BET Isotherm Research Articles

Adsorption, chemisorption, and catalysis

AbstractA short history of the relationships among adsorption, chemisorption, and catalysis with solid catalysts is reviewed. A special focus is on the development of quality and descriptions accuracy using computers, both for the modeling of elementary physical phenomena and adsorption, as well as for the solution of more complex problems like quantum chemical approach to chemisorption, kinetics over solid catalysts, and reactor systems. Modern approaches to the characterization of solid catalysts from the adsorption-desorption data based mainly on n-layer adsorption and non-linear three parameter BET isotherm regarding the volume of micropores as one of the parameters are demonstrated. Instrumentation techniques like infrared spectroscopy or NMR techniques for the analysis of the strength of component chemisorption are mentioned. As for the kinetics, a vague capability of the Langmuir-Hinshelwood-Hougen-Watson models to describe a reaction system in more complicated cases, e.g. bimolecular surface reactions, is discussed. In this context, the simplest model with a minimum number of parameters is advised. To estimate the most realistic values, intrinsic reaction kinetic and mass transport phenomena are taken into account. Usefulness of quantum mechanistic models for better understanding of the catalytic phenomena and more efficient design of catalysts are outlined.

Application of a heterogeneous adsorbent (HA) for the removal of hexavalent chromium from aqueous solutions: Kinetic and equilibrium modeling

In the present study, a heterogeneous adsorbent material (HA) was used as an adsorbent for the removal of Cr(VI) from aqueous solutions at laboratory scale. Cr(VI) containing water was treated with heterogeneous adsorbent(HA).Chromium solutions of various initial concentrations were treated with adsorbent in batch mode experiments to investigate the adsorption characteristics of heterogeneous adsorbent (HA). Kinetics of adsorption of Cr(VI) ions on adsorbent was investigated by using pseudo first order and second order kinetic models. Removal processes were found to be governed by pseudo second order model. Intraparticle diffusion model was also analyzed for this system. Removal was found to be increased by increasing the temperature from 298 to 318K which indicates the endothermic nature of the process. Various two parameter isotherm models viz. Langmuir, Freundlich, Elovich, Tempkin, Dubnin–Raduskevich (D–R), Harkin–Jura and BET isotherm were applied on resultant data for equilibrium modeling. It was observed that heterogeneous adsorbent (HA) particles were highly efficient for the removal of Cr(VI).

Chromatography and the Hotly Debated Enigma of Aqueous Surface’s Acid–Base Character

How charges are accommodated on the water surfaces is a 100 year mystery. In the past decade the question whether the surface of water is acidic or basic has arisen and has deepened the debate about the presence of ions at dielectric boundaries. Experimental results are scarce and scattered. Arguments stemming from macroscopic experiments support a basic interface but are in stark contrast to those probing it at the molecular level. For the very first time we placed chromatography in the armory for tackling the water surface acid–base character. Chromatography is a formidable tool to ascertain the extent to which H+ and OH– adsorb onto an hydrophobic surface. Frontal analysis gave unequivocal quantitative experimental evidence of the higher surface affinity of OH– compared to that of H+. Pulse chromatography independently confirmed this outstanding result. Adsorption isotherm data were obtained from breakthrough curves, and the isotherm parameters were estimated by fitting them to the BET isotherm model. ...

Multilayer adsorption on fractal surfaces

Multilayer adsorption is often observed in liquid chromatography. The most frequently employed model for multilayer adsorption is the BET isotherm equation. In this study we introduce an interpretation of multilayer adsorption measured on liquid chromatographic stationary phases based on the fractal theory. The fractal BET isotherm model was successfully used to determine the apparent fractal dimension of the adsorbent surface. The nonlinear fitting of the fractal BET equation gives us the estimation of the adsorption equilibrium constants and the monolayer saturation capacity of the adsorbent as well. In our experiments, aniline and proline were used as test molecules on reversed phase and normal phase columns, respectively. Our results suggest an apparent fractal dimension 2.88–2.99 in the case of reversed phase adsorbents, in the contrast with a bare silica column with a fractal dimension of 2.54.

