Kinds Of Adsorption Sites Research Articles

Bioinspired polydopamine-sheathed carbon nanotubes as environmentally safe, efficient, and durable adsorbents for organic pollutant capturing via hydrogen bonding

While the potential nanotoxicity of carbon nanomaterials likely renders them unsafe for environmental purification applications, it is still a daunting challenge to clarify the specific driving force for water pollutant capturing. We here employ the well-documented, mussel-inspired, environmentally safe, and biocompatible polydopamine (PDA) to modify the surface of carbon nanotubes (CNTs) which are typical carbon nanomaterials with potential toxicity. The PDA modification extents are finely tuned by adjusting the dopamine-to-CNT ratio, and the optimized modification is achieved in terms of the homogenous formation of the PDA sheath on the CNT surface (corresponding to the 0.4 PC sample). While a limited modification likely causes the surface of bare CNTs to be insufficiently covered by PDA, the excessive modification lowers the adsorption capacity per unit mass of adsorbent. The homogenous modification with PDA bearing different kinds of adsorption sites at the atomic scale makes the isothermal adsorption of Rhodamine B (RhB) suitable to be described by both the Langmuir and Freundlich models, with the maximum monolayer adsorption capacity estimated to be 266.7 mg/g. Besides, the 0.4 PC-based adsorption of RhB is dictated by chemisorption and verified to be a spontaneous, endothermic, entropy-gaining, anti-interference (against common metal cations in tap water), and repeatable process. Importantly, we disclose that hydrogen bonding is the pivotal driving force in the 0.4 PC-based adsorption process, thus enabling 0.4 PC to capture methyl orange (MO) bearing a charge state opposite to RhB, with equilibrium adsorption capacities calculated to be 135 and 159 mg/g for MO and RhB, respectively.

Enhanced CO Adsorption by Modulating the Electron Density Distribution of Graphite-Copper Porphyrin Sorbents with Light.

The photo-responsive adsorption has emerged as a vibrant area, but its current methodology is limited by the well-defined photochromic units and their molecular deformation driven by photo-stimuli. Herein, a methodology of nondeforming photo-responsiveness is successfully exploited. With the exploiting agent of Cu-TCPP framework assembled on the graphite and strongly interacted with it, the sorbent generates two kinds of adsorption sites, over which the electron density distribution of the graphite layer can be modulated at the c-axis direction, which can further evolve due to photo-stimulated excited states. The excited states are stable enough to meet the timescale of microscopic adsorption equilibrium. Independent of the ultra-low specific surface area of the sorbent (20 m2 g-1 ), the CO adsorption capability can be improved from 0.50 mmol g-1 at the ground state to 1.24 mmol g-1 (0 °C, 1 bar) with the visible light radiation, rather than the photothermal desorption.

Enhanced CO Adsorption by Modulating the Electron Density Distribution of Graphite‐Copper Porphyrin Sorbents with Light

AbstractThe photo‐responsive adsorption has emerged as a vibrant area, but its current methodology is limited by the well‐defined photochromic units and their molecular deformation driven by photo‐stimuli. Herein, a methodology of nondeforming photo‐responsiveness is successfully exploited. With the exploiting agent of Cu‐TCPP framework assembled on the graphite and strongly interacted with it, the sorbent generates two kinds of adsorption sites, over which the electron density distribution of the graphite layer can be modulated at the c‐axis direction, which can further evolve due to photo‐stimulated excited states. The excited states are stable enough to meet the timescale of microscopic adsorption equilibrium. Independent of the ultra‐low specific surface area of the sorbent (20 m2 g−1), the CO adsorption capability can be improved from 0.50 mmol g−1 at the ground state to 1.24 mmol g−1 (0 °C, 1 bar) with the visible light radiation, rather than the photothermal desorption.

