Trace O2 Research Articles

Effect of trace oxygen on plasma nitriding of titanium foil

Titanium nitride films are prepared by plasma enhanced chemical vapor deposition method on titanium foil using N2 as precursor. In order to evaluate the effect of oxygen on the growth of titanium nitride films, a small amount of O2 is introduced into the preparation process. The study indicates that trace O2 addition into the reaction chamber gives rise to significant changes on the color and micro-morphology of the foil, featuring dense and long nano-wires. The as-synthesized nanostructures are characterized by various methods and identified as TiN, Ti2N, and TiO2 respectively. Moreover, the experiment results show that oxide nanowire has a high degree of crystallinity and the nitrides present specific orientation relationships with the titanium matrix.

Does deprotonated benzoic acid lose carbon monoxide in collision-induced dissociation?

Decarboxylation is known to be the major fragmentation pathway for the deprotonated carboxylic acids in collision-induced dissociation (CID). However, in the CID mass spectrum of deprotonated benzoic acid (m/z 121) recorded on a Q-orbitrap mass spectrometer, the dominant peak was found to be m/z 93 instead of the anticipated m/z 77. Based on theoretical calculations, 18 O-isotope labeling and MS3 experiments, we demonstrated that the fragmentation of benzoate anion begins with decarboxylation, but the initial phenide anion (m/z 77) can react with trace O2 in the mass analyzer to produce phenolate anion (m/z 93) and other oxygen-containing ions. Thus oxygen adducts should be considered when annotating the MS/MS spectra of benzoic acids.

Impact of environmental oxygen on nanoparticle formation and agglomeration in aluminum laser ablation plumes.

The role of ambient oxygen gas (O2) on molecular and nanoparticle formation and agglomeration was studied in laser ablation plumes. As a lab-scale surrogate to a high explosion detonation event, nanosecond laser ablation of an aluminum alloy (AA6061) target was performed in atmospheric pressure conditions. Optical emission spectroscopy and two mass spectrometry techniques were used to monitor the early to late stages of plasma generation to track the evolution of atoms, molecules, clusters, nanoparticles, and agglomerates. The experiments were performed under atmospheric pressure air, atmospheric pressure nitrogen, and 20% and 5% O2 (balance N2), the latter specifically with in situ mass spectrometry. Electron microscopy was performed ex situ to identify crystal structure and elemental distributions in individual nanoparticles. We find that the presence of ≈20% O2 leads to strong AlO emission, whereas in a flowing N2 environment (with trace O2), AlN and strong, unreacted Al emissions are present. In situ mass spectrometry reveals that as O2 availability increases, Al oxide cluster size increases. Nanoparticle agglomerates formed in air are found to be larger than those formed under N2 gas. High-resolution transmission electron microscopy demonstrates that Al2O3 and AlN nanoparticle agglomerates are formed in both environments; indicating that the presence of trace O2 can lead to Al2O3 nanoparticle formation. The present results highlight that the availability of O2 in the ambient gas significantly impacts spectral signatures, cluster size, and nanoparticle agglomeration behavior. These results are relevant to understanding debris formation in an explosion event, and interpreting data from forensic investigations.

Formation mechanism of Al4SiC4 in Al–SiC composite under flowing nitrogen at 1300 °C

