Multilayer Adsorption Behavior Research Articles

Pseudocapacitive Storage in Nanolayered Ti2NTxmxene Using Mg-Ion Electrolyte

Electrochemical supercapacitors are hybrids of a capacitor and battery that rely on materials capable of storing charges via faradaic redox reactions or pseudocapacitive reactions in addition to conventional electrostatic double-layer charge storage. MXenes, a relatively new class of two-dimensional (2D) transition metal carbides and nitrides, are ideal candidates for supercapacitors due to their high electronic conductivity, high surface area, and ability to store charges viapseudocapacitive mechanisms. Nitride MXenes such as Ti2NTxare predicted to have higher pseudocapacitance than carbide MXenes but have not been explored experimentally. Here, we report on the synthesis, characterization, and pseudocapacitive charge storage mechanism in the Ti2NTxnitride MXene. Successful formation of nanolayered Ti2NTxMXene is characterized by XRD, SEM, and N2physisorption analyses. The identity of the surface terminating groups Txare assigned to primarily O and/or OH based on Raman, FTIR, and STEM-EELS. When tested in various electrolytes, the nanolayered Ti2NTxMXene exhibits pronounced reversible redox peaks and high areal capacitances (~1350 uF cm–2 in 1 M MgSO4aqueous electrolyte) well exceeding that expected from a double-layer charge storage (~50 uF cm–2) indicating that charge is stored in the material viaa pseudocapacitive mechanism. We report a trend in the capacitance as a function of cation as follows: Mg2+> Al3+> H+> Li+> Na+> K+, that matches theoretical predictions. Remarkably, nanolayered Ti2NTxMXene exhibits >200 F g–1 capacitance over a 1.0 V range in the Mg-ion electrolyte, and the capacitance increases to 160% of its initial value after 1000 cycles owing to the two-electron reaction and the unique multilayer adsorption behavior of the Mg2+cation on the Ti2NTxMXene. These findings identify Ti2NTxMXene as a new pseudocapacitive material that possesses high capacitance and wide working voltage in a safe and environmentally friendly Mg-ion electrolyte.

Equilibria of lignocellulose biosorbents for gas dehydration

Gas dehydration is a critical process in petro-chemical industries. High energy consumption and costs associated with the current industrial dehydration processes lead to an incentive to develop alternative solutions. Recently, it was demonstrated that dehydration of natural gas was successfully done using the oat hull based biosorbent. However, in-depth analysis of equilibrium of water adsorption by the biosorbent is essential to be further done to elucidate the adsorption characteristics. In this work, water vapor adsorption data was analyzed using various isotherm models in order to obtain key equilibrium parameters such as surface affinity, monolayer adsorption capacity, adsorption heat and the lateral interactions between the adsorbates at various temperatures and pressures, and the effect of these operating conditions on the equilibrium parameters were investigated. The system showed multilayer adsorption behavior at 24 °C and 300 kPa (type III isotherm), while ideal Langmuir behavior (type I isotherm) was seen at higher temperatures (35–50 °C). Redhead isotherms were considered as multilayer adsorption models, while Langmuir, Toth, and Fowler-Guggenhein models were used to fit the rest of the experimental data. Site energy distribution was also determined using the density functional theory. In addition, the isosteric heat of adsorption was determined using various models, and as a function of surface heterogeneity and surface loading. Finally, the specific surface area available for water vapor adsorption were determined. These findings can be very useful for the design and control of the pressure swing adsorption systems, and provide insights on the adsorption mechanisms of lignocellulose biosorbents in dehydration processes.

Redox synergistic Mn-Al-Fe and Cu-Al-Fe ternary metal oxide nano adsorbents for arsenic remediation with environmentally stable As(0) formation.

Arsenic mitigation behavior in aqueous systems is being evaluated for Mn-Al-Fe, Cu-Al-Fe nano adsorbents. Morphological, and vibrational spectroscopy analysis are observed with As-OH, and As-O surface complexes. XPS study of individual As(3d) spectra at different parameters is observed with multiplet peak behavior attributed to redox behavior of Mn-Al-Fe, Cu-Al-Fe. Significant proportions of As(0) signal (∼25 at.% in pH 7, ∼78 at.% in pH 2, ∼58 at.% in pH 12) implicate an environmentally stable behavior of these adsorbents to address the arsenic leaching issue. Adsorption kinetics are observed with Pseudo Second Order (PSO) model, and Freundlich model supported multilayer adsorption behavior is observed for adsorption isotherms. Trace metal voltammetry studies are observed with 75-90 % of As(III) mitigation in aliquot samples. Detailed study of Mn(2p), Cu(2p), Fe(2p), and O(1 s) spectra explains redox active, and surface ligand exchange synergism in arsenic adsorption. Low equilibrium concentrations (Ce < 10 ppb) in As(V) systems (Ci ∼ 100 and 500 ppb) indicate the drinking water application of these systems. Cyclic-voltammetry (CV) studies implicate the mitigation and immobilization of arsenic species onto adsorbent by both reduction, and sorption phenomenon.

