Energy Distribution Of Adsorption Sites Research Articles

Thermodynamic characteristics of CO2 adsorption on β-cyclodextrin based porous materials: Equilibrium capacity function with four variables

This work focuses on establishing a kind of extended adsorption model equation including four variables relevant to preparation conditions (activation temperature and heating rate) except adsorption situations (adsorption temperature and adsorption pressure). And the equation is used for thermodynamic analysis of CO2 adsorption on a promising cyclodextrin based porous material. Novelty, those isotherms of CO2 adsorption on the porous materials prepared at different activation temperatures and heating rates are experimentally obtained to investigate the effects of activation temperature and heating rate on CO2 adsorption. Then the parameters of the model equation are quantified based on 589 data points from the isotherms by the least squares regression method combined with the steepest descent mode. Research results indicate that high performance for CO2 uptake appears on the porous material prepared at high activation temperature and low heating rate. Differently with the previous studies, the adsorption site energy distribution is found to exhibit an asymmetrical peak and the peak site energy increases with an increase of activation temperature but the decrease of heating rate. Moreover, a series of interesting phenomena of those thermodynamic state parameters via adsorption temperature and pressure are verified based on both extended Slip model and D-R model, which have not been reported before.

Computational study on the effect of steric hindrance in functionalised Zr-based metal-organic frameworks on hydrocarbon storage and separation

ABSTRACT Light hydrocarbon gases, such as methane, ethane, propane, ethylene, and propene, are commonly used in manufacturing processes as raw material or energy fuel. Traditionally, these gases are mainly separated by cryogenic distillation, which requires massive amounts of energy because of high-pressure and low-temperature operating conditions. Many studies have shown that metal–organic frameworks (MOFs) are potential materials for gas storage and separation because of their unique properties, such as high surface area, nontoxicity, and large gas storage capacity. UiO-66 and its modified materials are considered ideal candidates for gas storage and separation and have attracted increased attention since they can be easily synthesized and are thermally and chemically stable. In this study, we used grand canonical Monte Carlo (GCMC) to simulate and understand the gas adsorption behaviour of UiO-66 and its variants, including UiO-66-NO2, UiO-66-NH2, UiO-66-Br, and UiO-66-(CH3)2, for pure light hydrocarbon gases and their binary mixtures. The adsorption isotherm and adsorption site energy distribution are analysed and discussed to identify the best candidate for gas adsorption and separation. It is found that the modified form of UiO-66-NH2 has good selectivity, large gas storage potential, and high working capacity compared to UiO-66. Further insight into favourable gas adsorption positions is discussed.

Adsorption of fulvic acid onto polyamide 6 microplastics: Influencing factors, kinetics modeling, site energy distribution and interaction mechanisms

Information on the interactions of microplastics (MPs) with dissolved organic matter (DOM) is essential for understanding their environmental impacts. This study selected fulvic acid (FA) as a typical DOM to investigate the influence of contact time, temperature, dosage, solution pH, salinity, and coexisting metal ions on the adsorption of FA onto polyamide 6 (PA6) MPs. The adsorption kinetic and isotherm can be successfully described by mixed-order (MO) and Freundlich models. The adsorption site energy distribution based on the Freundlich equation was applied to analyze the interaction between FA and PA6-MPs and the adsorption site heterogeneity. Thermodynamic analysis demonstrated that the values of parameters (ΔGads°, ΔSads°, ΔHads°) were significantly affected by initial solution concentrations and the adsorption process was spontaneous, endothermic, and randomness-increased. Fourier transform-infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) revealed the importance of amide functional groups of PA6-MPs in controlling FA adsorption. Hydrogen bonds, hydrophobic, electrostatic, and n-π electron donor-acceptor (n-π EDA) interactions played different roles on adsorption of FA under different conditions of solution chemistry. These findings are beneficial to provide new insights involving the adsorption behavior and interaction mechanisms of FA onto PA6-MPs for the environmental risk assessment of MPs.

