Slow Diffusion Of Oxygen Research Articles

Photocatalytic H2 O2 Generation Reaction with a Benchmark Rate at Air-Liquid-Solid Joint Interfaces.

The rapid charge recombination, low selectivity for two-electron oxygen reduction reaction (ORR), and limited O2 diffusion rate hinder the practical applications of photocatalytic H2 O2 generation. Herein, a triphase photocatalytic system in which the H2 O2 generation occurs at the air-liquid-solid joint interfaces is developed, using polymeric carbon nitride (PCN). The introduction of pyrrole units and cyano group into PCN can promote the activation of oxygen molecules and facilitate the spatial separation of HOMO and LUMO orbits, hence improving the charge carrier separation efficiency and enhancing the formation of H2 O2 . Importantly, the gas-liquid-solid triphase interface system allows for the rapid transport of oxygen from the air to the reaction interface, overcoming the low solubility and slow diffusion of oxygen in the water in conventional liquid reaction systems. The triphase system shows a benchmark H2 O2 generation rate over PCN-based materials in pure water (2063.21µmol g-1 h-1 ), which is an approximate tenfold enhancement as compared to powder photocatalyst (215.44µmol g-1 h-1 ). Simulation and electrochemical tests reveal that the rapid oxygen diffusion rate of triphase interface can promote charge separation and provide more O2 to generate H2 O2 . This work provides a promising strategy for constructing an efficient and sustainable H2 O2 production system.

Atomistic simulation on the nano-scale oxidation of Fe-Cr alloy surfaces and the diffusion behavior of oxygen

The oxidation of Fe-Cr alloy surfaces was performed by reactive molecular dynamics (MD) simulations. The results shown that the addition of Cr caused a slow internal diffusion of oxygen with a thinner oxide layer in the Fe-Cr alloy, indicating improved oxidation resistance. Furthermore, the achievement of the saturation point in the diffusion flux of oxygen in Fe-Cr alloy was closely linked to the oxide structure, the formation of FeO may inhibit further oxidation by preventing more oxygen ions from reaching the unoxidized interior. Therefore, the evolution of oxide structure played a crucial role in improving the oxidation resistance of Fe-Cr alloys.

Microbial Communities Associated with Alternative Fuels in Model Seawater-Compensated Fuel Ballast Tanks

The biocorrosion of carbon steel poses a risk for ships combining seawater and fuel in metal ballast tanks. Ballast tanks were simulated by duplicate reactors containing carbon steel coupons and either petroleum F76 (petro-F76), Fischer–Tropsch F76 (FT-F76), or a 1:1 mix of both fuels, to investigate whether the alternative fuel FT-F76 influenced this risk. The polycarbonate reactors were inoculated with seawater, and the control reactors did not receive fuel. The reactors were monitored for 400 days, and they all reached a pH and open circuit potential where elemental iron was oxidized, indicating corrosion. The reactors containing petro-76 or fuel mix had higher levels of dissolved iron; one of each replicate had lower concentrations of sulfate than the original seawater, while the sulfate concentration did not decrease in the other incubations. The high sulfate reactors, but not the low sulfate reactors, had a high relative abundance of microaerophilic sulfide-oxidizing bacteria. The FT-F76 and the no-fuel reactors had a high relative abundance of iron-sequestering Magnetovibrio. Although dissolved iron and loss of sulfate under anoxic conditions are associated with biocorrosion, our results suggest that in our reactors these indicators were altered by iron-sequestering and sulfide-oxidizing microbes, which is consistent with the slow diffusion of oxygen across the polycarbonate reactors.