Resin-bonded Al–SiC composite was sintered in an electric heating shuttle kiln to prepare aluminum-carbon sliding gates at 1300 °C with nitrogen gas, which synthesized ternary carbide Al4SiC4 in situ and a three-dimensional space interlocking structure of Al4SiC4rods/SiCparticles grown on the surface of SiC particles. The phase composition and microstructure of sintered sample were characterized by XRD and SEM, combined with thermodynamics to investigate the formation mechanism of Al4SiC4. It was found that as temperature increases, trace O2(g) in N2(g) causes active oxidation of carbon, with the composition of gas in environment as N2(g)+CO(g) +trace O2(g). A dense layer (Al4C3+Al2OC-AlNss) was formed on the outer layer of sample preventing the diffusion of ambient gas into inner. Under high temperature and low oxygen partial pressure conditions, the oxygen partial pressure inside the sample was lowered again by the active oxidation of Al(l), and the low oxygen partial pressure induced the active oxidation of SiC(s) so that N2(g)+CO(g)+trace O2(g) outside the sample was transformed to Al2O(g)+SiO(g)+CO(g) inside. Subsequently, Al4SiC4 nuclei were formed on the highly reactive surface of SiC particles by gas-gas reactions as well as releasing trace amounts of O2(g); surrounding SiC(s) and Al(l) rapidly convert O2(g) into reactive gas to participate a second time in the growth process of Al4SiC4 nuclei. A self-circulating system with O2(g) as a catalytic medium has been established to "catalyze" the nucleation and growth of Al4SiC4, which realizes a refined modification of Al4SiC4 morphology at the micro-scale.

Insight into Iron Leaching from an Ascorbate‐Oxidase‐like Fe−N−C Nanozyme and Oxygen Reduction Selectivity**

AbstractAscorbate (H2A) is a well‐known antioxidant to protect cellular components from free radical damage and has also emerged as a pro‐oxidant in cancer therapies. However, such “contradictory” mechanisms underlying H2A oxidation are not well understood. Herein, we report Fe leaching during catalytic H2A oxidation using an Fe−N−C nanozyme as a ferritin mimic and its influence on the selectivity of the oxygen reduction reaction (ORR). Owing to the heterogeneity, the Fe‐Nx sites in Fe−N−C primarily catalyzed H2A oxidation and 4 e− ORR via an iron‐oxo intermediate. Nonetheless, trace O2⋅− produced by marginal N−C sites through 2 e− ORR accumulated and attacked Fe‐Nx sites, leading to the linear leakage of unstable Fe ions up to 420 ppb when the H2A concentration increased to 2 mM. As a result, a substantial fraction (ca. 40 %) of the N−C sites on Fe−N−C were activated, and a new 2+2 e− ORR path was finally enabled, along with Fenton‐type H2A oxidation. Consequently, after Fe ions diffused into the bulk solution, the ORR at the N−C sites stopped at H2O2 production, which was the origin of the pro‐oxidant effect of H2A.

Insight into Iron Leaching from an Ascorbate-Oxidase-like Fe-N-C Nanozyme and Oxygen Reduction Selectivity.

Ascorbate (H2 A) is a well-known antioxidant to protect cellular components from free radical damage and has also emerged as a pro-oxidant in cancer therapies. However, such "contradictory" mechanisms underlying H2 A oxidation are not well understood. Herein, we report Fe leaching during catalytic H2 A oxidation using an Fe-N-C nanozyme as a ferritin mimic and its influence on the selectivity of the oxygen reduction reaction (ORR). Owing to the heterogeneity, the Fe-Nx sites in Fe-N-C primarily catalyzed H2 A oxidation and 4 e- ORR via an iron-oxo intermediate. Nonetheless, trace O2 ⋅- produced by marginal N-C sites through 2 e- ORR accumulated and attacked Fe-Nx sites, leading to the linear leakage of unstable Fe ions up to 420 ppb when the H2 A concentration increased to 2 mM. As a result, a substantial fraction (ca. 40 %) of the N-C sites on Fe-N-C were activated, and a new 2+2 e- ORR path was finally enabled, along with Fenton-type H2 A oxidation. Consequently, after Fe ions diffused into the bulk solution, the ORR at the N-C sites stopped at H2 O2 production, which was the origin of the pro-oxidant effect of H2 A.

