Nonpolar Gases Research Articles

MEPO-ML: a robust graph attention network model for rapid generation of partial atomic charges in metal-organic frameworks

Accurate computation of the gas adsorption properties of MOFs is usually bottlenecked by the DFT calculations required to generate partial atomic charges. Therefore, large virtual screenings of MOFs often use the QEq method which is rapid, but of limited accuracy. Recently, machine learning (ML) models have been trained to generate charges in much better agreement with DFT-derived charges compared to the QEq models. Previous ML charge models for MOFs have all used training sets with less than 3000 MOFs obtained from the CoRE MOF database, which has recently been shown to have high structural error rates. In this work, we developed a graph attention network model for predicting DFT-derived charges in MOFs where the model was developed with the ARC-MOF database that contains 279,632 MOFs and over 40 million charges. This model, which we call MEPO-ML, predicts charges with a mean absolute error of 0.025e on our test set of over 27 K MOFs. Other ML models reported in the literature were also trained using the same dataset and descriptors, and MEPO-ML was shown to give the lowest errors. The gas adsorption properties evaluated using MEPO-ML charges are found to be in significantly better agreement with the reference DFT-derived charges compared to the empirical charges, for both polar and non-polar gases. Using only a single CPU core on our benchmark computer, MEPO-ML charges can be generated in less than two seconds on average (including all computations required to apply the model) for MOFs in the test set of 27 K MOFs.

Unveiling the Dominant Influence of First-Order Rotational Defects in Molybdenum Ditelluride: Insights into Adsorption of Nonpolar Gas Molecules

Unveiling the Dominant Influence of First-Order Rotational Defects in Molybdenum Ditelluride: Insights into Adsorption of Nonpolar Gas Molecules

A novel ionic-liquid-mediated covalent organic framework as a strong electrophile for high-performance iodine removal

The research on the adsorption and conversion of non-polar gases (carbon dioxide, hydrogen, iodine, etc.) has attracted global attention. Extensive work has revealed the intuitive impact of the heteroatom effect on the adsorption performance of covalent organic framework (COF) adsorbents for non-polar gases. However, more influencing factors must be studied to more precisely design and construct target-specific COF adsorbents. In this work, an underlying influencing factor, local polarity, is discovered, which is defined as the polarity of the functional moiety. Due to the substitution of strong electrophile, the electron cloud distribution of the COF framework is regulated, and the local polarity that better matches the adsorption of target electrophilic gas (iodine) has been observed. The local polarity of COF has been controlled through several strong electrophilic ionic liquids, dramatically improving adsorption performance. The saturated adsorption capacity increases from 1.5 to 5.2 g·g−1, and the adsorption kinetics index k80% value increases from 0.51 to 2.69 g·g−1·h−1. The insight would support precise chemical regulation of target-specific COF in energy and environment science.

A broad-spectrum gas sensor based on correlated two-dimensional electron gas

Designing a broad-spectrum gas sensor capable of identifying gas components in complex environments, such as mixed atmospheres or extreme temperatures, is a significant concern for various technologies, including energy, geological science, and planetary exploration. The main challenge lies in finding materials that exhibit high chemical stability and wide working temperature range. Materials that amplify signals through non-chemical methods could open up new sensing avenues. Here, we present the discovery of a broad-spectrum gas sensor utilizing correlated two-dimensional electron gas at a delta-doped LaAlO3/SrTiO3 interface with LaFeO3. Our study reveals that a back-gating on this two-dimensional electron gas can induce a non-volatile metal to insulator transition, which consequently can activate the two-dimensional electron gas to sensitively and quantitatively probe very broad gas species, no matter whether they are polar, non-polar, or inert gases. Different gas species cause resistance change at their sublimation or boiling temperature and a well-defined phase transition angle can quantitatively determine their partial pressures. Such unique correlated two-dimensional electron gas sensor is not affected by gas mixtures and maintains a wide operating temperature range. Furthermore, its readout is a simple measurement of electric resistance change, thus providing a very low-cost and high-efficient broad-spectrum sensing technique.

