Mixed Gas Research Articles

A Novel Calibration Scheme of Gas Sensor Array for a More Accurate Measurement Model of Mixed Gases.

Gas sensor arrays (GSAs) usually encounter challenges due to the cross-contamination of mixed gases, leading to reduced accuracy in measuring gas mixtures. However, with the advent of artificial intelligence, there is a promising avenue for addressing this issue effectively. In pursuit of more accurate mixed gas measurements, we proposed a measurement model leveraging neural networks. Our approach involved employing the encoder of an autoencoder network (AEN) to extract features from experimental data, while fully connected layers were utilized for predicting concentrations of mixed gases. To refine the neural network parameters, we employed a variational autoencoder to generate additional data resembling the distribution of experimental data. Subsequently, we designed a domain difference maximum entropy technique to identify optimal concentration points for the calibration data. These calibration points were instrumental in training the fully connected layers, enhancing the model's accuracy. During practical usage, with the AEN configuration fixed, the model can be fine-tuned by using a small subset of test points across large-scale GSA deployments. Simulation and practical measurement results demonstrated the efficacy of our proposed measurement model, boasting high accuracy, with confidence intervals for relative errors of the four gas measurements below 3% at the 95% confidence level. Besides, the calibration scheme reduced the number of test points compared with traditional methods, reducing the cost of labor and equipment.

Zeolite‐Based “Particle Board” Membranes for Rapid and Selective Hydrogen Permeation

AbstractZeolites, with their well‐defined pore sizes and outstanding stability, are regarded as the quintessential material for precise molecular separation tasks. However, the leap from easily produced zeolite powders to synthesizing free‐standing zeolite membranes, which can fully leverage their ordered porosity, presents substantial technical hurdles. Addressing this challenge, this study introduces polymers of intrinsic microporosity (PIM‐1) as adhesives through a wrapping‐compression method for the facile fabrication of high‐performance and free‐standing zeolite (FAU) “particle board” membranes. The 3D tomogram reconstruction from the focused ion beam‐scanning electron microscope images reveals the “particle board” membranes exhibit hierarchical porous structures. The non‐interconnecting macropores between zeolite particles facilitate the rapid H2 permeation, and the zeolites’ ordered micropores sieve the gas molecules. The optimized FAU@PIM‐1 free‐standing membrane with hierarchically pore structures demonstrated hydrogen permeance (2240 GPU) alongside H2/CH4 selectivity (31.68) for mixed gas and maintained this performance level under pressure ranges from 1.2 to 4.0 bar and over protracted operational periods (up to 3200 h). Meanwhile, EMT and MFI‐type zeolites have also been successfully applied to validate the versatility of this method, demonstrating its widespread potential for the fabrication of various free‐standing zeolite membranes.

Polyoxometalate Confined Synthesis of BiVO4 Nanocluster for Urea Production with Remarkable O2/N2 Tolerance.

Urea electrosynthesis from flue gas and NO3- under operating conditions represents a promising alternative technology to traditional energy-intensive industrial process. Herein, we explore a polyoxometalate confined synthesis strategy to prepare ultrafine BiVO4 nanocluster by pre-incorporating [V10O28]6- into NH2-MIL-101-Al (MIL) framework. The resulting BiVO4@MIL-n can efficiently drive co-reduction of NO3- and CO2 to urea. A record urea yield of 63.4 mmol h-1 gcat-1 was achieved under CO2/O2 mixed gases (33% O2) atmosphere, and comparable performance can be obtained by feeding flue gas, demonstrating remarkable O2/N2 tolerance and potential feasibility for urea production under operating conditions. Systematic investigations revealed that MIL carrier with -NH2 group can enrich CO2, and BiVO4 nanocluster can reduce both NO3- and CO2 to ensure efficient urea synthesis even in the presence of O2. This work demonstrates the key role of in situ growth of BiVO4 nanocluster within a NH2-framework in facilitating urea electrosynthesis with exceptional tolerance to O2/N2.