A rutile and titanite record of subduction fluids: Integrated oxygen isotope and trace element analyses in Franciscan high‐pressure rocks

AbstractIn situ oxygen analysis of garnet in eclogite and related rocks is increasingly being used to probe the composition of subduction fluids. However, in many cases, these samples contain textural signs of both fluid flow and retrograde metamorphism, some of which may take place outside the garnet stability field. In order to test the connection between polymetamorphism and fluid infiltration, rutile rimmed by titanite from high‐grade tectonic blocks of the Franciscan Formation (California, USA) was analysed for oxygen isotope ratios and trace element concentrations. Zirconium concentrations in rutile yield temperatures of ~600°C for eclogite and hornblende eclogite from three well‐studied localities (Junction School, Tiburon and Ward Creek). Rutile trace element concentrations are generally low and consistent with a mafic protolith. Titanite surrounding rutile has inherited much of its trace element content from rutile, and Zr‐in‐titanite temperatures are spuriously high. Titanite in rutile‐free samples (blueschist and eclogite from Jenner beach) have similar compositions suggesting that they were formed at the expense of rutile as well. Oxygen isotope ratios from rutile and titanite in the same sample are fortuitously similar, indicating disequilibrium between these minerals, which formed at different times and temperatures but in equilibrium with the same oxygen reservoir. Rutile in blocks with garnets zoned in oxygen isotopes are generally in equilibrium with the rims rather than the cores. Slow oxygen diffusion in rutile and the low temperatures of formation require that rutile recrystallized after fluid interaction and before blueschist facies metamorphism. External fluid interaction of Franciscan eclogites took place near the peak of metamorphism.

Periclase deforms more slowly than bridgmanite under mantle conditions

Transport of heat from the interior of the Earth drives convection in the mantle, which involves the deformation of solid rocks over billions of years. The lower mantle of the Earth is mostly composed of iron-bearing bridgmanite MgSiO3 and approximately 25% volume periclase MgO (also with some iron). It is commonly accepted that ferropericlase is weaker than bridgmanite1. Considerable progress has been made in recent years to study assemblages representative of the lower mantle under the relevant pressure and temperature conditions2,3. However, the natural strain rates are 8 to 10 orders of magnitude lower than in the laboratory, and are still inaccessible to us. Once the deformation mechanisms of rocks and their constituent minerals have been identified, it is possible to overcome this limitation thanks to multiscale numerical modelling, and to determine rheological properties for inaccessible strain rates. In this work we use 2.5-dimensional dislocation dynamics to model the low-stress creep of MgO periclase at lower mantle pressures and temperatures. We show that periclase deforms very slowly under these conditions, in particular, much more slowly than bridgmanite deforming by pure climb creep. This is due to slow diffusion of oxygen in periclase under pressure. In the assemblage, this secondary phase hardly participates in the deformation, so that the rheology of the lower mantle is very well described by that of bridgmanite. Our results show that drastic changes in deformation mechanisms can occur as a function of the strain rate.

Controlled Adsorption Boosting the Cumulation of H2O2 Based on the Chain Mail Catalyst

AbstractSynthesis of H2O2 through oxygen electro‐reduction is a green sustainable route but suffers from the slow kinetics and limited cumulation of H2O2, because of the exceedingly slow oxygen diffusion and oxygen being directly reduced to water. Here, it has been proposed voltage pulse‐regulated catalysis achieves the continuous accumulation of H2O2 concentrations up to 50 times greater than constant catalysis based on the chain mail catalyst, which is capable of catalysis steadily for 3.6 million pulses (2000 h) at one current level. When using a flow cell, the preparation rate of 39 977 mmol gcat−1 h−1 is achieved, exceeding previous records. Because the applied voltage regulates the adsorption of O2/ H2O2 from the catalyst, as verified by molecular dynamics simulations, the chain mail catalyst exhibits a lower barrier to oxygen adsorption than the direct reduction of oxygen to water, achieving rapid and continuous cumulation. In addition, the direct use of in‐situ produced H2O2 achieves complete sterilization in two minutes demonstrating the potential for application at an industrial level.

Modulating Nucleus Oxygen Concentration by Altering Intramembrane Cholesterol Levels: Creating Hypoxic Nucleus in Oxic Conditions.

