Showing papers on "Oxygen published in 2019"
TL;DR: In this paper, the authors investigated Co-Zn oxyhydroxide electrocatalysts, and suggested that the mechanism depends on the amount of Zn2+ they contain, and found that Zn0.2Co0.8 has the optimum activity.
Abstract: The oxygen evolution reaction (OER) is a key process in electrochemical energy conversion devices. Understanding the origins of the lattice oxygen oxidation mechanism is crucial because OER catalysts operating via this mechanism could bypass certain limitations associated with those operating by the conventional adsorbate evolution mechanism. Transition metal oxyhydroxides are often considered to be the real catalytic species in a variety of OER catalysts and their low-dimensional layered structures readily allow direct formation of the O–O bond. Here, we incorporate catalytically inactive Zn2+ into CoOOH and suggest that the OER mechanism is dependent on the amount of Zn2+ in the catalyst. The inclusion of the Zn2+ ions gives rise to oxygen non-bonding states with different local configurations that depend on the quantity of Zn2+. We propose that the OER proceeds via the lattice oxygen oxidation mechanism pathway on the metal oxyhydroxides only if two neighbouring oxidized oxygens can hybridize their oxygen holes without sacrificing metal–oxygen hybridization significantly, finding that Zn0.2Co0.8OOH has the optimum activity. Oxygen evolution is one half of the overall water splitting reaction to produce hydrogen. Although this reaction is well studied, there remains debate over the particulars of the catalytic mechanism. Here, the authors investigate Co–Zn oxyhydroxide electrocatalysts, and suggest that the mechanism depends on the amount of Zn2+ they contain.
798 citations
239 citations
TL;DR: The insights on the primary active oxygen specie and its relation with oxygen vacancy is essential for the design of low-temperature oxidation catalysts in this article, where oxygen vacancy-rich La0.8Sr0.2CoO3 is considered.
Abstract: The insights on the primary active oxygen specie and its relation with oxygen vacancy is essential for the design of low-temperature oxidation catalysts. Herein, oxygen vacancy-rich La0.8Sr0.2CoO3 ...
228 citations
TL;DR: In this article, a Li-gradient structure of the oxides, obtained by a selective LiO leaching process via a molten salt treatment, displaying virtually zero oxygen loss was presented.
Abstract: Lithium-rich transition metal oxide (Li1+XM1−XO2) cathodes have high energy density above 900 Wh kg−1 due to hybrid anion- and cation-redox (HACR) contributions, but critical issues such as oxygen release and voltage decay during cycling have prevented their application for years. Here we show that a molten molybdate-assisted LiO extraction at 700 °C creates lattice-coherent but depth (r)-dependent Li1+X(r)M1−X(r)O2 particles with a Li-rich (X ≈ 0.2) interior, a Li-poor (X ≈ −0.05) surface and a continuous gradient in between. The gradient Li-rich single crystals eliminate the oxygen release to the electrolyte and, importantly, still allow stable oxygen redox contributions within. Both the metal valence states and the crystal structure are well maintained during cycling. The gradient HACR cathode displays a specific density of 843 Wh kg−1 after 200 cycles at 0.2C and 808 Wh kg−1 after 100 cycles at 1C, with very little oxygen release and consumption of electrolyte. This high-temperature immunization treatment can be generalized to leach other elements to avoid unexpected surface reactions in batteries. Critical issues such as oxygen release during battery cycling plague the development of high-energy Li-rich oxide cathodes. Here the authors report a Li-gradient structure of the oxides, obtained by a selective LiO leaching process via a molten salt treatment, displaying virtually zero oxygen loss.
206 citations
TL;DR: To reveal the mechanism of water oxidation, structures of PSII in the S1, S2, and S3 states by x-ray free-electron laser serial crystallography reveal details of photosynthetic oxygen evolution.