Empirical Modal Decomposition Combined with Deep Learning for Photoacoustic Spectroscopy Detection of Mixture Gas Concentrations.

In photoacoustic spectroscopy based multicomponent gas analysis, the overlap of the absorption spectra among different gases can affect the measurement accuracy of gas concentrations. We report a multicomponent gas analysis method based on empirical modal decomposition (EMD), convolutional neural networks (CNN), and long short-term memory (LSTM) networks that can extract the exact concentrations of mixed gases from the overlapping wavelength-modulated spectroscopy with second harmonic (WMS-2f) detection. The WMS-2f signals of 25 different concentration combinations of acetylene-ammonia mixtures are detected using a single distributed feedback laser (DFB) at 1531.5 nm. The acetylene concentrations range from 2.5 to 7.5 ppm and the ammonia concentrations from 12.5 to 37.5 ppm. The data set is enhanced by cyclic shifting and adding Gaussian noise. The classification accuracy of the test set reaches 99.89% after tuning. The mean absolute errors of the five additional sets of data measured under different conditions are 0.092 ppm for acetylene and 1.902 ppm for ammonia, within the above concentration ranges.

Computational Fluid Dynamics Analysis and Optimization of a Doublesuction Turbine Agitator

Background: As one of the essential pieces of chemical equipment, a reactor provides the necessary reaction space and conditions for the materials involved in the reaction during the stirring process. However, under typical operating conditions, issues such as uneven gas distribution, suboptimal gas-liquid mixing, and low product yield often arise in gas-liquid phase reactors. Purpose: To address the issues prevalent in current stirred reactors, a new design for a stirred reactor equipped with a double-suction turbine agitator was developed. Method: In this paper, a stirred reactor equipped with a double-suction turbine agitator was designed, and its three-dimensional modeling was conducted using SolidWorks. Computational Fluid Dynamics (CFD) simulations, based on the Euler-Euler two-phase approach with the RNG k −ε turbulence model, were performed to assess variables such as stirring speed, installation height, blade diameter and agitator inner diameter. The dispersion characteristics and flow field behaviors of the gas-liquid two-phase under varying conditions were comparatively analyzed. Optimizations were conducted across various parameters to enhance the gas mixing efficiency in the liquid phase. Result: The results show that a diameter of 370mm for the double-suction turbine agitator, an installation height of 640mm, a blade diameter of 500mm, and an inner hole diameter of 200mm yield optimal gas-liquid two-phase mixing performance. This configuration results in a broad and uniform gas distribution within the reactor, maintaining a desired high level of gas holdup at specific positions. Conclusion: The double suction turbine agitator is a type of radial agitator. During operation, it induces significant centrifugal forces in the liquid, exerts a robust shear effect, and enhances the mixing of the gas-liquid phases, thereby increasing the production efficiency of the product.

Thermal Runaway Warning of Lithium Battery Based on Electronic Nose and Machine Learning Algorithms

Characteristic gas detection can be an efficient way to predict the degree of thermal runaway of a lithium battery. In this work, a sensor array consisting of three commercial MOS sensors was employed to discriminate between three target gases, CO, H2 and a mixture of the two, which are characteristic gases released during the thermal runaway of lithium batteries. In this work, an integrated model that makes the classification stage results one of the feature inputs for the concentration regression stage was employed, successfully reducing the RMSE of the concentration regression results. In addition, we also explored the influence of the selection of the response time length on the classification and regression tasks, achieving the best results in a short time through the optimum algorithm. To assess the impact of time duration sensor data on the results, we selected four time windows of different length and extracted the corresponding sensor response data for subsequent processing. Initially, principal component analysis (PCA) was used to visualise the clustering of the three target gas samples at room temperature, providing a preliminary data analysis. For the classification phase, we chose three classification algorithms—MLP (Multilayer Perceptron), ELM (Extreme Learning Machine), and SVM (Support Vector Machine)—and performed a comprehensive comparison of their classification and generalisation abilities using grid search for hyperparameter optimisation and five-fold cross-validation. The results demonstrated that MLP achieved 99.23% classification accuracy during the 20 s response period. In the concentration regression phase, we combined the classification results with the raw features to create a new feature set, which was then input into a multi-output MLP regression model. The root mean square error (RMSE) employing the new feature set was used to measure the prediction error. Ultimately, the findings showed that the input of combined features significantly reduced the regression error for the mixed gas.