Iodine nanotrap for highly efficient iodine capture under high temperature

Covalent organic frameworks have been recognized as promising porous materials for the radioactive iodine capture. However, most COFs often suffer from low adsorption capacity under practical conditions (typically ≥ 150 °C, ≤ 150 ppmv I2) due to the lack of strong binding sites and low affinity toward iodine, which restricts their application in industrial relevant conditions. Here, we develop a strategy of constructing highly efficient I2 nanotraps by manipulating two kinds of adsorption sites (ionic binding sites and Lewis binding sites) located at adjacent spatial positions, thereby realizing the synergistic binding toward I2. The obtained I2 nanotrap 4F-iCOF-TpBpy-I- delivers a remarkable I2 uptake capacity of 37 wt% at 150 °C and 150 ppmv of I2, which illustrate a record-high value for all COFs reported so far. This work opens a new avenue for the rational design of COF materials toward highly efficient I2 capture and related application.

Nonlinear regression for treating adsorption isotherm data to characterize new sorbents: Advantages over linearization demonstrated with simulated and experimental data

This paper demonstrates that determining adsorption capacity and affinity through data fitting of adsorption isotherms by nonlinear regression (NLR) is more accurate than linearized Langmuir equations. Linearization errors and the subjective choice of data points used to apply the linear regression analysis may deviate the fitted adsorption parameters (constants and adsorption capacities) from the expected values. The deviation magnitude increases for heterogeneous sorbents such as environmental particles and molecularly imprinted polymers, which adsorb by more than one sorption mechanism or adsorption sites of diverse chemical natures. For instance, Lineweaver-Burk linearization of isotherms simulated considering the presence of two adsorption sites (distinct adsorption energies) provides excellent linear regression fittings but for only one kind of adsorption site. Contrary, Scatchard and Eadie-Hoffsiee's equations indicate the presence of more than one kind of adsorption site, but if the difference between the adsorption constants is not significant, the choice of points used to perform the computation becomes subjective. On the contrary, NLR analysis considers all the adsorption points (experimental or simulated), providing objective criteria to define if more than one kind of site or retention mechanism rules the adsorbed amounts of analyte. The fitted constants have smaller deviations from the expected values than those obtained by linearization. In addition to the simulated data, the enhanced robustness of the NLR was demonstrated in the determination of the adsorption capacity and adsorption affinity of a humic acid sample towards Cu2+ at different pH.

Adsorption Mechanism of Congo Red on Mg–Al-layered Double Hydroxide Nanocompound

In this work, congo red (CR) was removed by applying carbonate intercalated Mg-Al-layered double hydroxide (Mg-Al-LDH) nanocompound as an adsorbent. Batch adsorption experiments performed under various temperatures, ionic strengths, initial CR concentrations, alkalinities and shaking rates. The maximum adsorption capacities of Mg-Al-LDH for CR were 100, 105 and 86.8 mg g-1 at 308, 318 and 328 K, respectively. Adsorption sites of Mg-Al-LDH for CR were -OH groups attached to Al atoms of adsorbent layers. Adsorption isotherms of the process were studied by the ARIAN model and analysis of obtained data showed that there were two kinds of adsorption sites on the surface of Mg-Al-LDH. Results of instrumental analysis showed that these adsorption sites were -OH groups located on the surface of mesopores and micropores of adsorbent and were named MP and 003 sites, respectively. The kinetic data were studied by the KASRA model and ISO and intraparticle diffusion (pore-diffusion) equations which showed that CR molecules were adsorbed at first on the MP sites. Also, during the adsorption of CR on MP sites the interaction of CR with adsorbent surface was rate-controlling step. Furthermore, during CR adsorption on 003 sites, adsorption kinetics was diffusion-controlled.

Simultaneous removal of methylene blue and Pb2+ from aqueous solution by adsorption on facile modified lignosulfonate