THE SENSING PERFORMANCE OF SURFACE-MODIFIED POROUS SILICON GAS SENSORS FOR NON-POLAR GAS DETECTION

Gas sensors are important devices in various industrial and environmental monitoring applications. Toluene and chloroform are harmful non-polar gases that are produced in various combustion processes and are associated with air pollution and respiratory diseases. Porous silicon (PS) has shown promising results as a material for ammonia and ethanol gas sensing applications. However, there is potential for further improvement by optimizing their surface properties for non-polar gas sensing applications. Chemical treatment has been widely utilized to modify the surface characteristics of materials, including semiconductors, for various applications. We have deposited nickel (Ni) layer on PS surface using chemical treatment. In comparison to the PS sample, it was discovered that the Ni-deposited PS sample was more sensitive to 0.1 ppm concentrations of non-polar toluene and chloroform vapours, increasing from 1% to 39% and 32.6%, respectively. This study provides valuable insights into the surface modification techniques for enhancing the performance of gas sensors, which can have a significant impact on the development of advanced sensing technologies for environmental and industrial applications.

Source and Migration of Fluids in a Meso-Tethyan Subduction Zone: Fluid Inclusion Study of Syn-Mélange Veins from the Mugagangri Accretionary Complex

The Mugagangri Group (MG), located at the southern margin of the Qiangtang terrane in Tibet, is a crucial research target for understanding the subduction and accretion history of the Meso-Tethys Ocean. Extensional crack-seal veins restricted within sandstone blocks from the broken formation in the MG (Gaize) formed synchronously in the mélange formation. The primary inclusions trapped in the veins recorded multiple pieces of information during the formation of the accretionary wedge. To precisely constrain the MG subduction–accretion processes, we investigated the trapping temperature, salinity, density, and composition of the fluid inclusions within the crack-seal veins derived from the broken formation in the MG (Gaize). The primary inclusions indicate that the crack was sealed at ~151–178 °C. The salinity of the primary inclusions exhibited a well-defined average of 3.3 ± 0.7 wt% NaCl equivalent, slightly lower than the average of seawater (3.5 wt%). There were no nonpolar gases, and only H2O (low salinity) was detectable in the primary inclusions. These characteristics suggest that the syn-mélange fluids were a type of pore fluid in the shallow subduction zone, with the principal source being pore water from sediments overlying the oceanic crust. Because of mineral dehydration and compaction, the pore fluids became more diluted with H2O and fluid overpressure owing to a pore fluid pressure that was greater than the hydrostatic pressure. Subsequently, the creation of cracks through hydraulic fracturing provided a novel pathway for the flow of fluids which, in turn, contributed to the décollement step-down and underthrusting processes. These fractures acted as conduits for fluid movement and played a crucial role in facilitating these peculiar occurrences of quartz veins. The depth (~5 km) and temperature estimates of the fluid expulsion align with the conditions of the décollement step-down, thereby leading to the trapping of fluids within the sandstone blocks and their subsequent underplating to the accretionary complex. In our preferred model, such syn-mélange fluids have the potential to provide valuable constraints on the subduction–accretion processes occurring in other accretionary complexes.

Catalytic green energy production based on engineered active water innovatively prepared using sunlight-illuminated gold nanoparticles

Water is the most common solvent in our daily lives. This solvent has advantages of green environmental protection and low cost. Water is conventionally considered to be an inert solvent, yet it is relatively polar and can form a large number of hydrogen bonds (HBs). But this property also inhibits the dissolution of many non-polar gases, causing many important electrocatalytic reactions performed in it to have low efficiencies. In this work, we propose an innovative application of solar energy-generated active pure water (APW) with reduced HBs to enhance chemical reactions and physical procedures. Compared to conventional deionized water (DIW), the generated APW possessed a lower specific heat of ca. 0.96. The swelling degree of artificial skin in APW significantly increased by ca. 29%. Moreover, the density of an ethanol/APW solution significantly increased by 0.21% due to more free water molecules being available in APW to form stronger HBs with ethanol. Encouragingly, efficiencies of hydrogen evolution reactions performed in an APW-based acidic solution and oxygen evolution reactions performed in an APW-based basic solution significantly increased by 42% and 17%, respectively, compared to DIW-based solutions. The developed APW based on utilizing solar energy can create a more-effective green process.