Synthesis mechanism of α-Si3N4 whiskers via SiO vapor in reaction bonded Si3N4-SiC composite

An indirect nitridation mechanism was designed to realize a high conversion rate to α-Si3N4 whiskers in reaction bonded Si3N4-SiC composite via VS mechanism, in which the dominant vapor-phase source of reactants is SiO vapor produced by the active oxidation of Si. The role played by the temperature-time program and addition of ultra-fine Si powder on the formation of SiO vapor and the resulting Si3N4 whiskers was clarified based on the experimental results and the thermodynamic calculations. The result shows that during sintering, in the presence of trace O2, ultra-fine Si particles preferentially undergo active oxidation and volatilized completely in the form of SiO(g). While an equilibrium of SiO(g) and Si(s) is formed on the surface of the coarse Si particle meanwhile. By stepped holding temperature further, this equilibrium is constantly upgraded, so the coarse Si particles is evaporated as SiO(g) shell layer by layer. Massive α-Si3N4 whiskers is formed by SiO(g) nitridation while the stepped holding temperature sintering. A reasonable model of the controllable indirect nitridation mechanism was established.

Upgrade the high-load anaerobic digestion and relieve acid stress through the strategy of side-stream micro-aeration: biochemical performances, microbial response and intrinsic mechanisms

In high-load anaerobic digestion such as in kitchen waste, side-stream micro-aeration (SMA) shows excellent operational performance to direct micro-aeration (DMA). It immediately restores the acidification to stability. Methanogenic performance remained stable when organic load ratios (OLR) was further increased to 5.5 g VS/L. Enhanced enzyme activity, microbial aggregation, and proliferation of bacteria and archaea were observed in SMA. The results indicates that SMA enriched Methanosaeta (relative abundance exceeded 93%) and induced the change of the main methanogenic pathway to acetoclastic methanogenesis. Mechanisms was further explored by using metagenomic analysis, and the results show SMA avoids mass formation of ROS (reactive oxygen species) by cycling the aerated slurry, and retains benefits of trace O2 on material and energic metabolism, which poses great application potentials and deserves further investigation.

Revealing the absence of carbon in aprotic Li–CO2 batteries: a mechanism study toward CO2 reduction under a pure CO2 environment

We find out that the reaction at a potential higher than 2 V should be the reduction reaction of trace O2. The reduction reaction of CO2 itself occurs below 2 V. CO, rather than C, is formed as the production of the CO2 reduction reaction.

Improving the Optical Quality of MoSe2 and WS2 Monolayers with Complete h-BN Encapsulation by High-Temperature Annealing.

We improved the optical quality and stability of an exfoliated monolayer (ML) MoSe2 and chemical vapor deposition (CVD)-grown WS2 MLs by encapsulating and sealing them with both top and bottom few-layer h-BN, as tested by subsequent high-temperature annealing up to 873 K and photoluminescence (PL) measurements. These transition-metal dichalcogenide (TMD) MLs remained stable up to this maximum temperature, as seen visually. After the heating/cooling cycle, the integrated photoluminescence (PL) intensity at 300 K in the MoSe2 ML was ∼4 times larger than that before heating and that from exciton and trion PL in the analogous WS2 ML sample was ∼14 times and ∼2.5 times larger at 77 K and the exciton peak was ∼9.5 times larger at 300 K. This is attributed to the reduction of impurities, the lateral expulsion of contamination leading to clean and atomically flat surfaces, and the sealing provided by the h-BN layers that prevents the diffusion of molecules such as trace O2 and H2O to the TMD ML. Stability and optical performance are much improved compared to that in earlier work using top h-BN only, in which the WS2 ML PL intensity decreased even for an optimal gas environment. This complete encapsulation is particularly promising for CVD-grown TMD MLs because they have relatively more charge and other impurities than do exfoliated MLs. These results open a new route for improving the optical properties of TMD MLs and their performance and applications both at room and higher temperatures.

Understanding promotional effects of trace oxygen in CO2 methanation over Ni/ZrO2 catalysts

Feed composition is crucial for gas-phase catalytic reactions but has rarely been studied in CO2 methanation. In this work, we studied the effect of impurities (i.e., N2, steam, and O2) on the activity of Ni/ZrO2 in CO2 methanation. The reducible ZrO2 favored the formation of oxygen vacancies, which enhanced CO2 adsorption and subsequent hydrogenation into methane. Interestingly, trace O2 enhanced CO2 methanation over Ni/ZrO2 due to the generation of more *OH groups facilitating the conversion of intermediates to methane.