Abstract: Photosynthetic water oxidation is catalyzed by the Mn4CaO5 cluster of photosystem II (PSII) with linear progression through five S-state intermediates (S0 to S4). To reveal the mechanism of water oxidation, we analyzed structures of PSII in the S1, S2, and S3 states by x-ray free-electron laser serial crystallography. No insertion of water was found in S2, but flipping of D1 Glu189 upon transition to S3 leads to the opening of a water channel and provides a space for incorporation of an additional oxygen ligand, resulting in an open cubane Mn4CaO6 cluster with an oxyl/oxo bridge. Structural changes of PSII between the different S states reveal cooperative action of substrate water access, proton release, and dioxygen formation in photosynthetic water oxidation.
205 citations
TL;DR: It is shown that bulk Ni sites in Ni and NiCo oxides are active and evolve oxygen via a NiOO− precursor, and Fe incorporation not only dramatically increases the activity, but also changes the nature of the active sites.
Abstract: Nickel iron oxide is considered a benchmark nonprecious catalyst for the oxygen evolution reaction (OER). However, the nature of the active site in nickel iron oxide is heavily debated. Here we report direct spectroscopic evidence for the different active sites in Fe-free and Fe-containing Ni oxides. Ultrathin layered double hydroxides (LDHs) were used as defined samples of metal oxide catalysts, and 18 O-labeling experiments in combination with in situ Raman spectroscopy were employed to probe the role of lattice oxygen as well as an active oxygen species, NiOO- , in the catalysts. Our data show that lattice oxygen is involved in the OER for Ni and NiCo LDHs, but not for NiFe and NiCoFe LDHs. Moreover, NiOO- is a precursor to oxygen for Ni and NiCo LDHs, but not for NiFe and NiCoFe LDHs. These data indicate that bulk Ni sites in Ni and NiCo oxides are active and evolve oxygen via a NiOO- precursor. Fe incorporation not only dramatically increases the activity, but also changes the nature of the active sites.
201 citations
Northeastern University (China)1, Lawrence Berkeley National Laboratory2, Geballe Laboratory for Advanced Materials3, Peking University4, Shandong University5, Binghamton University6, Zhengzhou University7, Tianjin Normal University8, Stanford University9, SLAC National Accelerator Laboratory10, Tsinghua University11
TL;DR: In this paper, a mapping of resonant inelastic X-ray scattering (mRIXS) was used to quantitatively quantify the evolving redox states of both cations and anions in Na2/3Mg1/3mn2/ 3O2 and Li1.17Ni0.21Co0.08Mn0.54O2 systems.
192 citations
TL;DR: It is found that ZnO and cocatalyst could concertedly activate CH4 and O2 into methyl radical and mildly oxidative intermediate in water, which are two key precursor intermediates for generating oxygenated liquid products in direct CH4 oxidation.
Abstract: Direct conversion of methane into methanol and other liquid oxygenates still confronts considerable challenges in activating the first C-H bond of methane and inhibiting overoxidation. Here, we report that ZnO loaded with appropriate cocatalysts (Pt, Pd, Au, or Ag) enables direct oxidation of methane to methanol and formaldehyde in water using only molecular oxygen as the oxidant under mild light irradiation at room temperature. Up to 250 micromoles of liquid oxygenates with ∼95% selectivity is achieved for 2 h over 10 mg of ZnO loaded with 0.1 wt % of Au. Experiments with isotopically labeled oxygen and water reveal that molecular O2, rather than water, is the source of oxygen for direct CH4 oxidation. We find that ZnO and cocatalyst could concertedly activate CH4 and O2 into methyl radical and mildly oxidative intermediate (hydroperoxyl radical) in water, which are two key precursor intermediates for generating oxygenated liquid products in direct CH4 oxidation. Our study underlines two equally significant aspects for realizing direct and selective photooxidation of CH4 to liquid oxygenates, i.e., efficient C-H bond activation of CH4 and controllable activation of O2.
189 citations
TL;DR: In this paper, an oxygen defect dependent peroxymonosulfate (PMS) activation mechanism over perovskite with the singlet oxygen (1O2) as the dominant ROS was proposed.