Nitrogenated holey graphene C2N monolayer anodes for lithium- and sodium-ion batteries with high performance

The experimentally synthesized layered nitrogenated holey graphene (C2N) nanosheets possess intriguing properties, having broad applications such as in metal-ion batteries and catalysis. However, its impurity and lack of C2N monolayers in experiments and the failed classic method of one single atom searching the stable adsorption site(s) of metal ions on C2N in simulations greatly hinder their mechanistic study, limiting their further development. Herein, by proposing a new particle pair adsorption model and employing first-principles calculations, we investigated mechanisms of the C2N monolayer as a high-performance anode material for sodium-(Na-) and lithium-(Li-) ion batteries. The maximum theoretical capacities of Na and Li ions approach to 2469 and 2939 mA h g−1, respectively, which are 6–8 times larger than those of graphite. A stable multilayer adsorption behavior was found. Based on the results of charge density difference, electron localization function and Bader charge analysis, the microscale mechanism is that the channels between C2N and metal atoms are built up for electron transfer, increasing their interactions. We proposed a new particle (atom) pair diffusion model and found that the metal ions have two stage diffusions. The metallic feature of the monolayer/metal ions complex and the low diffusion barriers of metal ions on the monolayer are the origins of the fast charge/discharge process in the battery. The C2N monolayer also possesses advantages of low average open-circuit voltages (~ 0.45 V) and excellent cycling stability. The work provides fundamental insights for the design and innovation of high-performance energy storage materials.

Ono-Kondo lattice model for propane multilayer adsorption in organic nanopores in relation to shale gas

Accurate characterization of shale gas adsorption capacity is of great importance to the gas-in-place estimation and prediction of well productivity. Propane is one of major constituents in shale gases which can have multilayer adsorption behavior on pore surfaces. While experiments can measure excess adsorption, the absolute adsorption, which describes adsorbed gas capacity, has to be converted from the excess adsorption. In previous works, absolute adsorption is usually calculated from excess adsorption with adsorbed phase volume or density based on the single-layer adsorption model. However, for heavier alkanes, such as ethane and propane, which can have multilayer adsorption behavior, single-layer adsorption model becomes invalid. While molecular simulation can characterize the multilayer adsorption behavior, it comes with expensive computational costs. Currently, there is no viable model to characterize the absolute adsorption of hydrocarbons which can have multilayer adsorption behavior. In this work, we conduct grand canonical Monte Carlo (GCMC) simulation to study propane adsorption in shale nanopores over a wide range of pressures at temperature higher than the critical temperature. We find that propane can form multiple adsorption layers, thus, using single-layer adsorption model to obtain the absolute adsorption becomes unjustifiable. Based on the propane density distribution characteristics, we propose a multilayer adsorption model and use Ono-Kondo (OK) lattice model with multilayer structures to regress propane excess adsorption and subsequently obtain the absolute adsorption in each layer. For propane, we take into account the correlation effect arising from strong adsorbate–adsorbate interactions beyond mean field theory (MFT) and the only predetermined parameters are number of layers and adsorption layer width. The proposed OK model shows an excellent agreement with GCMC simulations on the excess adsorption and absolute adsorption in each layer, with discrepancies less than 6% above 50 bar. The proposed OK model can readily take into account the propane multilayer adsorption behavior, while significantly reducing calculation time. Our method presents a reliable and highly-efficient approach for accurate characterization of adsorption of hydrocarbons with multilayer structures and provide important insights into the gas-in-place estimation in shale.