Metal-Coordinated, Dual-Crosslinked PIM Polymer Membranes for Upgraded CO2 Separation: Aging and Plasticization Resistance.

The practical use of polymers of intrinsic microporosity (PIMs) in CO2 separation is often hindered by their moderate selectivity, performance instability over time, and pressure constraints. To address these limitations, a straightforward approach is presented to enhance the CO2 separation capability of PIM-1 by incorporating metal ions into uniformly hydrolyzed PIM-1 (cPIM). This dual linking strategy, achieved via ionic and coordination bonding of metal ions with the polymeric side chains including ─COOH and ─CONH2, restructures the polymer, disrupting hydrogen bonds between cPIM chains and creating active sites for CO2 via π-complexation. This modification enhances gas permeability while maintaining high selectivity. The optimized zinc-coordinated membrane achieves an impressive CO2 permeability of ≈2,500 Barrer with CO2/N2 and CO2/CH4 selectivities of 27.1 and 23, respectively, outperforming pristine cPIM (700 Barrer; CO2/N2=27; CO2/CH4=19). Notably, this performance surpasses the 2008 Robeson upper-bound limits for both gas pairs. Additionally, the metal-coordinated membranes exhibit remarkable long-term stability, resisting aging effects for up to 20 days and maintaining anti-plasticization properties at pressures up to 20 bar. These dual-crosslinked membranes demonstrate promising potential for mixed gas separation, indicating their suitability for real-world industrial applications.

Numerical modeling and parameter optimization of the combustion chamber in a tower-type zinc refining furnace

With the implementation of “coal-to-gas” projects, the tower-type zinc refining furnace (TZRF) has encountered a mismatch between high-calorific value fuel and the primary combustion system. To address this issue, this study conducted a full-scale modeling of the combustion and heat transfer processes in the TZRF combustion chamber using computational fluid dynamics. An in-depth analysis and condition diagnosis of the TZRF were performed, elucidating the key causes of deteriorating furnace conditions, reduced efficiency, and decreased production capacity following fuel replacement. This study shows that the flow ratios of gas outlets are unevenly distributed, leading to poor mixing of natural gas and air, resulting in an uneven temperature field in the TZRF. A low-temperature zone is clearly observed below the second-layer burner, where significant accumulation of natural gas fails to combust completely. Although tilting the first-layer burner upward slightly enhances the overall heat transfer performance of the tray, it detrimentally affects the overall combustion of natural gas in the TZRF. The deployment of third-layer air outlets substantially eliminates low-temperature zones, expands the range of high-temperature zones, and significantly improves the furnace condition. The optimal flow ratio of the third-layer air outlets is 29.57%.

Systematic analysis of mixing and segregation patterns of binary mixtures in fluidised beds for multi-functional processes

Fluidized beds are increasingly used in renewable energy and chemical production due to their versatility in handling different solids for multi-functional industrial applications. The diversity in size and density of solid particles impacts fluidization, influencing mixing and segregation behaviours critical for optimizing chemical processes and reactor design. This study investigates the expansion and segregation behaviours of mixed Geldart group powders in binary systems, simulating polydispersed beds with different materials and catalysts. By applying a modified Cheung equation and an adapted Gibilaro-Rowe model, the study analyzes segregation behaviours of Geldart Group A and B materials at varying mixing rates and gas flow velocities. Results show a good match between experimental data and model predictions. Using novel non-invasive X-ray imaging, the study provides real-time analysis of mixing and segregation at different fluidization regimes and temperatures. These findings aid in designing and optimizing advanced thermochemical conversion technologies, enhancing process efficiency and resilience.

