Showing papers on "Oxygen published in 1999"
TL;DR: The purpose of this review was to compile data from many different sources to provide a convenient resource for biological scientists with interest in this area and is essentially an updated subset of the excellent comprehensive review on singlet oxygen yields by Wilkinson et af.
Abstract: Singlet molecular oxygen, 02(’Ag) , has been intensively investigated over the last few decades, primarily in relation to photosensitization reactions. In combination with the increasing awareness and interest in singlet oxygen has been the development of various methodologies for the detection and quantification of singlet oxygen production. A large body of literature now exists concerning the singlet oxygengenerating abilities of a large variety of endogenous and exogenous compounds that can be photoactivated in the UV and visible regions of the electromagnetic spectrum. The purpose of this review was to compile data from many different sources to provide a convenient resource for biological scientists with interest in this area. This compilation is essentially an updated subset of the excellent comprehensive review on singlet oxygen yields by Wilkinson et af. (l), which, unlike this compilation, was not limited to biological molecules. The data are current up to the early months of 1999. The following tables include various related parameters in addition to the central parameter of the quantum yield of singlet oxygen generation (QA) for a given photosensitizer. The methodology or technique used to measure the QA value for each entry is given and ranges from direct detection of the luminescence produced on relaxation of singlet oxygen (time-resolved [TRILIT or steady-state [SSIL]), calorimetric
984 citations
TL;DR: Oxygen-18 isotope labeling showed that water is the source of the oxygen atoms in the molecular oxygen evolved, and so this system is a functional model for photosynthetic water oxidation.
Abstract: The formation of molecular oxygen from water in photosynthesis is catalyzed by photosystem II at an active site containing four manganese ions that are arranged in di-mu-oxo dimanganese units (where mu is a bridging mode). The complex [H2O(terpy)Mn(O)2Mn(terpy)OH2](NO3)3 (terpy is 2,2':6', 2"-terpyridine), which was synthesized and structurally characterized, contains a di-mu-oxo manganese dimer and catalyzes the conversion of sodium hypochlorite to molecular oxygen. Oxygen-18 isotope labeling showed that water is the source of the oxygen atoms in the molecular oxygen evolved, and so this system is a functional model for photosynthetic water oxidation.
644 citations
TL;DR: In this article, the influence of the oxygen content on the metal-insulator transition and the corresponding magnetic susceptibility anomaly is studied for LnBaCo2O5+δ with various oxygen stoichiometries.
503 citations
Patent•
17 Sep 1999TL;DR: In this article, a converter containing a NOx absorbing and reducing catalyst is disposed in the exhaust passage of an internal combustion engine, where the upstream half portion (portion of the inlet side) of the substrate carries the oxygen storage component that absorbs oxygen in exhaust gas and releases the absorbed oxygen when the air-fuel ratio of the exhaust gas flowing in is rich.
Abstract: A converter containing a NOx absorbing and reducing catalyst is disposed in the exhaust passage of an internal combustion engine. The upstream half portion (portion of the inlet side) of the substrate of the NOx absorbing and reducing catalyst in the converter carries the oxygen storage component that absorbs oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas is lean and releases the absorbed oxygen when the air-fuel ratio of the exhaust gas flowing in is rich in addition to carrying the NOx absorbing and reducing catalyst. After NOx is absorbed by the NOx absorbing and reducing catalyst as a result of operating the engine at a lean air-fuel ratio, the engine is operated at a rich air-fuel ratio, so that NOx is released from the NOx absorbing and reducing catalyst and is purified by reduction. Here, oxygen is released from the oxygen storage component carried by the upstream half portion of the substrate and is reacted with the H 2 and CO components in the exhaust gas, so that the temperature of the NOx absorbing and reducing catalyst is raised within short periods of time due to the heat of reaction. Therefore, the catalyst exhibits increased activity and the NOx absorbing and reducing catalyst exhibits improved NOx purification capability.
494 citations
TL;DR: In this paper, a spin-polarized DFT calculation was used to estimate the probability of an adsorbed O2 molecule following the molecular and dissociative channels in a 1:1 ratio with each vacancy site.