We propose a novel mechanism by which cancer cells can modulate the oxygen concentration within the nucleus, potentially creating low nuclear oxygen conditions without the need of an hypoxic micro-environment and suited for allowing cancer cells to resist chemo- and radio-therapy. The cells ability to alter intra-cellular oxygen conditions depends on the amount of cholesterol present within the cellular membranes, where high levels of cholesterol can yield rigid membranes that slow oxygen diffusion. The proposed mechanism centers on the competition between (1) the diffusion of oxygen within the cell and across cellular membranes that replenishes any consumed oxygen and (2) the consumption of oxygen in the mitochondria, peroxisomes, endoplasmic reticulum (ER), etc. The novelty of our work centers around the assumption that the cholesterol content of a membrane can affect the oxygen diffusion across the membrane, reducing the cell ability to replenish the oxygen consumed within the cell. For these conditions, the effective diffusion rate of oxygen becomes of the same order as the oxygen consumption rate, allowing the cell to reduce the oxygen concentration of the nucleus, with implications to the Warburg Effect. The cellular and nucleus oxygen content is indirectly evaluated experimentally for bladder (T24) cancer cells and during the cell cycle, where the cells are initially synchronized using hydroxeaurea (HU) at the late G1-phase/early S-phase. The analysis of cellular and nucleus oxygen concentration during cell cycle is performed via (i) RT-qPCR gene analysis of hypoxia inducible transcription factors (HIF) and prolyl hydroxylases (PHD) and (ii) radiation clonogenic assay every 2 h, after release from synchronization. The HIF/PHD genes allowed us to correlate cellular oxygen with oxygen concentration in the nucleus that is obtained from the cells radiation response, where the amount DNA damage due to radiation is directly related to the amount of oxygen present in the nucleus. We demonstrate that during the S-phase cells can become hypoxic in the late S-phase/early G2-phase and therefore the radiation resistance increases 2- to 3-fold.

A new strategy to accelerate co-deposition of plant polyphenol and amine for fabrication of antibacterial nanofiltration membranes by in-situ grown Ag nanoparticles

Due to the far cheaper price than polydopamine, plant polyphenol and amine have been widely used for mussel-inspired chemistry. However, co-deposition of them still suffers the problem of long co-deposition time owning to the slow dissolution and diffusion of oxygen in solution. Herein, we reported an interesting strategy regarding addition of AgNO3 as both oxidant and Ag precursor into co-deposition of tannic acid and diethylenetriamine for nanofiltration (NF) membrane preparation. It was found that, AgNO3 addition not only shortened co-deposition time, but also in-situ generated Ag nanoparticles and rendered membrane surface antibacterial activity. The effect of AgNO3 addition on co-deposition behavior, physicochemical properties, separation and antibacterial performance of the as-prepared NF membrane were systematically investigated. The results showed that, when 0.15 wt% AgNO3 was added, the prepared NF membrane possessed a pure water flux of 5.36 L m−2h−1 bar−1, a MgCl2 rejection of 86.5% and an excellent antibacterial activity. This study not only provided a facile and effective strategy to prepare antibacterial NF membrane, but also offered new insights into co-deposition of polyphenol and amine for more widespread applications.

Enhanced Mass Transfer of Oxygen through a Gas–Liquid–Solid Interface for Photocatalytic Hydrogen Peroxide Production

AbstractSolar‐driven photocatalytic oxygen reduction is a potentially sustainable route for the production of hydrogen peroxide (H2O2). However, this approach suffers from the limited solubility and slow diffusion of oxygen in water. Another problem is that most photocatalytic oxygen reduction systems do not work well with just water. They often require the addition of sacrificial agents such as alcohols. Here, a covalent organic framework (COF)‐based photocatalyst that can reduce O2 to H2O2 efficiently in pure water under visible‐light irradiation is reported. A solar‐to‐chemical conversion of 0.76% is achieved for H2O2 generation. More importantly, the hydrophobic and mesoporous properties of triphenylbenzene‐dimethoxyterephthaldehyde‐COF allow the formation of a triphase interface (gas–liquid–solid) when loading this catalyst onto a porous substrate. The H2O2 production rate reaches ≈2.9 mmol gcat−1 h−1 at the triphase interface by overcoming the mass‐transfer limitation of O2 in water. Notably, this rate is 15 times higher than that in a diphase system (liquid–solid). The photoelectrochemical tests reveal that the increase in yield is closely related to the enhanced mass‐transfer rate and the higher interfacial O2 concentration. Furthermore, the triphenylbenzene part is identified as the reactive site based on theoretical calculations.

