Gas-phase Adsorption Research Articles

Directly measuring octanol-air partition ratios using a gas chromatography retention time (GC-RT) method

The octanol-air partition ratio (KOA) is a fundamental property used for screening chemicals for concern, depending on their potential to bioaccumulate and harm living systems. With millions of chemicals used in commerce, unfortunately, less than 800 compounds currently have experimentally measured KOA values due to limitations in traditional measurement techniques. We aimed to develop a direct gas chromatography retention time (GC-RT) method using a custom-packed column, with octanol as the stationary phase, for rapidly measuring KOA. We installed the column into a GC-mass spectrometry system and isothermally measured retention times of 15 volatile organic compounds. We calculated KOA values at experimental temperatures from the retention times and extrapolated the results to values at 25 °C using the Van't Hoff equation. Our directly measured log KOA values at 25 °C spanned 3.66 log units (0.820–4.48). Except for alcohols, our measured values agree with literature log KOA values (root mean square error, RMSE = 0.50). The discrepancy probably arose from differences in how well our method versus the literature methods capture measurement artifacts, specifically, gas-phase adsorption to the octanol phase and hydrogen bonding interactions. This proof-of-concept study demonstrates that our direct GC-RT method is promising for rapidly measuring KOA to support chemical risk assessment.

Gaseous Nitric Oxide As Probe Molecule for Fe-N-C ORR Electrocatalysts

Iron nitrogen carbon (Fe-N-C) electrocatalysts remain the most promising platinum group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs). Although significant progress has been made in recent years in improving both the ORR activity and stability of the Fe-N-C catalysts, further enhancement in the catalyst durability while maintaining the activity is imperative for the practical applications. One of the major factors that hinders the further enhancement of the catalysts is the lack of clear understanding of the nature of the active sites and density of the active sites in the Fe-N-C catalysts due to the complexity of the Fe-N-C catalysts’ composition and structure. Gaseous nitric oxide was proven to be a suitable probe molecule that can bond to Fe and impede the ORR of the Fe-N-C catalysts. In recent years, we have studied the effects of gas-phase adsorption and desorption of nitric oxide on the redox features and ORR activity on Fe-N-C catalysts synthesized by different methods. We have used various characterization tools to quantify the amount of nitric oxide adsorbed and to identify the adsorption sites and geometry, including electrochemical stripping, temperature programmed desorption, and in situ X-ray techniques. The corresponding results obtained will be summarized in this talk. The implication of these results on the nature of the ORR active sites of the Fe-N-C catalysts will be discussed.AcknowledgementsThis work was supported by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO) under the auspices of the Electrocatalysis Consortium (ElectroCat 2.0). This work utilized the resources of the Advanced Photon Source, a U.S. DOE Office of Science user facility operated by Argonne National Laboratory for DOE Office. This work was partially authored by Argonne, a U.S. Department of Energy (DOE) Office of Science laboratory operated for DOE by UChicago Argonne, LLC under contract no. DE-AC02-06CH11357.

Synthesis of Carboxyl-Functionalized COFs with Alternate Stable β-Ketoenamine and Benzimidazole Linkages: Unraveling Exceptional Solvent Effects for Efficient Uranium Separation.

The prevalent π-π interactions in 2D covalent organic frameworks (COFs) impart a certain flexibility to the structures, making the stacking of COF layers susceptible to external stimuli and introducing some structural disorder. Recent research indicates that the flexibility between COF layers and the associated disorder significantly influence their selective adsorption performance toward gas molecules. However, the adsorption process in a solution environment is more complex compared to gas-phase adsorption, involving interactions between adsorbents and adsorbates, as well as the solvation effects of flexible 2D COFs. Therefore, the inherent flexibility and disorder in 2D COFs under solution conditions and their impact on the adsorption performance of metal ions have not been observed yet. Herein, the synthesis of a novel carboxyl-functionalized COF featuring stable β-ketoenamine and benzimidazole linkages, named DMTP-COOH, is presented. DMTP-COOH exhibits excellent selective adsorption capability for uranium, with significantly different adsorption capacities observed after treatment with different solvents. This notable difference in adsorption capacity is observed under varying pH, concentration, time, and even in the presence of multiple competing ions. This work represents the first observation of the significant impact of solvent soaking treatment on the selective adsorption performance of COFs for uranium under liquid conditions.

