O2-rich Atmosphere Research Articles

Atomistic Picture of the Catalytic Activity of Platinum Nanoparticles through Coherent Diffraction Imaging and Reactive Molecular Dynamics Simulations

Platinum nanoparticles are among the most widely used catalysts due to their high chemical stability, surface reactivity and electron transfer rate. They find applications in chemical and bio-sensing, fuel cells, photocatalysis and methanol formation. Elucidating the catalytic mechanism under in-situ conditions is vital to the structural and chemical design of more efficient catalysts that are urgently needed to meet the energy needs of today. Coherent X-ray diffraction imaging (CDI) is a powerful technique for operando characterization with the ability to provide evolving snapshots of defect structure, lattice dynamics and structural evolution at ~10 nm resolution, while reactive molecular dynamics (MD) simulations provide atomistic details of lattice dynamics, bond breakage and formation as well as reaction kinetics at experimentally inaccessible length and time scales. We present results from combined CDI experiments and reactive MD simulations on the atomistic processes underlying methanol formation from methane on Pt nanoparticles. We observe the preferential oxidation of edge and corner sites on the Pt nanoparticle in an O2 rich atmosphere, which when followed by the introduction of methane, leads to the binding of CH4 to disassociated O atoms. Fig. 1 a) shows the partially oxidized Pt particle in a methane atmosphere. O2 on surface sites is not seen to disassociate while oxygen at corner and edge sites disassociates due to the higher site reactivity at edges and corners. Fig 1. b)-c) shows a close-up of the surface during the binding of CH4 to the disassociated oxygen. Finally, the mechanism predicted by the reactive MD simulations is validated through CDI experiments, with the observation of local lattice strains at edge and corner sites, which arise due to the binding of O2 and CH4 molecules. Figure 1

Structure and stoichiometry evolution of sputtered Nb doped TiO2 films induced by O2 pressure variation during postannealing process

Nb doped TiO2 (TiO2:Nb) thin films are prepared by direct current magnetron sputtering on glass substrates followed by different O2 pressure postanneal at 300 °C. The proper postannealing O2 pressure (PO2−anneal) prompts the crystallization of the as-deposited amorphous TiO2:Nb films and improves the electron mobility. High vacuum postanneal leads to weak crystalline TiO2:Nb films, while the TiO2:Nb films remain amorphous at high PO2−anneal. The average transmittance in the visible region increases monotonically with PO2−anneal due to the reduction of oxygen vacancy and reaches 81% when PO2−anneal is 20 Pa. However, an oxygen interstitial can be formed in O2 rich annealing atmosphere, which would result in the suppression of carrier concentration. The carrier concentration maximum (∼1.55 × 1021 cm−3) of the TiO2:Nb films is obtained when the PO2−anneal is 0.5 Pa.

DISTINGUISHING A HYPOTHETICAL ABIOTIC PLANET–MOON SYSTEM FROM A SINGLE INHABITED PLANET

ABSTRACT It has recently been suggested that an exomoon with a CH4 atmosphere, orbiting an abiotic Earth-mass planet with an O2-rich atmosphere, can produce a false positive biosignature at a low–moderate spectral resolution (R = λ/Δλ ≤ 2000). If this were true, inferring the presence of life on exoplanets will be beyond our reach in the next several decades. Here we use a line-by-line radiative transfer model to compute the relevant reflection spectrum between 1 and 3.3 μm. We show that it is possible to separate the combined spectra of such planet–moon systems from an inhabited planet by multiple-band NIR observations. We suggest that future observations near the 2.3 μm CH4 absorption band at a resolution of 100 and an SNR of 10 or more may be a good way to distinguish an abiotic planet–moon system from a inhabited single planet.

Impedance Spectroscopic Indication for Solid State Electrochemical Reaction in (CH3NH3)PbI3 Films.

Halide perovskite-based solar cells still have limited reproducibility, stability, and incomplete understanding of how they work. We track electronic processes in [CH3NH3]PbI3(Cl) ("perovskite") films in vacuo, and in N2, air, and O2, using impedance spectroscopy (IS), contact potential difference, and surface photovoltage measurements, providing direct evidence for perovskite sensitivity to the ambient environment. Two major characteristics of the perovskite IS response change with ambient environment, viz. -1- appearance of negative capacitance in vacuo or post-vacuo N2 exposure, indicating for the first time an electrochemical process in the perovskite, and -2- orders of magnitude decrease in the film resistance upon transferring the film from O2-rich ambient atmosphere to vacuum. The same change in ambient conditions also results in a 0.5 V decrease in the material work function. We suggest that facile adsorption of oxygen onto the film dedopes it from n-type toward intrinsic. These effects influence any material characterization, i.e., results may be ambient-dependent due to changes in the material's electrical properties and electrochemical reactivity, which can also affect material stability.

