Adsorbate-adsorbate Interactions Research Articles

Harnessing Adsorbate-adsorbate Interaction to Activate C-N Bond for Exceptional Photoelectrochemical Urea Oxidation.

The photoelectrochemical (PEC) urea oxidation reaction (UOR) presents a promising half-reaction for green hydrogen production, but the stable resonance structure of the urea molecule results in sluggish kinetics for breaking the C-N bond. Herein, we realize the record PEC UOR performance on a NiO-modified n-Si photoanode (NiO@Ni/n-Si) by harnessing the adsorbate-adsorbate interaction. We quantificationally unveil a dependence of the UOR activation barrier on the coverage of photogenerated surface high-valent Ni-oxo species (NiIV=O) by employing operando PEC spectroscopic measurements and theoretical simulations. The strong attraction between NiIV=O and adsorbed urea facilitates their N-O coupling while weakening the C-N bonding within urea, manifesting as the decreased UOR activation energy from 0.74 to 0.41 eV when the surface coverage of NiIV=O is enhanced from zero to full, corresponding to more than two orders of magnitude enhancement for the UOR rate. Furthermore, an industrial-grade photocurrent density of 100 mA cm-2 is achieved at a potential as low as 1.08 VRHE by stimulating the NiIV=O accumulation under 10 Suns, which is 300 mV lower than the potential required for most reported electrochemical counterparts. This work opens new prospects for achieving high-performance PEC urea oxidation via adsorbate-adsorbate interaction.

Unravelling the pH-dependent mechanism of ferroelectric polarization on different dynamic pathways of photoelectrochemical water oxidation.

Ferroelectric polarization is considered to be an effective strategy to improve the oxygen evolution reaction (OER) of photoelectrocatalysis. The primary challenge is to clarify how the polarization field controls the OER dynamic pathway at a molecular level. Here, electrochemical fingerprint tests were used, together with theoretical calculations, to systematically investigate the free energy change in oxo and hydroxyl intermediates on TiO2-BaTiO3 core-shell nanowires (BTO@TiO2) upon polarization in different pH environments. We demonstrate that the adsorbate evolution mechanism (AEM) dominated in acidic environments, and both positive and negative polarization resulted in a reduction in the oxo-free energy, which inhibited the reaction kinetics. In the oxide path mechanism (OPM) that occurs in alkaline conditions, the ferroelectric polarization exhibits repulsive adsorbate-adsorbate interaction for OH- coverage and free energy shift of the OH- groups. We elucidate that a weakly alkaline electrolyte is the optimal environment for ferroelectric polarization because the positive polarization promotes OH- coverage and facilitates reaction pathway transfer from AEM to OPM; therefore, BTO@TiO2 exhibited a record polarization enhancement to 0.52 mA cm-2 at 1.23 VRHE in pH = 11. This work provides a more accurate insight into the pH-dependent effect of ferroelectric polarization on the OER dynamic pathway than conventional models that are based solely on the regulation of band bending.

Extraction of heavy metal ions from aqueous solutions by frame sorbents based on benzene-1,3,5-tricarboxylate (MBTC) and benzene-1,4-dicarboxylates (MB DC) of various metals