Mixing effects on NO reduction by NH3 at intermediate and elevated temperatures in flow reactors

Ammonia is an efficient NO reducing agent and a promising carbon-free fuel. The selection of NH3 transformation in the NH3/NO/O2 system is closely dependent on the local temperature level and atmosphere condition, which is dramatically influenced by the mixing process. A significant mixing-reaction interaction has been observed previously in different laboratory flow reactors with distinct mixing strategies, while, the mechanism was not fully illustrated. Thus, in the present work, 3D reacting flow simulations combined with a detailed chemical kinetic model were conducted to give an insight into the complex mixing-reaction interaction at elevated temperatures inside the reactors. Calculation results agree fairly well with the experimental data. The main reaction zone shifts upstream and becomes narrower with temperature increase leading to the considerably different experimental results with different mixing strategies. In one reactor, NH3 and NO are separated in the mixing section due to the specific mixing strategy, and the DeNOx process is inhibited at elevated temperatures. While, in the other reactor, efficient NO reduction proceeds in the NH3/NO premixing channel upstream of the O2-containing jets, resulting in the second decline of the outlet NO concentration in the high temperature range. Moreover, the efficient NO reduction with trace O2 addition was verified experimentally.

Microaerobic Lifestyle at Nanomolar O2 Concentrations Mediated by Low-Affinity Terminal Oxidases in Abundant Soil Bacteria.

ABSTRACTHigh-affinity terminal oxidases (TOs) are believed to permit microbial respiration at low oxygen (O2) levels. Genes encoding such oxidases are widespread, and their existence in microbial genomes is taken as an indicator for microaerobic respiration. We combined respiratory kinetics determined via highly sensitive optical trace O2 sensors, genomics, and transcriptomics to test the hypothesis that high-affinity TOs are a prerequisite to respire micro- and nanooxic concentrations of O2 in environmentally relevant model soil organisms: acidobacteria. Members of the Acidobacteria harbor branched respiratory chains terminating in low-affinity (caa3-type cytochrome c oxidases) as well as high-affinity (cbb3-type cytochrome c oxidases and/or bd-type quinol oxidases) TOs, potentially enabling them to cope with varying O2 concentrations. The measured apparent Km (Km(app)) values for O2 of selected strains ranged from 37 to 288 nmol O2 liter−1, comparable to values previously assigned to low-affinity TOs. Surprisingly, we could not detect the expression of the conventional high-affinity TO (cbb3 type) at micro- and nanomolar O2 concentrations but detected the expression of low-affinity TOs. To the best of our knowledge, this is the first observation of microaerobic respiration imparted by low-affinity TOs at O2 concentrations as low as 1 nM. This challenges the standing hypothesis that a microaerobic lifestyle is exclusively imparted by the presence of high-affinity TOs. As low-affinity TOs are more efficient at generating ATP than high-affinity TOs, their utilization could provide a great benefit, even at low-nanomolar O2 levels. Our findings highlight energy conservation strategies that could promote the success of Acidobacteria in soil but might also be important for as-yet-unrevealed microorganisms.IMPORTANCE Low-oxygen habitats are widely distributed on Earth, ranging from the human intestine to soils. Microorganisms are assumed to have the capacity to respire low O2 concentrations via high-affinity terminal oxidases. By utilizing strains of a ubiquitous and abundant group of soil bacteria, the Acidobacteria, and combining respiration kinetics, genomics, and transcriptomics, we provide evidence that these microorganisms use the energetically more efficient low-affinity terminal oxidases to respire low-nanomolar O2 concentrations. This questions the standing hypothesis that the ability to respire traces of O2 stems solely from the activity of high-affinity terminal oxidases. We propose that this energetically efficient strategy extends into other, so-far-unrevealed microbial clades. Our findings also demonstrate that physiological predictions regarding the utilization of different O2 concentrations based solely on the presence or absence of terminal oxidases in bacterial genomes can be misleading.

