Showing papers on "Oxygen published in 1980"

A molecular beam study of the catalytic oxidation of CO on a Pt(111) surface

Abstract: The oxidation of carbon monoxide catalyzed by Pt(111) was studied in ultrahigh vacuum using reactive molecular beam–surface scattering. Under all conditions studied, the reaction follows a Langmuir–Hinshelwood mechanism: the combination of a chemisorbed CO molecule and an oxygen adatom. When both reactants are at low coverage, the reaction proceeds with an activation energy E*LH =24.1 kcal/mole and a pre‐exponential υ4 =0.11 cm2 particles−1 sec−1. At very high oxygen coverage, E*LH decreases to about 11.7 kcal/mole and υ4 to about 2×10−6 cm2 particles−1 sec−1. This is largely attributed to the corresponding increase in the energy of the adsorbed reactants. When a CO molecule incident from the gas phase strikes the surface presaturated with oxygen, it enters a weakly held precursor state to chemisorption. Desorption from this state causes a decrease in chemisorption probability with temperature. Once chemisorbed, the CO molecule then has almost unit probability of reacting to produce CO2 below 540 K. The CO2 product angular distribution varies from cosγ to cos4γ depending sensitively upon the adsorbed reactant concentrations.

Distribution of oxygen in marine sediments measured with microelectrodes1

Abstract: . Abstract Membrane-covered platinum electrodes with a tip diameter of 2-8 pm were used for an amperometric assay of dissolved oxygen in marine sediments. The oxygen profile extended to 3-5mm depth in nonilluminated sediment; even at high light intensities and at low tem- peratures it did not extend below lo-mm depth in a homogeneous sandy sediment. Oxygen profiles recorded during light-dark cycles were used to estimate the rates of oxygen produc- tion and consumption and also to calculate the apparent diffusion coefficient for oxygen in the sediment. Apparently macrofaunal activity, rather than molecular diffusion and water turbulence, was important for the occasional transport of oxygen into deeper layers and thus for the provision of oxidized conditions (positive redox potential) down to 5-10 cm below the sediment surface.

Normobaric oxygen toxicity of the lung

Abstract: ALTHOUGH oxygen therapy has been of clear benefit in many clinical settings, it also carries a risk of tissue damage. All tissues of the body can be injured by sufficiently high oxygen concentrations, but the lung is exposed directly to the highest partial pressure of inspired oxygen. The precise concentration of oxygen that is toxic to the lung probably depends on a large number of variables in the exposed person, including age, nutrition, endocrine status, and previous exposure to oxygen or other oxidants.1 2 3 Hyperbaric oxygen accelerates the effects of oxygen toxicity and also damages the central nervous system, probably because . . .

Responses of superoxide dismutase and glutathione reductase activities in cotton leaf tissue exposed to an atmosphere enriched in oxygen.

Abstract: Responses of superoxide dismutase (EC 1.15.1.1) and glutathione reductase (EC 1.6.4.2) activities were evaluated in leaf tissue from intact cotton plants (Cotton Branch 1697) which were exposed to 75% O(2), 350 microliters per liter CO(2) for 48 hours. Soluble protein was extracted from O(2)-treated and control tissue, and enzyme levels were determined. Superoxide dismutase activity in cotton leaf tissue was high (26 units per milligram protein) under normal conditions of 21% O(2), saturating light, and limiting CO(2), and neither qualitative nor quantitative differences in the cyanide-sensitive or -insensitive forms of the enzyme occurred in response to hyperoxic conditions. Glutathione reductase activity, however, was 2- to 3-fold higher in extracts from tissue exposed to 75% O(2). No increase in activity was observed for the peroxisomal enzymes, glycolate oxidase (EC 1.1.3.1) and catalase (EC 1.11.1.6). Results are consistent with an integrated pathway involving superoxide dismutase and glutathione reductase for protection of sensitive leaf components against detrimental effects of intermediate reduction products of O(2).

The ultraviolet absorption spectrum of liquid water

Abstract: The absorptivities of carefully purified water have been measured at 1 nm intervals in the wavelength range 196 to 320 nm. The measured absorptivities fell monotonically from a value of (1.26±0.03) m−1 at 196 nm, to a value of (0.0100±0.0006) m−1 at 320 nm. The water was purified by ion–exchange, followed by four subsequent distillations, the first being carried out under mild oxidizing conditions, and the last being performed in all‐silica apparatus, the water vapor being heated to a temperature of 870 K in the presence of oxygen to remove residual organic impurities. The absorptivities were measured using a differential pathlength method and a correction for the effect of double reflections in the absorption cells was derived and applied to the data. The measured absorptivities were much lower than all previously reported values in the 200–300 nm region, but gave considerably better agreement with theory and aligned well with the edge of a set of recent vacuum UV measurements on high purity water. It ap...