185 citations
TL;DR: An advanced trifunctional electrocatalyst based on a series of composites composed of TiO2-encapsulated carbon-nitride (CNx) is developed, which is derived from the Ti3C2Tx and melamine-cyanuric acid calcinated at different temperatures.
Abstract: An advanced trifunctional electrocatalyst based on a series of composites composed of TiO2-encapsulated carbon-nitride (CNx) (denoted as TiO2C@CNx) is developed, which is derived from the Ti3C2Tx and melamine–cyanuric acid calcinated at different temperatures. Among the series of TiO2C@CNx nanosheets, the TiO2C@CNx,950 (obtained by calcination at 950 °C) hybrid exhibits robust trifunctional electrocatalytic activity toward the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) by combining the excellent electrochemical activity of the graphene-like nanostructure and the high electrocatalytic performances of TiO2 nanoparticles. When TiO2C@CNx,950 is used as the electrocatalyst for water splitting, a current density of 10 mA cm−2 (Ej = 10) is achieved at a low cell voltage of 1.50 V vs. reversible hydrogen electrode. Meanwhile, the overall oxygen activity of the TiO2C@CNx,950 exhibits good reversible oxygen reaction, giving a small potential difference between the Ej = 10 for OER and the half-wave potential for ORR (0.75 V). Moreover, a simply equipped Zn-air battery is assembled using a homemade cathode, showing open-circuit potential of 1.344 V, which also can supplied an electrical power and produced H2 at the cathode and O2 at the anode. Consequently, this work can pave a path for developing multifunctional electrocatalysts for water spitting and liquid Zn–air battery.
182 citations
TL;DR: In this paper, the interaction of reactant CO2 molecules on Ceria (CeO2)-based photocatalysts was investigated in comparison with Cu2O nanoparticles, and the results showed that the synergistic effects of CO2 reactant on Cu2 nanoparticles were beneficial for more generated carboxylate and CO2− radicals, instead of carbonate species which promoted CO2 reduction to CO.
Abstract: Ceria (CeO2) with abundant oxygen defects, surface alkalinity, low cost effectiveness and admirable redox ability could be used in the photoreduction of CO2. However, little attention has been paid to the interaction of reactant CO2 molecules on CeO2–based photocatalysts. In this work, Cu2O nanoparticles were applied to the modification of the properties of Lewis acid/base, surface oxygen defect content and visible light adsorption of CeO2, and the adsorption/activation abilities of CO2 reactant on Cu2O/CeO2 and CeO2 photocatalysts were investigated in comparison. The photocatalytic performance showed that Cu2O/CeO2 had better activity than CeO2. And the loading of Cu2O resulted in more oxygen defects and Ce3+ species, which was helpful for available visible light adsorption and higher charge-separation efficiency. Furthermore, CO2-TPD, CO2-adsorption DRIFTS and in-situ ESR results demonstrated that the synergistic effects of Cu2O/CeO2 were beneficial for more generated carboxylate and CO2− radicals, instead of carbonate species which promoted CO2 reduction to CO.
TL;DR: The layered copper manganese oxide (LCMO) with a bridged monoclinic/tetragonal phase structure was successfully prepared by one-step hydrothermal redox-precipitation method and demonstrated highly efficient and stable catalytic capability to CO and VOCs oxidation.
TL;DR: In this paper, hierarchical mixed oxides perovskite microspheres were synthesized for catalytic ozonation over oxalic acid and benzotriazole.