Development of ReaxFFSFOH Force Field for SF6‐H2O/O2 Hybrid System Based on Synergetic Optimization by CMA‐ES and MC Methodology

AbstractThe reactive force field (ReaxFF) based molecular dynamic (MD) simulation provides a new perspective for studying the microscopic decomposition mechanism of sulfur hexafluoride (SF6) under fault conditions. Since none of the published ReaxFF force fields contains proper interaction relationship between S, F, O and H elements, the development of ReaxFF parameters for SF6‐H2O/O2 hybrid system is urgently needed. In this work, a universal and efficient covariance matrix adaptation evolutionary strategy (CMA‐ES) and Monte‐Carlo FF (MCFF) synergetic (CMS) optimization strategy for ReaxFF parameter is proposed. The optimization results for two representative force fields (disulfide and silica) from literature show that under the same number of evaluations, the CMS optimization strategy significantly reduces the optimization error by 22.08 % (disulfide) and 10.3 % (silica) respectively. By applying CMS strategy, the ReaxFFSFOH force field for SF6‐H2O/O2 hybrid system is efficiently obtained. Through multi‐dimensional force field performance verification, we find that the bond length calculation error between ReaxFF and density functional theory (DFT) calculation results is mainly concentrated within ±0.05 Å, and the bond angle error is mostly within ±5°. In addition, the trends of relative energy changes during chemical reaction processes calculated by ReaxFF are basically consistent with the DFT results. This work lays a solid foundation for the in‐depth study of the microscopic influence mechanism of trace H2O and trace O2 in the decomposition process of SF6.

Decomposition Characteristics of SF6 under Arc Discharge and the Effects of Trace H2O, O2, and PTFE Vapour on Its By-Products

The research on decomposition characteristics of SF6 and its by-products have great significance to the operation, maintenance, condition assessment and fault diagnosis of power equipment. In this paper, the particle composition models of SF6, SF6/polytetrafluoroethylene (PTFE), SF6/PTFE/O2, SF6/PTFE/H2O, and SF6/PTFE/O2/H2O were established by using Gibbs free energy minimization method, and the effects of trace H2O and O2 impurities and PTFE vapour on SF6 by-products were studied by the models. In order to verify the correctness of the simulation results, a series of breaking experiments were carried out on a 40.5 kV SF6 circuit breaker, and a gas chromatograph was used to detect and analyse the SF6 by-products. It was found that when PTFE vapour is involved in the arc plasma, the main by-product after arc quenching is CF4, and the molar fractions of C2F6 and C3F8 are very low. When O2 is involved, the main by-products are SOF2, SO2 and SO2F2, and a small amount of CO and CO2 was also produced. When H2O is involved, the main by-products in simulation are SOF2, SO2 and HF, and a small amount of SO2, CO2, CO, SO2F2 and H2 was also produced. The experimental results are in good agreement with the above results.

Metal-free visible-light-promoted C(sp3)-H functionalization of aliphatic cyclic ethers using trace O2.

Presented is a light-promoted C-C bond forming reaction yielding sulfone and phosphate derivatives at room temperature in the absence of metals or photoredox catalyst. This transformation proceeds in neat conditions through an auto-oxidation mechanism which is maintained through the leaching of trace amounts of O2 as sole green oxidant.

Key Roles of Trace Oxygen Treatment for High‐Performance Zn‐Doped CuI p‐Channel Transistors

AbstractThe development of transparent and high‐performance p‐type semiconductors as a counterpart of n‐type metal oxide semiconductors has attracted significant interest for the integration of complementary circuits and p–n junction devices. This study investigates the effect of trace O2 for high‐performance and solution‐processed inorganic p‐channel Zn‐doped copper iodide (CuI) thin‐film transistors (TFTs) via a combined computation–experiment approach. The absorbed O2 molecules in the CuI film can occupy iodine vacancies, acting as trap passivator. Meanwhile, the strong electronegativity of O2 enables electron capture from the CuI matrix, leading to p‐doping. Trace O2‐treated Zn‐doped CuI TFTs exhibit significantly improved electrical performance compared to untreated devices. Optimized TFTs exhibit a high field‐effect hole mobility of 4.4 cm2 V−1 s−1, high on/off current ratio of ≈107, and small hysteresis. These findings provide a clear basis for realizing reproducible and high‐performance metal‐halide (e.g., CuI and perovskite) optoelectronic devices using low‐cost solution process.