Pollution of soils and water sources with heavy metals leads to negative consequences for the environment associated with disruption of ecosystem balance, harm to the health of living organisms and humans. To solve this problem, organometallic frame sorbents capable of efficiently extracting heavy metal ions from aqueous solutions have been synthesized. During the conducted research, the regularities of the adsorption of cadmium, lead, copper, cobalt and nickel ions were studied using synthesized frame sorbents based on benzene-1,3,5-tricarboxylates (MBTC) and benzene-1,4-dicarboxylates (MBDC). The identification analysis performed on diffractograms of CoBTC and NiBTC powders showed the presence of structures [Co3(BTC)2·12H2O] and [Ni3(BTC)2·12H2O]. Unlike NiBTC, NiBDC dicarboxylate crystallizes in triclinic syngony (spatial group P1¯, Z = 1) and corresponds to the crystal structure [Ni3(OH)2(BDC)2··4H2O], the sample of the CuBTC compound crystallizes in cubic symmetry with the space group Fm3¯m (Z = 16) and corresponds to the crystal structure [Cu3(BTC)2·3H2O], and the CuBDC compound has a structure belonging to the monoclinic symmetry. The results of the analysis of isotherms of low-temperature nitrogen adsorption using synthesized MOFs made it possible to determine important textural characteristics of sorbents. It was noted that a strong adsorbate-adsorbent interaction is realized for CoBTC in the micropore region. It is shown that the specific surface area of synthesized sorbents, calculated by the Brunnauer-Emmett-Teller (BET) method, varies widely. Thus, for CoBTC and NiBTC compounds, it was 276.0 and 9.0 m2/g, respectively. The noted differences are due to the presence of a large number of micropores in the sorbent CoBTC. In most cases, the kinetic patterns of the adsorption of heavy metal ions can be described by a pseudo-second-order equation. The only example of the process proceeding according to the kinetic equation of the pseudo-first order is the adsorption of copper ions on the NiBDC sorbent. It is noted that cobalt, nickel and copper ions are better absorbed by sorbents containing the corresponding ions of the same name according to the Paneta-Faience rule. The linear relationship found between the sorption capacity and the logarithm of the ratio of the radius of ions to their electronegativity implies that the mechanism of adsorption of metal ions on MOFs is determined by the physicochemical properties of the ions themselves. The developed organometallic frame compounds can be effectively used in technologies for purification of water resources from toxic heavy metal ions.

(Invited) First Principles Treatments of Oxygenate Electrooxidation – Structure Sensitivity and Influence of Solvent on Chemically Diverse Elementary Reaction Steps

Advances in the theoretical understanding of electrochemical systems have, over the past decade, led to growing use of periodic Density Functional Theory (DFT) studies to treat a surprisingly large ensemble of electrocatalytic reactions, ranging from carbon dioxide electroreduction to oxygen evolution. Many such studies have employed simplified models of the electrochemical environment to determine reactivity trends across a broad space of catalytic materials, while other efforts have focused on developing detailed descriptions of electrochemical phenomena, such as the structure of electrochemical double layers, on model catalyst structures. An emerging challenge is to combine these approaches to ultimately enable theoretical design of electrocatalysts for reactions of significantly expanded chemical and materials complexity.In this talk, we begin by discussing how we develop and apply DFT-based methods to study the electrooxidation of ethanol and related oxygenated species on Pt surfaces. We show how explicit double layer models and ab initio molecular dynamics provide exciting insights beyond the traditional computational hydrogen electrode treatments, leading to improved descriptions of elementary reaction steps involving adsorption/desorption of reaction intermediates and proton-coupled electron transfer processes. By combining the aggregate results with detailed microkinetic models and explicit descriptions of adsorbate-adsorbate interactions, we further demonstrate how these effects can influence both the predicted overpotentials and the selectivities to acetic acid, acetaldehyde, and carbon dioxide. We conclude with some perspectives on how these insights may be used to enhance the search for improved oxygenate electrooxidation catalysts.

Efficiency and ecotoxicity of activated biochar in the treatment of artificial wastewater contaminated by pharmaceuticals

Pharmaceuticals are emerging contaminants of global concern due to potential ecotoxicity and persistence in wastewater. Since conventional wastewater treatment plants are not designed to remove micropollutants and the removal efficiency varies compound-specifically, pharmaceuticals pose a risk in the recipient aquatic environments. Adsorption by solid materials such as activated biochar has been suggested to offer a practical removal method. However, not much is known about the environmental risks of the adsorbents used in wastewater treatment. This study aimed to study the efficiency of activated biochar (ACB) to remove low and high concentration of specific pharmaceuticals including diclofenac (DI), tetracycline (TE), and cephalexin (CEP) from Milli-Q water (MQ) and artificial wastewater (AWW). Furthermore, the study evaluated the ecotoxicity of these pharmaceuticals, as well as pristine ACB and ACB loaded with pharmaceuticals (ACB-LP), in both MQ and AWW using Daphnia magna. The adsorbate concentration and matrix affected ACB's removal efficiency. Weaker adsorbent-adsorbate interactions and mass transfer resistance at lower adsorbate concentrations, along with interactions between wastewater constituents and pharmaceuticals were the leading factors contributing to this reduction. These experimental observations indicate practical considerations for using adsorbents in operational wastewater settings. Furthermore, ACB-LPs generally exhibited lower toxicity compared to ACB, attributed to the saturation of free binding sites and reduced adhesion to daphnids. This study highlights the importance of examining the environmental risks of adsorbent materials used in wastewater treatment, particularly given their anticipated future use.