A Miniaturized Method for Evaluating the Dynamic Gas-Phase Adsorption and Degradation of Sarin on Porous Adsorbents at Different Humidity Levels.

Metal organic frameworks based on zirconium nodes (Zr-MOFs) have impressive adsorption capacities, and many can rapidly hydrolyze toxic organophosphorus nerve agents. They could thus potentially replace commonly used adsorbents in respiratory filters. However, current test methodologies are poorly adapted to screen the large number of available MOFs, and data for nerve agent adsorption by MOFs are scarce. This paper presents a miniaturized method for assessing the capacity of Zr-MOFs for dynamic gas phase adsorption and degradation of sarin (GB) into the primary hydrolysis product isopropyl methyl phosphonic acid (IMPA). The method was validated by comparing the dynamic adsorption capacities of activated carbon (AC) and NU-1000 for GB under dry and humid conditions. Under dry conditions, unimpregnated AC had a greater capacity for GB uptake (0.68 ± 0.06 g/g) than pelletized NU-1000 (0.36 ± 0.03 g/g). At 55% relative humidity (RH), the capacity of AC was largely unchanged (0.72 ± 0.10 g/g) but that of NU-1000 increased slightly, to 0.46 ± 0.10 g/g. However, NU-1000 exhibited poor water retention at 55% RH. For both adsorbents, the degree of hydrolysis of GB into IMPA was significantly greater at 55% RH than under dry conditions, but the overall degree of hydrolysis was limited in both cases. Further tests at higher relative humidities are needed to fully evaluate the ability of NU-1000 to degrade GB after adsorption from the gas phase. The proposed experimental setup uses very small amounts of both adsorbent material (20 mg) and toxic agent, making it ideal for assessing new MOFs. However, future methodological challenges are reliable generation of sarin at higher RH and exploring sensitive methods to monitor degradation products from nerve agents in real-time.

Cross-Polarization Solid-State NMR Quantification of Species within Pores of Metal-Organic Frameworks: A Case Study of α-Mg3(HCOO)6.

The quantitative measurement of adsorbed guest species within metal-organic framework (MOF) pores is of fundamental importance for evaluating the adsorption performance of MOFs. However, routine analytic techniques such as thermogravimetric analysis cannot distinguish the contribution from species adsorbed within pores, species adsorbed on the surface, and gas phase or liquid phase encapsulated in the inter-crystalline space. Herein, we developed a new quantification method based on the cross-polarization (CP) solid-state nuclear magnetic resonance (ssNMR) technique, in which only the species within MOF pores are selectively probed due to the dramatically reduced mobility. Using the commercialized MOF α-Mg3(HCOO)6 as an example, a good linear correlation between Areaguest/Areaframework (i. e., the integrated area of guest and framework 13C NMR signals) and guest loading can be observed for several representative molecules such as benzene, tetrahydrofuran (THF), and 1,4-dioxane, clearly revealing the feasibility of CP quantification approach. The effects of guest molecule and corresponding residual mobility on the CP quantification are further discussed by varying the geometry and size of guest molecules. This methodology thus provides an effective and irreplaceable route to evaluate the adsorption performance of porous materials in-depth, especially for liquid-phase adsorption and gas-phase adsorption in which the capillary condensation is not negligible.

Utilizing Activated Carbon Nanopores for Electrochemical Oxidation of Perylene to Redox-Active 3,10-Perylenedione: Application in Electrochemical Capacitor Electrodes.