Analysis of the performance of zirconia-multiwalled carbon nanotube nanoheterostructures in adsorbing As(V) from potable water from the aspects of physical chemistry with an emphasis on adsorption site energy distribution and density functional theory calculations

Abstract This study probes the performance of zirconia-multiwalled carbon nanotube nanoheterostructures in adsorbing As(V) from potable water as an adsorbent, and at the same time investigates into the physical chemistry governing the adsorption process. Zirconia (ZrO2) present in this heterostructure is in the monoclinic phase and is supposed to contain oxygen vacancies. The cumulative pore volume of the adsorbent is 0.602 cm3 g−1. Here the adsorption of As(V) is endothermic and is extremely favourable with more than 99.4% removal efficacy for initial As(V) concentrations between 50 and 1000 ppb at 30, 40, and 50 °C temperatures with 0.7 g L−1 dose. The Toth and the D-R isotherm models are found describing the adsorption process decently. The approximate adsorption site energy is found to increase with the increase in temperature for a certain initial As(V) concentration, and decrease with the increase in initial As(V) concentration at a given temperature. Both the Toth and D-R isotherm based approximate adsorption site energy distribution ( f ( E ∗ ) ) follows a unimodal quasi-normal pattern. Furthermore, it is found that the surface heterogeneity-related parameter ( n T ) in the Toth isotherm based f ( E ∗ ) should only be altered with ( 1 − n T ) when n T

Evaluation of the performance of zirconia-multiwalled carbon nanotube nanoheterostructures in adsorbing As(III) from potable water from the perspective of physical chemistry and chemical physics with a special emphasis on approximate site energy distribution.

In this study, the performance of zirconia-multiwalled carbon-nanotube nanoheterostructure in adsorbing the highly toxic water-contaminant As(III) from water has been probed from the perspective of physical chemistry and chemical physics. The adsorbent found extremely efficient in adsorbing As(III) from potable water. Moreover, its ability to oxidize As(III) to As(V) in the aqueous solution has been evinced by the XPS studies. The values of the maximum adsorption capacities (qm) depend on the isotherm studied and in this study, no wonder different values of qm are obtained for different adsorption isotherms. The thermodynamic studies advocate the exothermic and spontaneous nature of the adsorption process. Calculation on density functional theory (DFT) also suggested the exothermic nature of the adsorption process. DFT calculation further revealed the role of the Zr-O and Zr-OH bridges in binding As(III) species on the zirconia surface. However, this study finds an adverse effect of visible light-irradiation on the adsorption process. Furthermore, this study propounds an approach to estimate the maximum solubility of As(III) in water combining the Cerofolini's condensation-approximation and Polanyi adsorption potential. Detailed analysis on the approximate adsorption site energy distribution (f(E*)) further finds an inconsistency in the formula used to estimate qm using f(E*), which underestimates qm. The inconsistency, for the very first time, has successfully been resolved by modifying the heterogeneity related parameter in f(E*).

Prediction of hydrogen adsorption in nanoporous materials from the energy distribution of adsorption sites

We present a fast and accurate, semi-analytical method for predicting hydrogen adsorption in nanoporous materials. For any temperature and pressure, the adsorbed amount is calculated as an integral over the energy density of adsorption sites (guest-host interactions) plus an average guest-guest term. The guest-host interaction energy is calculated using a classical force field with hydrogen modelled as a single-site probe. The guest-guest interaction energy is approximated using an average coordination number, which is regressed using Gaussian Process Regression (GPR). Local adsorption at each site is then modelled using a Langmuir isotherm, which when weighted with its probability density gives an accurate description of hydrogen adsorption. The method is tested on 933 metal-organic frameworks (MOFs) from the Computation-Ready Experimental (CoRE) MOF database at 77 K from to 100 bar, and the results are compared against GCMC predictions. To demonstrate the utility of the method, we calculated hydrogen adsorption isotherms for 12,914 existing MOF structures, at two different temperatures at a speed about 100 times that of GCMC simulations and analyzed the results. We found 13 MOFs with predicted deliverable capacities exceeding the DOE target of 50 g/L for adsorption at 100 bar, 77 K and desorption at 5 bar, 160 K.

Synthesis, characterization and application of surface-modified biochar synthesized from rice husk, an agro-industrial waste for the removal of hexavalent chromium from drinking water at near-neutral pH

This study reports the synthesis, characterization and applicability of surface-modified biochar, an eco-friendly and economic adsorbent for the removal of highly toxic hexavalent chromium from drinking water at a pH suitable for drinking water. The acid modification of the pristine biochar prepared by the pyrolysis of rice husk substantially enhanced the carbon content percentage by 25.29% and active sites by 10 times in the acid-modified biochar and produced a BET surface area of 142.78 ± 2.57 m2 g−1. Furthermore, the alteration of the surface of pristine biochar resulted in the substantial increase in C=C, C=O, C–O, phenolic and alcoholic –OH bonds on the surface of the biochar which further facilitated the adsorption of Cr. The batch adsorption study was carried out to comprehend the process of adsorption. The Freundlich isotherm model has been found to be describing the adsorption process most precisely. The maximum adsorption capacity has been found to be 4109 μg g−1 at 30 °C and 4536 μg g−1 at 50 °C from the analysis of approximate adsorption site energy distribution function. The analysis of approximate adsorption site energy distribution function further showed the availability of more energetic sites to the adsorbate Cr(VI) oxyanions at higher temperatures. Further, the endothermic nature of the adsorption apart from the experimental outcomes has been evinced from the D–R isotherm model and from the values of ∆G° and ∆H°. The negative values of ∆G° at all the experimental temperatures indicate the spontaneity of the adsorption process. Furthermore, the values of ∆G° more than − 20 kJ mole−1 indicate the physisorption of Cr(VI) oxyanions on the adsorbent surface.