ABSTRACTIn this paper, simultaneous removal of methylene blue (MB) and Pb2+ from the binary component system by an easily prepared cross-linked lignosulfonate bio-adsorbent (CLLS) was described. CLLS was characterized by FTIR, SEM/EDS and TGA. The influences of pH, temperature, contact time and initial MB and Pb2+ concentrations on the adsorption performance were investigated. The results demonstrated a good ability of CLLS to remove MB and Pb2+ simultaneously. Using of 1.0 g L−1 loading, removal efficiency of MB and Pb2+ reached 98.0% and 97.8%, respectively, in the MB (100 mg.L−1)-Pb2+ (50 mg.L−1) system. Moreover, the adsorption isotherms and adsorption kinetics indicated that the results were fitting well with the Langmuir and pseudo-second-order model, respectively, for both MB and Pb2+. Based on the Langmuir model, the maximum adsorption capacity of MB and Pb2+ reached 132.6 and 64.9 mg g−1, respectively, in the MB-Pb2+ system, which was much lower than that in the single component system (358.4 mg g−1 100.9 mg g−1 for MB and Pb2+, respectively). Hence, simultaneous adsorption of MB and Pb2+ onto CLLS was an antagonistic adsorption. In addition, an apart-sequential adsorption method was used to study the action of adsorption sites on CLLS for MB and/or Pb2+ with the help of an efficient self-made apparatus. Rudimental results showed that there would be three different kinds of adsorption sites on CLLS: MB-site, Pb2+-site and MB/Pb2+- shared sites. Furthermore, in the MB (100 mg.L−1)-Pb2+(50.0 mg.L−1) system, the simultaneous removal efficiency of MB and Pb2+ still maintained 91.8% and 85.0%, respectively, after 6 cycles.

Chitosan/polyaniline hybrid for the removal of cationic and anionic dyes from aqueous solutions

ABSTRACTNanostructured chitosan/polyaniline (CH/PANI) hybrid was synthesized via in situ polymerization of aniline in the presence of chitosan. The CH/PANI hybrid was characterized by FTIR spectroscopy, X‐ray diffraction, thermogravimetric analysis, and scanning electron microscopy. The CH/PANI hybrid had a nanofibrous structure with an average diameter of 70 nm. This hybrid was employed as an ecofriendly and efficient adsorbent with high adsorption capacity for the removal of Acid Green 25 (AG) and methylene blue (MB) from aqueous solutions. AG and MB were used as anionic and cationic model dyes, respectively. The CH/PANI adsorbent showed high dependence on the pH of the medium with an excellent adsorption performance and regeneration manner. The kinetics and adsorption isotherms were studied. The CH/PANI hybrid follows the pseudo second‐order adsorption kinetics and Temkin isotherm model for the adsorption of both AG and MB dyes. This assumes that the enthalpy of dyes molecules decreases with the adsorption on heterogeneous surface with various kinds of adsorption sites and as well as the ability to form multilayers of the dye. Also, intraparticle diffusion was found to play an important role in the adsorption mechanism. The maximum adsorption capacity was found to be 240.4 mg g−1 of AG at pH 4 and 81.3 mg g−1 of MB at pH 11. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47056.

Synthesis and evaluation of different adsorbents based on poly(methacrylic acid–trimethylolpropane trimethacrylate) and poly(vinylimidazole–trimethylolpropane trimethacrylate) for the adsorption of tebuthiuron from aqueous medium

In the present study a new cross-linked copolymer poly(methacrylic acid–trimethylolpropane trimethacrylate) (named PMA) and poly(vinylimidazole–trimethylolpropane trimethacrylate) (named PVI) was synthesized through precipitation polymerization and applied to the adsorption of tebuthiuron. Adsorbent materials were characterized by using FT-IR, SEM, TGA, textural data, and elemental analysis. Kinetic study showed that the tebuthiuron adsorption was very quick and the equilibrium time was achieved at 20min for both polymers. Experimental kinetic data for both polymers were very well described by the second-order kinetic model, thus indicating that tebuthiuron adsorption involves chemical adsorption in different binding sites, which could control the reaction rate. Adsorption equilibrium data were better fitted to the dual-site Langmuir–Freundlich model, which recognizes the existence of two kinds of adsorption sites on the polymer surface ascribed to the presence of carboxyl and carbonyl groups from PMA and imidazole ring and carbonyl group from PVI. The maximum adsorption capacities of PMA and PVI were found to be 186.76 and 213.89mgg−1, respectively, which are much higher than other adsorbents.

CO2 adsorption on silver nanoparticle/carbon nanotube nanocomposites: A study of adsorption characteristics

In this work, we present the analysis of adsorption properties of CO2 on Ag nanoparticle/carbon nanotube nanocomposites compared to those of N2 and O2. We observe for all gases an identical kinetic behavior, indicating that the nonwetting properties of CO2 on a carbon surface do not influence the gas adsorption. We perform the analysis in conditions of low pressure and temperature to better observe the physical properties of the system. We observe a first kinetic order for all gases, indicating the absence of gas–gas interactions on the composite surface. The desorption curves for all gases present several desorption temperatures, indicating the presence of different kinds of adsorption sites.