We demonstrate that nanopores of activated carbon (AC) function as nanoreactors that oxidize perylene (PER) to a redox-active organic compound, 3,10-perylenedione (PERD), without any metal catalysts or organic solvents. PER is first adsorbed on AC in the gas phase, and the PER-adsorbed AC is subjected to electrochemical oxidation in aqueous H2SO4 as the electrolyte. Because gas-phase adsorption is solvent-free, PER is completely adsorbed on AC as long as the amount of PER does not exceed the saturated adsorption capacity of the AC, which enables accurate control of the amount adsorbed. PER is electrochemically oxidized to PERD in the nanopores of AC at above 0.7 V vs Ag/AgCl. The hybridized PERD undergoes a rapid reversible two-electron redox reaction in the nanopores owing to the large contact interface between the conductive carbon pore surfaces and PERD. The resulting AC/PERD hybrids serve as electrodes for electrochemical capacitors, utilizing the rapid redox reaction of PERD. The hybridization method is advantageous for quantitatively optimizing electrochemical capacitor performance by adjusting the amount of adsorbed PER. Moreover, because PERD hybridization in the AC nanopores does not expand the electrode volume, the volumetric capacitance increases with increasing hybridized PERD content. In three-electrode cell measurements, the volumetric capacitance at 0.05 A g-1 reaches 299 F cm-3, and 61% of this capacitance is retained at 10 A g-1 when 5 mmol of PER is used per gram of AC. Meanwhile, pristine AC delivers 117 F cm-3 at 0.05 A g-1 with a capacitance retention of 46% at 10 A g-1. Two-electrode cell measurements reveal that self-discharge is significantly suppressed by the hybridized PERD when AC/PERD hybrids and AC are used as cathodes and anodes, respectively, compared to that of a symmetrical AC cell. Moreover, PERD does not undergo cross-diffusion in the asymmetrical cells during self-discharge tests for 24 h.

Schottky-Functionalized Type-II heterojunction of Ag/CuNb2O6/g-C3N4: Efficient photoredox capability of CO2 to valuable fuel products via gas phase adsorption

Owing to encyclopedic energy crises and ecological concerns, the conversion of solar energy into sustainable value-added fuel products using a reasonable photocatalyst has received a lot of interest. The crucial challenge of the photoreduction of CO2 into fuel products such as CO and CH4 is the minor output and poor selectivity. Herein, a novel synthesized schottky-functionalized type-II heterojunction, Ag/CuNb2O6/g-C3N4 (Ag/CNO/g-CN), is extensively characterized to provide insights regarding its photocatalytic performance in reducing CO2. More significantly, electron paramagnetic resonance was employed to assist in understanding the inclusion of Schottky-junction and type II heterojunction charge transfer. The CO2 photoreduction to CO (2.78 μmol g−1h−1) with Ag/CNO/g-CN was 5- and 3-fold higher than single CNO and single g-CN, and the CO2 photoreduction to CH4 was 0.15 μmol g−1h−1 under simulated solar irradiation. This enhanced CO2 photoreduction was attributed to the large surface area and type II heterojunction, which promoted the separation as well as the transformation of photoinduced e−/h+ pairs and the superior redox ability of charge carriers. The composite's excellent photocatalytic efficiency towards CO2 was exceptionally enhanced by depositing Ag on CNO/g-CN. This study paves the way for immediate needs to explore the selective conversion of CO2 into CO and CH4 via systematic designing and effective schottky-functionalized type-II heterojunction.