Abstract: vacancy population), presumably due to dissociative filling of the vacancy sites in a 1:1 ratio. Above a coverage of 4 10 13 molecules/cm 2 , a first-order O2 TPD peak appears at 410 K. Oxygen molecules in this peak do not scramble oxygen atoms with either the surface or with other coadsorbed oxygen molecules. Sequential exposures of 16 O2 and 18 O2 at 120 K indicate that each adsorbed O2 molecule, irrespective of its adsorption sequence, has equivalent probabilities with respect to its neighbors to follow the two channels (molecular and dissociative), suggesting that O 2 adsorption is not only precursor-mediated, as the sticking probability measurements indicate, but that all O2 molecules reside in this precursor state at 120 K. This precursor state may be associated with a weak 145 K O2 TPD state observed at high O2 exposures. ELS measurements suggest charge transfer from the surface to the O2 molecule based on disappearance of the vacancy loss feature at 0.8 eV, and the appearance of a 2.8 eV loss that can be assigned to an adsorbed O2 - species based on comparisons with Ti-O2 inorganic complexes in the literature. Utilizing results from recent spin-polarized DFT calculations in the literature, we propose a model where three O2 molecules are bound in the vicinity of each vacancy site at 120 K. For adsorption temperatures above 150 K, the dissociation channel completely dominates and the surface adsorbs oxygen in a 1:1 ratio with each vacancy site. ELS measurements indicate that the vacancies are filled, and the remaining oxygen adatom, which is apparent in TPD, is transparent in ELS. On the basis of the variety of oxygen adsorption states observed in this study, further work is needed in order to determine which oxygen-related species play important roles in chemical and photochemical oxidation processes on TiO2 surfaces.
452 citations
TL;DR: A novel combination of Pd(OAc)(2)/pyridine/MS3A catalyzes the aerobic oxidation in toluene of a variety of primary and secondary alcohols into the corresponding aldehydes and ketones in high yields.
Abstract: A novel combination of Pd(OAc)2/pyridine/MS3A catalyzes the aerobic oxidation in toluene of a variety of primary and secondary alcohols into the corresponding aldehydes and ketones in high yields. Various substituents and protecting groups are compatible with this oxidation. The ca. 2:3 ratio of O2 uptake to product yield is observed, whereas in the absence of MS3A, the ratio is ca. 1:1, suggesting the in situ formation of H2O2 and its decomposition by MS3A into water and oxygen. A catalytic cycle including the formation of a Pd(II)-alcoholate followed by β-elimination of a Pd(II)H species and a carbonyl compound and then the formation of a Pd(II)OOH species is proposed.
409 citations
TL;DR: In this article, a spray-pyrolytic method was used to synthesize nanocrystalline thin films of n-Fe2O3 for the photoelectrochemical splitting of water to hydrogen and oxygen gases.
Abstract: Semiconducting nanocrystalline thin films of n-Fe2O3 were synthesized by a spray−pyrolytic method. These films were used for the photoelectrochemical splitting of water to hydrogen and oxygen gases. The rates of photoelectrochemical splitting of water at these thin-film electrodes were found to depend on the spray time, substrate temperature, solvent composition in the spray solution, and the concentration of the spray solution. The maximum photocurrent density of 3.7 mA cm-2 at 0.7 V/saturated calomel electrode (SCE) was obtained at the n-Fe2O3 film synthesized using the optimum condition of substrate temperature of 350 °C, the spray time of 60 s, and the spray solution of 0.11 M FeCl3 in 100% ethanol. The band gap energy of this film was found 2.05 eV. The flatband potential of −0.74 V/SCE and the apparent donor density of 2.2 × 1020 cm-3 were found from the Mott−Schottky plots at the AC frequency of 1000 Hz. The n-Fe2O3 films synthesized using the optimum conditions gave rise to a total conversion effi...
370 citations
TL;DR: In this article, a simple explanation for seawater chemistry dependent stable oxygen isotope variations in the planktonic foraminifera Orbulina universa was provided, which is derived from oxygen isotopes partitioning during inorganic precipitation.
354 citations
TL;DR: In this paper, the influence of oxidative and reductive treatments of indium-tin-oxide (ITO) on the performance of electroluminescent devices is presented.
Abstract: The influence of oxidative and reductive treatments of indium–tin–oxide (ITO) on the performance of electroluminescent devices is presented. The improvement in device performance is correlated with the surface chemical composition and work function. The work function is shown to be largely determined by the surface oxygen concentration. Oxygen-glow discharge or ultraviolet–ozone treatments increase the surface oxygen concentration and work function in a strongly correlated manner. High temperature, vacuum annealing reduces both the surface oxygen and work function. With oxidation the occupied, density of states (DOS) at the Fermi level is also greatly reduced. This process is reversible by vacuum annealing and it appears that the oxygen concentration, work function, and DOS can be cycled by repeated oxygen treatments and annealing. These observations are interpreted in terms of the well-known, bulk properties of ITO.