Self-Assembly of Peptide-Amphiphile Forming Helical Nanofibers and in Situ Template Synthesis of Uniform Mesoporous Single Wall Silica Nanotubes

A lysine based peptide amphiphile (PA) is designed and synthesized for efficient water immobilization. The PA with a minimum gelation concentration (MGC) of 1% w/v in water shows prolonged stability and can also efficiently immobilize aqueous mixtures of some other organic solvents. The presence of a free amine induced pH dependency of the gelation as the PA could form hydrogel at a pH range of 1-8 but failed to do so above that pH. Various spectroscopic and microscopic experiments such as steady state fluorescence, NMR, IR, CD, and FESEM reveal the presence of hydrophobic interaction, hydrogen bond, and π-π stacking interaction in the self-assembly process. The self-aggregation has been correlated with the design of the molecule to show the involvement of supramolecular forces and the hierarchical pathway. While the L analogue formed left-handed helical nanofibers, the other enantiomer showed opposite helicity. Interestingly the equimolar mixture of the isomers failed to form any fibrous aggregate. Although fibers formed at a subgel concentration, no helical nature was observed at this stage. The length and thickness of the fibers increased with increase in the gelator concentration. The nanofibers formed by the gelator are used as a template to prepare mesoporous single wall silica nanotubes (SWSNTs) in situ in plain water without the requirement of any organic solvent as well as any external hydrolyzing agent. The SWSNTs formed are open at both ends, are few micrometers in length, and have an average diameter of ~10 nm. The BET isotherm showed a type IV hysteresis loop suggesting mesoporous nature of the nanotubes.

Synthesis and photocatalytic properties of bismuth titanate with different structures via oxidant peroxo method (OPM)

Bismuth titanate (Bi4Ti3O12 and Bi12TiO20) powders were synthesized by the Oxidant Peroxide Method (OPM), and the effect of temperatures on physical and chemical properties of particles was investigated. The results showed that the morphology and average particle size of materials can be successfully controlled by adjusting the temperature. The samples after calcination were characterized by X-ray diffractometry, transmission electron microscopy, diffuse reflectance spectroscopy, Raman spectroscopy, and BET isotherms. The photocatalytic activity of materials was also evaluated by studying the degradation of 10ppm aqueous rhodamine B dye under ultraviolet radiation.

Unified water isotherms for clayey porous materials

[1] We provide a unified model for the soil-water retention function, including the effect of bound and capillary waters for all types of soils, including clayey media. The model combines a CEC-normalized isotherm describing the sorption of the bound water (and the filling of the trapped porosity) and the van Genuchten model to describe the capillary water sorption retention but ignore capillary condensation. For the CEC-normalized isotherm, we tested both the BET and Freundlich isotherms, and we found that the Freundlich is more suitable than the BET isotherm in fitting the data. It is also easier to combine the Freundlich isotherm with the van Genuchten model. The new model accounts for (1) the different types of clay minerals, (2) the different types of ions sorbed in the Stern layer and on the basal planes of 2:1 clays, and (3) the pore size distribution. The model is validated with different data sets, including mixtures of kaolinite and bentonite. The model parameters include two exponents (the pore size exponent of the van Genuchten model and the exponent of the Freundlich isotherm), the capillary entry pressure, and two critical water contents. The first critical water content is the water content at saturation (porosity), and the second is the maximum water content associated with adsorption forces, including the trapped nonbound water.

Removal of reactive black 5 from aqueous solution using chitosan beads: optimization by Plackett–Burman design and response surface analysis

Contaminations due to dyes pose not only a severe public health concern, but also many serious environmental problems. The present project aims to prepare an inexpensive and efficient sorbent that could remove dyes that are commonly used in the industry. Chitosan has been seen as a promising sorbent due to its significant properties, such as biocompatibility, biodegradability and non-toxicity. Batch studies were performed under various experimental conditions and the parameters studied included pH, contact time, initial concentration and agitation rate. The kinetics of dye sorption fitted a pseudo-second-order rate expression. Equilibrium data could be fitted into both Langmuir and BET isotherms. Plackett–Burman design was applied to identify the most significant factors in the removal of Reactive Black 5 (RB5) by chitosan. The effect of operating parameters on RB5 uptake was studied and a mathematical model showing the influence of each variable was obtained. The interaction between the factors and their optimum levels for maximum percentage uptake of RB5 were determined using Response Surface Methodology. The optimum sorption conditions were determined as initial pH 7.00 and at contact time greater than 200 min.