1,2-dichlorobenzene gas-phase adsorption onto Nd3+ exchanged faujasite 13X beads - Influence of the exchange rate on adsorbed amounts and desorption temperatures

Aromatic chlorinated compounds, such as 1,2-dichlorobenzene, are considered as precursors of dioxin formation and their removal from flue gas using adsorption technology could be a solution. Faujasite type zeolites seem to be the most promising adsorbent, as the size of its porosity matches the size of the molecule. Even though the cationic faujasites are hydrophilic, the strength of interaction of chlorinated aromatic compounds adsorbed in porosity are higher than with more silicic faujasite such as FAU-NaY.FAU-NaX commercial beads have been Nd3+ exchanged at different rates. Dichlorobenzene dynamic gas adsorption has been performed via a thermobalance with vapours at concentration of 938 ppm in nitrogen gas flow and the quantities absorbed have been recorded during time. Then, from the coupling of a thermobalance with an in-line mass spectrometer, the desorbed quantities and desorption temperatures have been measured for both water and dichlorobenzene.Total quantities of 1,2-dichlorobenzene adsorbed onto partially exchanged FAU-Nd-NaX zeolites increases with the exchange rate: it has been measured a capacity of 23.3 wt % (relative to activated zeolite) for the most exchanged FAU-Nd-NaX at exchange rate of 53.6 %. For that zeolite, 74.7 % of the 1,2-dichlorobenzene is adsorbed at high-energy bonds with cations, desorbable at temperatures higher than 160 °C. The desorption temperature of dichlorobenzene decreases from 380 to 310 °C for FAU-NaX and the FAU-Nd-NaX at exchange rate of 53.6 % and remains higher than that of water. So that, thermal-desorption remains a separative process between water and chlorinated molecules previously adsorbed on cationic faujasites.

A sustainable approach to combat industrial air pollution using commercially available and engineered gas-phase adsorbents

The occurrence of benzene, toluene, ethylbenzene, and xylene (BTEX) in the environment has human health and ecological consequences; hence, it is essential to remediate the gas-phase industrial plume before its release. A laboratory-scale experimental apparatus was designed to investigate single-component gas-phase adsorption of BTEX compounds on commercially available granular activated carbon (CGAC) and laboratory-developed Mesquite-derived granular activated carbon (MDAC). The physical properties of CGAC and MDAC show that these adsorbents have a high N2 BET surface area, with CGAC exhibiting microporosity features. The single-component adsorption of BTEX was modeled by considering the transport of BTEX by axial dispersion, convective transport, and accumulation in the adsorbent bed at isothermal conditions. A linear driving force model was used to examine the mass transfer process. The experimental breakthrough curves showed good agreement with the modeled breakthrough curves. The modeling parameters demonstrated the dominance of intraparticle diffusion with a negligible external diffusion of BTEX onto CGAC and MDAC. The analysis of experimental data validated the modeled dynamic behavior of BTEX adsorption onto CGAC and MDAC. Overall, BTEX compounds followed Langmuir isotherms for both adsorbents, with intra-particle diffusion as a dominant gas phase adsorption mechanism.

Fluidity comparison of biomass-derived activated carbon and TiO(OH)2 and its improving toward promoted low-temperature CO2 capture in gaseous medium

Biowaste-derived activated carbons (ACs) and TiO(OH)2 are considered promising materials for effective CO2 capture. TiO(OH)2 has not been employed in gas phase adsorption process; moreover, the comparison of fluidization of these materials had not been investigated. Hence, this study not only did evaluate the CO2 adsorption performance of both synthetic TiO(OH)2 and biomass-derived AC, but also assessed their fluidity aimed to implement at pilot scales. The samples were examined using XRD, BET, SEM, EDX, FTIR, and TGA techniques. The adsorption of the adsorbents was evaluated through TGA under several conditions. The AC demonstrated an average CO2 capture capacity of 3.54 mmol/g, surpassing that of the TiO(OH)2 (0.971 mmol/g) during three successive cycles at 25/120 ℃ under a CO2/N2 flow of 90/10 vol% and 1 bar. Adding 5 wt% of auxiliary SiO2 nanoparticles enhanced bed expansion of AC and TiO(OH)2 by 1.83 and 1.5 times respectively, due to dwindling interparticle cohesive interactions.