High-temperature behaviour of HVOF (Co,Ni)O coated Cu-Ni-Fe anodes

Cu-Ni-Fe alloys are promising inert anodes for green Al production but their corrosion resistance must be improved. Here, protective (Co,Ni)O coatings are deposited by high velocity oxygen fuel (HVOF) process on Cu-Ni-Fe alloys. The influence of the coating and substrate compositions on their behavior is studied at 1000 °C under argon and air. On Cu-rich alloy, CoxNi1−xO coatings with higher nickel content slow down oxygen diffusion to the substrate, as well as copper diffusion to the sample surface. On Ni-rich alloy, the formation of a NiFe2O4 scale is observed, whose thickness decreases as the Ni content of the CoxNi1−xO coatings increases.

Open-circuit dissolution of platinum from the cathode in polymer electrolyte membrane water electrolysers

Platinum is the state-of-the-art catalyst for hydrogen evolution in polymer electrolyte membrane (PEM) water electrolysers; however, its stability has only been characterized to a limited extent in situ. This study measures platinum dissolving from the cathode during intermittent operation of a 3-electrode PEM electrolyser cell, using a differential pulse voltammetry technique that provided detection limits for platinum of less than 2 ng L−1. Water samples were periodically taken during on-off current cycling, and during periods of open-circuit voltage (OCV) platinum dissolution was detected when the cathode potential rose above 0.85 V NHE due to diffusion of oxygen from the anode. This reached a maximum dissolution rate at the highest cathode potential of 1.02 V NHE, and gradually decayed over a 90 h period. The average total amount of platinum dissolved per 90 h OCV period was estimated to be 152 ng cm−2 or 0.005% of the initial electrode catalyst mass. The dissolution mechanism was predicted to be the same as that occurring in PEM fuel cell cathodes, although being kinetically hindered in PEM electrolysers by the slow diffusion of oxygen from the anode to the cathode.

Self-heating effects in the thermogravimetric analysis of wood char oxidation

Thermogravimetric curves are measured, by means of a flexible home-made (HM) system, of the oxidation of wood chars generated from packed-bed pyrolysis at 800 K. A careful control of the sample temperature and a small mass/size (4 mg mass corresponding to 140micron thick particle layer) allow a kinetic regime to be established for heating rates between 5 and 10 K/min. Good quantitative predictions are obtained based on a two-step mechanism, with activation energies affected by the hardwood or softwood origin of chars (125–143 kJ/mol and 186–207 kJ/mol for the devolatilization and oxidation reaction, respectively). The same conditions applied for a standard commercial (SC) thermogravimetric system result into remarkable self-heating leading to uncontrolled ignition and too slow oxygen diffusion rates. Only extremely small sample masses (≤1mg) and slow heating rates (≤5K/min) permit to completely eliminate heat and mass transfer intrusions but the actual limits are strictly related to the specific char reactivity.

New insight into the formation and oxygen barrier mechanism of carbonaceous oxide interlayer in a multicomponent carbide

ABSTRACTEarly transition metal carbides are considered to be superior candidate materials for oxidizing environments at temperatures exceeding 2000°C. Generally, the remarkable oxidation resistance is largely attributed to a carbonaceous oxide interlayer (eg, Hf–O–C, Zr–O–C, and Ta–O–C), located at the interface between the external oxide layer and internal carbide (eg, HfC, ZrC, and TaC), acting as the primary oxygen barrier. However, the oxygen barrier mechanism of the carbonaceous oxide interlayer remains unclear. Herein, through studying the oxidation behavior of a novel multicomponent carbide Hf0.5Zr0.3Ti0.2C in oxidizing environments up to 2500°C, the oxygen barrier mechanism of the carbonaceous oxide was recently revealed. We found that the oxygen barrier resulted from the slow oxygen diffusion through the inner grains of Hf‐Zr–Ti–O due to the presence of carbon formed at the grain boundaries because of the existence of compact external oxide layer, beneath which the Hf–Zr–Ti–O–C interlayer possesses much lower oxygen activity and temperature that allow carbon to exist stably. This as‐formed carbon strongly retarded the fast diffusion of oxygen along the grain boundaries of oxides. Additionally, desirable synergisms of the designed multicomponent system, particularly, the outward short‐circuit diffusion of Ti, lead to the self‐healing of the external oxide layer, evidently enhancing integral protection performance against oxidizing environments.