Silica conversion of polysilazanes by low-temperature plasma jet generated from Ar and water-vapor mixed gas

Perhydropolysilazane (PHPS), which contains no organic catalyst, chemically reacts with H2O in the atmosphere by heating it at 450 °C for more than 1 h to turn it into a SiO2 film. If silica conversion can be achieved at temperatures below 100 °C, which plastics can withstand, it may be applicable to a wide range of applications, such as flexible electronics. Here, we report a technique for forming SiO2 films with leakage current characteristics very close to those of thermally oxidized (THOX) films that works at a very low temperature of 52 °C with high speed. Using a 9-kV, 30-kHz power supply, a low-temperature plasma jet containing a gas mixture of Ar and water vapor irradiated an 8 nm thick PHPS thin film, inducing silica conversion at a maximum substrate temperature of 52 °C. The current density-electric field strength (J-E) characteristics of metal oxide semiconductor capacitors fabricated with this SiO2 film showed characteristics very close to those of THOX films. In addition, the mechanism of silica conversion of PHPS through low-temperature plasma jet irradiation using this gas mixture was clarified in real-time FT-IR measurements.

Efficient membrane separation of SO2 by choline chloride-based natural deep eutectic solvents: The role of hydrogen bonding sites

Natural deep eutectic solvents (NDESs) are recognized as promising membrane materials for sustainable SO2 separation. Evaluating their separation performances is necessary, as well as understanding of the role that each hydrogen bonding site (including anion and functional groups in NDESs) plays in SO2 separation. Herein, three choline chloride (ChCl)-based NDESs bearing different number and type of functional groups were firstly incorporated into Pebax matrix to fabricate blended membranes for removing SO2 from CO2 and N2. The Pebax/NDES membranes show SO2 permeability up to 10502 Barrer (0.20 bar and 40 °C), with excellent SO2/N2 and SO2/CO2 selectivity of 1808 and 62.5 obtained, respectively, which are enhanced by 156 %, 61.5 % and 56.1 % compared with those in neat Pebax membrane. By means of gas solubility and diffusivity measurements, quantum chemical calculations and spectral characterizations, the highly-reversible multi-site interactions between SO2 and hydrogen bonding sites in NDESs (Cl−···SO2, carbonyl O···SO2 and hydroxyl O···SO2) were revealed. Meanwhile, the strength of hydrogen bonding network inside the NDESs, which is governed by the hydrogen bonding sites, is found to significantly influence the gas diffusion. More importantly, SO2/CO2/N2 (2.5/15/82.5 mol%) mixed gas separation experiment also displays the superior separation performance and long-term stability of Pebax/NDES membrane.

A La1.85Mg0.15Ce2O7-δ-based ceramic hydrogen pump for stable hydrogen separation out of the H2-CO2 mixtures

Currently, hydrogen is largely produced from steam methane reforming with the H2-and CO2-containing stream as the process gas. It is highly relevant to explore new pathways to efficiently separate hydrogen out of the H2-CO2 mixed gases. In the present work, a novel ceramic hydrogen pump for H2 purification is designed with thin La1.85Mg0.15Ce2O7-δ (LMCO) electrolytes supported by symmetrical Ni-LMCO electrodes, and is fabricated by using the tape casting, tape lamination and co-sintering techniques. Chemical stability and proton conductivities are evaluated for fluorite LMCO oxides in humidified 10 % H2-90 % CO2. Superior stability of LMCO in CO2 allows for stable and selective hydrogen separation out of humidified 10 % H2–90 % CO2. The Faraday’s efficiency decreases pronouncedly with an increase in the operation temperature, e.g., ≈ 30 % at 700 ℃ vs. ≈ 76 % at 500 ℃, due to increasingly high reduction susceptibility of Ce4+ to Ce3+ and the resultant electronic conductivities. A reasonable H2 flux of 0.57 mL·min−1·cm−2 is achieved at 500 °C and 0.25 V with electricity consumption of 0.72 kWh·Nm−3 H2. Operation at a higher pumping voltage of 0.49 V yields a larger H2 flux of 1.16 mL·min−1·cm−2, but simultaneously results in higher electricity consumption of 1.42 kWh·Nm−3 H2. Impedance analysis reveals that the H2 fluxes for the present ceramic hydrogen pump are predominantly limited by high resistances against bulk transport of protons through the LMCO electrolytes as well as sluggish hydrogen oxidation kinetics in humidified 10 % H2–90 % CO2.