Effect of Gas-Phase Nitric Oxide Adsorption on ORR Activity of Fe-N-C Catalysts

Significant progress has been made in recent years in developing platinum group metal-free catalysts, particularly so-called Fe-N-C materials, for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells.1 However, further improvement in both activity and, especially, durability are still necessary for practical applications. One of the major factors that hinder the further enhancement of the catalysts is the lack of clear understanding of the nature of the active sites in the Fe-N-C catalysts due to the complexity of the Fe-N-C catalysts’ composition and structure. Gaseous nitric oxide was reported to be a suitable probe molecule that can bond to Fe and impede the ORR of the Fe-N-C catalysts.2,3 In this work, the effect of nitric oxide gas-phase adsorption on Fe-N-C catalyst redox feature and ORR activity for a variety of Fe-N-C catalysts from different origin will be investigated. The implication of these results on the nature of the ORR active sites of the Fe-N-C catalysts will be discussed.AcknowledgementsThis work was supported by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO) under the auspices of the Electrocatalysis Consortium (ElectroCat 2.0). This work was partially authored by Argonne, a U.S. Department of Energy (DOE) Office of Science laboratory operated for DOE by UChicago Argonne, LLC under contract no. DE-AC02-06CH11357.References X.X. Wang, M.T. Swihart, and G. Wu, "Achievements, challenges, and perspectives on cathode catalysts in proton exchange membrane fuel cells for transportation", Nature Catalysis, 2 (2019) 578-589.J.L. Kneebone, et al., "A Combined Probe-Molecule, Mossbauer, Nuclear Resonance Vibrational Spectroscopy, and Density Functional Theory Approach for Evaluation of Potential Iron Active Sites in an Oxygen Reduction Reaction Catalyst", J. Phys. Chem. C 121 (2017) 16283-16290.P. Boldrin, D. Malko, A. Mehmood, U.I. Kramm, S. Wagner, S. Paul, N. Weidler, A. Kucernak, “Deactivation, reactivation and super-activation of Fe-N/C oxygen reduction electrocatalysts: Gas sorption, physical and electrochemical investigation using NO and O2”, Applied Catalysis B: Environmental 292 (2021) 120169.

THEORETICAL BASICS OF MONITORING THE CONDITION OF THE ELECTRODES OF CHEMICAL CURRENT SOURCES BY THE METHOD OF ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY

On the basis of the conducted critical review of modern physical models of the porous electrode, it сan be stated that under the conditions of non-uniform mass transfer taking into account the depth of the electrode, it is possible not only to develop new porous electrodes for a certain application, but also to control the state of electrochemical systems as a whole using the non-destructive method of electrochemical impedance spectroscopy. The presence of a macroscopic model of porous electrode allows one to use the integration of parameters over the surface of the electrode and obtain the average values of current, resistance and capacity within the electrode using the method of averaging in the volume ele­ment within the electrode, where porosity is the volume fraction of the void within the element, which is filled with electrolyte solution. This is the theoretical basis for using electrochemical impedance spectroscopy to assess the state of electrodes in electrochemical current sources. To take into account the influence of the aqueous electrolyte, it is possible to use a model taking into account the area of the effective wetted surface, which makes it possib­le to relate the wetting of the electrode pores with the electrolyte solution to the change in electrical conductivity and polarization of the electrode surface. In this case, when usingelectrochemical impedance spectroscopy, it is possible to obtain information about the following changes in primary current sources: 1– the effect of temperature, which leads to a decrease in the areas of the electrode wetted by the electrolyte, which affect the value of the capacity of the DEL, 2 – chemical processes that lead to the destruction of hydrophilic pores and pores with hydrophilic-hydrophobic walls, an increase in the hydrophobic component on the surface of the electrode, 3 – mechanical destruction of the electrodes. The use of models that take into account the geometry of pores makes it possible to obtain correct data for the analysis of the porous surface in the presence of an electrolyte and in cases of gas phase adsorption in presence of closed pores, as well as to use the value of the capacity on the surface of electrodes to assess the state of their performance.