Air tightness of hBN encapsulation and its impact on Raman spectroscopy of van der Waals materials

Raman spectroscopy is a precious tool for the characterization of van der Waals materials, e.g. for the determination of the layer number in thin exfoliated flakes. For sensitive materials, however, this method can be dramatically invasive. In particular, the light intensity required to obtain a significant Raman signal is sufficient to immediately photo-oxidize few-layer thick metallic van der Waals materials. In this work we investigated the impact of the environment on Raman characterization of thin NbSe2 crystals. We show that in ambient conditions the flake is locally oxidized even for very low illumination intensity. Based on this extreme sensitivity to the presence of light and oxygen, we could study the air-tightness of the hBN encapsulation method, the most common passivation method for a wide range of 2D material-based devices. We find that only fully encapsulated devices are reliably air-tight. On the contrary, a simple hBN cover from the top does not prevent a slow diffusion of oxygen between the SiO2 surface and the flake itself.

Catalytic Decomposition of Hypochlorite Anions by Ceria Nanoparticles Visualized by Spectroscopic Techniques

Hypochlorous acid (HOCl) and hypochlorite anions (−OCl) are nonselective oxidants produced by activated neutrophils and monocytes and play an important role in the formation of immune response of a living organism. At the same time, because of the strong oxidative properties of hypochlorites, their concentrations in cells need to be under strict control. In this paper, cerium oxide nanoparticles (nanoceria) were used for spectroscopically controlled catalytic decomposition of hypochlorite anions. The processes of scavenging of hypochlorite anions by ceria nanoparticles and regeneration of nanoceria were monitored using the changes in hypochlorite absorption and 5d → 4f luminescence of Ce3+ ions during oxidation–reduction cycles. The high oxygen deficiency of ceria nanoparticles provides sufficient mobility of oxygen ions, leading to the involvement of both surface and bulk Ce3+ ions into oxidation, whereas the recovery of initial Ce3+/Ce4+ ratio is terminated by the slow diffusion of oxygen out of ceria nanoparticles.

Electro‐chemomechanical Contribution to Mechanical Actuation in Gd‐Doped Ceria Membranes

AbstractGd‐doped ceria (CGO), one of the most extensively studied oxygen ion conductors, is a low dielectric constant/low mechanical compliance material exhibiting large nonclassical electrostriction. The electromechanical response of the micro‐electromechanical devices with CGO films as an active material described previously can not be attributed exclusively to electrostriction. Here it is shown that, below 1 Hz, in addition to electrostriction (second‐harmonic response), there is a strong contribution of the electro‐chemomechanical effect (ECM, first harmonic response). ECM is the change in mechanical dimensions of ionic and mixed ionic‐electronic conductors as a result of a change in chemical composition induced by an electric field. In batteries, the presence of ECM is highly detrimental. In ceria at room temperature, it was considered to be negligible because of slow oxygen diffusion. This work demonstrates ECM actuation at ambient temperature and moderate electric field (<5 V µm−1). ECM‐induced strain is attributed to reversible oxidation/ reduction of TiO2 layers at the Ti‐CGO interface. At 25 °C, the ECM bending strain is 1.2 × 10−6, increasing exponentially with temperature. These data suggest that with a proper choice of materials, ECM‐type response can be a viable mechanism for mechanical actuation at ambient and also at slightly elevated temperatures.