Competitive Adsorption of Carbon Monoxide and Water Vapour on MIL-100(Fe) Prepared Using a Microwave Method

Competitive adsorption of CO and water vapour on MIL-100(Fe) synthesized using a microwave method was systematically investigated. Adsorption equilibrium, kinetics of CO adsorption and the competitive adsorption of CO/H2O(V) within MIL-100(Fe) were evaluated. Results show that the unsaturated Fe(II) can enhance the adsorption amount of CO on MIL-100(Fe). The adsorbed amount of CO on MIL-100(Fe) was 2.56 mmol/g at 298 K and 10 bar with isosteric adsorption heat of 28.31–38.43 kJ/mol. MIL-100(Fe) had two kinds of adsorption sites with desorption activation energy of 53.38 and 109.5 kJ/mol for CO, respectively. The diffusion coefficient of CO through MIL-100(Fe) was approximately 2.85 times higher than that of water vapour (10.54 × 10–16 cm2/s vs. 2.735 × 10–16 cm2/s). The working adsorption amount of CO at 40% relative humidity was approximately 0.631 mmol/g, retaining 75% of the original adsorption amount, indicating that CO is more competitive than H2O on the surface of MIL-100(Fe) under humid conditions.

Fixed-bed adsorption of toluene on high silica zeolites: experiments and mathematical modelling using LDF approximation and a multisite model

The adsorption of toluene (TOL) as a target volatile organic compound has been studied experimentally and modelled on various hydrophobic zeolites: Faujasite (FAU), ZSM-5 (Z) and Mordenite (MOR). The influence of the nature of the compensating cation (H+ or Na+) has also been investigated for ZSM-5 zeolite, which is known to possess three kinds of adsorption sites (sinusoidal channels, straight channels and intersections). Type I isotherms observed on FAU, Na-Z and MOR fitted well with the Langmuir model. A deviation from a type I isotherm was observed for H-Z, because of the structure of this zeolite. The Successive Langmuir Model was more successful to fit the ‘bump’ of the experimental curve than the Double Langmuir. Classical shapes were found for MOR, FAU and Na-Z breakthrough curves that were fitted with good accuracy using the Linear Driving Force (LDF) approximation. In the case of H-Z, a change of profile was observed during the dynamic adsorption and the differences seen between the Na-Z and H-Z behaviours were explained by the strong interactions between Na+ and adsorbed TOL at the intersection sites. The Na+ cations prevented reorientation of TOL molecules at the intersection and thereby avoided the filling of the sinusoidal channel segments. Thus, a specific model was developed for fitting the breakthrough curve of H-Z. The model developed took into account these two types of adsorption sites with the overall uptake for each site being given by an LDF approximation.

Improved selective cholesterol adsorption by molecularly imprinted poly(methacrylic acid)/silica (PMAA–SiO2) hybrid material synthesized with different molar ratios

The present paper describes the synthesis of molecularly imprinted polymer — poly(methacrylic acid)/silica and reports its performance feasibility with desired adsorption capacity and selectivity for cholesterol extraction. Two imprinted hybrid materials were synthesized at different methacrylic acid (MAA)/tetraethoxysilane (TEOS) molar ratios (6:1 and 1:5) and characterized by FT-IR, TGA, SEM and textural data. Cholesterol adsorption on hybrid materials took place preferably in apolar solvent medium, especially in chloroform. From the kinetic data, the equilibrium time was reached quickly, being 12 and 20min for the polymers synthesized at MAA/TEOS molar ratio of 6:1 and 1:5, respectively. The pseudo-second-order model provided the best fit for cholesterol adsorption on polymers, confirming the chemical nature of the adsorption process, while the dual-site Langmuir–Freundlich equation presented the best fit to the experimental data, suggesting the existence of two kinds of adsorption sites on both polymers. The maximum adsorption capacities obtained for the polymers synthesized at MAA/TEOS molar ratios of 6:1 and 1:5 were found to be 214.8 and 166.4mgg−1, respectively. The results from isotherm data also indicated higher adsorption capacity for both imprinted polymers regarding to corresponding non-imprinted polymers. Nevertheless, taking into account the retention parameters and selectivity of cholesterol in the presence of structurally analogue compounds (5-α-cholestane and 7-dehydrocholesterol), it was observed that the polymer synthesized at the MAA/TEOS molar ratio of 6:1 was much more selective for cholesterol than the one prepared at the ratio of 1:5, thus suggesting that selective binding sites ascribed to the carboxyl group from MAA play a central role in the imprinting effect created on MIP.