Spectroscopic properties of Nd3+-doped tungsten–tellurite glasses

In this work, we investigate the spectroscopy properties of neodymium doped tungsten–tellurite glasses prepared in ambient and O2-rich atmospheres. A conversion of TeO4 to TeO3 units was caused by the addition of Nd3+ into the glass, which was confirmed by absorption spectra and by Judd–Ofelt parameter behavior. The relaxation of the 4F3/2 level is dominated by radiative decay and cross-relaxation between Nd3+ and Nd3+ ions. The energy transfer from Nd3+ to the hydroxyl group is negligible when compared to the cross-relaxation. The luminescence quantum efficiency values of the 4F3/2 level decreases as the Nd3+ concentration increases, independently if determined by the Judd–Ofelt method or by the thermal lens technique. The observed reduction in the IR absorption associated to OH groups was not effective to improve the luminescence quantum efficiency.

Detection of Callisto’s oxygen atmosphere with the Hubble Space Telescope

We report the result of a search for evidence of an O2-dominated atmosphere on Callisto, using the high far-ultraviolet sensitivity of the Hubble Space Telescope Cosmic Origins Spectrograph (COS). Observations of Callisto’s leading/Jupiter-facing hemisphere show, for the first time, variable-strength atomic oxygen (O I) emissions with brightness up to 4.7±0.7Rayleighs for the O I 1304Å triplet and 1.9±0.4 Rayleighs for the O I 1356Å doublet, averaged over the 2.5arcsec. diameter COS aperture. Because the observations were made in Earth’s shadow, and are brighter than expected emission from nighttime geocoronal airglow or other plausible sources, we are confident that they originate from Callisto or its immediate vicinity. In addition, COS’s limited (∼1arcsec) spatial resolution implies a 2σ detection of excess 1356Å emission concentrated on the disk of Callisto itself, with brightness 3.2±1.6 Rayleighs. The (O I 1356Å)/(O I 1304Å) emission ratio from Callisto’s disk favors dissociative excitation of O2, suggesting that O2 is the dominant atmospheric component rather than other possible oxygen-bearing alternatives. Photoelectrons, rather than magnetospheric electrons, are the most likely source of the dissociative excitation. This detection yields an O2 column density of ∼4×1015cm−2 on the leading/Jupiter facing hemisphere, which implies that Callisto’s atmosphere is collisional and is the fourth-densest satellite atmosphere in the Solar System, in addition to being the second-densest O2-rich collisional atmosphere in the Solar System, after Earth. Longitudinal variations in published densities of ionospheric electrons suggest that O2 densities in Callisto’s trailing hemisphere, which we did not observe, may be an order of magnitude greater. The aperture-filling emissions imply that there is also an extended corona of predominantly O I 1304Å emission around Callisto, with observed strength of 1–4 Rayleighs, likely due to solar resonance scattering from sputtered atomic O, with a density of up to 104cm−3 at the exobase.

Heterojunction between Al2O3 and SnO2 thin films for application in transparent FET

Alternative materials for use in electronic devices have grown interest in the past recent years. In this paper, the heterojunction SnO2/Al2O3 is tested concerning its use as a transparent insulating layer for use in FETs. The alumina layer is obtained by thermal annealing of metallic Al layer, deposited by resistive evaporation technique. Combination of undoped SnO2, deposited by sol-gel-dip-coating technique, and Al thermally annealed in O2-rich atmosphere, leads to fair insulation when the number of aluminum oxide layers is 4, with 0.3% of the current lost through the gate terminal as leakage current. This insulation is not obtained for devices with alumina layer treated for long time, under room atmosphere, due to degradation of the insulating film and interfusion with the conduction channel even using Sb-doped SnO2. The annealing of Al deposited on soda-lime glass substrate leads also to the formation of a Si layer, crystallized at Substrate/Al2O3 interface. The conclusion is that for an efficient insulation the thermal annealing must be short and then, O2-rich atmospheres are preferred.

