Differences In Adsorption Characteristics Research Articles

Two-Dimensional MXene Flakes with Large Second Harmonic Generation and Unique Surface Responses.

MXenes are a family of two-dimensional (2D) materials with broad and varied applications in biology, materials science, photonics, and environmental remediation owing to their layered structure and high surface area-to-volume ratio. MXenes have exhibited significant nonlinear optical characteristics, which have been primarily explored in the context of photonics applications, yet the second-harmonic generation (SHG) behavior of MXenes remains an unexplored aspect of their optical properties. Herein, we demonstrate and quantify large second-order responses of 2D Ti3C2Tx MXenes both in aqueous solutions and on a silicon substrate for the first time. MXene flakes showed strong second-harmonic scattering (SHS) in a dilute suspension with a sensitivity of less than 0.1 μg/mL. Angle-dependent SHS experiments further found that the second-order responses originate from coherent 2D dipole radiation. Through confocal and atomic force microscopies, we found that the intense SHG signal from free-standing MXene flakes increases exponentially with decreasing thickness, while two-photon fluorescence increases linearly with thickness. The second-order susceptibility of the MXenes was determined to be 3.6 pm V-1 with a thickness of 10 nm, almost twice of that for an often-used SHG crystal, beta barium borate. We further explored surface properties of the MXene sheets by investigating the SHS responses upon addition of organic dye molecules to the system. It was found that the adsorption of crystal violet (CV) obeys a Langmuir adsorption model while the addition of malachite green (MG) resulted in almost no change in SHG intensity, even though the adsorption capacities for both CV (61.3 ± 1.7 mg/g) and MG (54.8 ± 2.8 mg/g) are similar. Such a stark difference in adsorption characteristics between cationic organic CV and MG dyes is likely due to their distinct orientational orderings on the MXene surfaces. This work opens many possibilities for the further employment of the family of 2D materials in photonics, optics, and surface catalysis applications.

Thermodynamic analysis of the difference in adsorption characteristics of CH4 and CO2 on continental shale

In order to explain the difference in adsorption characteristics of CH4 and CO2 on continental shale from the perspective of thermodynamics, the isothermal adsorption experiments of CH4 and CO2 adsorbed by shale in Yanchang Formation in Ordos Basin were carried out, and the excess adsorption capacity was corrected to absolute adsorption capacity. Then the Clausius-Clapeyron equation was used to analyze the isosteric heat of adsorption of CH4 and CO2 on shale. The results show that, for calculating the absolute adsorption capacity, Ozawa empirical formula or Van der Waals approximation method should be used to calculate the adsorption phase density. The absolute adsorption capacity should be selected as the basic data for calculating the isosteric heat of adsorption. The reason is that the excess isosteric heat of adsorption has a negative value in the low adsorption capacity stage, which is contradictory to the fact that the adsorption process is exothermic, and is significantly higher than the absolute isosteric heat of adsorption. There is a good linear positive correlation between the isosteric heat of adsorption and the adsorption amount of CH4 and CO2 adsorbed by continental shale, and the isosteric heat of adsorption of CH4 is greater than that of CO2. The absolute initial isosteric heat of adsorption of CH4 and CO2 adsorbed by shale is 52.04 kJ/mol and 27.71 kJ/mol, indicating that the adsorption force of CH4 on Yanchang Formation shale is stronger than that of CO2.

Adsorption of singlet and triplet oxygen on B-doped graphene: adsorption and electronic characteristics.

The density functional calculations of electronic and structural properties of the adsorption of dioxygen on boron-doped graphene surfaces are conducted using spin-polarized density functional theory methods, including van der Waals correction. The results show significant differences in the adsorption characteristics of singlet and triplet oxygen on boron-doped graphene surfaces. Both triplet and singlet show only weak attraction to intrinsic and singly doped graphene. The singlet oxygen adsorption on doped graphene shows fascinating features involving chemisorption with dioxetane ring formation with appreciable charge transfer. In contrast, the triplet oxygen is only weakly physisorbed on the boron-doped surfaces. Chemisorption of singlet oxygen occurs with noticeable charge transfer and leads to almost featureless band structures, while the triplet oxygen physisorption proceeds with a well-defined band structure. Chemisorption of the singlet oxygen is attributed to the enormous mixing of π* of dioxygen and the p-orbitals of dopant and carbon. Because of the difference in adsorption characteristics, chemically modified graphene can find use in detecting and trapping singlet oxygen, which has potential applications in photodynamic therapy.