Analyses of tetracycline adsorption on alkali-acid modified magnetic biochar: Site energy distribution consideration

As a widely used antibiotic, tetracycline has a huge hidden danger to human health. Municipal sludge rich in organic substances has the potential to produce biochar. In this work, the municipal sludge biochar from solid waste was modified by the alkali-acid binding method, and tetracycline was efficiently removed from the aqueous solution, the adsorption removal efficiency reached to 86% at initial concentration of 200 mg/L. The activation energy was determined by analyzing the adsorption kinetics at different temperatures and tetracycline concentrations. The results showed that tetracycline adsorption on modified biochar was endothermic reaction. Presenting the Langmuir-Freundlich model, adsorption site energy distributions was reckoned. The average adsorption site energy and corresponding standard deviation of the adsorption site energy distribution were deduced emphatically to inquiry the strength of tetracycline adsorption on modified biochar and the adsorption site heterogeneity. The method proposed of research further proves that modified biochar from sewage sludge remove tetracycline from contaminated water has great potential, and exploration of tetracycline adsorption mechanisms by quantifying average site energy. The results and methods of this work can be transferred to study water treatment systems.

Energy distribution function based universal adsorption isotherm model for all types of isotherm

Based upon the adsorbate–adsorbent interactions due to pore size distribution and surface heterogeneity, as characterized by the adsorption isotherms, the adsorption phenomenon has many industrial and environmental applications. These adsorption isotherms are very important to define the information related to the equilibrium uptake of adsorbate–adsorbent pair. Due to the presence of different energy distribution of adsorption sites, pore size distribution, surface area availability and surface heterogeneity, of each of the adsorbent–adsorbate pair, these isotherms are categorized into six types by the International Union of Pure and Applied Chemistry and so far, in the literature, there is no generalized adsorption isotherm model available that can define and predict the behavior of all adsorption isotherm types. In this study, a universal adsorption isotherm model is developed based upon the energy distribution function of the available adsorption sites and the pore size. The proposed model is able to define all adsorption isotherm characteristics, irrespective of their multi- or monolayer formations and micro- or meso-pore distribution.

CO₂ Capture with Mesoporous Silicas Modified with Amines by Double Functionalization: Assessment of Adsorption/Desorption Cycles.

CO2 adsorption on mesoporous silica modified with amine by double functionalization was studied. Adsorption microcalorimetry was used in order to investigate the influence of increasing the nitrogen surface density on double functionalized materials with respect to the only grafted materials. The distribution of sites and the rate-controlling mechanism of adsorption were evaluated. A Tian Calvet microcalorimeter coupled to a manometric setup was used to evaluate the energy distribution of adsorption sites and to calculate the thermokinetic parameters from the differential enthalpy curves. CO2 and N2 adsorption equilibrium isotherms at 50 and 75 °C were measured with a magnetic suspension balance, allowing for the computation of working capacity and selectivity at two temperatures. With these data, an Adsorbent Performance Indicator (API) was calculated and contrasted with other studied materials under the same conditions. The high values of API and selectivity confirmed that double functionalized mesoporous silica is a promising adsorbent for the post combustion process. The adsorption microcalorimetric study suggests a change in active sites distribution as the amine density increases. Maximum thermokinetic parameter suggests that physisorption on pores is the rate-controlling binding mechanism for the double-functionalized material.