Porous silica preconcentrator for selective ambient volatile organic compounds detection: Effects of surface functionalization and humidity

With the increased demand for hand-held and ambient gas sensors, it is imperative to develop sensors that can offer both selective and sensitive detection. Gas preconcentration is a widely tried and tested method to increase the sensitivity of gas detectors. While it effectively lowers the limit of detection, it does not impact the selectivity of the detector. Therefore, preconcentrator materials have mostly been used in conjunction with selective detectors. In this work, we use the preconcentration method with a nonselective small and portable photoionization detector to introduce selectivity. For this purpose, we use a relatively slow heating rate, that allows for gradual desorption and analytes from the preconcentrator material–nanoporous silica. The characteristic desorption temperature of the volatile organic compounds (VOC) from the preconcentrator allows selective detection of the VOC. In this work, we study the effect of surface functionalization, to make it hydrophobic, and observe the adsorption–desorption behavior of polar (isopropyl alcohol) and non-polar (octane) gas molecules. The hydrophobic silica surface was found to improve the adsorption of non-polar octane, while it reduced the adsorption of polar isopropyl alcohol. The desorption temperature for isopropanol remained unchanged for both functionalized and non-functionalized preconcentrators; however, the desorption temperature for octane increased by 10 °C when the functionalized hydrophobic pSiO2 was used. We also observed the presence of humidity, a known interferent, did not heavily impact the sensing performance. These results are promising evidence that functionalized porous silica integrated with a photoionization detector can be used for selective gas detection in the ambient atmosphere.

Thermodynamic Properties of van der Waals Dimers and Trimers of Nonpolar Gases and Their Correlation with Lennard-Jones Potential Well Depths.

A method of unifying the equilibrium thermodynamic properties ΔHo and ΔGo relating to the van der Waals dimers and trimers of 15 nonpolar gases (He-Xe, H2, D2, CH4, CF4, ethene, ethane, CO2, SF6, propane, and neopentane) is described. Values of ΔHo and ΔGo, obtained at the reduced temperature Tr = 0.7, show good correlation with the respective Lennard-Jones pair potential well depth, calculated from the monomer critical temperature and the acentric factor. Such relationships present the opportunity to estimate the van der Waals dimer and trimer thermodynamic properties of other nonpolar molecules, and examples of seven such applications are given. It is found that the enthalpies of dimerization and trimerization of the 15 gases are about 21 and 29% of the respective condensation enthalpies, providing information about the thermodynamics of small clusters in relation to liquefaction.

Exploring the Potential of a Highly Scalable Metal-Organic Framework CALF-20 for Selective Gas Adsorption at Low Pressure.

In this study, the ability of the highly scalable metal-organic framework (MOF) CALF-20 to adsorb polar and non-polar gases at low pressure was investigated using grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. The results from the simulated adsorption isotherms revealed that the highest loading was achieved for SO2 and Cl2, while the lowest loading was found for F2 molecules. The analysis of interaction energies indicated that SO2 molecules were able to form the strongest adsorbent-adsorbate interactions and had a tight molecular packing due to their polarity and angular structure. Additionally, Cl2 gas was found to be highly adsorbed due to its large van der Waals surface and strong chemical affinity in CALF-20 pores. MD simulations showed that SO2 and Cl2 had the lowest mobility inside CALF-20 pores. The values of the Henry coefficient and isosteric heat of adsorption confirmed that CALF-20 could selectively adsorb SO2 and Cl2. Based on the results, it was concluded that CALF-20 is a suitable adsorbent for SO2 and Cl2 but not for F2. This research emphasizes the importance of molecular size, geometry, and polarity in determining the suitability of a porous material as an adsorbent for specific adsorbates.