Characteristics of Gaseous/Liquid Hydrocarbon Adsorption Based on Numerical Simulation and Experimental Testing.

Hydrocarbon vapor adsorption experiments (HVAs) are one of the most prevalent methods used to evaluate the proportion of adsorbed state oil, critical in understanding the recoverable resources of shale oil. HVAs have some limitations, which cannot be directly used to evaluate the proportion of adsorbed state oil. The proportion of adsorbed state oil from HVA is always smaller than that in shale oil reservoirs, which is caused by the difference in adsorption characteristics of liquid and gaseous hydrocarbons. The results of HVA need to be corrected. In this paper, HVA was conducted with kaolinite, an important component of shale. A new method is reported here to evaluate the proportion of adsorbed state oil. Molecular dynamics simulations (MDs) of gaseous/liquid hydrocarbons with the same temperature and pressure as the HVAs were used as a reference to reveal the errors in the HVAs evaluation from the molecular scale. We determine the amount of free state of hydrocarbons by HVAs, and then calculate the proportion of adsorbed state oil by the liquid hydrocarbon MD simulation under the same conditions. The results show that gaseous hydrocarbons adsorptions are monolayer at low relative pressures and bilayer at high relative pressures. The liquid hydrocarbons adsorption is multilayer adsorption. The adsorption capacity of liquid hydrocarbons is over 2.7 times higher than gaseous hydrocarbons. The new method will be more effective and accurate to evaluate the proportion of adsorbed state oil.

Adsorption characteristics and controlling factors of marine deep shale gas in southern Sichuan Basin, China

Deep shale gas (3500–4500 m) is the important replacement field of shale gas production growth in the future China. The research on key parameters of deep shale-gas reservoir is critical to determine its basic geological characteristics and establish a suitable development mode. In order to clarify the adsorption characteristics and controlling factors of deep shale gas in Longmaxi Formation, the main tests such as high-pressure methane adsorption, low-temperature nitrogen and carbon dioxide adsorption coupled with the adsorption fitting model and comparative analysis were conducted. The results show that the adsorption isotherms of deep shale gas also have a downward trend in spite of the higher pressure, and there is no obvious difference in adsorption characteristics, which is mainly due to the similar characteristics of microscopic pore-structure between deep shale and shallower shale. It is found that different adsorption models can well fit the experimental adsorption curve of deep shale gas, but the absolute adsorption capacity converted from excess adsorption capacity shows the same fitting result, i.e., DA-LF model > DR model > Langmuir model. Furthermore, DR model based on micropore filling theory is more suitable for characterizing the adsorption law of deep shale gas combined with the correlation analysis between pore structure and adsorbed-gas capacity. In addition, TOC is the key material factor controlling the adsorption capacity, and specific surface area of micropore is the key spatial factor. Compared to shallower shale, the deep shale shows higher siliceous content, lower calcite content, lower TOC content and lower adsorbed-gas content (the proportion of adsorbed-gas is about 30%).

Adsorption characteristics and mechanisms of Pb2+ and Cd2+ by a new agricultural waste-Caragana korshinskii biomass derived biochar.

In order to explore the comprehensive utilisation and recycling technology of Caragana korshinskii resources, a new agricultural biomass waste, 15 kinds of Caragana korshinskii biochar (CB) were prepared by controlling the pyrolysis temperature and time at the anaerobic environment. Moreover, we pay more attention to deriving the adsorption mechanisms and exploring the difference in adsorption characteristics of Pb2+ and Cd2+. The optimal preparation conditions and the batch adsorption experiments were evaluated, and the adsorption characteristics and mechanisms were discussed using 8 theoretical adsorption models and multiple characterisation methods. The results showed that the CB prepared at 650°C for 3h presented the best performance. The Langmuir and Freundlich models can well simulate the isotherm adsorption process of CB for Pb2+ and Cd2+, respectively. The adsorption kinetics of CB for Pb2+ and Cd2+ were best fitted by the pseudo-second-order model. The adsorption equilibrium for Pb2+ and Cd2+ was reached within 3h, and their maximum adsorption capacity reached 220.94mgg-1 and 42.43mgg-1, respectively. In addition, the best addition amount was 3gL-1 and 2.2gL-1 for Pb2+ and Cd2+, respectively. The optimum pH range was 3-6 for Pb2+ and 6-7.5 for Cd2+. The adsorption mechanisms of CB for Pb2+ and Cd2+ were physicochemical composite adsorption processes, mainly including physical sorption on surface sites, intraparticle diffusion, electrostatic adsorption, ion/ligand exchange, cationic-π interactions, surface complexation and precipitation. Furthermore, the ash of CB also presented a positive effect on the adsorption of Pb2+. Compared with other cellulose- and lignin-based biomass materials, CB showed low cost and efficient performance without complicated modification conditions. Therefore, this study demonstrates that CB is a promising raw material in water pollution control to immobilise heavy metals.