Artificial Intelligence based Leak Detection in Blended Hydrogen and Natural Gas Pipelines

In the transition towards a hydrogen-based energy system, the strategic use of pipelines is crucial for efficient hydrogen distribution. Leveraging existing natural gas pipelines to carry a mix of hydrogen and natural gas offers a cost-effective alternative to building new infrastructure. This study explores the development of a leak detection system compatible with existing pipelines, specifically tailored for the blended hydrogen and natural gas mix. Given the scarcity of leak data for blended hydrogen-natural gas pipelines, the study introduces a Real-Time Transient Model (RTTM) for blended gases, simulating leak dynamics to generate necessary data. Additionally, a leak detection system (LDS) is developed using a fusion of Convolutional Neural Network (CNN) and Explainable Artificial Intelligence (XAI) through Adaptive Neuro-Fuzzy Inference Systems (ANFIS). This LDS framework overcomes the “black box” issue common in AI-driven systems, enabling reliable detection. The integration of Explainable and traditional AI techniques holds promising implications for blended hydrogen pipelines by enhancing the safety and efficiency of hydrogen transportation, thereby mitigating economic and environmental impacts, and addressing public concerns.

Mild hypercapnia before reperfusion reduces ischemia-reperfusion injury in hyperacute ischemic stroke rat model.

Endovascular thrombectomy has a recanalization rate over 80%; however, approximately 50% of ischemic stroke patients still experience dependency or mortality. Recently, clinical trials demonstrated the benefits of administering neuroprotective agents prior to endovascular thrombectomy. Additionally, recent studies showed neuroprotective effects of mild hypercapnia in patients resuscitated after cardiac arrest. However, its efficacy in ischemic stroke remains unclear. We aimed to investigate whether carbon dioxide (CO2) per-conditioning has neuroprotective effects in rat models with middle cerebral artery occlusion (MCAO). Rat models received intermittent inhalation of mixed gas during the MCAO period. After surgery, behavioral assessments, infarct size measurement, immunohistochemistry, and western blot analysis were performed. We found CO2 per-conditioning reduced infarct size and neurological deficit. The number of 8-hydroxy-2-deoxyguanosine (8-OHdG) positive cells and matrix metalloproteinase 9 (MMP-9)/platelet derived growth factor receptor beta (PDGFRβ) double positive cells were significantly decreased after CO2 per-conditioning. The expressions of tight junction protein and pericytes survival were preserved. This study underscores mild hypercapnia before reperfusion not only reduces neurologic deficit and infarct size, but also maintains the integrity of the blood-brain barrier and neurovascular unit, alongside mitigating oxidative stress in hyperacute stroke rat models. Therapeutic mild hypercapnia before reperfusion is promising and requires further clinical application.

Experimental study on the effect of discharge parameters of high-frequency nSDBD on the multi-point ignition characteristics of NH3/air mixture

Multi-point ignition by high-frequency nanosecond surface dielectric barrier discharge (nSDBD) offers the potential to enhance the combustion speed of NH3/air mixtures. This paper investigates the ignition process of high-frequency nSDBD in NH3/air mixture. Firstly, the morphology of the discharge channels is studied, and it is found that before the formation of a clear flame kernel, with the increase of the pulse number, the discharge filament length increases first and then stabilized, while the discharge filament number decreases from about 20 to around 12. Secondly, the influence of the mixed gas state on discharge characteristics is studied, and it is observed that as the gas pressure increases, the single pulse energy decreases and the discharge filament length decreases, but the deposition energy per unit length did not change significantly. The increase in NH3 concentration leads to a slight decrease in discharge energy and length. Finally, the effect of the flame kernel on discharge morphology is studied, and it is found that when the flame kernel is small, the discharge filament trees change from dispersed to coincident, and the discharge energy is concentrated. When the flame kernel is large, the discharge filament trees mostly transform into overlapping types, and the number decreases.