Abstract: La-based perovskites are catalytically active owing to the oxygen vacancies, redox metal centers of B sites and surface hydroxyl groups. Nevertheless, the insights into these active centers on environmental catalysis are still insufficient. In this study, hierarchical mixed oxides perovskite microspheres were synthesized for catalytic ozonation over oxalic acid and benzotriazole. LaMn4Ox, with LaMnO3-δ as the dominant crystal phase, demonstrated superior catalytic activity to Mn2O3 and LaMnO3 synthesized from citric acid sol-gel method. Temperature-programmed desorption of NH3 (NH3-TPD) and pyridine-Fourier transform infrared spectroscopy (pyridine-FTIR) tests proved Lewis acid as the main acid type. Temperature-programmed reduction of H2 (H2-TPR), O2-TPD and X-ray photoelectron spectroscopy (XPS) analysis indicated the presence of oxygen vacancies and mixed valences of Mn in the crystal structure facilitated the catalytic process. Moreover, the content of oxygen vacancy was calculated by iodometric titration method. With the aid of theoretical calculations, oxygen vacancies were found to exhibit a strong affinity toward ozone adsorption, where ozone molecules spontaneously dissociated into reactive oxygen species (ROS) such as O2 − and 1O2. The B site of Mn facilitated ozone decomposition by extending the O O bond of ozone due to the electron transfer from Mn3+/Mn4+ redox cycle. In-situ EPR and quenching tests confirmed the contribution of O2 − and 1O2 in benzotriazole degradation along with OH. This study stepped further to unveil the ozone adsorption/decomposition and ROS generation on nanoscale perovskite-based composites.
TL;DR: In this article, the authors compared two nonthermal air plasma sources, streamer corona and transient spark, interacting with water in open and closed reactors, and by enhancing the plasma-liquid interaction by water electrospray through these discharges, demonstrating that the plasma gaseous products strongly depend on the discharge regime, its deposited power and gas flow conditions.
Abstract: When cold atmospheric plasma comes into contact with water and biological media, antimicrobial or antitumor effects are induced, representing great potential for applications in biomedicine and agriculture. The need to control and tune the chemical composition and biomedical effects of plasma activated water/media (PAW/PAM) is emerging. By comparing two nonthermal air plasma sources, streamer corona and transient spark, interacting with water in open and closed reactors, and by enhancing the plasma–liquid interaction by water electrospray through these discharges, we demonstrate that the plasma gaseous products strongly depend on the discharge regime, its deposited power and gas flow conditions. The streamer corona strongly leads to the formation of ozone and hydrogen peroxide, while the more energetic transient spark leads to nitrogen oxides and hydrogen peroxide. The gaseous products then determine the chemical properties of the PAW and the dominant aqueous reactive oxygen and nitrogen species (RONS). The production of hydrogen peroxide depends on water evaporation and hydroxyl radical formation, which is determined by the discharge power. A transient spark produces higher concentrations of gaseous and aqueous RONS and induces stronger antibacterial effects than a streamer corona; however, the RONS production rates per joule of deposited energy are comparable for both studied discharge regimes. The net production rate per joule of gaseous nitrogen oxides strongly correlates with that of aqueous nitrites and nitrates. The antibacterial effects of the PAW tested on Escherichia coli bacteria are determined by the aqueous RONS: in the lower power streamer corona, this is ascertained mainly by the dissolved ozone and hydrogen peroxide, and in the higher power transient spark, by the combination of hydrogen peroxide, nitrite and acidic pH, while in the transient spark in the closed reactor it is determined by the acidified nitrites present.
TL;DR: Inspired by the active Ni3+ intermediates involved in electrocatalytic water oxidation by nickel‐based materials, NiBx was prepared as a catalyst and used water as the oxygen source for the oxygenation of various organic compounds and hydrogenation of organic compounds.
Abstract: The use of water as an oxygen and hydrogen source for the paired oxygenation and hydrogenation of organic substrates to produce valuable chemicals is of utmost importance as a means of establishing green chemical syntheses. Inspired by the active Ni3+ intermediates involved in electrocatalytic water oxidation by nickel-based materials, we prepared NiBx as a catalyst and used water as the oxygen source for the oxygenation of various organic compounds. NiBx was further employed as both an anode and a cathode in a paired electrosynthesis cell for the respective oxygenation and hydrogenation of organic compounds, with water as both the oxygen and hydrogen source. Conversion efficiency and selectivity of ≥99 % were observed during the oxygenation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid and the simultaneous hydrogenation of p-nitrophenol to p-aminophenol. This paired electrosynthesis cell has also been coupled to a solar cell as a stand-alone reactor in response to sunlight.