Monolayer–multilayer adsorption phenomenological model: Kinetics, equilibrium and thermodynamics

A phenomenological model for multilayer liquid phase adsorption in closed and batch system was developed to assess the diffusional resistances and the adsorption rate-limiting steps. Equilibrium, kinetic and thermodynamic results indicated a multilayer adsorption behavior, wherein two adsorption steps were identified: (i) monolayer adsorption, with chemisorption characteristic binding energy due to synergistically electrostatic and H-bond intermolecular interactions; (ii) multilayer adsorption, favored by high dye concentrations and temperatures, however presenting a physical biding energy, related to less intense interactions such as H-bond and van der Waals forces. Additionally, dye aggregation in the liquid phase was observed and investigated. The proposed adsorption model allowed to investigate physically the kinetics, equilibrium and thermodynamics through a unique approach and to elucidate the mechanisms in distinct operational conditions. Hence, this mechanistic and predictive model can be used for the optimization and scale-up of the multilayer adsorption processes.

Comparison on physical, chemical, and adsorption properties of activated carbon derived from different solid wastes

Four activated carbons (ACs) were prepared from four different solid wastes (SWs), including Enteromorpha prolifera, Kraft lignin, hair, and petroleum coke, using KOH as the activating agent. The fabricated ACs were characterized by N2 adsorption, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transformed infrared and X-ray photoelectron spectroscopy to compare their pore structures and surface chemical properties of the ACs. It was found that ACs produced from E. prolifera and Kraft lignin were rich in mesopores, whereas petroleum coke and hair were inclined to produce ACs with more micropores. Meanwhile, ACs derived from E. prolifera possessed the largest surface area of 3,471 m2 g−1 and pore volume of 2.681 cm3 g−1. In addition, ACs prepared from various SWs displayed different chemical nature. Adsorption behavior of hexavalent chromium on the four fabricated ACs was studied as well, and the adsorption data were all well fitted by Freundlich equation, indicating the similar multilayer adsorption behavior of hexavalent chromium on the heterogeneous surface of the ACs.

Polypropylene non-woven meshes with conformal glycosylated layer for lectin affinity adsorption: The effect of side chain length

The unique characteristics of polypropylene non-woven meshes (PPNWMs), like random network of overlapped fibers, multiple connected pores and overall high porosity, make them high potentials for use as separation or adsorption media. Meanwhile, carbohydrates can specifically recognize certain lectin through multivalent interactions. Therefore glycosylated PPNWMs, combing the merits of both, can be regarded as superior affinity membranes for lectin adsorption and purification. Here, we describe a versatile strategy for the glycosylation of PPNWMs. Two hydrophilic polymers with different side chain length, poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(oligo(ethylene glycol) methacrylate) (POEGMA), were first conformally tethered on the polypropylene fiber surface by a modified plasma pretreatment and benzophenone (BP) entrapment UV irradiation process. Then glucose ligands were bound through the reaction between the hydroxyl group and acetyl glucose. Chemical changes of the PPNWMs surface were monitored by FT-IR/ATR. SEM pictures show that conformal glucose ligands can be achieved through the modified process. After deprotection, the glycosylated PPNWMs became superhydrophilic and had high specific recognition capability toward Concanavalin A (Con A). Static Con A adsorption experiments were further performed and the results indicate that fast adsorption kinetics and high binding capacity can be accomplished at the same time. We also found that increasing the side chain length of polymer brushes had positive effect on protein binding capacity due to improved chain mobility. Model studies suggest a multilayer adsorption behavior of Con A.

Waste biomass adsorbents for copper removal from industrial wastewater—A review

Copper (Cu2+) containing wastewaters are extensively released from different industries and its excessive entry into food chains results in serious health impairments, carcinogenicity and mutagenesis in various living systems. An array of technologies is in use to remediate Cu2+ from wastewaters. Adsorption is the most attractive option due to the availability of cost effective, sustainable and eco-friendly bioadsorbents. The current review is dedicated to presenting state of the art knowledge on various bioadsorbents and physico-chemical conditions used to remediate Cu2+ from waste streams. The advantages and constraints of various adsorbents were also discussed. The literature revealed the maximum Cu adsorption capacities of various bioadsorbents in the order of algae>agricultural and forest>fungal>bacterial>activated carbon>yeast. However, based on the average Cu adsorption capacity, the arrangement can be: activated carbon>algal>bacterial>agriculture and forest-derived>fungal>yeast biomass. The data of Cu removal using these bioadsorbents were found best fit both Freundlich and Langmuir models. Agriculture and forest derived bioadsorbents have greater potential for Cu removal because of higher uptake, cheaper nature, bulk availability and mono to multilayer adsorption behavior. Higher costs at the biomass transformation stage and decreasing efficiency with desorption cycles are the major constraints to implement this technology.