Adsorption Thermodynamics for Process Simulation.

Adsorption has rapidly evolved in recent decades and is an established separation technology extensively practiced in gas separation industries and others. However, rigorous thermodynamic modeling of multicomponent adsorption equilibrium remains elusive, and industrial practitioners rely heavily on expensive and time-consuming trial-and-error pilot studies to develop adsorption units. This article highlights the need for rigorous adsorption thermodynamic models and the limitations and deficiencies of existing models such as the extended Langmuir isotherm, dual-process Langmuir isotherm, and adsorbed solution theory. It further presents a series of recent advances in the generalization of the classical Langmuir isotherm of single-component adsorption by deriving an activity coefficient model to account for the adsorbed phase adsorbate-adsorbent interactions, substituting adsorbed phase adsorbate and vacant site concentrations with activities, and extending to multicomponent competitive adsorption equilibrium, both monolayer and multilayer. Requiring a minimum set of physically meaningful model parameters, the generalized Langmuir isotherm for monolayer adsorption and the generalized Brunauer-Emmett-Teller isotherm for multilayer adsorption address various thermodynamic modeling challenges including adsorbent surface heterogeneity, isosteric enthalpies of adsorption, BET surface areas, adsorbed phase nonideality, adsorption azeotrope formation, and multilayer adsorption. Also discussed is the importance of quality adsorption data that cover sufficient temperature, pressure, and composition ranges for reliable determination of the model parameters to support adsorption process simulation, design, and optimization.

Utilization of desilication products as efficient adsorbents for the removal of basic fuchsine

Desilication products (DSPs) are one of the main components of bauxite residue, which is currently discharged without further usage. The present study reports on the use of DSPs as adsorbents for basic fuchsine dye. Using artificial spent liquor, we synthesized not only neat DSP but also solids in the presence of various organics, to account for their likely occurrence during the Bayer process. The physico-chemical properties of all DSPs are similar, except for the specific surface area (SSA), which decreases as the organic content in the final product increases. Further, we compared the adsorption characteristics of DSP to those of Y-type faujasite (FAUY). Strikingly, both time-dependent adsorption measurements and adsorption isotherms showed that DSP, despite its 28-fold smaller SSA, binds 4–5 times more dye molecules. Computational modelling for sodalite (as a model for DSP) and FAUY indicates not only more favourable adsorbent-adsorbate interactions, but also more available free Si-OH sites for binding of fuchsine in the case of sodalite. Finally, we find that organic impurities present in the Bayer liquor do not alter the adsorption capacity of neat DSP to any significant degree; therefore, this adsorbent tolerates numerous organic contaminants without decreasing its affinity to the binding of fuchsine.

Energy-Efficient Separation of Xylene Isomers through Stacked 1-Aminopyrene Polymer Composite.

Developing high-performance adsorbents for energy-efficient separation of xylene isomers has important research and application value. Identification and separation of xylene isomers (PX, MX, and OX) at room temperature based on the different relative positions of two methyl groups on the benzene ring is an unprecedented attempt. Herein, 1-aminopyrene polymer (PAP) is designed and electro-synthesized composited with a supporting electrolyte as an adsorbent for the separation of xylene isomers using a multi-stage dispersed liquid-solid adsorption process at room temperature. Each -NH-pyrene-NH- unit can form a force field to identify and discriminate xylene isomer through π-π interaction combined with -CH3···-NH- interactions of different strength, thereby achieving separation of PX, MX, and OX isomers by amplifying the slight differences in the adsorption capacity and diffusion rates. The density functional theory (DFT) simulations provide a more quantitative analysis of the adsorbate-adsorbent interactions to illustrate PX > MX > OX order of adsorption selectivity. This study may offer an alternative strategy for the precise designing of energy-efficient and adsorption-based separation materials.