TL;DR: In this article, the effects of oxygen species on the DRM over Ni/ZrO2 catalysts were investigated, where the treatment to ZrO 2 in the reducing gas such as H2 was able to promote the formation of adsorbed oxygen species over the surface.
Abstract: Carbon deposition on the catalyst surface has been considered as one of the main reasons to cause Ni catalyst deactivation during methane reforming with carbon dioxide (DRM). Generally, it is thought that the properties and levels of coke formation are significantly affected by the ability of CO2 adsorption and activation, which is closely related to the oxygen species over catalyst surface. In this study, the effects of oxygen species on the DRM over Ni/ZrO2 catalysts were investigated. Wherein, ZrO2 support was further treated under H2, N2, and O2 atmospheres respectively to obtain different oxygen species distribution over the surface, and 3 wt.% Ni was introduced by the deposition-precipitation method. The representative samples were characterized using XRD, EPR, XPS, FTIR, TPR, TPD, TPSR, TG, Raman and TPH techniques, etc. It was found that the treatment to ZrO2 in the reducing gas such as H2 was able to promote the formation of adsorbed oxygen species over the surface. CO2 DRIFTS and CH4-TPSR experiments further confirmed that adsorbed oxygen species were favorable for enhancing both CO2 adsorption and activation and CH4 dissociation. As a result, the catalyst with more adsorbed oxygen species exhibited relatively outstanding performance for the DRM and promotional ability in removal of the deposited carbon.
Journal Article•
TL;DR: This review included the most relevant measuring methods of PO2 as well as a practical overview of its reference values in several tissues, including a facilitated description of the gradient of pressure from the atmosphere to the cells.
Abstract: The human body is a highly aerobic organism, in which it is necessary to match oxygen supply at tissue levels to the metabolic demands. Along metazoan evolution, an exquisite control developed because although oxygen is required as the final acceptor of electron respiratory chain, an excessive level could be potentially harmful. Understanding the role of the main factors affecting oxygen availability, such as the gradient of pressure of oxygen during normal conditions, and during hypoxia is an important point. Several factors such as anaesthesia, hypoxia, and stress affect the regulation of the atmospheric, alveolar, arterial, capillary and tissue partial pressure of oxygen (PO2). Our objective is to offer to the reader a summarized and practical appraisal of the mechanisms related to the oxygen's supply within the human body, including a facilitated description of the gradient of pressure from the atmosphere to the cells. This review also included the most relevant measuring methods of PO2 as well as a practical overview of its reference values in several tissues.
TL;DR: In this paper, the authors describe the selective late-stage oxygenation of sulfur-containing complex molecules with ground-state oxygen under ambient conditions, which enabled the efficient synthesis of sulfones.
Abstract: Oxygenation is a fundamental transformation in synthesis. Herein, we describe the selective late-stage oxygenation of sulfur-containing complex molecules with ground-state oxygen under ambient conditions. The high oxidation potential of the active uranyl cation (UO2 2+ ) enabled the efficient synthesis of sulfones. The ligand-to-metal charge transfer process (LMCT) from O 2p to U 5f within the O=U=O group, which generates a UV center and an oxygen radical, is assumed to be affected by the solvent and additives, and can be tuned to promote selective sulfoxidation. This tunable strategy enabled the batch synthesis of 32 pharmaceuticals and analogues by late-stage oxygenation in an atom- and step-efficient manner.
TL;DR: In this article, a review of the relevant researches on the iron-based oxygen carrier is summarized, which include the characteristics of iron oxides, the preparations of the iron based oxygen carrier, and their applications in the continuous operated chemical looping conversion processes.
TL;DR: In this article, it is shown that it is possible to increase the charge capacity of transition metal (TM) oxide cathodes in alkali-ion batteries by invoking redox reactions on the oxygen.
