redox potential measurements. Oxygen losses from roots were much greater during the day than at night and reflected the substantial day-time throughflow convection of gases from culm to rhizome which occurs in this species. Lateral roots, borne at frequencies of ≤120/cm of adventitious root were identified as potentially the major source of oxygen release. Predictions of sediment oxygenation based on oxygen release from single adventitious roots plus laterals in a streaming oxygen-free water-flow system, yielded values of 5-12 g O2 m-2 d-1, based on 150 shoots m-2, the conservative estimate of 10 roots per shoot, and a mean rhizome oxygen concentration of 17%. Mathematical modelling predictions amounted to 4.6-9.6 g of O2 m-2 d-1 based on the same shoot and root numbers, with 600 laterals/root, soil oxygen demand of 50-500 ng cm-3 s-1, and soil oxygen diffusivities from 0.3(clay)-0.85(sand) x 10-5 cm2 s-1. It was estimated that the respective amounts of oxygen likely to diffuse into the sediment from the soil surface would be be 1.0-7.3 g m-2 d-1. It is emphasised that sediment oxygenation by roots may be enhanced proportionally only by greater root numbers, not greater sediment oxygen demand.

Thin films of Co3O4 and NiCo2O4 obtained by the method of chemical spray pyrolysis for electrocatalysis III. The electrocatalysis of oxygen evolution

Abstract: The electrocatalytic properties of thin Co3O4 and NiCo2O4 films prepared on CdO-nonconductive glass by the method of chemical spray pyrolysis were investigated for oxygen evolution in varying KOH concentrations. The Tafel slopes were close to (2.3RT/F) V per decade for both oxides. The reaction order with respect to [OH−] was found to be approximately 1.3. It was observed that Co3O4 is more active than NiCo2O4 towards oxygen evolution. A mechanism for oxygen evolution involving the electrochemical adsorption of OH− as a fast step and the electrochemical desorption of OH forming an intermediate H2O2 as the rate-determining step has been suggested.

Low and high temperature TiO2 oxygen sensors

Abstract: We have studied two types of TiO2-based oxygen sensors operating at different temperatures with different detection principles At high temperatures, TiO2 devices can be used as thermodynamically controlled bulk defect sensors to determine oxygen over a large range of partial pressures Their intrinsic behaviour can be controlled by carefully directed doping with tri- or pentavalent cations At low temperatures, we find that Pt/TiO2 Schottky diodes make extremely sensitive oxygen detection possible The latter show reversible shifts of current-voltage curves, which are determined by interface states formed by chemisorbed oxygen