Kinetics and thermodynamics investigations of efficient and eco-friendly removal of alizarin red S from water via acid-activated Dalbergia sissoo leaf powder and its magnetic iron oxide nanocomposite.

The present work aimed to highlight an efficient, readily accessible, and cost-effective adsorbent derived from Dalbergia sissoo (DS) leaf powder for removing the environmentally hazardous dye "alizarin red S" (ARS) from hydrous medium. A variant of the adsorbent is activated via sulfuric acid and composited with magnetic iron oxide nanoparticles (DSMNC). Both adsorbents are thoroughly characterized using techniques such as Fourier transform infrared spectroscopy, point of zero charge, energy-dispersive X-ray spectroscopy, and scanning electron microscopy, which show that they have a porous structure rich in active sites. Different adsorption conditions are optimized with the maximum removal efficiency of 76.63% for DS and 97.89% for DSMNC. The study was highlighted via the application of various adsorption isotherms, including Freundlich, Langmuir, Temkin, and Dubinin-Radushkevich, to adsorption data. Pseudo-first-order, pseudo-second-order, and intra-particle diffusion models were utilized to investigate the kinetics and mechanism of adsorption. The Freundlich model and pseudo-second-order kinetics exhibited the best fit, suggesting a combination of physical interactions, as confirmed by the D-R and Temkin models. The dominant adsorbate-adsorbent interactive interactions responsible for ARS removal were hydrogen bonding, dispersion forces, and noncovalent aromatic ring adsorbent pi-interactions. Thermodynamic parameters extracted from adsorption data indicated that the removal of the mutagenic dye "ARS" was exothermic and spontaneous on both DS and DSMNC, with DSMNC exhibiting higher removal efficiency.

Core-shell V2O5- Gum ghatti grafted poly (acrylamide-co-methacrylic acid) adsorbent for the removal of methylene blue dye in water: Kinetic, equilibrium and thermodynamic studies

Herein, vanadium pentoxide (V2O5) encapsulated Gum ghatti grafted poly(acrylamide-co- methacrylic acid) adsorbent was synthesized to remove methylene blue dye from water. The materials were characterized by FTIR, TEM, SEM, Raman and XRD analysis. Batch adsorption studies were performed on the materials. Several variables' effects on methylene blue removal, including pH, contact time, initial dye concentration, and adsorbent dosage, were examined. Kinetics and thermodynamic analysis were also carried out for the adsorbent-adsorbate interaction to determine the maximum adsorption and mechanism for adsorption. By using UV-Vis spectrophotometric analysis, the dye concentration was evaluated both before and after adsorption. Langmuir, Freundlich, and Dubini-Radushkevic's isotherm models were used to analyze the adsorption data. Results showed a maximum adsorption efficiency of 92 % at a pH of 9 and 0.2 g as the maximum adsorbent dosage. The study followed the Langmuir isotherm model with a correlation coefficient (R2) of 0.9995. The results from the kinetic studies show a pseudo-second-order mechanism. The negative values of the Gibbs free energy change ΔG° ranging from −10.57 KJmol−1 to −9.64 Kmol−1 within the temperatures of 298 K to 313 K is an indication of a spontaneous process. A negative enthalpy change (ΔH° = −29.05 KJmol−1) shows an exothermic process and a negative entropy change (ΔS° = −0.06 KJmol−1) represents a highly ordered system. ANOVA in Microsoft Excel and other statistical analyses were used to evaluate the effects of time, pH, concentration, dose, temperature, and other factors on the effectiveness of dye adsorption. Importantly, every parameter that was investigated showed statistically significant impacts on the removal of dye (p < 0.05), revealing their important impact on the adsorption process.