Molecular Motions of Adsorbed CO2 on a Tetrazole-Functionalized PIM Polymer Studied with 13C NMR

The CO2 adsorption in a polymer of intrinsic microporosity (PIM) functionalized by tetrazole (TZPIM) has been studied with in situ 13C nuclear magnetic resonance (NMR) spectroscopy at variable temperature and loading conditions. The CO2 molecules are seen to interact with tetrazole sites through physisorption. The adsorbed system was studied from 8 to 385 K at two loadings of CO2. The 13C NMR resonance lineshapes and relaxation times have been analyzed to give insights into the adsorption process. The CO2 molecules undergo site to site hopping with accompanying CO2 reorientations while adsorbed on TZPIM, resulting in line narrowing starting near 100 K. Correspondingly, the spin echo T2 passes through a minimum at 100 K. The analysis indicates that two kinds of adsorption sites are available.

Bimetallic Pt–Pd/silica–alumina hydrotreating catalysts. Part II: Structure–activity correlations in the hydrogenation of tetralin in the presence of dibenzothiophene and quinoline

The catalytic hydrogenation of tetralin in the absence and presence of quinoline and dibenzothiophene was studied on bimetallic Pt–Pd catalysts supported on silica and amorphous silica–alumina (ASA). The proportion of Pt on the surface determined the activity given that the Pt–Pd catalyst with the highest proportion of surface Pt was the most active. In the absence of poisons, the electronegativity of the support correlated with the hydrogenation. In the presence of quinoline, the activity of the catalysts increased with the dispersion of the metal particles, whereas in the presence of dibenzothiophene, the acidity of the support controlled the activity. The observed effects of the poisons indicated the presence of two kinds of adsorption sites, that is, metal particles and Brønsted acid sites at the perimeter. The key contribution of acid sites for hydrogenation was confirmed by removing the Brønsted acid sites of the support. Reference Pt catalysts were more active than the Pt–Pd counterparts in poison-free feed. In the presence of poisons, the Pt catalysts were also more active than the bimetallic formulations with the remarkable exception of the bimetallic catalyst with the higher proportion of Pt on the surface. The bimetallic catalysts are more resistant to sulfur and nitrogen poisoning as well as to sintering. The poison resistance of bimetallic catalysts originates from the electron transference from Pt to Pd that yields weak adsorption of poisons on electron deficient Pt atoms. However, the surface coverage of Pt must be maximized to compensate the low activity of Pd-enriched catalysts.

Phloxine B as a Probe for Entrapment in Microcrystalline Cellulose

The photophysical behaviour of phloxine B adsorbed onto microcrystalline cellulose was evaluated by reflectance spectroscopy and laser induced time-resolved luminescence in the picosecond-nanosecond and microsecond-millisecond ranges. Analysis of the absorption spectral changes with concentration points to a small tendency of the dye to aggregate in the range of concentrations under study. Prompt fluorescence, phosphorescence and delayed fluorescence spectral decays were measured at room temperature and 77 K, without the need of sample degassing because cellulose protects triplet states from oxygen quenching. In all cases, spectral changes with time and lifetime distribution analysis were consistent with the dye coexisting in two different environments: dyes tightly entrapped between polymer chains in crystalline regions of cellulose showed longer fluorescence and phosphorescence lifetimes and more energetic triplet states, while dyes adsorbed in more amorphous regions of the support showed shorter lifetimes and less energetic triplet states. This behaviour is discussed in terms of the different dye-support interactions in both kinds of adsorption sites.