Investigating trimethylphosphine oxide interactions with Brønsted and Lewis acid sites in zeolites: A comparative solid-state NMR study of wet-phase and gas-phase adsorption techniques

This study examines the differences in using two primary methods for adsorbing trimethylphosphine oxide (TMPO) onto acid catalysts and their influence on the NMR spectra of TMPO-loaded zeolites. The findings show that TMPO molecules interact with both Brønsted and Lewis acid sites, and that the amount of (TMPO)2H+ dimers formed increases with the amount of TMPO added. The use of dichloromethane as a solvent inhibits the formation of dimers and crystalline TMPO but may lead to inaccurate estimations of acid site strength and quantification due to interactions with residual solvent molecules. The gas-phase method is preferred for adsorbing TMPO onto zeolites, but caution is needed to avoid forming a high number of dimers that ca pose challenges in the interpretation of the NMR spectra.

Recent Progress and Perspectives of Direct Ink Writing Applications for Mass Transfer Enhancement in Gas-Phase Adsorption and Catalysis.

Conventional adsorbents and catalysts shaped by granulation or extrusion have high pressure drop and poor flexibility for chemical, energy, and environmental processes. Direct ink writing (DIW), a kind of 3D printing, has evolved into a crucial technique for manufacturing scalable configurations of adsorbents and catalysts with satisfactory programmable automation, highly optional materials, and reliable construction. Particularly, DIW can generate specific morphologies required for excellent mass transfer kinetics, which is essential in gas-phase adsorption and catalysis. Here, DIW methodologies for mass transfer enhancement in gas-phase adsorption and catalysis, covering the raw materials, fabrication process, auxiliary optimization methods, and practical applications are comprehensively summarized. The prospects and challenges of DIW methodology in realizing good mass transfer kinetics are discussed. Ideal components with a gradient porosity, multi-material structure, and hierarchical morphology are proposed for future investigations.

The Halsey isotherm for water contaminant adsorption is fake

We present a critique of the Halsey isotherm model (also known as Frenkel–Halsey–Hill), which is used by many researchers to interpret water contaminant adsorption data. We first convert the original Halsey isotherm for gas-phase adsorption to a form appropriate for liquid-phase adsorption. We call this version the “genuine Halsey isotherm.” We then show that the Halsey isotherm currently used in water contaminant adsorption research bears only a partial resemblance to the genuine Halsey isotherm. As such, we label the former the “fake Halsey isotherm.” We go on to prove that the fake Halsey isotherm is mathematically equivalent to the Freundlich isotherm. Consequently, the modeling results obtained by the fake Halsey isotherm reported in numerous previously published studies have been mostly misleading. Many such reports can be found across a broad spectrum of major journals. Finally, we highlight the data-fitting attributes of the genuine Halsey isotherm. It is shown to be capable of describing adsorption isotherms having types I, II, and III curve shapes.

Improved photocatalytic activity of novel NiAl2O4/g-C3N4 binary composite for photodegradation of 2,4-dinitrophenol and CO2 reduction via gas phase adsorption

Immense interest has been expressed in the efficient conversion of solar to chemical energy using promising semiconductor-based photocatalysts, seen as a prospective solution for energy problems and environmental remediation. In particular, employing photocatalytic technology to reduce CO2 into carbon fuels and degrade 2,4-dinitrophenol has become a much-discussed topic in renewable energy. In this study, binary NiAl2O4/g-C3N4 (NAO/g-CN) composites were synthesized via a process of calcination followed by sonication, which enhanced the transfer of photogenerated electrons from g-C3N4 to NiAl2O4, creating a much greater excited reductive electron charge on the surface of NiAl2O4. The crystallographic, electron-microscopy, photoemission spectroscopy, electrochemical and spectroscopic characterizations of the prepared composites allowed insights into their photocatalytic activity in the photoreduction of CO2 and photodegradation of 2,4-dinitrophenol (2,4-DNP). The most active photocatalyst, 40% NAO/g-CN, produced 10.73 μmol g−1h−1 of CO and 99.29% degradation of 2,4-DNP, representing the effective elimination of these pollutants under visible light. This 10.73 μmol g−1h−1 photocatalytic conversion of CO2 into CO is 10- and 12-fold that of NAO and g-CN, respectively. The 40% NAO/g-CN produced 1.58 times more 2,4-DNP degradation than bare g-CN, and 1.81 times more than bare NAO, at constant rate (k) values of 15.52 × 10−3, 5.33 × 10−3, and 6.27 × 10−3, respectively. This study opens new avenues to accurately modifying photogenerated charge separation directions by developing chemical bonding in direct scheme NiAl2O4/g-C3N4 structures for CO2 photoreduction.