Chlorobenzene, chloroform, and carbon tetrachloride adsorption on undoped and metal-doped sol–gel substrates (SiO 2, Ag/SiO 2, Cu/SiO 2 and Fe/SiO 2)

Adsorption isotherms of chlorobenzene, chloroform and carbon tetrachloride vapors on undoped SiO 2, and metal-doped Ag/SiO 2, Cu/SiO 2 and Fe/SiO 2 substrates were measured in the temperature range of 398–593 K. These substrates were prepared from a typical sol–gel technique in the presence of metal dopants that rendered an assortment of microporous–mesoporous solids. The relevant characteristic of these materials was the different porosities and micropore to mesopore volume ratios that were displayed; this was due to the effect that the cationic metal valence exerts on the size of the sol–gel globules that compose the porous solid. The texture of these SiO 2 materials was analyzed by X-ray diffraction (XRD), FTIR, and diverse adsorption methods. The pore-size distributions of the adsorbents confirmed the existence of mesopores and supermicropores, while ultramicropores were absent. The Freundlich adsorption model approximately fitted the chlorinated compounds adsorption data on the silica substrates by reason of a heterogeneous energy distribution of adsorption sites. The intensity of the interaction between these organic vapors and the surface of the SiO 2 samples was analyzed through evaluation of the isosteric heat of adsorption and standard adsorption energy; from these last results it was evident that the presence of metal species within the silica structure greatly affected the values of both the amounts adsorbed as well as of the isosteric heats of adsorption.

A New Adsorption Isotherm for Heterogeneous Sorbents

An adsorption isotherm has been suggested for the intended description of the experimental isotherms for the adsorption of gases and vapours on heterogeneous surfaces. Two parameters of the suggested equation, which characterize the heterogeneity of a sorbent, were expressed in the form of explicit functions of temperature as well as of variance and the excess coefficient for the corresponding energy distribution of adsorption sites. The presence of such explicit expressions allowed the application of the suggested equation for the description of the dependence of the adsorption not only on pressure but also on temperature. The adsorption of N2 on TiO2 was considered as an example. It was shown that the suggested equation could be used as the thermal equation of state for a real adsorption system over quite wide ranges of temperature and pressure.

Simulation of photodesorption at the Photon Factory

A model of photodesorption which takes place in the electron storage ring and experimental apparatus in a photon beam line is presented. The model is constructed, based on the kinetics of adsorption, thermal desorption, photodesorption and the evacuation of gas phase molecules, taking account of the energy distribution of adsorption sites and the photon flux distribution on the chamber wall. The calculated results are compared with the experimental photodesorption yield obtained at the Photon Factory. The calculated decay curve of the photodesorption yield against the photon dose agrees well with the experimental ones.

Adsorption of CO2 on Glass Fibers

Isotherms of adsorption of CO2 on two different glass fibers are measured at 158 and 185 K by the volumetric method. Heats of adsorption are determined. Experimental data are analyzed in terms of the Langmuir model of independent adsorption sites. Distributions of adsorption sites in free energy (not in energy) are determined at two temperatures, and their temperature dependence is discussed. This dependence makes it possible to obtain the entropy and energy distribution of adsorption sites and the relation of entropy to energy for a site. The latter enables one to assess the reliability of the distributions.

Sorption hysteresis of pyrene on zeolite

Systematic studies were performed to investigate the conditions and the extent of irreversible sorption and sorption hysteresis of a hydrophobic compound on a mineral surface. Adsorption and desorption isotherms of pyrene on zeolite were established. Various solvents were used, such as surfactant solution, 2-propanol, 2-propanol/water (70/30) or a microemulsion. Different adsorption and desorption isotherms were found for pyrene physisorbed on the mineral surface. Dependence of sorption on the adsorbent particle density was observed. Kinetic experiments were carried out in order to determine desorption rates. The curves obtained by fluorescence spectroscopy were fitted to the Henry model (monoexponential fit) which can be obtained from the Langmuir kinetics and to modified models (biexponential fit). The latter fitted the curves perfectly. The dynamics of the equilibrated system were studied by radio-tracer experiments. After the preceding desorption process a portion of pyrene containing solution was replaced by an equal amount of radio-labelled pyrene solution of the same concentration. No exchange could be detected on the time scale of the desorption experiments. The results can be explained in terms of metastable equilibrium. They suggest energy distribution of adsorption sites and high activation energies for at least one of the processes (adsorption or desorption).

Adsorption of 1-olefin/n-paraffin liquid mixtures on NaX and NaY zeolites

The isotherm equation for adsorption of binary liquid mixtures of nonelectrolytes, involving differences in molecular sizes of components and quasi-Gaussian energy distribution of adsorption sites, is used to describe the excess adsorption data for 1-olefin/n-paraffin liquid mixtures on NaX and NaY zeolites at 293 K. This equation gives a good representation of the above adsorption data. Analysis of these data showed that the heterogeneity parameter is slightly dependent on the difference in molecular sizes of both components.