Effects of feeding on metabolic rate, and the Specific Dynamic Action in plaice, Pleuronectes platessa L.

Abstract: The rate of oxygen consumption of plaice increases after feeding and declines to a resting level after 24–72 h. The maximum increase corresponds to a level which is approximately twice the resting rate of oxygen consumption. This increase corresponds to the Specific Dynamic Action (SDA) and increases in magnitude with increase in food intake. The magnitude is greatest with high protein content diets. The duration of the SDA effect is reduced with increase in temperature and increases with the percentage of protein in the diet.

On High Temperature Oxidation of Chromium I . Oxidation of Annealed, Thermally Etched Chromium at 800°–1100°C

Abstract: Oxidation of annealed, thermally‐etched chromium has been studied at different oxygen pressures ranging from in the temperature range 800°–1100°C. A varied oxidation behavior is observed: at and 800°C the oxidation is approximately logarithmic, while above 900°C it is parabolic followed by repeated breakdowns and protective stages. At reduced oxygen pressures the oxidation behavior changes and becomes linear at the lower oxygen pressures. The different kinetics are correlated with properties of the scales which may crack, wrinkle, or balloon depending upon the reaction conditions. Large compressive stresses are built up in the scales during oxidation and the ability of the scale to deform increases dramatically with decreasing partial pressure of oxygen. Under most conditions the oxide scales become detached from the metal substrate. It is concluded that volume diffusion and transport along cracks and grain boundaries contribute to the growth of scales.

Shock compression of liquid argon, nitrogen, and oxygen to 90 GPa (900 kbar)

Abstract: Dynamic equation‐of‐state data for liquid Ar, N2, and O2 were measured in the shock pressure range 21–91 GPa (210–910 kbar) by means of a two‐stage light‐gas gun. The liquids were shocked from initial states near their saturation curves at 0.1 MPa (1 bar) and 80 K. The cryogenic target system is described. The initial liquid densities were obtained by measuring the temperature and pressure of the specimens and calculating the densities from published equations of state. Shock velocities were measured to 0.5–1.1% accuracy with an electronic detection system with subnanosecond time resolution. Impactor velocities in the range 4–7 km/s were measured to 0.1% accuracy with a flash radiographic technique. Mass velocities were obtained by the method of shock impedance matching. The data are discussed in terms of the statistical–mechanical theories of Ross and Ree.

Influence of oxygen on silicon resistivity

Abstract: The influence of oxygen on the resistivity of dislocation‐free silicon wafers was studied after annealing at various temperatures under a nitrogen ambient. Significant resistivity shifts were observed in the temperature range 600–900 °C and related to oxygen precipitation. This phenomenon is independent of the well‐known donor generation observed around 450 °C and is more difficult to cure.

Oxygen uptake, acid-base status, and performance with varied inspired oxygen fractions

Abstract: Six subjects rode a bicycle ergometer on three occasions breathing 17, 21, or 60% oxygen. In addition to rest and recovery periods, each subject worked for 10 min at 55% of maximal oxygen uptake (VO2 max) and then to exhaustion at approximately 90% VO2 max. Performance time, inspired and expired gas fractions, ventilation, and arterialized venous oxygen tension (PO2), carbon dioxide tension (PCO2), lactate, and pH were measured. VO2, carbon dioxide output, [H+]a, and [HCO3-]a were calculated. Performance times were longer in hyperoxia than in normoxia or hypoxia. However, VO2 was not different at exhaustion in normoxia compared with hypoxia or hyperoxia. During exercise, hypoxia was associated with increased lactate levels and decreased [H+]a, PCO2, and [HCO3-]a. The opposite trends were generally associated with hyperoxia. At exhaustion, [H+]a was not different under any inspired oxygen fraction. These results support the contention that oxygen is not limiting for exercise of this intensity and duration. The results also suggest that [H+] is a possible limiting factor and that the effect of oxygen on performance is perhaps related to control of [H+].