The Improved Non-Polar Gas Sensing Performance of Surface-Modified Porous Silicon-Based Gas Sensors

The present article studied gas sensor sensing characteristics based on surface-modified porous silicon (PS) by depositing the metal oxide semiconductor layer. The PS layer was prepared through the electrochemical etching of crystalline silicon in an HF-based solution. DC magnetron sputtering technology was used to obtain the p-CuO layer on the surface of the p-PS. The obtained material’s structural, morphological, and sensing behavior were investigated using SEM, XRD, Raman spectra, and the current–voltage characteristics. For the detection of toluene and chloroform vapors, a planar structure was used. The sensing response value revealed that the CuO/PS-based gas sensors have good sensitivity for toluene and chloroform vapors. The sensing mechanism is explained using schematic energy band diagrams. Therefore, this approach is helpful for the development of a simple, cost-effective sensor for detecting non-polar chemical analytes.

Adsorption Capacity and Desorption Efficiency of Activated Carbon for Odors from Medical Waste.

Five types of odor-emitting exhaust gases from medical waste were selected, and their adsorption capacity and desorption efficiency were investigated using activated carbon. The selected gases included polar gases (hydrogen sulfide (H2S) and ammonia (NH3)) and non-polar gases (acetaldehyde (AA), methyl mercaptan (MM), and trimethylamine (TMA))). Commercial activated carbon with a specific surface area of 2276 m2/g was used as the adsorbent. For the removal of odor from medical waste, we investigated: (1) the effective adsorption capacity of a single gas (<1 ppm), (2) the effect of the adsorbed NH3 gas concentration and flow rate, and (3) the desorption rate using NH3 gas. The values of the effective adsorption capacity of the single gas were in the following order: H2S < NH3 < AA < MM < TMA, at 0.2, 4.2, 6.3, 6.6, and 35.7 mg/g, respectively. The results indicate that polar gases have a lower effective adsorption capacity than that of non-polar gases, and that the size of the gas molecules and effective adsorption capacity exhibit a proportional relationship. The effective adsorption performance of NH3 gas showed an increasing trend with NH3 concentration. Therefore, securing optimal conditions for adsorption/desorption is imperative for the highly efficient removal of odor from medical waste.

Guest-adaptive molecular sensing in a dynamic 3D covalent organic framework

Molecular recognition is an attractive approach to designing sensitive and selective sensors for volatile organic compounds (VOCs). Although organic macrocycles and cages have been well-developed for recognising organics by their adaptive pockets in liquids, porous solids for gas detection require a deliberate design balancing adaptability and robustness. Here we report a dynamic 3D covalent organic framework (dynaCOF) constructed from an environmentally sensitive fluorophore that can undergo concerted and adaptive structural transitions upon adsorption of gas and vapours. The COF is capable of rapid and reliable detection of various VOCs, even for non-polar hydrocarbon gas under humid conditions. The adaptive guest inclusion amplifies the host-guest interactions and facilitates the differentiation of organic vapours by their polarity and sizes/shapes, and the covalently linked 3D interwoven networks ensure the robustness and coherency of the materials. The present result paves the way for multiplex fluorescence sensing of various VOCs with molecular-specific responses.

First-Principles Study of Adsorption of Methane on Defective Silicene

Considering the significance of natural gas, such as methane, and the difficulties in storing it, research is increasingly focusing on developing materials for solid-state methane storage. Two-dimensional materials have a large number of possible adsorption sites for gas molecules due to their high surface-to-volume ratio. However, the two-dimensional structure is chemically inactive and attracts nonpolar gases rather weakly. We investigated the methane gas adsorption capabilities of silicene by activating it with different defects. According to our density functional theory calculations, the mono-vacancy (MV) defect is advantageous in increasing the binding strength of energy-carrying gases such as methane. In MV defective silicene, methane adsorption energy is detected in the order of the internationally specified energy regime.