Abstract: It is possible to increase the charge capacity of transition metal (TM) oxide cathodes in alkali-ion batteries by invoking redox reactions on the oxygen. However, oxygen loss often occurs. To explo ...
TL;DR: In this paper, the extent of lattice oxygen participation in the oxygen evolution and associated degradation of rutile and hydrous iridium oxide catalysts, and correlate this mechanism with the atomic-scale structures of the catalytic surfaces.
Abstract: Understanding the fundamentals of iridium degradation during the oxygen evolution reaction is of importance for the development of efficient and durable water electrolysis systems. The degradation mechanism is complex and it is under intense discussion whether the oxygen molecule can be directly released from the oxide lattice. Here, we define the extent of lattice oxygen participation in the oxygen evolution and associated degradation of rutile and hydrous iridium oxide catalysts, and correlate this mechanism with the atomic-scale structures of the catalytic surfaces. We combine isotope labelling with atom probe tomography, online electrochemical and inductively coupled plasma mass spectrometry. Our data reveal that, unlike rutile IrO2, Ir hydrous oxide contains –IrIIIOOH species which directly contribute to the oxygen evolution from the lattice. This oxygen evolution mechanism results in faster degradation and dissolution of Ir. In addition, near surface bulk regions of hydrous oxide are involved in the oxygen catalysis and dissolution, while only the topmost atomic layers of rutile IrO2 participate in both reactions. Overall our data provide a contribution to the fundamental understanding of the exceptional stability of Ir-oxides towards the oxygen evolution reaction. The proposed approach to a quantitative assessment of the degree of lattice oxygen participation in the oxygen evolution reaction can be further applied to other oxide catalyst systems.
TL;DR: A novel strategy is developed that creating a spinel Co3O4/perovskite La0.3Sr0.7CoO3 interface by in-situ reconstruction of surface Sr enrichment region in LSC to activate surface lattice oxygen to provide a fundamental understanding to explain activation mechanism of surface oxygen sites.
Abstract: Surface lattice oxygen in transition-metal oxides plays a vital role in catalytic processes. Mastering activation of surface lattice oxygen and identifying the activation mechanism are crucial for the development and design of advanced catalysts. A strategy is now developed to create a spinel Co3 O4 /perovskite La0.3 Sr0.7 CoO3 interface by in situ reconstruction of the surface Sr enrichment region in perovskite LSC to activate surface lattice oxygen. XAS and XPS confirm that the regulated chemical interface optimizes the hybridized orbital between Co 3d and O 2p and triggers more electrons in oxygen site of LSC transferred into lattice of Co3 O4 , leading to more inactive O2- transformed into active O2-x . Furthermore, the activated Co3 O4 /LSC exhibits the best catalytic activities for CO oxidation, oxygen evolution, and oxygen reduction. This work would provide a fundamental understanding to explain the activation mechanism of surface oxygen sites.
TL;DR: A novel synchronous illumination X-ray photoelectron spectroscopy technique finds that the surface Vos (surf-Vos) exhibit a strong selectivity for binding to the adsorbed water molecules, and sequentially capture an oxygen atom from water molecule to achieve the anisotropic self-healing of surface lattice oxygen.
Abstract: Oxygen vacancy (Vo) on transition metal oxides plays a crucial role in determining their chemical/physical properties. Conversely, the capability to directly detect the changing process of oxygen vacancies (Vos) will be important to realize their full potentials in the related fields. Herein, with a novel synchronous illumination X-ray photoelectron spectroscopy (SI-XPS) technique, we found that the surface Vos (surf-Vos) exhibit a strong selectivity for binding with the water molecules, and sequentially capture an oxygen atom to achieve the anisotropic self-healing of surface lattice oxygen. After this self-healing process, the survived subsurface Vos (sub-Vos) promote the charge excitation from Ti to O atoms due to the enriched electron located on low-coordinated Ti sites. However, the excessive sub-Vos would block the charge separation and transfer to TiO2 surfaces resulted from the destroyed atomic structures. These findings open a new pathway to explore the dynamic changes of Vos and their roles on catalytic properties, not only in metal oxides, but in crystalline materials more generally.