High utilization efficiencies of alkylbenzokynones hybridized inside the pores of activated carbon for electrochemical capacitor electrodes.

Benzoquinone derivatives (BQDs) are hybridized inside activated carbon (AC) pores via gas-phase adsorption to prepare electrochemical capacitor materials. In this study, 2 mmol of BQDs are hybridized with 1 g of AC. The hybridization of alkylbenzoquinones (ABQs) with AC enhances the volumetric capacitances of the hybrids from 117 to 201 F cm-3 at 0.05 A g-1 and the capacitances are retained up to 73% at 10 A g-1. Meanwhile, the volumetric capacitances are increased to 163 F cm-3 at 0.05 A g-1 by the hybridization of halobenzoquinones (HBQs) and the capacitance retentions at 0.05 A g-1 are ∼62%, which are higher than that of AC (46%). The results of electrochemical measurements suggest that HBQs exist as agglomerates while ABQs are finely dispersed inside the pores. The ABQs have good contact with the conductive carbon pore surface compared to the HBQs. Consequently, most of the ABQ molecules undergo reversible redox reactions (i.e., high utilization efficiencies), and a large contact area facilitates charge transfer at the large contact interface, thereby endowing the hybrids of ABQs with fast charging and discharging characteristics. HBQ molecules can be finely dispersed by liquid-phase adsorption, but the finely dispersed HBQ molecules are mobile inside the pores at room temperature and gradually form agglomerates. The difference in the existing form of BQDs is explained by the dominant interaction affecting the BQD molecules. ABQs have a strong interaction with the carbon pore surface while the intermolecular interaction is dominant for HBQs.

Adsorption of the hydrophobic organic pollutant hexachlorobenzene to phyllosilicate minerals

Hexachlorobenzene (HCB), a representative of hydrophobic organic chemicals (HOC), belongs to the group of persistent organic pollutants (POPs) that can have harmful effects on humans and other biota. Sorption processes in soils and sediments largely determine the fate of HCB and the risks arising from the compound in the environment. In this context, especially HOC–organic matter interactions are intensively studied, whereas knowledge of HOC adsorption to mineral phases (e.g., clay minerals) is comparatively limited. In this work, we performed batch adsorption experiments of HCB on a set of twelve phyllosilicate mineral sorbents that comprised several smectites, kaolinite, hectorite, chlorite, vermiculite, and illite. The effect of charge and size of exchangeable cations on HCB adsorption was studied using the source clay montmorillonite STx-1b after treatment with nine types of alkali (M+: Li, K, Na, Rb, Cs) and alkaline earth metal cations (M2+: Mg, Ca, Sr, Ba). Molecular modeling simulations based on density functional theory (DFT) calculations to reveal the effect of different cations on the adsorption energy in a selected HCB-clay mineral system accompanied this study. Results for HCB adsorption to minerals showed a large variation of solid–liquid adsorption constants Kd over four orders of magnitude (log Kd 0.9–3.3). Experiments with cation-modified montmorillonite resulted in increasing HCB adsorption with decreasing hydrated radii of exchangeable cations (log Kd 1.3–3.8 for M+ and 1.3–1.4 for M2+). DFT calculations predicted (gas phase) adsorption energies (− 76 to − 24 kJ mol−1 for M+ and − 96 to − 71 kJ mol−1 for M2+) showing a good correlation with Kd values for M2+-modified montmorillonite, whereas a discrepancy was observed for M+-modified montmorillonite. Supported by further calculations, this indicated that the solvent effect plays a relevant role in the adsorption process. Our results provide insight into the influence of minerals on HOC adsorption using HCB as an example and support the relevance of minerals for the environmental fate of HOCs such as for long-term source/sink phenomena in soils and sediments.