Kinetic and equilibrium modeling of single and binary adsorption of methyl tert-butyl ether (MTBE) and tert-butyl alcohol (TBA) onto nano-perfluorooctyl alumina

Wide spread use of methyl tert-butyl ether (MTBE) has led to pollution of drinking water sources and has received great attention. In this study, single-component and binary competitive adsorptions of MTBE and tert-butyl alcohol (TBA) onto two novel adsorbents, i.e., nano-perfluorooctyl alumina prepared by using nano-γ-alumina (nano-PFOALG) and nano-boehemite (nano-PFOALB) as the supports were investigated. In single-component experiments, experimental data were modeled with five isotherm equations (Langmuir, Freundlich, modified BET for liquid phase, Sips and Dubinin–Radushkevich). Results revealed that the adsorption isotherms of both components on two adsorbents were best described by the modified BET isotherm. The multicomponent data were fitted to seven different predictive models, especially an extended form of modified BET, has been used in the present work. For the binary adsorption system, it is found that the quality of respective single-component isotherms for each component has significant effect onto the predicting multicomponent adsorption via Ideal Adsorbed Solution Theory (IAST). Accordingly, IAST-BET provided an accurate description of MTBE/TBA binary adsorption experimental data, hence being a reliable isotherm for the design of industrial adsorption equipment for wastewater treatment especially in high concentration ranges. The kinetic study was also conducted and two common kinetic models (pseudo first-order and pseudo second-order) were used to determine kinetics parameters. The mechanism of the adsorption process was described from the intraparticle diffusion model. The kinetic data showed a closer fitness to the pseudo-second order model. Maximum values for the rate constant, k2, were obtained for MTBE and TBA adsorption, as 0.252 and 1.494 (gmg−1min−1), respectively.

Removal of Cr(VI) from water by cork waste

The biosorption by cork powder is considered as a promising method for heavy metal removal from industrial waste waters such as chromium tanning factories. The aim of this study is to evaluate the efficiency extent of this method using cork powder as a biosorbent for Cr(VI). The Fourier Transform Infrared spectroscopy (FTIR) analysis permits to distinguish the type of functional groups likely to participate in metal binding. A linear form of BET isotherms for all the three used temperatures (i.e., 25, 35 and 45°C) and a pseudo-second-order equation of adsorption kinetics are obtained. Other experimental results highlight the meaningful influence of parameters such as contact time, pH, concentration of Cr(VI) and the adsorbent particle size on Cr(VI) adsorption. 97% of Cr(VI) has been removed under definite conditions particularly a particle size of diameter d<0.08mm and pH of 2–3 values.

The kinetics of selective oxidation of propene on bismuth vanadium molybdenum oxide catalysts

We report the results of a systematic investigation of the kinetics of propene oxidation to acrolein over Bi1−x/3V1−xMoxO4. BET isotherms were measured to determine catalyst surface area, and powder X-ray diffraction was used to characterize the bulk structure. Further characterization by X-ray absorption near-edge spectroscopy (XANES) was used to determine the oxidation states of Bi, Mo, and V before and after exposure of the catalyst to propene at 713K. We find that, contrary to previous discussions of the mechanism of propene oxidation on Bi1−x/3V1−xMoxO4, Bi remains in the 3+ state and only V and Mo undergo reduction and oxidation during reaction. The kinetics of propene oxidation were examined to establish the activation barrier for acrolein formation, and how the partial pressure dependences on propene and oxygen change with the value of x. The data obtained from this study were then used to propose a generalized model for the kinetics of propene oxidation over Bi1−x/3V1−xMoxO4 that is consistent with our findings about the reducibility of the three metallic elements in the oxide. According to this model, vanadium and molybdenum are randomly distributed to form three types of sites each associated with its own rate parameters. MoV sites are found to exhibit the highest activity. The proposed model provides a good description of the experimental data for all catalyst formulations examined, for a range of propene and oxygen partial pressures, and for temperatures above 653K.

Removal of reactive yellow dye by adsorption onto activated carbon using simulated wastewater

Reactive yellow dye was investigated as an adsorbate to be removed onto activated carbon during batch and continuous processes. A Fourier transform infrared (FTIR) analysis was carried out before and after onto the activated carbon adsorption. Thermodynamic parameters: Gibbs free energy, enthalpy change, and entropy change were found at different temperatures. Isotherm experiments were carried out at different conditions: adsorbent dosage (0.1–0.5 g), temperature (20–50°C), pH (2.5–9), and initial concentration (50–150 mg/l). Langmuir, Freundlich, and (Brunauer, Emmet, and Teller) BET isotherm model were used to fit the results. Kinetics mechanism of mass transfer pseudo-first-order, pseudo-second-order, and intraparticale diffusion models were studied. Fixed bed column experiments were carried out at different bed heights (0.05–0.15 m), initial concentration (10–50 mg/l), and flow rate (4–12) 10−6 m3/min to find empty bed contact time at break though point. Bed depth service time model was used as a shortcut model to predicate the breakthrough curves comparing with experimental data.