345 citations
TL;DR: In this paper, the effect of key parameters on the characteristics of barium oxide-based NOxstorage catalysts was systematically investigated, and the influence of temperature, storage and regeneration times, NOxsource (NO or NO2), oxygen concentration, reducing agent (C3H6, C3H8, CO, or H2), and carbon dioxide concentration on NOx storage capacity was studied.
344 citations
TL;DR: This study shows that hydrogen peroxide-induced apoptosis in T-cells did not require tyrosine kinase p56lck, phosphatase CD45, the CD95 receptor and its associated Caspase-8, and that pharmacological and genetic inhibition of transcription factor NF-κB protected cells from hydrogenperoxide-elicited cell death.
Abstract: Hydrogen peroxide-induced apoptosis is CD95-independent, requires the release of mitochondria-derived reactive oxygen species and the activation of NF-κB
TL;DR: In this paper, the perovskite-like SrCoO 3− δ -based solid solutions were found to exhibit highest permeation fluxes compared to other membranes.
TL;DR: Oxygen transport and surface reaction measurements have been obtained for La0.6Sr0.4Co0.2Fe0.8O3−δ using the conductivity relaxation technique as discussed by the authors.
TL;DR: Cerium−zirconium mixed oxides (CexZr(1-x)O2), precalcined at 900 °C in dry air, were supplied by Rhodia Terres Rares as monophasic solid solutions as mentioned in this paper.
Abstract: Cerium−zirconium mixed oxides (CexZr(1-x)O2), precalcined at 900 °C in dry air, were supplied by Rhodia Terres Rares as monophasic solid solutions. Introduction of some zirconium atoms in the ceria...
TL;DR: In this article, the selective CO oxidation on a Au/α-Fe2O3 catalyst in simulated reformer gas (low concentrations of CO and O2, 75 kPa H2, balance N2) at atmospheric pressure was investigated over almost two orders of magnitude in CO partial pressure (0.025-1.5 kPa) and over a large range of pO2/pCOratios ( 0.25-10).
TL;DR: An explicit oxygen permeation model has been developed for ion-conducting membranes with a high ratio of electronic to ionic conductivity, which makes it possible to correlate the permeation flux to directly measurable variables as discussed by the authors.
TL;DR: In this paper, the active oxygen species and mechanism for catalytic CO oxidation with O2 on a highly active TiO2-supported Au catalyst (denoted as Au/Ti(OH)*4), which was prepared by supporting a Au-phophine complex on as-precipitated wet titanium hydroxide followed by calcination at 673 K, have been studied by means of oxygen isotope exchange, O2 temperature-programmed desorption (O2 TPD), electron spin resonance (ESR), and Fourier-transformed infrared spectrosc
TL;DR: Primary alcohols such as ethanol or benzyl alcohol are selectively and catalytically oxidized by the mononuclear copper(II) radical complex 1-a functional model of the metalloenzyme galactose oxidase-with oxygen from air at 20°C to give the corresponding aldehydes and H2 O2 in about 60 % yield.
Abstract: Primary alcohols such as ethanol or benzyl alcohol are selectively and catalytically oxidized by the mononuclear copper(II) radical complex 1-a functional model of the metalloenzyme galactose oxidase-with oxygen from air at 20°C to give the corresponding aldehydes and H2 O2 in about 60 % yield.
TL;DR: In this article, a 1-10-ML thin film was grown on the Ru(0001) and Ni(111) surfaces with an orientation determined by the orientation of the substrate on which they were grown.
TL;DR: In this article, the authors examined the relationship between the two parameters measured by the 18O/16O Isotope Exchange Depth Profile (IEDP) method, the oxygen surface exchange coefficient, k*, and the oxygen tracer diffusion coefficient, D*, and interpreted the strong correlation observed between k* and D* in terms of oxygen vacancies playing a major role in the surface exchange process for these materials.
TL;DR: In this article, the authors explored the adsorption of oxygen at the wurtzite (0001) and (0001)-GaN surfaces employing density-functional theory and found that both surface orientations are very active towards oxygen adsorbing, explaining the high oxygen concentrations typically observed in GaN.
Abstract: A critical point in device fabrication based on GaN is the controlled doping and the incorporation of impurities like, e.g., oxygen. We have explored the adsorption of oxygen at the wurtzite (0001) and (0001) GaN surfaces employing density-functional theory. Our results show that both surface orientations are very active towards oxygen adsorption, explaining the high oxygen concentrations typically observed in GaN. However, the (0001) and (0001) surfaces behave differently and oxygen incorporation is expected to be higher at the (0001) surface. The different reactivity is explained in terms of the specific structural configurations.