Silane‐Modified Mesoporous Silica Nanocoatings for Selective Mid‐Infrared Waveguide‐Based Gas Sensing

AbstractThis paper reports the use of silane monolayers of different polarities to enable recognition and selective interfacial enrichment of nonpolar versus polar gases for spectroscopic mid‐infrared (MIR) on‐chip sensing. However, detection of gases using on‐chip waveguides (WGs) is challenging because of the low concentrations of gas molecules within the evanescent field surrounding the WGs. To solve this issue, high‐surface‐area, precisely‐controlled‐thickness nanocoatings are introduced whose surface modification with strategically selected silanes enables control of preferential adsorption and concentration of polar versus nonpolar gas analytes. Conformal nanocoatings are constructed using layer‐by‐layer deposition of mesoporous silica nanoparticles (MSNs) at substrate withdrawal speeds controlled within the capillary regime. Surface modification of the WG‐immobilized MSNs with hydrophobic triethoxy(octyl)silane enables, for the first time, enhanced sensitivity and selectivity towards nonpolar methane gas as compared to its polar counterpart acetone. On the other hand, modification of the nanocoatings with (3‐aminopropyl)triethoxysilane leads to preferential binding of polar acetone vapors. This work demonstrates the capability of interfacial modification for controlling the selection and enhancement of the spectroscopic features of gas analytes with different polarities. The developed interfacial assemblies can enable diverse spectroscopic gas sensing platforms for applications in health diagnostics, environmental monitoring, and process control.

Fit-for-Purpose Assessment of QuEChERS LC-MS/MS Methods for Environmental Monitoring of Organotin Compounds in the Bottom Sediments of the Odra River Estuary.

Organotin compounds (OTCs) are among the most hazardous substances found in the marine environment and can be determined by either the ISO 23161 method based on extraction with non-polar organic solvents and gas chromatography analysis or by the recently developed QuEChERS method coupled to liquid chromatography-mass spectrometry (LC-MS/MS). To date, the QuEChERS LC/MS and ISO 23161 methods have not been compared in terms of their fit-for-purpose and reliability in the determination of OTCs in bottom sediments. In the case of ISO 23161, due to a large number of interferences gas chromatography-mass spectrometry was not suitable for the determination of OTCs contrary to more selective determination by gas chromatography with an atomic emission detector. Moreover, it has been found that the derivatization of OTCs to volatile compounds, which required prior gas chromatography determination, was strongly affected by the sediments’ matrices. As a result, a large amount of reagent was needed for the complete derivatization of the compounds. Contrary to ISO 23161, the QuEChERS LC-MS/MS method did not require the derivatization of OTC and is less prone to interferences. Highly volatile and toxic solvents were not used in the QuEChERS LC-MS/MS method. This makes the method more environmentally friendly according to the principles of green analytical chemistry. QuEChERS LC-MS/MS is suitable for fast and reliable environmental monitoring of OTCs in bottom sediments from the Odra River estuary. However, determination of di- and monobutyltin by the QuEChERS LC-MS/MS method was not possible due to the constraints of the chromatographic system. Hence, further development of this method is needed for monitoring di- and monobutyltin in bottom sediments.

Plasmonically enhanced electrochemistry boosted by nonaqueous solvent.

Plasmon excitation of metal electrodes is known to enhance important energy related electrochemical transformations in aqueous media. However, the low solubility of nonpolar gases and molecular reagents involved in many energy conversion reactions limits the number of products formed per unit time in aqueous media. In this Communication, we use linear sweep voltammetry to measure how electrochemical H2O reduction in a nonaqueous solvent, acetonitrile, is enhanced by excitation of a plasmonic electrode. Plasmonically excited electrochemically roughened Au electrodes are found to produce photopotentials as large as 175 mV, which can be harnessed to lower the applied electrical bias required to drive the formation of H2. As the solvent polarity increases, by an increase in the concentration of H2O, the measured photopotential rapidly drops off to ∼50 mV. We propose a mechanism by which an increase in the H2O concentration increasingly stabilizes the photocharged plasmonic electrode, lowering the photopotential available to assist in the electrochemical reaction. Our study demonstrates that solvent polarity is an essential experimental parameter to optimize plasmonic enhancement in electrochemistry.