Unraveling the Origin of the Repulsive Interaction between Hydrogen Adsorbates on Platinum Single-Crystal Electrodes.

Hydrogen adsorption on platinum (Pt) single-crystal electrodes has been studied intensively in both experiments and computations. Yet, the precise origin and nature of the repulsive interactions observed between hydrogen adsorbates (Hads) have remained elusive. Here, we use first-principles density functional theory calculations to investigate in detail the interactions between Hads on Pt(111), Pt(100), and Pt(110) surfaces. The repulsive interaction between Hads on Pt(111) is deconvoluted into three different physical contributions, namely, (i) electrostatic interactions, (ii) surface distortion effect, and (iii) surface coordination effect. The long-range electrostatic interaction, which is generally considered the most important source of repulsive interactions in surface adsorption, was found to contribute less than 30% of the overall repulsive interaction. The remaining >70% arises from the other two contributions, underscoring the critical influence of surface-mediated interactions on the adsorption process. Surface distortion and coordination effects are found to strongly depend on the coverage and adsorption geometry: the effect of surface distortion dominates when adsorbates reside two or more Pt atoms apart; the effect of surface coordination dominates if hydrogen is adsorbed on neighboring adsorption sites. The above effects are considerably less pronounced on Pt(100) and Pt(110), therefore resulting in weaker interactions between Hads on these two surfaces. Overall, the study highlights the relevance of surface-mediated effects on adsorbate-adsorbate interactions, such as the often-overlooked surface distortion. The effect of these interactions on the hotly debated adsorption site for the adsorbed hydrogen intermediate in the hydrogen evolution reaction is also discussed.

Key Role of Oxidising Species for Water Oxidation on Iridium Oxides Revealed By Time-Resolved Optical and X-Ray Spectroscopies

Iridium oxide is the state-of-the-art electrocatalyst for water oxidation in polymer electrolyte membrane (PEM) electrolysers, crucial for green hydrogen production. Despite its extensive industrial use, the water oxidation mechanism on this metal oxide and the key factors controlling the reaction rate remain unclear. Understanding these controls at a fundamental level on such benchmark metal oxides is vital for designing more active and stable electrocatalysts for water oxidation in PEM electrolysers.In this talk, I will present our research on probing oxygen evolution reaction (OER) relevant species on iridium-based catalysts. This involves a combination of time-resolved operando optical spectroscopy, soft and hard X-ray absorption spectroscopy (XAS), and electrochemical mass spectrometry. Initially, I will discuss the intermediate and catalytically active states of iridium oxides. This is based on correlating optically detected states with oxygen molecular products identified through on-chip electrochemical mass spectrometry. Subsequently, I will demonstrate how we used optical spectroscopy to quantify these states as a function of potential. The nature of these states, including the Ir oxidation state and surface adsorbates on iridium sites, will be elucidated using a combination of time-resolved Ir-L edge XAS, O-K edge XAS, and supported by Density Functional Theory (DFT).With these quantification results of active species for OER, I will compare the intrinsic kinetics of two state-of-the-art iridium oxide structures - amorphous IrOx versus crystalline rutile IrO2.[1] The effect of the electrolyte on the intrinsic activity of iridium oxides will also be discussed.[2] Finally, based on this molecular-level understanding, I will present a modified volcano model for OER catalyst design. This model considers the impact of adsorbate-adsorbate interactions on binding energetics, demonstrating critical design principles for high intrinsic activity OER catalysts.The authors acknowledge the funding support from the Imperial College-Chinese Scholarship Council Studentship and bp-ICAM 92, which made this research possible.[1] Caiwu Liang, Reshma Rao, Karine Svane et al. Unravelling the effects of active site densities and energetics on the water oxidation activity of iridium oxides, 07 March 2023, PREPRINT (Version 1) available at Research Square [https://doi.org/10.21203/rs.3.rs-2605628/v1][2] Caiwu Liang, Yu Katayama, Yemin Tao, Asuka Morinaga, Benjamin Moss, Verónica Celorrio, et al. Role of electrolyte pH on water oxidation for iridium oxides. ChemRxiv. Cambridge: Cambridge Open Engage; 2023