Asymmetric transmembrane transfer caused by a difference in adsorption characteristics at interfaces

A surface adsorption mechanism is proposed for the effect of asymmetric diffusion permeability of a modified membrane. A model is developed to describe diffusion transfer through the modified membrane with regard to the finiteness of the rate of adsorption on external membrane surfaces. Expressions are derived for diffusion fluxes as functions of transfer direction, and asymmetry coefficient is calculated for the diffusion permeability of the membrane. It is shown that the difference in the kinetic properties of the external surfaces of the membrane, which results from the modification of one of the surfaces, may induce the effect of asymmetric diffusion permeability.

Thermodynamic limits to thermal regeneration in adsorption cooling cycles

Energy and exergy models for ideal adsorption cycles with isothermal beds and no mass recovery are developed to predict the limits to COP enhancement using thermal regeneration. The models are applied to compare the performance of zeolite–water and silica gel–water adsorbent–refrigerant pairs over a range of maximum bed temperatures. The thermodynamic consistencies of several alternate adsorption property assumptions are quantified. Differences in adsorption characteristics between zeolite–water and silica gel–water result in a significantly larger potential to enhance COP by implementing thermal regeneration for zeolite–water. Based on COP, the zeolite–water pair is preferred when both thermal regeneration and a high temperature thermal energy source (>150 °C) are used, while the silica gel–water pair is preferred when thermal regeneration is not used and/or a low temperature thermal energy source (<100 °C) is used.

Flexibility in metal-organic framework materials: Impact on sorption properties

Recent years have seen the development of a new class of porous coordination polymers known collectively as metal organic framework materials (MOFs). This review outlines recent progress in understanding how adsorption characteristics of these systems differ from rigid classical sorbents such as activated carbon and zeolites. Gas/vapor adsorption studies for characterization of the porous structures of MOF materials are reviewed and differences in adsorption characteristics based on detailed measurement of equilibrium and dynamical sorption behavior, compared with previous generations of sorbents, are highlighted. The role of framework flexibility and specific structural features, such as windows and pore cavities, within the MOF porous structures are discussed in relation to adsorption mechanisms.

Contrasting nitrate adsorption in Andisols of two coffee plantations in Costa Rica.

Fertilizer use in coffee plantations is a suspected cause of rising ground water nitrate concentrations in the ground water-dependent Central Valley of Costa Rica. Nitrate adsorption was evaluated beneath two coffee (Coffea arabica L.) plantations in the Central Valley. Previous work at one site had identified unsaturated zone nitrate retardation relative to a tritium tracer. Differences in nitrate adsorption were assessed in cores to 4 m depth in Andisols at this and one other plantation using differences in KCl- and water-extractable nitrate as an index. Significant adsorption was confirmed at the site of the previous tracer test, but not at the second site. Anion exchange capacity, X-ray diffraction data, extractable Al and Si, and soil pH in NaF corroborated that differences in adsorption characteristics were related to subtle differences in clay mineralogy. Soils at the site with significant nitrate adsorption showed an Al-rich allophane clay content compared with a more weathered, Si-rich allophane and halloysite clay mineral content at the site with negligible adsorption. At the site with significant nitrate adsorption, nitrate occupied less than 10% of the total anion adsorption capacity, suggesting that adsorption may provide long-term potential for mitigation or delay of nitrate leaching. Evaluation of nitrate sorption potential of soil at local and landscape scales would be useful in development of nitrogen management practices to reduce nitrate leaching to ground water.