Mesoporous boron nitride in contact with water - Chemical stability and adsorption properties

In the context of adsorptive water treatment, exhaust gas treatment, gas storage, or catalysis, porous boron nitride is gaining in scientific attention due to its versatile properties. The material is above all characterized by its high chemical and thermal stability. However, it has recently been shown that porous boron nitride is highly water-susceptible, which severely limits its application in non-inert atmospheres. So far, solely the influence of chemical composition on water stability has been investigated in literature. In the present work, the influence of different pore sizes is considered. Two different boron nitrides, a mesoporous and a micro-mesoporous one, are investigated with regard to their stability in air, water vapor, and liquid water for multiple weeks. Using different analytical methods, it can be shown that the stability of porous boron nitride strongly depends on pore size and structure. We demonstrate that the mesoporous boron nitride synthesized in this work has much higher water stability than micro-mesoporous boron nitrides. The mesoporous material showed minor to no changes in its chemical structure and isothermal shape when exposed to water vapor and air. We suggest that a higher amount of crystalline hexagonal boron nitride leads to increased stability. This conclusion opens pathways to improve the water stability of porous boron nitride in order to make it suitable for non-inert gas phase adsorption applications.

Redox Behavior and ORR Catalytic Activity of Fe-N-C Electrocatalysts in Various Electrolytes and in the Presence of Probe Molecules

Materials in the Fe-N-C family are the most promising platinum group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs). Significant progress has been made in recent years in improving both the ORR activity and durability of the Fe-N-C catalysts. However, further improvements are needed, especially in long-term performance durability in hydrogen-air PEFCs, to enable use of these catalysts in practical applications, such as powering light and heavy-duty vehicles. One of the major factors that hinder the further enhancement of the catalysts is the lack of clear understanding of the nature of the active sites in the Fe-N-C catalysts that catalyze the ORR reaction, due to the complexity of the Fe-N-C catalysts composition and structure. The most active Fe-N-C catalysts have been synthesized by pyrolyzing iron salts or other iron-containing compounds with zinc-based zeolitic imidazolate frameworks (ZIFs) and/or phenanthroline (as carbon and nitrogen sources), or by heat treating iron-substituted ZIFs. It has been shown that many synthesis variables, such as type of iron precursors, the metal and carbon-nitrogen macrocycle ratio, the heat treatment temperature, atmosphere, and temperature profile, all affect the properties and structure and thus the activity and stability/durability of the resulting catalysts. In order to further improve the Fe-N-C catalysts, it is critical to gain insights into the origins of ORR activity and the reaction mechanism. In this work, the redox behavior of different Fe-N-C catalysts, synthesized by a variety of techniques mentioned above, and correlation of this redox behavior with ORR catalytic activity in different electrolytes will be discussed and the effect of nitric oxide gas-phase adsorption on the catalyst redox feature and ORR activity will be presented. The implication of these results on the identity or identities of the ORR active sites of the Fe-N-C catalysts will be discussed.This work was supported by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO) under the auspices of the Electrocatalysis Consortium (ElectroCat 2.0). This work utilized the resources of the Advanced Photon Source, a U.S. DOE Office of Science user facility operated by Argonne National Laboratory for DOE Office and was authored by Argonne, a U.S. Department of Energy (DOE) Office of Science laboratory operated for DOE by UChicago Argonne, LLC under contract no. DE-AC02-06CH11357.