Up-conversion effect of Er- and Yb-doped ZnO thin films

Visible up-conversion in ZnO:Er and ZnO:Er:Yb thin films deposited by RF magnetron sputtering under different O2-rich atmospheres has been studied. Conventional photoluminescence (325nm laser source) and up-conversion (980nm laser source) have been performed in the films before and after an annealing process at 800°C. The resulting spectra demonstrate that the thermal treatment, either during or post-deposition, activates optically the Er3+ ions, being the latter process much more efficient. Moreover, the atmosphere during deposition was also found to be an important parameter, as the deposition under O2 flow increases the optical activity of Er+3 ions. In addition, the inclusion of Yb3+ ions into the films has shown an enhancement of the visible up-conversion emission at 660nm by a factor of 4, which could be associated to either a better energy transfer from the 2F5/2 Yb level to the 4I11/2 Er one, or to the prevention of having Er2O3 clustering in the films.

On the variations of optical property and electronic structure in heavily Al-doped ZnO films during double-step growth process

We have investigated the variations of optical property and electronic structure in heavily Al-doped ZnO (AZO) films during the growth process, which were formed by first creating Zn vacancies in O2-rich atmosphere and second filling the vacancies with Zn atoms in Zn-vapor atmosphere. After the first step, the high-resistance AZO films have the same optical bandgap with nominally undoped ZnO, indicating that negligible variations in the fundamental bandgap happened to the AZO films although Al atom was incorporated into the ZnO lattice. After the second step, once free electrons were brought into the lattice by Zn-filling, the optical transition energy blueshifts due to the band-filling effect. X-ray absorption fine structure measurements suggest that Zn-filling process decreased the unoccupied states of the conduction band, but not raised the conduction band minimum.

Deposition of Al2O3 by resistive evaporation and thermal oxidation of Al to be applied as a transparent FET insulating layer

Alumina thin films have been obtained by resistive evaporation of Al layer, followed by thermal oxidation by means of annealing in appropriate atmosphere (air or O2-rich), with variation of annealing time and temperature. Optical and structural properties of the investigated films reveal that the temperature of 550°C is responsible for reasonable oxidation, which is accelerated up to 8 times for O2-rich atmosphere. Results of surface electrical resistivity and Raman spectroscopy are in good agreement with these findings. Surprisingly, X-ray and Raman data suggest also the crystallization of Si nuclei at glass substrate–alumina interface, which would come from the soda-lime glass used as substrate.

Cobalt–iron red–ox behavior in nanostructured La0.4Sr0.6Co0.8Fe0.2O3−δ cathodes

Nano-sized La0.4Sr0.6Co0.8Fe0.2O3−δ (LSCF) perovskite samples (prepared by a conventional acetate route and a novel acetate synthesis with HMTA additives), were tested simulating a red–ox cycle. The crystallography was studied by X-ray Powder Diffraction (XPD) and the changes in the oxidation state of the perovskite B-site were evaluated by synchrotron X-ray Absorption Near Edge Spectroscopy (XANES). After a reducing treatment, LSFC particles show the appearance of a new phase that coexists with the original one. The structural change is accompanied by a Co and Fe formal oxidation states decrease, although Fe remains always closer to 4+ and Co closer to 3+. The treatment produces a B-site valence average reduction from 3.52+ to 3.26+ and the formation of oxygen vacancies. A re-oxidation treatment under O2 rich atmosphere at 800°C for 10h shows that the change is reversible and independent of the two chemical methods used to synthesize the LSCF nano-particles.

Effect of support for Pt[sbnd]Cu bimetallic catalysts synthesized by electron beam irradiation method on preferential CO oxidation

Supported bimetallic catalysts have been shown to improve properties and catalytic performance of monometallic catalysts in various fuel conversion reactions. To investigate the effect of support on bimetallic PtCu system, PtCu supported on γ-Fe2O3 and CeO2 were synthesized using electron beam irradiation method and preferential CO oxidation (PROX) performance was investigated in relation to structural and chemical properties of the catalysts. The difference in performance among the catalysts existed in both oxygen transport ability and selectivity. While oxygen transport ability was influenced by support property such as crystallite size (within the comparison among catalysts supported on CeO2 and γ-Fe2O3), selectivity was rather influenced by support material which led to clear differences in reducibility, reduction degree of catalyst, and alloying extent of PtCu. CeO2-supported catalysts are unique in the view point that they were able to convert CO selectively in high CO conversion region compared to Fe2O3-supported catalyst. PtCu with small CeO2 crystallite maintained high O2 conversion (high capacity for oxygen transport) in O2-rich atmosphere in addition to high selectivity. Compared to monometallic Cu which lost its activity in the O2-rich wet condition, or compared to monometallic Pt which exhibited poor selectivity, PtCu realized both high activity and selectivity on CeO2 of small crystallite size, leading to more than 99% of CO conversion at 100°C. By modifying the structure, it was demonstrated that PtCu/CeO2 system can be improved to have low-temperature activity comparable to monometallic Pt while keeping high selectivity of the original PtCu catalyst in the practical PROX condition.