Effects of nitro-functionalization on the gas adsorption properties of isoreticular metal-organic framework-eight (IRMOF-8)

A study was done to determine the effects of nitro-functionalization on the gas sorption properties of IRMOF-8. The IRMOF-8 and IRMOF-8-NO2 were prepared by a rapid solvothermal method. BET isotherm analysis showed that nitro-functionalization of IRMOF-8 caused the surface area to decrease from 1599 to 832m2/g while the pore volume decreased from 0.639 to 0.322cm3/g. However, these large decreases in surface area and pore volume did not result in a proportionate decrease in gas sorption capability. For example, the IRMOF-8 adsorbed 3.2 wt% hydrogen while IRMOF-8-NO2 adsorbed 2.2 wt% hydrogen. This is a relatively small reduction in hydrogen adsorption (∼30%) compared to the large decrease in surface areas (∼50%). The amounts of methane and carbon dioxide adsorbed by the IRMOF-8-NO2 were also disproportionately high. A sticking factor (defined as the percent gas adsorbed per m2 of surface area per gram) was determined for the adsorption of each gas on the two MOFs at various temperatures. In all cases, the sticking factors for gases adsorbed on the IRMOF-8-NO2 were greater than those for gas adsorbed on the IRMOF-8. Enthalpies were also measured for the adsorption of each gas on the two MOFs at various temperatures. It was found that, in all cases, the enthalpies for gas adsorption on IRMOF-8-NO2 were greater than those for IRMOF-8. Thus it appears that nitro-functionalization strengthens the bonding of the gases to the surface thus resulting in greater sticking efficiency and larger than expected gas adsorption.

Hydrogen adsorption properties of platinum decorated hierarchically structured templated carbons

In this report, the possibility of Pt catalytic activity for the dissociation of hydrogen molecules and subsequent hydrogen adsorption on sucrose templated carbon at ambient temperature has been studied. In order to investigate Pt catalytic effect for hydrogen storage solely, 6.8wt.% Pt-doped (Pt/TC) and pure templated carbon (TC) possessing almost identical specific surface area (SSA) and pore volume (Vp) have been successfully synthesized. Since both Pt/TC and TC shares for their textural properties (e.g. SSA and Vp), any difference of hydrogen adsorption characteristic and storage capacity can be ascribed to the presence of Pt nanoparticles. Both samples are characterized by various techniques such as powder X-ray diffraction, ICP-OES, Ranman spectroscopy, transmission electron microscopy, cryogenic thermal desorption spectroscopy, low-pressure high-resolution hydrogen and nitrogen BET and high-pressure hydrogen adsorption isotherms in a Sieverts’ apparatus. By applying hydrogen and deuterium isotope mixture, cryogenic thermal desorption spectroscopy point to a Pt catalytic activity for the dissociation of hydrogen molecules. Furthermore, the hydrogen adsorption isotherms at RT indicate an enhancement of the initial hydrogen adsorption kinetics in Pt-doped system. However, the hydrogen storage capacity of Pt/TC exhibits a negligible enhancement with a strong hysteresis, suggesting no connection between the spillover effect and a feasible hydrogen storage enhancement.

Removal of organic contaminants from river and reservoir waters by three different aluminum-based metal salts: Coagulation adsorption and kinetics studies

Surface waters from two different sources, river and reservoir, with different dissolved organic matter (DOM) concentrations and characters were investigated for removals using three metal coagulants, alum, a high performance aluminum polymer (HPAC) and polyaluminum chloride (PACl). Jar tests were conducted with coagulation pH ranging between 5.5 and 8.5 and doses between 5 and 16mgAl/L. The adsorption mechanisms of DOM onto Al hydroxide precipitates of the three coagulants were investigated based on Freundlich, Langmuir and BET isotherm models. Coagulation kinetics and floc formation were studied using a photometric dispersion analyzer (PDA). For the three coagulants tested, optimum DOM removal was achieved at a coagulant dose of 11mgAl/L and pH 6 for both waters. The lowest coagulant dose (5mgAl/L) removed most organics with apparent molecular weights of between 2000 and 10,000Da for both waters. DOM adsorption is best described by the BET isotherm model, indicating that DOM sorption occurred through multilayer formation in an arbitrary distribution of adsorbed particles. PACl showed the highest DOM adsorption capacity, determined by the Freundlich model. By PDA analyses, the flocculation index, average growth rate and variance values from the use of HPAC were highest in comparison with alum and PACl. The study of the coagulation kinetics suggested that HPAC produced the largest and most heterogeneous floc sizes with highest settling rates.