Application of a Taguchi L 16 orthogonal array for optimizing the removal of Acid Orange 8 using carbon with a low specific surface area

The conditions for the adsorption of the dye Acid Orange 8 onto carbon were optimized using the Taguchi method. Four samples of activated carbon were obtained from guava seeds, and their textures were characterized by nitrogen adsorption isotherms. The equilibrium adsorption isotherm on mesoporous carbon (114 m 2 g −1) showed multilayer adsorption behavior with a kinetic curve that could be described by the Ho equation. To achieve maximum dye removal, the effects of temperature (10–40 °C), specific surface area (67–143 m 2 g −1), initial concentration (250–1500 mg L −1), pH (2–13), and the mass–volume ratio (10–150 g L −1) were studied using an L 16 orthogonal array. The percent dye removal was transformed into an accurate S/N ratio for a “high is better” response. pH was found to be the most effective factor for promoting dye removal, followed by the mass–volume ratio. A temperature of 10 °C, a specific surface area of 114 m 2 g −1, an initial concentration of 500 mg L −1, pH 2, and a mass/volume ratio of 150 g L −1 were the best conditions determined by the Taguchi method. Finally, the carbon was tested using samples of real and simulated wastewater solutions under the optimal conditions, and color removal efficiencies of 60% and 40%, respectively, were obtained.

Understanding the adsorption mechanism of chitosan onto poly(lactide-co-glycolide) particles

Polyelectrolyte-coated nanoparticles or microparticles interact with bioactive molecules (peptides, proteins or nucleic acids) and have been proposed as delivery systems for these molecules. However, the mechanism of adsorption of polyelectrolyte onto particles remains unsolved. In this study, cationic poly(lactide-co-glycolide) (PLGA) nanoparticles were fabricated by adsorption of various concentrations of a biodegradable polysaccharide, chitosan (0–2.4 g/L), using oil-in-water emulsion and solvent evaporation techniques. The particle diameter, zeta-potential, and chitosan adsorption of chitosan-coated PLGA nanoparticles confirmed the increase of polyelectrolyte adsorption. Five adsorption isotherm models (Langmuir, Freundlich, Halsey, Henderson, and Smith) were applied to the experimental data in order to better understand the mechanism of adsorption. Both particle diameter and chitosan adsorption increased with chitosan concentration during adsorption. A good correlation was obtained between PLGA-chitosan nanoparticle size and adsorbed chitosan on the surface, suggesting that the increased particle size was primarily due to the increased chitosan adsorption. The zeta-potential of chitosan-coated PLGA nanoparticles was positive and increased with chitosan adsorbed until a maximum value (+55 mV) was reached at approximately 0.4–0.6 g/L; PLGA nanoparticles had a negative zeta-potential (−20 mV) prior to chitosan adsorption. Chitosan adsorption on PLGA nanoparticles followed a multilayer adsorption behavior, although the Langmuir monolayer equation held at low concentrations of chitosan. The underlying reasons for adsorption of chitosan on PLGA nanoparticles were thought to be the cationic nature of chitosan, high surface energy and microporous non-uniform surface of PLGA nanoparticles.

Density functional theory of adsorption and phase behavior of the Lennard–Jones fluids confined in MCM-41 with a finite thickness

The adsorption and phase behavior of the Lennard–Jones (LJ) fluid confined in MCM-41 with a finite thickness were calculated by using density functional theory (DFT). In the DFT calculation, a potential model representing the interaction between a fluid molecule and MCM-41 pore of finite thickness was used, and the Tarazona's recipe of weighted density approximation was adopted. We reported the effect of different variables, including pore size, temperature, potential well depth, pore wall thickness and fluid molecular parameters, on adsorption isotherm of the LJ fluid confined in MCM-41 pore. The capillary condensation, hysteresis loop and layering transition of the confined LJ fluids were also investigated. Based on the calculated grand potential of the system studied, the phase transition point was determined. The competition of capillary condensation and layering transition of a LJ fluid in confined space was observed. When the interaction between fluid molecule and pore wall is weak, the capillary condensation is of dominance, and the layering transition vanishes on desorption isotherm. In the case of reduction of molecular size, the multi-layer adsorption behavior of the confined fluid appears. In short, the calculated results give a good insight into the adsorption and phase behavior of the confined fluid.