Application of modified atmosphere packaging (gas flushing and active packaging) for extending the shelf life of Beauveria bassiana conidia at high temperatures

Shelf life determinations under non-refrigerated conditions, especially high temperature regimes characteristic of tropical/subtropical regions, deserve more attention. In this study, we investigated effects of modified atmosphere packaging (MAP) on longevity of conidia of Beauveria bassiana (Bb) strain GHA. Similar rates of conidial survival were observed after storage for 60days at 50°C in atmospheres of pure CO2, N2, H2, or He (49–51% viability), but few conidia (⩽2%) survived storage in O2-rich atmospheres. Viability of conidia stored in an atmosphere of 20% CO2/80% N2 decreased to <80% within 180days at 40°C and within 30days at 50°C but remained high (87%) after a 16-month storage period at 25°C. O2 concentrations in the storage containers ranged from 0.3% at the start to as high as 12.4% at the end of experiments (due to container leakage). When active packaging (hermetically sealed packages with O2/moisture scavengers) was employed, shelf lives were substantially improved. Viabilities ⩾80% were consistently recorded after 6months at 40°C or 2months at 50°C when a dual O2/moisture absorber or a combination of sachets (dual O2 absorber/CO2 generator+desiccant) were used. Water activities (aw) supporting greatest survival were ⩽0.030, suggesting that optimal aw for long-term storage under anaerobic conditions is lower than determined in previous studies of storage in the presence of O2. Additionally, we have shown that actively packaged conidia with higher than desirable initial aw should be allowed an equilibration period at a moderate temperature before exposure to high storage temperatures. Active packaging of dried conidia (aw⩽0.032) preserved 71% viability for 16months at 40°C and 63–65% for 3months at 50°C. To our knowledge, these are the longest survival times yet reported for Bb conidia under high-temperature conditions.

Ambient Dependence of Photoluminescence for ZnO Nanocrystalline Films Synthesized in a Sonochemical Method

We investigated the photoluminescence (PL) spectra of solution-based synthesized ZnO nanocrystalline (NC) films under various ambient conditions. While the PL spectra typically showed a sharp peak near 375 nm (near-band-edge, NBE) and a broad structure near 550 nm (deep level, DL), the NBE peak was enhanced and the DL emission was significantly suppressed in an O2-free atmosphere at room temperature. Interestingly, the PL spectra changed in an opposite manner in an O2-rich atmosphere. These findings indicate that the light emission from ZnO NC films is easily controlled by the O2 concentration in the atmosphere at room temperature, which is clearly distinguished from the post-annealing effect.

Oscillatory Behavior in the CO-oxidation over Bulk Ruthenium Dioxide — the Effect of the CO/O2 Ratio

Abstract CO oxidation over polycrystalline ruthenium dioxide was monitored in an in-situ XRD setup. The evolution of the bulk state of the catalyst was followed by in-situ XRD during reaction, while the surface morphology and chemical state before and after reaction were investigated by HRSEM and EDX. The commercial RuO2 powder was calcined prior reaction to ensure the formation of completely oxidized RuO2. This pre-calcined RuO2 is initially inactive in CO oxidation regardless of the CO/O2 feed ratio and requires an induction period, the length of which strongly depends whether the catalyst is diluted with boron nitride or not. After this induction period oscillations in the CO2 yield occur under O2-rich conditions only. These oscillations exhibit two time constants for the diluted catalyst, while the low frequency oscillations were not observed in the case of undiluted RuO2. Furthermore, the state of the catalyst after activation in O2-rich feed conditions differs dramatically from the state after activation in CO-rich feed conditions. Firstly, the catalyst activated in an O2-rich atmosphere remains inactive under CO-rich conditions in contrast to the catalyst activated in CO-rich conditions which is afterwards active under all feed ratios examined. Secondly, the surface morphology of the catalyst is quite different. While the apical surfaces of the RuO2 crystals become roughened upon activation in the CO-rich feed, they become facetted under O2 rich activation conditions. Therefore, we conclude that at least two different active surface states on the bulk RuO2 catalyst exist.