Regulation of citric acid production by oxygen: Effect of dissolved oxygen tension on adenylate levels and respiration in Aspergillus niger

Abstract: The mechanism of the control of citric acid accumulation by oxygen was investigated by means of pilot plant fermentation using Aspergillus niger. The critical dissolved oxygen tension (DOT) for oxygen uptake of this fungus was about 18–21 and 23–26 mbar for trophophase and idiophase, respectively. Minimal DOT for citric acid production was about 25 mbar. Citric acid production increased steadily between 40–150 mbar. Short time changes in the DOT produced immediate, irreversible changes in the rate of product formation. Adenine nucleotides paralleled growth but showed no evidence for control function in the oxygen effect on citric acid fermentation. A branched respiratory system was identified by experiments using specific inhibitors (antimycin, cyanide, azide, rotenone, amytal and salicylhydroxamic acid). Growth was sensitive towards inhibitors of the standard respiratory chain, but only slightly sensitive towards salicylhydroxamic acid (SHAM). Citric acid synthesis was highly sensitive towards SHAM during trophophase, but sensitive towards antimycine during idiophase. Interruptions in aeration cause an impairment of the SHAM sensitive oxidase during trophophase, and of the antimycin sensitive oxidase during idiophase.

Oxidation of polycrystalline indium studied by x‐ray photoelectron spectroscopy and static secondary ion mass spectroscopy

Abstract: The chemisorption of oxygen on clean polycrystalline indium surfaces is investigated by the combined techniques of x‐ray photoelectron spectroscopy (XPS) and the static mode of secondary ion mass spectroscopy (SIMS). Oxygen uptake is characterized by a moderately rapid formation of In2O3, with no preoxidation adsorbed O2 phase discernable in the XPS spectra. A high binding energy O (1s) peak becomes prominent after one monolayer of In2O3 has formed, which we ascribe to adsorbed oxygen on the oxide. Formation of In2O3 continues into the bulk either via migration of the adsorbed oxygen through the oxide layer to the metal‐In2O3 interface or by the outward migration of indium cations to the surface. Both the formation of oxide and the onset of adsorption are reflected in the secondary ion yield ratios InO−2/InO− and O−2/O−. The behavior of SIMS molecular cluster ion yields with oxygen exposure are consistent with recent theoretical calculations of the cluster formation process, in which considerable local atomic order of the surface is present in the cluster.

Electronic and structural properties of plasma-deposited a-Si:O:H - The story of O2

Abstract: Measurements of luminescence, optical absorption and infrared vibrational spectra are reported for thin films plasma-deposited from SiH4/O2/Ar mixtures. The addition of oxygen causes the optical absorption edge and luminescence peak position to shift monotonically to higher photon energy. Increases in the luminescence peak width and in the temperature of thermal quenching result in room temperature luminescence in the visible at high oxygen contents.

Respiratory Function of the Hemocyanins

Abstract: Recent studies clearly demonstrate the respiratory importance of the hemocyanins in each of the three animal phyla in which they occur. Despite their generally low oxygen affinity, hemocyanins can be highly oxygenated at the site of gas exchange with the medium as well as deoxygenated at the tissues. The functional range of a hemocyanin oxygen transport system is severely limited however by environmental change. These systems function under incipient hypoxia due largely to responses of blood pH which are not fully understood a normal Bohr shift is accompanied by a rise in blood pH and a reverse Bohr shift by a decrease in blood pH. In both instances blood oxygen affinity increases and its oxygenation state at the gill remains high in spite of its lower Po2. Dilution of the blood at low salinity generally alters its oxygenation properties both oxygen affinity and cooperativity. These properties may or may not be restored by concomitant changes in blood pH, which depend on the various mechanisms of osmotic adaptation. Within a homogeneous taxon the oxygenation properties of a hemocyanin appear to be highly conservative showing little interspecific adaptation except to extreme changes in the mode of gas exchange. Unlike that in vertebrates air-breathing in crustaceans is accompanied by an increase in blood oxygen affinity. Similar oxygen affinities in latitudinally separated species result in optimal functioning of the system at the same temperature, corresponding to different seasons. In eurythermal species a temperature acclimation of oxygen affinity extends the operating range of the crustacean hemocyanins but they cannot deoxygenate at very low temperatures. Unsolved problems of hemocyanin function include specific effects of pH and CO2 the basis of which is not entirely clear, and the postulated occurrence in native blood of both dialyzable and non-dialyzable substances that modify oxygen affinity the identity of which is unknown. With the exception of the crustacean oxygen carrier the hemocyanins confer a respiratory advantage over their predecessors. But the oxygen carrying capacity of crustacean blood never reaches the levels found in the annelids and molluscs due to the colloid osmotic pressure of the relatively low molecular weight hemocyanin and to the drop in blood hydrostatic pressure accompanying the loss of a fluid skeleton. The selection of a blood oxygen carrier with an apparently limiting combination of respiratory and osmotic properties is obscured by the uncertain phylogenetic position of the phylum.