A first-principles study of dihydroazulene as a possible optical molecular switch

By applying nonequilibrium Green’s function formalism combined with first-principles density functional theory, we investigate the electronic transport properties of the dihydroazulene optical molecular switch. Three kinds of adsorption sites including the hollow, bridge and top sites are studied. The two forms of this molecule, namely the open form and the closed form, can reversibly switch from each other upon photoexcitation. Their transmission spectra are remarkably distinctive. Theoretical results show that the current of the closed form is always significantly larger than that of the open form for all three adsorption sites, which promises this system as possibly one of the good candidates for optical switches due to its unique advantage, and which may have some potential applications in the future molecular circuit.

Adsorption of cyclohexene and its dehydrogenation intermediates on nAu/Pt(100)( n = 0, 1, 2) surfaces: A DFT study

Abstract Adsorption of cyclohexene and its dehydrogenation intermediates on the nAu/Pt(100) (n = 0, 1, 2 means clean Pt, one monolayer and two layers of Au covered Pt surfaces, respectively.) has been investigated by self-consistent (GGA-PW91) density functional theory combined with periodic slab model. It is found that on the clean platinum, there are two kinds of favorable adsorption sites, i.e., hollow sites and bridge sites, and the adsorption energy at the hollow site is larger than that at the bridge site. However, on the Au/Pt and 2Au/Pt surfaces, there are three kinds of adsorption sites, and the adsorption energies are alike at both the bridge site and the top site. The magnitude order of the adsorption energies is as follows: clean Pt > Au/Pt > 2Au/Pt. The configurations of cyclohexene molecule have been distorted a little during the geometry optimizations. The lengths of C-M (M = Pt or Au, on the top layer of the slab) bonds are closely related to the corresponding adsorption energies.

Removal of arsenate from water by using an Fe–Ce oxide adsorbent: Effects of coexistent fluoride and phosphate

The Langmuir two-site equation, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure spectroscopy have been employed to study the competitive behaviors of fluoride (F) and phosphate (P) in relation to arsenate adsorption on an Fe–Ce adsorbent as well as the mechanisms involved. The two-site isotherm revealed the presence of two kinds of adsorption sites with different binding affinities for arsenate. Both the total and low-binding-energy maximum adsorption capacities ( Q and Q 1) of arsenate decreased significantly even at a molar ratio of As/P = 1:0.1. The coexistence of F, only influenced the total Q of arsenate at high simultaneous F concentrations. The fact that Fe–Ce released 0.15–0.24 mmol sulfate for every mmol arsenate adsorbed suggested that, while sulfate groups might have played a role for adsorption, surface hydroxyl groups should be the major active sites. The XPS results indicated that arsenate and P are mainly adsorbed through the substitution of Fe surface active sites, while F is mainly adsorbed through substitution of Ce surface active sites. The As k-edge EXAFS data show that the second peak of Fe–Ce after arsenate adsorption is As–Fe shell, which further supported that arsenate adsorption occurs mainly at the Fe surface active sites.

Structures of D 2 layers on LiF(0 0 1)

Classical Monte Carlo (MC) simulations of D 2 molecules physisorbed on LiF(0 0 1) surfaces are reported and show a series of interesting commensurate structure forms, viz., p(2 × 2) → p(8 × 2) → p(4 × 2), with coverages Θ = 0.5, 0.625, and 0.75, respectively, and are stable up to 8 K. These structures are consistent with recent helium atom scattering (HAS) results (the p(4 × 2) is not observed) in terms of coverage and stability, but disagree in terms of symmetry. The p(2 × 2) structure contains two D 2 molecules per unit cell, with each molecule lying parallel to the plane of the surface directly above every other cationic site. For the p(4 × 2) structure, there are two kinds of adsorption sites: a parallel site, as in the case of p(2 × 2), and a tilted site, where the D 2 molecules sit between cationic and anionic sites with the molecular axis directed toward the anionic site, with a tilt angle of θ ∼ 63°. Perturbation theory calculations show that the adsorbed D 2 molecules are azimuthally delocalized and hence the structures are indeed c-type. Our calculations also indicate that o-D 2 and helicoptering p-D 2 species prefer cationic sites, compared to cartwheeling p-D 2 species.