A Comprehensive Study of Open Cluster Chemical Homogeneity Using APOGEE and Milky Way Mapper Abundances

Stars in an open cluster are assumed to have formed from a broadly homogeneous distribution of gas, implying that they should be chemically homogeneous. Quantifying the level to which open clusters are chemically homogeneous can therefore tell us about interstellar medium pollution and gas mixing in progenitor molecular clouds. Using Sloan Digital Sky Survey (SDSS)-V Milky Way Mapper and SDSS-IV Apache Point Observatory Galaxy Evolution Experiment DR17 abundances, we test this assumption by quantifying intrinsic chemical scatter in up to 20 different chemical abundances across 26 Milky Way open clusters. We find that we can place 3σ upper limits on open cluster homogeneity within 0.02 dex or less in the majority of elements, while for neutron capture elements, as well as those elements having weak lines, we place limits on their homogeneity within 0.2 dex. Finally, we find that giant stars in open clusters are ∼0.01 dex more homogeneous than a matched sample of field stars.

Research on aerodynamic performance of centrifugal compressors for hydrogen-mixed natural gas.

Natural gas-doped hydrogen transportation has been widely used in industrial engineering. The change of the physical parameters of the conveying medium affects the working performance of the centrifugal compressor. This study aims to explore the aerodynamic performance of centrifugal compressors for hydrogen mixed natural gas, including the effects of hydrogen blending ratio (HBR) and inlet temperature on the pressure ratio and isentropic efficiency of compressors. The numerical simulation method is used and the results are compared with the experimental results to determine the reliability of the numerical simulation method. The results show that the pressure ratio decreases with the increase of HBR where the inlet temperature and rotation speed are constant. The pressure ratio and efficiency of the compressor are the highest when the conveying medium is pure natural gas (HBR = 0). The maximum pressure ratio reduction is 4.8% when the HBR is 5% and the inlet temperature is increased by 20 K. In summary, when the conveying medium of the compressor changes from pure natural gas to hydrogen-mixed natural gas, the working range and efficiency of the compressor will be reduced. Therefore, it is necessary to increase the rotation speed of the compressor or redesign the centrifugal compressor for hydrogen-mixed natural gas in order to achieve a constant flow rate at the outlet of the compressor.

High-sensitivity multi-gas detection using dual-ridge metasurface emitters with polarization-distinguishable emission spectra

The metasurface thermal emitter offers an energy-efficient, compact, and sensitive solution as a radiation source for non-contact gas detection, enabling the “molecular fingerprint” technique to be widely applied, from medical diagnostics to environmental monitoring. However, most narrowband emitters are designed for a single target gas, hindering the miniaturization of multi-gas detection systems. In this work, a one-dimensional dual-ridge grating emitter is employed, achieving dual-band and tri-band polarization-distinguishable emission spectra through the excitation of Fabry-Perot (FP) resonances and quasi-bound states in the continuum (qBICs). These emission spectra can be readily matched to multiple non-overlapping absorption peaks of gases such as CH4, CO2, CO, NO, and NH3 within the 3–6 µm range, thereby reducing the impact of mixed gases on measurements. Compared to conventional metal-dielectric-metal structures, the use of a single metal layer results in lower material losses, enabling higher Q-factors and more pronounced directional radiation intensity variations. Furthermore, adjusting the asymmetry to modulate the qBIC-excited absorption peaks does not affect the Q-factor of the FP resonance absorption, thus achieving high-sensitivity multi-band gas detection. This work provides a promising approach for the miniaturization and integration of multi-gas channel detection, facilitating more accurate and sensitive sensing strategies.