Capture and removal of nanoplastics using ZIF-derived defective nanoframework: Structure-performance correlation, theoretical calculation and application

Nanoplastics, as a novel pollutant, are widely found in the environment and water, which also can be migrated to vegetables and fruits through irrigation water and the growing process, posing threat to the human health. In this study, post-modification strategy was implemented to synthesize a series of fine-tuning hierarchically porous ZIF-derived nanoframework (Zn-Co-Ni-ZIF, Zn-Co-TA-ZIF and ZIF67-Ni-ZIF) with various morphologies and structures to investigate the relationship between the polystyrene nanoplastics (PS-NPs) adsorption performance and structure property. The correlation degree between PS-NPs adsorption property and structure parameter followed the order of the intensity of Co2p > the intensity of M-N > the number of linker deficiencies > crystal defect > zeta potential. The gray models (GM) (1,1) between adsorption capacity and evaluation items were successfully established (C < 0.342; RSD < 18.108 %) but zeta potential. A highly linked association (R2 = 0.9561) between the PVmeso/PVmicro and equilibrium constants kt2 may be discovered. The combined findings from experimental analysis and Density Functional Theory calculations demonstrated that the adsorption capacity of adsorbents for PS-NPs was greatly enhanced by the introduction of additional defects and the reinforcement of adsorbent-adsorbate interactions, including electrostatic and coordination interactions. Among the selected materials, Zn-Co-ZIF and Zn-Co-Ni-(60 mg)-ZIF exhibited better resistance to acid, alkali, and salt. Moreover, the PS-NPs removal efficiency of Zn-Co-ZIF and Zn-Co-Ni(60 mg)-ZIF were > 83.55 % and > 87.80 %, respectively, in irrigation water, watermelon, strawberry, and carrot, meeting the environmental and food safety. Therefore, this work provides valuable insights into the modification of MOFs to improve the elimination of micropollutants in water, fruit, vegetable, as well as a comprehensive comprehension of the correlations between structure and performance.

Experimental and modeling of potassium diclofenac uptake on activated carbon.

The presence of pharmaceuticals in wastewater resulting from human activities has driven researchers to explore effective treatment methods such as adsorption using activated carbon (AC). While AC shows promise as an adsorbent, further studies are essential to comprehend its entire interaction with pharmaceuticals. This article investigates the adsorption of potassium diclofenac (PD) onto AC using experimental and modeling approaches. Batch adsorption studies coupled with Fourier transform infrared spectroscopy (FTIR) were employed to clarify the adsorption mechanism of PD on AC. Various kinetic and isotherm adsorption models were applied to analyze the adsorbent-adsorbate interaction. The kinetics were best described by Avrami's fractional order (AFO) nonlinear model. Also, the intraparticle diffusion (IP) model reveals a three-stage adsorption process. The experimental equilibrium data fitted well with the three-parameter nonlinear Liu model, indicating a maximum adsorption capacity (Qmax) of 88.45mgg-1 and suggesting monolayer or multilayer adsorption. Thermodynamic analysis showed favorable adsorption (ΔG° < 0), with an enthalpy change (ΔH° = -30.85kJmol-1) characteristic of physisorption involving hydrogen bonds and π-π interactions. The adsorption mechanism was attributed to forming a double layer (adsorbate-adsorbent and adsorbate-adsorbate).