Image of O2 dynamics released by oak wood submerged in model wine with nanoparticle sensors

Oxidation of wine while in contact with oak wood is a well-known fact and recognized as an important process in wine ageing. The slow and continuous diffusion of oxygen from the oak wood entrapped in its porosity occurs and encourages the reactions usually related with wine aging. In this work, oxygen sensitive particles were used to study the oxygen dynamics from different anatomical regions of the oak wood and simultaneously the oxygen increasement because of the deoxygenation of the wood and the oxygen depletion due to its consumption by the tannins and other compounds released from the wood. To this end, core-shell-type PSPVP nanoparticles stained with Macrolex Fluorescence Yellow and Pt-TFPP were employed as reference and oxygen-sensitive particles respectively. Moreover, a Guppy Pro RGB camera was employed to monitor the particles performing ratiometric oxygen imaging, using the green and red channels to acquire the light emitted from the reference and the oxygen-sensitive particles respectively. Because the volume of nanosensors corresponding to each surface unit is very different from that of the wood region, different exposure times were chosen to obtain the images at each of the times. The results show the wood degassing process during the first minutes of the experiment, the oxygen release from different structural elements of the wood, its consumption by the released compounds from wood and the diffusion of the oxygen through the model wine.

A Polyextreme Hydrothermal System Controlled by Iron: The Case of Dallol at the Afar Triangle.

One of the latest volcanic features of the Erta Ale range at the Afar Triangle (NE Ethiopia) has created a polyextreme hydrothermal system located at the Danakil depression on top of a protovolcano known as the dome of Dallol. The interaction of the underlying basaltic magma with the evaporitic salts of the Danakil depression has generated a unique, high-temperature (108 °C), hypersaline (NaCl supersaturated), hyperacidic (pH values from 0.1 to −1.7), oxygen-free hydrothermal site containing up to 150 g/L of iron. We find that the colorful brine pools and mineral patterns of Dallol derive from the slow oxygen diffusion and progressive oxidation of the dissolved ferrous iron, the iron-chlorine/-sulfate complexation, and the evaporation. These inorganic processes induce the precipitation of nanoscale jarosite-group minerals and iron(III)-oxyhydroxides over a vast deposition of halite displaying complex architectures. Our results suggest that life, if present under such conditions, does not play a dominant role in the geochemical cycling and mineral precipitation at Dallol as opposed to other hydrothermal sites. Dallol, a hydrothermal system controlled by iron, is a present-day laboratory for studying the precipitation and progressive oxidation of iron minerals, relevant for geochemical processes occurring at early Earth and Martian environments.

Probing Colossal Ion Conductivity Hypotheses

Fuel cells have long promised to bring high efficiency to the production of energy. In practice, fuel cells have been limited to hydrogen systems, where the high diffusivity of hydrogen through membranes allows high current densities. Hydrogen based fuel cells pose three major limitations: the requirement of expensive catalysts, the expensive or dangerous shipping of hydrogen, and the limitation to the single fuel that is hydrogen. Alternatives are present in the form of solid oxide fuel cells (SOFCs), which are composed of earth abundant materials, and may utilize arbitrary fuels. Despite the prospects of SOFCs, their deployment has been limited by low current densities caused by the slow diffusion of oxygen through SOFC membranes. A scientific solution to the problem of low current density was found in the form of Colossal Ion Conductivity (CIC), where large changes in ion conductivity were observed in heterolayer environments. Both because of the complexity of this problem, and the importance of it as a technological solution, CIC immediately became hotly debated, with several researchers offered mechanistic hypotheses.

SIMS study of oxygen diffusion in monoclinic HfO2

The diffusion of oxygen in dense ceramics of monoclinic HfO2 was studied by means of (18O/16O) isotope exchange annealing and subsequent determination of isotope depth profiles by Secondary Ion Mass Spectrometry. Anneals were performed in the temperature range of 573 ≤T/K≤ 973 at an oxygen partial pressure of pO2=200 mbar. All measured isotope profiles exhibited two features: the first feature, closer to the surface, was attributed mainly to slow oxygen diffusion in an impurity silicate phase; the second feature, deeper in the sample, was attributed to oxygen diffusion in bulk monoclinic HfO2. The activation enthalpy of oxygen tracer diffusion in bulk HfO2 was found to be ΔHD∗≈0.5 eV.