Cost-effective manufacture and synthesis mechanism of ferrosilicon nitride porous ceramic with interlocking structure

To realize cost-effectively manufacture of high-performance Si3N4 porous ceramic, a ferrosilicon nitride porous ceramic with an optimized interlocking structure was synthesized by flash combustion synthesis using FeSi75 powder as raw material. And the technology has been improved in many ways to ensure stable industrial production. The theoretical combustion temperature of FeSi75 in N2(g) is up to 4608K, while Si3N4 is unstable. Both adding diluent and designing the preheat temperature of nitrogen are taken to control synthesis temperature below 1600 °C. During synthesis, the Fe–Si liquid phase and SiO(g), which are essential for the selective growth of elongated columnar β-Si3N4 and whisker α-Si3N4 respectively, are formed firstly. Then, nitriding proceed in multiple ways. N diffuses through Fe–Si(l) and reacts with Si to form β-Si3N4, and the growth of elongated β-Si3N4 in Fe–Si liquid follows the dynamic ripening model, which is very fast and effective. Thus, an interlocking structure composed of elongated β-Si3N4 with an aspect ratio above 20 is reached. There is also an indirect nitridation reaction, that is, FeSi75 preferentially reacts with trace O2 in atmosphere to form SiO(g), which is further nitrided to form needle-like α-Si3N4. Needle-like α-Si3N4 is interspersed in the well-developed columnar β-Si3N4, making the structure stronger. Fe finally exists in the form of Fe3Si, which binds the surrounding elongated Si3N4 to form a sea-urchin like unit, making the structure more stable and strengthened. Through control of these reactions, optimizations in microstructure are reached, and the annual output of has reached 25,000 tons. The reaction model is established.

Plasmachemical Trace‐Oxygen Removal in a Coke Oven Gas with a Coaxial Packed‐Bed‐DBD Reactor

AbstractThe trace‐O2 removal in coke oven gas, which enables better utilization of its contained H2, is investigated with combinations of atmospheric nonthermal plasma and a Pt/γ‐Al2O3 catalyst. Herein it is shown that a coaxial packed‐bed dielectric barrier discharge (DBD) reactor removes up to 80 % O2 in a model coke oven gas. Along this line, the H2 content and the usage of Al2O3 granules in the plasma zone have been identified as major factors for the plasmachemical trace‐O2 conversion. In contrast to the Pt/γ‐Al2O3 catalyst, nonthermal plasma converts trace O2 at coke oven gas temperatures below 100 °C.

Synthesis of (Al2OC)x(AlN)1-x whiskers via Al2O(g) transient phase in Al-Al2O3 composite at 1000–1300°C in flowing N2

ABSTRACT In this paper, (Al2OC)x(AlN)1-x whiskers are successfully synthesized in resin-bonded Al-Al2O3 composite at 1000–1300°C in flowing N2 by forming highly active AlN in the composite with Al2O(g) as a transition phase. The formation of highly active AlN reduces the formation temperature of (Al2OC)x(AlN)1-x, while the diffusion and mass transfer of Al2O(g) construct a highly active Al2O3-Al4C3-AlN ternary system in the composite, and thus make the (Al2OC)x(AlN)1-x grow into a whisker morphology. The formation mechanism of the Al2O(g) can be expressed as: The uniformly dispersed C produced by phenolic resin enlarges the reaction temperature range of Al by forming an Al2O3-AlN-Al4C3-C composite shell wrapping the Al particle, and the reaction between continuously exposed fresh liquid Al and trace O2(g) in high-purity nitrogen achieves the continuous generation of the Al2O(g).