Polydopamine coating for enhanced electrostatic adsorption of methylene blue by multiwalled carbon nanotubes in alkaline environments

The removal of dye molecules in alkaline environments is an issue that should receive increased attention. In this study, the interaction mechanism between polydopamine-modified multiwalled carbon nanotubes (P-MWCNTs) and multiwalled carbon nanotubes (MWCNTs) with the cationic dye methylene blue (MB) in alkaline environments was explained in depth by adsorption, spectroscopy, and density functional theory (DFT). The mechanism of action and dominant forces between the adsorbent and adsorbate were analyzed graphically by introducing energy decomposition analysis (EDA) and an independent gradient model (IGM) into the DFT calculations. In addition, the force distribution was investigated through an isosurface. Moreover, batch adsorption studies were conducted to evaluate the performance of MWCNTs and P-MWCNTs for MB removal in alkaline environments. The maximum MB adsorption capacities of the MWCNTs and P-MWCNTs in solution were 113.3 mg‧g−1 and 230.4 mg‧g−1, respectively, at pH 9. The IGM and EDA showed that the better adsorption capacity of the P-MWCNTs originated from the enhancement of the electrostatic effect by the proton dissociation of polydopamine. Moreover, the adsorption of MB by MWCNTs and P-MWCNTs in alkaline environments was governed by dispersion and electrostatic effects, respectively. Through this study, it is hoped that progress will be made in the use of DFT to explore the mechanism of adsorbent-adsorbate interactions.

Adsorption-reduction behavior of hexavalent chromium on Fe3O4-graphene oxide surface with special implication on environmental remediation

Magnetic graphene hybrid has encouraging application in environmental remediation. Herein, adsorption-reduction behavior of Cr (Ⅵ) on Fe3O4-graphene oxide (Fe3O4-GO) surface was inspected via both theoretical and experimental methods. In the theoretical level, acidity impact on Cr (Ⅵ) species evolution and Cr (Ⅵ)→Cr (Ⅲ) reduction was investigated with the methodology of equilibrium thermodynamics. In the experimental level, adsorption fittings and spectroscopic tests (FTIR, UV–Vis, XPS) were performed to clarify the adsorbent-adsorbate interaction. Results indicate: (1) Under acidic condition, Cr (Ⅵ) is adsorbed by Fe3O4-GO in the form of H2CrO4. Afterwards, over three-fifths of H2CrO4 is reduced to Cr (Ⅲ), reaching adsorption percent and quantity 95.53 % and 382.10 mg g−1 in 10 min (2) Under basic condition, Cr (Ⅵ) is adsorbed in the form of CrO42− without reduction. In the context of environmental remediation, two special implications are thereby procured. (1) Under acidic condition, Cr (Ⅵ) removal efficiency at the first cycle is maximized, while magnetic recovery is deteriorated, resulting in unsatisfactory cyclic performance. (2) Under basic condition, Cr (Ⅵ) removal efficiency is inferior to that under acidic condition, while magnetic recovery is guaranteed, leading to satisfactory cyclic performance. Exploration of this work provides reference especially for developing Fe3O4-GO architecture towards efficient sequestering of hexavalent chromium.

Reaching Quantum Accuracy in Predicting Adsorption Properties for Ethane/Ethene in Zeolitic Imidazolate Framework-8 at Low Pressure Regime.

Nanoporous materials in the form of metal-organic frameworks such as zeolitic imidazolate framework-8 (ZIF-8) are promising membrane materials for the separation of hydrocarbon mixtures. To compute the adsorption isotherms in such adsorbents, grand canonical Monte Carlo simulations have proven to be very useful. The quality of these isotherms depends on the accuracy of adsorbate-adsorbent interactions, which are mostly described using force fields owing to their low computational cost. However, force field predictions of adsorption uptake often show discrepancies from experiments at low pressures, providing the need for methods that are more accurate. Hence, in this work, we propose and validate two novel methodologies for the ZIF-8/ethane and ethene systems; a benchmarking methodology to evaluate the performance of any given force field in describing adsorption in the low-pressure regime and a refinement procedure to rescale the parameters of a force field to better describe the host-guest interactions and provide for simulation isotherms with close agreement to experimental isotherms. Both methodologies were developed based on a reference Henry coefficient, computed with the PBE-MBD functional using the importance sampling technique. The force field rankings predicted by the benchmarking methodology involve the comparison of force field derived Henry coefficients with the reference Henry coefficients and ranking the force fields based on the disparities between these Henry coefficients. The ranking from this methodology matches the rankings made based on uptake disparities by comparing force field derived simulation isotherms to experimental isotherms in the low-pressure regime. The force field rescaling methodology was proven to refine even the worst performing force field in UFF/TraPPE. The uptake disparities of UFF/TraPPE improved from 197% and 194% to 11% and 21% for ethane and ethene, respectively. The proposed methodology is applicable to predict adsorption across nanoporous materials and allows for rescaled force fields to reach quantum accuracy without the need for experimental input.