Surface interaction changes in minerals for underground hydrogen storage: Effects of CO2 cushion gas

Hydrogen (H2) offers a promising solution for the energy transition but storing it on a large scale at the surface poses significant technical and environmental challenges. Underground formations provide a practical alternative for large-scale H2 storage, yet understanding H2 behavior under various subsurface conditions is crucial for optimizing storage capacity and efficiency, which are influenced by rock properties such as wettability. This study addresses the gap in literature regarding the wettability of different minerals when CO2 was injected with H2 as a cushion gas. We examined the wettability of various mineral rocks in realistic subsurface circumstances. A series of wettability measurements were carried out using a captive drop instrument under high-pressure and high-temperature conditions, employing seven different minerals.Our findings reveal that mixed gas contact angles are significantly affected by temperature and pressure, with CO2 playing a crucial role in minimizing hydrogen trapping and enhancing CO2 capture in UHS and Carbon Capture, Utilization, and Storage (CCUS) projects. Higher CO2 mole fractions lead in increased contact angles due to the enhanced density and solubility of the gases, especially in basalt. Increasing the CO2 mole fraction from 25 % to 75 % at a constant pressure of 100 bar and temperature of 60 °C caused the contact angle of basalt with brine to rise from 54° to 86°. Pressure has a more pronounced effect on contact angle changes than temperature. For quartz rock in a 50 % H2 + 50 % CO2 mixture, the contact angle changes by 18° when the temperature increases from 20 °C to 80 °C at a constant pressure of 70 bar in distilled water. In contrast, the contact angle changes by 36° when the pressure increases from 10 bar to 100 bar at a constant temperature of 40 °C.These insights are essential for selecting suitable rock formations for underground gas storage, thereby supporting the advancement of hydrogen storage technologies. Furthermore, utilizing CO2 improves technical performance and environmental sustainability while also supporting international efforts to reduce greenhouse gas emissions. Overall, our results offer insightful information about the planning and operation of underground hydrogen storage systems, emphasizing the potential of CO2 to increase storage effectiveness and support carbon sequestration initiatives.

Gas-phase metallicity gradients in galaxies at z ∼ 6–8

The study of gas-phase metallicity and its spatial distribution at high redshift is crucial to understand the processes that shaped the growth and evolution of galaxies in the early Universe. Here we study the spatially resolved metallicity in three systems at z ∼ 6 − 8, namely A2744-YD4, BDF-3299, and COSMOS24108, with JWST NIRSpec IFU low-resolution (R ∼ 100) spectroscopic observations. These are among the highest-z sources in which metallicity gradients have been probed so far. Each of these systems hosts several spatial components in the process of merging within a few kiloparsecs, identified from the rest-frame UV and optical stellar continuum and ionised gas emission line maps. The sources have heterogeneous properties, with stellar masses log(M*/M⊙) ∼7.6–9.3, star formation rates (SFRs) ∼1–15 M⊙ yr−1, and gas-phase metallicities 12+log(O/H) ∼7.7–8.3, which exhibit a large scatter within each system. Their properties are generally consistent with those of the highest-redshift samples to date (z ∼ 3 − 10), though the sources in A2744-YD4 and COSMOS24108 are at the high end of the mass-metallicity relation (MZR) defined by the z ∼ 3 − 10 sources. Moreover, the targets in this work follow the predicted slope of the MZR at z ∼ 6 − 8 from most cosmological simulations. The gas-phase metallicity gradients are consistent with being flat in the main sources of each system. Flat metallicity gradients are thought to arise from gas mixing processes on galaxy scales, such as mergers or galactic outflows and supernova winds driven by intense stellar feedback, which wash out any gradient formed in the galaxy. The existence of flat gradients at z ∼ 6 − 8 sets also important constraints on future cosmological simulations and chemical evolution models, whose predictions on the cosmic evolution of metallicity gradients often differ significantly, especially at high redshift, but are mostly limited to z ≲ 3 so far.