Adhesion of two bacteria onto dolomite and apatite: their effect on dolomite depression in anionic flotation

Bacteria can adhere to mineral surfaces and affect subsequent flotation of the minerals. In the present study, the adhesion of Bacillus subtilis and Mycobacterium phlei onto dolomite and apatite was studied by sorption measurements and scanning electron microscopy. The effect of the microorganisms on the oleate flotation of the minerals was then compared at several different pH values. It was found that both B. subtilis and M. phlei adhered to dolomite more readily than onto apatite at acidic and near neutral pH values . At more basic pH values B. subtilis adheres more readily onto and remains a better depressant for dolomite than for apatite. However, at basic pH values, M. phlei adsorbs more onto apatite than onto dolomite with the result that it is a weaker depressant for dolomite, but a stronger depressant for apatite than B. subtilis. The differences in adsorption characteristics are attributed to the different surface characteristics of the two bacteria species and of the two minerals. Both possess acidic isoelectric points. However, B. subtilis has a greater affinity for Mg(II) ions than does M. phlei. Thus, B. subtilis should adsorb more strongly onto dolomite through Mg sites than M. phlei. M. phlei, however, has a more hydrophobic surface. The result is that both species adsorb onto dolomite and function as dolomite depressants and also function as apatite depressants, albeit as weaker depressants. The net results show that, while both function as depressants in anionic collector flotation of dolomitic phosphate ores, B. subtilis functions as the stronger depressant, especially for dolomite. Implications in the anionic flotation of apatite from dolomite are discussed.

Adsorption characteristics of synthesized mordenite membranes

Nitrogen adsorption at 77 K was measured for a self-supporting mordenite membrane synthesized on and detached from, a polytetrafluoroethylene (PTFE) substrate, and for a simultaneously formed powder. The adsorption by the membrane saturated an amount which was about half that found for adsorption by the powder. To examine this difference in adsorption characteristics, both of powder and membrane were measured for adsorption for water, methanol,n-hexane andtert-butyl alcohol. For methanol,n-hexane andtert-butyl alcohol, the powder and membrane both showed the same adsorption behavior as for nitrogen. When water molecules were adsorbed, however, there was no difference in saturated adsorption amount between powder and membrane, suggesting the presence of “deformed pores” in the rectangular crystal layer of the mordenite membrane. The deformed pores are assumed to occur as gaps among crystallites which displaced along theab planes of the rectangular crystals, forming the rectangular crystal layer. The channel bonding sites resulting from such displacement were found to serve as pore walls against adsorbable molecules which have a cross-sectional area greater than that of the water molecule. and so to hinder their entrance into the deformed pores. X-ray diffraction analysis revealed that the rectangular crystals have no periodic, continuous structure along thec-axis; they show a slight distortion of the crystallite shift from the plane perpendicular to thec-axis.

Catalytic reaction and fouling effects on intraparticle diffusion

A pellet of gamma alumina with a unimodal pore structure and a mean pore radius of 47 Å was exposed to cycles involving counterdiffusing butenes and helium, counterdiffusing nitrogen and helium, and varying catalyst treatments. When butenes were present and the catalyst was active, the isomerization of 1-butene to cis- and trans-2-butene occurred. In the nonreacting nitrogen-helium system, the helium diffusion rate was about 11% less through an air-fouled catalyst than through the catalyst immediately after activation; it was about 13% less through the catalyst when butene-fouled than through the freshly activated catalyst. In the reacting butenes-helium system, the helium diffusion rate was about 17% less through the air-fouled catalyst than through the freshly activated catalyst; it was about 32% less through the butene-fouled catalyst than through the freshly activated catalyst. Only in the freshly activated catalyst was the helium diffusion rate in the reacting system predictable from the helium diffusion rate in the nonreacting system. It is proposed that the diffusion behavior differences between the reacting and nonreacting systems result from differences in adsorption characteristics of the fouled and active surfaces. The differences in adsorption characteristics in turn may cause different degrees of pore blockage. The experimental temperature was 154 °C, and the pressure range was 1–2.5 atm for the butenes-helium system, and 1–13 atm for the nitrogen-helium system.