Pressure dependence of electrochemical CO2 reduction to ethanol in flow cell

The electroreduction of carbon dioxide (CO2RR) to ethanol is an ideal approach for using released CO2 and producing fuel. The impact of CO2 partial pressure (PCO2) on ethanol synthesis was significant but was not well understood. The present study investigates the pressure dependence of ethanol formation from 0.25 bar to 15 bar in a high-pressure flow cell. It displayed a volcano type. The PCO2 associated with the peak Faradaic efficiency (FE) of ethanol increased as the current density increased. The pressure peak values were 0.5 bar, 0.75 bar at 100, 200, and 300 mA cm−2, respectively. Ethanol formation kinetics was affected by adsorbate-adsorbate interactions and the physical blocking of active sites on catalysts with excess CO2. Besides, ethanol synthesis using the configured flue gas, i.e., low-concentration CO2 (VCO2 = 5–30%), was realized via system pressurization from 5 to 20 bar at 200 mA cm−2. By optimizing the combination of CO2 concentration and total pressure (Ptot), the fuel efficiency (FE) of ethanol was maximized to 26.5%. This study uncovers the ideal CO2 partial pressure for ethanol synthesis under industrial current densities, hence presenting potential industrial applications for utilizing flue gas directly in ethanol production.

Hydrogen and NH3 co-adsorption on Pd–Ag membranes

Ammonia (NH3) represents a carbon-free hydrogen carrier that can be used as a zero-emission fuel in the maritime and heavy transport sector with hydrogen fuel cells. To use ammonia as feedstock, the hydrogen must be recovered through NH3 decomposition into H2 and N2. Ammonia decomposition by a membrane-enhanced reactor would inherently produce high-purity H2 through the membrane avoiding the need for a costly hydrogen separation/purification unit. Pd-based membrane reactors can obtain full ammonia decomposition and show significantly higher conversion than conventional reactors. However, the H2 permeability is found to be inhibited in the presence of NH3. A further fundamental understanding of the long-term stability and performance of the Pd-based membranes under exposure to NH3 is therefore required. In the current work, the adsorbate-adsorbate interactions during co-adsorption, the influence the presence of NH3 has on the hydrogen dissociation kinetics, and surface segregation effects in the presence of NH3 and/or hydrogen on the surface of Pd and Pd3Ag are investigated in detail using density functional theory calculations. We find that both the hydrogen surface coverage and dissociation kinetics are hindered by the presence of NH3 on the surface, and possible segregation of Ag towards the surface in the presence of NH3, which could explain the reduced hydrogen permeation in NH3.

A generic statistical model of absorbate diffusion in an alloy

The understanding of absorbate diffusion in alloys is now far from complete because this complex process is influenced by a multitude of factors. Herein, the dependence of the chemical diffusion coefficient, D, on the fraction of sites occupied by absorbate, θ, and fraction of the added metal, f, is scrutinized in the random-alloy case by using a statistical model taking the adsorbate-metal and adsorbate-adsorbate interactions in the ground and thermally activated transition states into account. The parameters employed in the calculations are chosen by aiming at the systems such as hydrogen in an alloy formed by Pd and another metal with f≤0.5. The dependence of D on θ is found to be complex if f is small (e.g., 0.1). In particular, D first decreases and then increases with increasing θ due to the interplay of the H-M and H-H interactions. For larger f, D is predicted to increase appreciably with increasing θ primarily due to the domination of the H-M interaction in the ground state.