Study of the effect of plasma-striking atmosphere on Fe-oxidation in thermal dc arc-plasma processing

The effect of plasma-striking atmosphere: air and air+Ar-gas on the crystallization of Fe-oxide phases was studied using dc thermal arc-plasma processing route. The powders were characterized by x-ray diffraction, vibrating sample magnetometry, transmission electron microscopy, and Mössbauer spectroscopy techniques. At room temperature and O2 rich atmosphere, arc-evaporated Fe2+ ions oxidize into either γ-Fe2O3 or Fe3O4 depending upon the combining ratio of Fe with molecular O2. Fe/O ratio could be adjusted using proper flow rate of Ar gas to crystallize the pure γ-Fe2O3.

Processing of Tellurite-Based Glass with Low OH Content

We report results on the processing and characterization of tellurite-based glass in the TeO2–Bi2O3–ZnO (TBZ) glass family and efforts to reduce their absorption loss due to residual hydroxyl (OH) content. We show that the introduction of alkaline or alkaline earth in the glass network, drying of the batch with fluorine-based raw materials before melting, and melting of the glass in O2-rich atmosphere can lead to a significant reduction in OH content. Employing these steps in TBZ glass processing resulted in a 93% reduction of OH content as compared with a reference composition. The reduction in OH content was verified by a decrease of the absorption band center at 3000 cm−1 as well as a decrease of the free volume in the glass. The addition of F in the glass network was found to reduce the fraction of nonbridging oxygen associated with OH. X-ray fluorescence spectrometry was used to determine the amount of remaining F.

Preparation and Functional Characterizations of Ta2O5 Deposits Organized at the Micro- and Nano-scale

The present communication deals with the coating of bundles of single wall carbon nanotube (SWCNTs) by nanocrystalline tantalum pentoxide (Ta2O5) deposits. Arrays of SWCNTs bundles are produced on Si, and AlN plates by means of a Hot Filament CVD apparatus, using as reactants carbon nanopowders and in-situ produced atomic hydrogen. A subsequent process, carried out in an O2-rich atmosphere, enables to cover the bundles by a uniform deposit of Ta2O5 nanograins. The coating process does not modify the organization of the pristine nanotube deposits. A series of SWCNT/Ta2O5 samples, characterized from both the morphological and structural point of view, have been tested as cathodes in Field Emission experiments (F.E.). The F.E. experiments proved that these hybrid SWCNT/dielectric systems are good field emitters and that their properties make them suitable for the realization of cold cathodes.

Effect of pretreatment and regeneration conditions of Ru/γ-Al2O3 catalysts for N2O decomposition and/or reduction in O2-rich atmospheres and in the presence of NOX, SO2 and H2O

The effect of the pretreatment (inert, oxidative, and reducing) of Ru/γ-Al2O3 catalyst on its activity and stability in the decomposition of N2O in the absence or presence of O2, SO2, H2O and NOX was studied in the present work. Decomposition of pure N2O was slightly enhanced by the H2-pretreated catalyst (metallic Ru) compared to the O2- or He-pretreated ones, owing to a cyclic oxidation–reduction pathway of metallic Ru. The observed decrease of activity by O2 or H2O addition was reversible compared to SO2 which caused a strong, irreversible deactivation of the catalyst, irrespective of the type of pretreatment. This was attributed to the formation of stable sulphates, mainly those on RuO2 surface, which could only be removed by regeneration under reducing (H2 in He) atmosphere at temperatures of ca. 500°C. Oxidative or inert regeneration required very high temperatures (i.e. >700°C) in order to decompose these sulphates. A method of retaining N2O conversion activity very high (≥98%) for long reaction times is suggested and is based on frequent and short-time (ca. 10min) regenerations of the catalyst under reducing atmosphere (ca. 5% H2 in He). The effect of co-feeding various reducing agents, such as CO or C3H6, on the N2O conversion activity in the presence of O2, SO2, H2O and NOX is negligible, mainly because they are oxidized at relatively low temperatures in the O2-rich feeds used in this study.

Stability and deactivation of unconditioned Au/TiO2 catalysts during CO oxidation in a near-stoichiometric and O2-rich reaction atmosphere

The deactivation of unconditioned Au/TiO2 catalysts during CO oxidation at different reaction temperatures (30 and 80 °C) and in different reaction atmospheres was investigated. Kinetic and in situ IR spectroscopic measurements revealed a close correlation between the exponentially decreasing CO2 formation rate and the COad coverage (decreasing) and carbonate coverage (increasing) during deactivation, whereas transmission electronic microscopy imaging revealed no significant changes in Au particle size. Consequences for the deactivation mechanism are discussed.