TL;DR: In this article, the authors measured the thermal expansion coefficients of the ceramic materials, calculated from dilatometric data, are in the range (10.5-13.2) × 10============ −6============ -1======
Abstract: The solid solutions La
2
Ni
1–x
Fe
x
O
4+δ
(x = 0.02 and 0.10), La
1.9
Sr
0.1
Ni
1 – x
Fe
x
O
4 + δ
(x = 0.02 and 0.10) and La
2
Ni
0.88
Fe
0.02
Cu
0.10
O
4+δ
with the tetragonal K
2
NiF
4
-type structure were prepared by a standard ceramic technique. The thermal expansion coefficients of the ceramic materials, calculated from dilatometric data, are in the range (10.5–13.2) × 10
–6
K
–1
at 300–1100 K. Oxygen permeation fluxes through dense La
2
NiO
4 + δ
-based membranes at 970–1170 K were found to be limited by both surface exchange and bulk ionic transport, whereas the limiting effect of the oxygen interphase exchange increases with decreasing oxygen pressure at the membrane permeate side and with decreasing temperature. Applying porous cermet layers of dispersed platinum and praseodymium oxide onto the membrane surface results in enhanced permeation fluxes. The maximum oxygen permeability was found for the La
2
Ni
0.98
Fe
0.02
O
4+δ
and La
2
Ni
0.88
Fe
0.02
Cu
0.10
O
4+δ
solid solutions; oxygen permeability data demonstrated that a vacancy diffusion mechanism and oxygen interstitial migration make significant contributions to the total ionic conductivity.
TL;DR: In this paper, a Si-B-C ceramic matrix has been developed to improve the oxidation resistance and the lifetime in an oxygen environment of carbon-fiber-reinforced ceramic-matrix composites.
TL;DR: The heme domain (iNOS(heme) of inducible nitric oxide synthase (NOS) was expressed in Escherichia coli and purified to homogeneity and provided the first direct evidence for an NOS-bound H(3)B.
Abstract: The heme domain (iNOS(heme)) of inducible nitric oxide synthase (NOS) was expressed in Escherichia coli and purified to homogeneity. Rapid freeze-quench (RFQ) EPR was used to monitor the reaction of the reduced iNOS(heme) with oxygen in the presence and absence of substrate. In these reactions, heme oxidation occurs at a rate of approximately 15 s(-)(1) at 4 degrees C. A transient species with a g = 2.0 EPR signal is also observed under these conditions. The spectral properties of the g = 2.0 signal are those of an anisotropic organic radical with S = (1)/(2). Comparison of the EPR spectra obtained when iNOS(heme) is reconstituted with N5-(14)N- and (15)N-substituted tetrahydrobiopterin (H(4)B) shows a hyperfine interaction with the pterin N5 nitrogen and identifies the radical as the one-electron oxidized form (H(3)B.) of the bound H(4)B. Substitution of D(2)O for H(2)O reveals the presence of hyperfine-coupled exchangeable protons in the H(4)B radical. This radical forms at a rate of 15-20 s(-)(1), with a slower decay rate that varies (0.12-0.7 s(-)(1)) depending on the substrate. At 127 ms, H(3)B. accumulates to a maximum of 80% of the total iNOS(heme) concentration in the presence of arginine but only to approximately 2.8% in the presence of NHA. Double-mixing RFQ experiments, where NHA is added after the formation of H(3)B., show that NHA does not react rapidly with H(3)B. and suggest that NHA instead prevents the formation of the H(4)B radical. These data constitute the first direct evidence for an NOS-bound H(3)B. and are most consistent with a role for H(4)B in electron transfer in the NOS reaction.
TL;DR: This review attempts to summarize recent developments and to suggest steps that might be taken to improve the quality of stored red blood cells (RBCs) to maximize their safety and their benefits for the transfused recipient.
TL;DR: In this article, the authors studied the catalytic activity of very tiny Au particles on TiO2 and found that they can prepare Au islands with controlled thicknesses from one to several monolayers.