Shock-wave structure in non-polar diatomic and polyatomic dense gases under rotation and vibration

This study investigates the effect of rotation and vibration on the structure of shock waves in moderately dense diatomic and polyatomic non-polar gases using the one-temperature Navier–Stokes–Fourier approach. The modified Enskog equation of state of the gas is taken to include the denseness and shielding effects. The specific heat at constant volume has been taken to be temperature-dependent. The shear viscosity, the bulk viscosity, and the thermal conductivity have been assumed to follow the temperature-dependent power-law model. Nitrogen and oxygen gas have been taken as the test cases for diatomic gases while carbon dioxide was taken for the polyatomic gases. The implicit system of equations is derived and solved numerically for density and temperature. The inclusion of denseness, rotational, and vibrational modes of molecular motion have a significant effect on the density and temperature profiles, the inverse shock thickness, the bulk to shear viscosity ratio, and the molar specific heat at constant pressure. The gas having a low characteristic vibrational temperature has been found to have a high value of inverse shock thickness. The inverse shock thickness, the bulk to shear viscosity ratio, and the molar specific heat at constant pressure for nitrogen and carbon dioxide are found to be in good agreement with the experimental values.

Determination of trace organic chlorides in hydrogen for fuel cell vehicles by gas chromatography coupled with ion mobility spectrometry.

A method for the determination of organic chlorides in hydrogen for fuel cell vehicles by gas chromatography coupled with ion mobility spectrometry was established. Organic chlorides were separated by a non-polar gas chromatography column and detected in the negative ion mode of the ion mobility spectrometer. The effect of operating parameters of ion mobility spectrometer including drift gas flow rate and drift tube temperature on sensitivity and resolution were evaluated. Under the optimized conditions, the detection limits of seven organic chlorides were from 0.65 to 6.73nmol/mol, which met the requirement of detection for the specification limit of 50nmol/mol of total halogen impurities in hydrogen for fuel cell vehicles. Compared with gas chromatography-mass spectrometry, and gas chromatography coupled with electron capture detector under the same gas chromatography conditions, gas chromatography coupled with ion mobility spectrometry method demonstrated higher sensitivity for detection of organic chlorides under study. Based on the portability of the device and its detection capabilities, gas chromatography coupled with ion mobility spectrometry has the potential to perform online detection of impurities in hydrogen for fuel cell vehicles.

Single-phase high-entropy oxide-based chemiresistor: Toward selective and sensitive detection of methane gas for real-time applications

Detecting methane (CH4) with good selectivity is one of the most important safety precautions to prevent catastrophic incidents in the current industrial environment. Detection of small molecular size, inert and nonpolar characteristic gases at trace levels using chemiresistive technique at room temperature is still challenging because of weak adsorption between gas and sensing material. Therefore, we fabricated a single-phase high entropy oxide based chemiresistor and tested it towards the various gases and several hydrocarbons at room temperature, but the sensor displayed a higher selectivity to CH4 gas than other gases. Further, the sensor characteristics revealed a significant response to CH4 gas, good response and recovery time, good long-term stability, and an experimental detection limit of 25 ppm. Besides, the as-fabricated sensor is low-cost, small in size and consumes 50 nW power, much lower than the other commercialized light-based sensors. As proof of concept, the fabricated sensor was utilized to measure CH4 gas in a real-time atmosphere. The sensor reflected response characteristics similar to the controlled environment and recovered without carrier gas. This facile approach sheds light on the rational design of high-entropy oxides, paving the way for mass production and commercialization of ultra-trace gas detection sensors with superior sensing capability.