01 Apr 2019
01 Sep 2019
TL;DR: Findings indicated that the oxygen vacancies promoted electronic transfer and participated in the redox cycle from Co3+/Fe3+ to Co2/Fe2+ to generate 1O2 and O2-.
TL;DR: In this article, Li/Na-ion cathodes with capacity exceeding the limitation of conventional intercalation-based oxide cathodes were shown to have reversible charge compensation of both lattice oxygen redox and transition metal (TM) redox.
Abstract: There is great interest in the discovery of Li/Na-ion cathode materials with capacity exceeding the limitation of conventional intercalation-based oxide cathodes. One plausible but challenging path is to reversibly use the charge compensation of both lattice oxygen redox and transition metal (TM) redox. Here, we report that lattice oxygen redox alone contributes over 190 mA h g−1 charge capacity (cut-off at 4.65 V vs. Na+/Na) for the newly synthesized P3-type Na2/3Mg1/3Mn(IV)2/3O2. Similar amounts of discharge capacity are reversibly achieved. The discharge capacity exceeds 220 mA h g−1 when Mn3+/Mn4+ redox is partially used in addition to the oxygen redox reaction. This represents one of the highest energy density sodium-ion cathodes with superior low-cost. Our results reveal that cations with strong ionic bonding nature with oxygen (such as Mg2+) are very effective in inducing the reversible oxygen redox reaction. We also identified the origin of voltage hysteresis to be a P3-to-O3 phase transition in concomitance with Mg2+ migration, suggesting further structure design that reduces the structure transition induced cation migration is critical for increasing the energy efficiency of the oxygen redox reactions.
TL;DR: This article hypothesizes that the radioprotecting effect of FLASH irradiation could be due to the specific sparing of hypoxic stem cell niches, which have been identified in several organs including the bone marrow and the brain.
Abstract: Recent results from animal irradiation studies have demonstrated the potential of ultra-high dose rate irradiation (also known as FLASH) for reducing radiation toxicity in normal tissues. However, despite mounting evidence of a "FLASH effect", a mechanism has yet to be elucidated. This article hypothesizes that the radioprotecting effect of FLASH irradiation could be due to the specific sparing of hypoxic stem cell niches, which have been identified in several organs including the bone marrow and the brain. To explore this hypothesis, a new computational model is presented that frames transient radiolytic oxygen depletion (ROD) during FLASH irradiation in terms of its effect on the oxygen enhancement ratio (OER). The model takes into consideration oxygen diffusion through the tissue, its consumption by metabolic cells, and its radiolytic depletion to estimate the relative decrease in radiosensitivity of cells receiving FLASH irradiation. Based on this model and the following parameters (oxygen diffusion constant Dalt;subagt;O2alt;/subagt; = 210alt;supagt;5alt;/supagt; cmalt;supagt;2alt;/supagt; salt;supagt;-1alt;/supagt;, oxygen metabolic rate m = 3 mmHg salt;supagt;-1alt;/supagt;, ROD rate alt;Iagt;Lalt;/iagt;alt;subagt;RODalt;/subagt; = 0.42 mmHg Gyalt;supagt;-1alt;/supagt;, prescribed dose Dp = 10 Gy, and capillary oxygen tension palt;subagt;0alt;/subagt; = 40 mmHg), several predictions are made that could be tested in future experiments: (1) the FLASH effect should gradually disappear as the radiation pulse duration is increased from alt;1 s to 10 s; (2) dose should be deposited using the smallest number of radiation pulses to achieve the greatest FLASH effect; (3) a FLASH effect should only be observed in cells that are already hypoxic at the time of irradiation; and (4) changes in capillary oxygen tension (increase or decrease) should diminish the FLASH effect.