Abstract: Very tiny Au particles on TiO2 show excellent activity and selectivity in a number of oxidation reactions. We have studied the vapor deposition of Au onto a TiO2(110) surface using XPS, LEIS, LEED and TPD and found that we can prepare Au islands with controlled thicknesses from one to several monolayers. In order to understand at the atomic level the unusual catalytic activity in oxidation reactions of this system, we have studied oxygen adsorption on Au/TiO2(110) as a function of Au island thickness, and have measured the titration of this adsorbed oxygen with CO gas to yield CO2, as function of Au island thickness, CO pressure and temperature. A hot filament was used to dose gaseous oxygen atoms. TPD results show higher O2 desorption temperatures (741 K) from ultrathin gold particles on TiO2(110) than from thicker particles (545 K). This implies that Oa bonds much more strongly to ultrathin islands of Au. Thus from Bronsted relations, ultrathin gold particles should be able to dissociatively adsorb O2 more readily than thick gold particles. Our studies of the titration reaction of oxygen adatoms with CO (to produce CO2) show that this reaction is extremely rapid at room temperature, but its rate is slightly slower for the thinnest Au islands. Thus the association reaction (COg + Oa → CO2,g) gets faster as the oxygen adsorption strength decreases, again as expected from Bronsted relations. For islands of about two atomic layers thickness, the rate increases slowly with temperature, with an apparent activation energy of 11.4 ± 2.8 kJ/mol, and shows a first‐order rate in CO pressure and oxygen coverage, similar to bulk Au(110).
TL;DR: In this paper, a method involving HNO3/NH3 treatments of a carbon support (Vulcan XC-72R), addition of Fe from inorganic precursors, and a thermal treatment at 900 °C under an inert atmosphere for the preparation of Fe-based oxygen reduction electrocatalysts for use in polymer electrolyte fuel cells has been investigated.
Abstract: A method involving HNO3/NH3 treatments of a carbon support (Vulcan XC-72R), addition of Fe from inorganic precursors, and a thermal treatment at 900 °C under an inert atmosphere for the preparation of Fe-based oxygen reduction electrocatalysts for use in polymer electrolyte fuel cells has been investigated. The prepared materials were evaluated in acidic medium by the rotating disk electrode technique using the potential at the oxygen reduction peak, Ep, as a measure of the electrocatalytic activity. Two sets of experiments were performed: (i) varying the conditions of the HNO3/NH3 treatments of Vulcan XC-72R, and (ii) varying Fe loadings added to the HNO3/NH3 pretreated C samples. In the first set of experiments, no Fe was adsorbed on the C support. It was shown that a combined HNO3 and NH3 treatment of the carbon support gave higher activity for oxygen reduction than either HNO3 or NH3 treatment alone. In the second set of experiments, it was shown that the activity of the HNO3/NH3-modified C support in...
TL;DR: In this paper, the use of mixed-oxides containing CeO2 as oxygen storage/release components is discussed with special focus on the applications of these materials in auto-exhaust catalytic converters.
Abstract: The use of mixed-oxides containing CeO2 as oxygen storage/release components is discussed with special focus on the applications of these materials in auto-exhaust catalytic converters. Ceria easily forms solid solutions with transition-metal/rare-earth oxides over a wide composition range. The incorporation of dopants like Zr4+, Pr3/4+, Tb3/4+ into the cubic fluorite lattice of CeO2 strongly affects the structural and energetic properties of the materials by lowering activation energy for oxygen migration and by increasing reducibility of the cerium cation. This augments both total and kinetic oxygen exchange between the solid and the gas phase under conditions typically encountered in real systems.
TL;DR: In this article, a kinetic model of the NO oxidation to NO2 over a Pt/Al2O3 model catalyst for the temperature range 250-450 degrees C was presented and validated with some transient experiments with either NO or NO2 and different oxygen concentrations in the gas feed.
Abstract: Laboratory tests and kinetic modeling were carried out in order to provide kinetic input data to a systematic investigation of the mechanism of nitrogen oxides (NO,) storage in catalysts used for lean-burn engines. In particular, we present a kinetic model of the NO oxidation to NO2 over a Pt/Al2O3 model catalyst for the temperature range 250-450 degrees C. Since the oxygen behavior at atmospheric pressure is critical for such a model, we have also studied the adsorption/desorption of oxygen by temperature-programmed desorption (TPD) experiments. The experiments show that oxygen starts to desorb at about 300 degrees C. Furthermore, the NO oxidation was studied in a temperature ramp with NO and oxygen in the gas feed. The data from this experiment and the above-determined values for the oxygen adsorption/desorption were used to construct a kinetic model for the NO oxidation. Finally, the model was validated with some transient experiments with either NO or NO2 and different oxygen concentrations in the gas feed. We found a good agreement between these experiments and the model.
TL;DR: In this article, the formation of oxygen structures in activated carbon is investigated using cherry stones (CS) as starting material and commonly air as activating agent, and the activation at 250°C in air results in activated carbons that contain different oxygen structures when CS is carbonized at 450 or 600°C.