Chromium(VI) Adsorption Behavior of Silk Sericin Beads

Silk sericin (SS) has been fabricated into beads using a 1 M LiCl/DMSO solvent and utilized as a heavy metal adsorbent. Among the various heavy metals, we targeted Cr(VI) for adsorption using SS beads and found that its adsorption depended on the coagulant used for the fabrication of the SS beads. When methanol was used as a coagulant, the beads had a better adsorption capacity than when ethanol was used except at pH 1. The adsorption behavior of Cr(VI) on the SS beads followed the BET isotherm. The maximum adsorption capacity was 33.76 mg/g at pH 2. The adsorption of Cr(VI) was confirmed by FT-IR and EDS analyses. Finally, the desorption was carried out using NaOH solution, and it was found that 73.19% of the adsorbed Cr(VI) could be detached.

3D-Graphitized and Iron Modified Carbon Aerogels for Sustainable Energy Applications

3D-carbon aerogel nanomatrix has been modified with iron nitrate in the amounts of 10 and 20 wt. % by wet incipient method. The crystal structure, morphology, and electrocatalytic activity of the formed nanocomposites have been studied in comparison to the unmodified carbon aerogels after their heat-treatment at 900oC, 1200oC and 1400oC in argon. The mass and specific surface area losses in these materials reduced with iron doping is clear from BET isotherms and BJH pore size analysis. The HRTEM analysis confirms the presence of uniformly distributed ~43.5nm iron nanoparticles surrounded by graphite layers. The XRD analysis show that the level of graphitization increases with the sintering temperature and the concentration of the iron-containing phase. The ORR electrocatalytic activity studies demonstrate the influence of the heat-treatment temperature of samples on the ORR activity. Temperature programed reduction is carried out to understand the reduction behavior of these catalysts to metallic iron, to use them as substrates for direct CNT growth.

Encapsulation of copper(I)-Schiff base complex in NaY nanoporosity: An efficient and reusable catalyst in the synthesis of propargylamines via A3-coupling (aldehyde-amine-alkyne) reactions

Nanodimensional microreactor containing bis[2-(phenylthio)benzylidene]-1,2-ethylenediamine copper(I) complexes, [Cu(N2S2)]X-Y (X=CN, Cl, Br, I) was successfully prepared. This nano composite was used as a catalyst in one-pot multicomponent coupling reaction of aldehydes, amines and alkynes toward synthesis of propargylamines. The copper(I) complex with N2S2 donor Schiff base ligand was readily trapped in the nanocavity of zeolite-Y through a template synthetic (TS) method. Two consecutive processes in the liquid phase were carried out: (i) preparation of a solution of [Cu(MeCN)4]X coordinated with 2-(phenylthio)-benzaldehyde in the presence of zeolite-Y. This process caused the inclusion of [Cu(MeCN)2(S2O2)]X complex into the nanopores of zeolite Y via pre-adsorption method; (ii) template synthesis of the [Cu(MeCN)2(N2S2)]X-Y, through Schiff condensation between [Cu(MeCN)2(S2O2)]X and 1,2-ethylenediamine moiety into the nanopores of NaY. The resulting catalysts were fully characterized by spectroscopic techniques such as AAS, FT-IR, BET isotherm, XRD, UV-Vis and TGA. The results confirmed encapsulation of Y-zeolite with copper(I) complex of N2S2 donor ligand. The catalytic activity of this nanocomposite material was investigated for preparation of propargylamines via three-component reaction of aldehydes, amines and alkynes. Also, the effective factors on this condensation reaction were discussed, such as reaction temperature, solvent, catalyst loading, leaching and reusability of the catalyst.

Adsorption kinetics and thermodynamics of hydrophobic natural organic matter (NOM) removal from aqueous solution by multi-wall carbon nanotubes

The objective of this research was to study the efficiency of Multi-Walled Carbon Nanotubes (MWCNTs) for the adsorption of hydrophobic natural organic matter (NOM) from aqueous solution under different operational conditions of contact time, pH, initial concentration of NOM and temperature. MWCNTs were synthesized via chemical vapor deposition (CVD) with average diameter of 10–50 nm. The results illustrated that both as-prepared and functionalized MWCNTs showed high adsorption capacity for the NOM studied. Functionalization of MWCNT affected the surface area and introduced oxygen-containing functional groups to the surface of MWCNT which depressed the adsorption of NOM onto MWCNTs-COOH. The obtained data were studied using Freundlich, Langmuir and BET isotherms; NOM adsorption data had the best conformity to the Freundlich isotherm. Kinetic studies were performed and the adsorption kinetics successfully followed the pseudo-second-order kinetic model. Thermodynamic parameters such as free energy change (ΔG°), enthalpy change (ΔH°) and entropy change (ΔS°) were determined and evaluation of them showed that the adsorption process for NOM is general spontaneous, endothermic and thermodynamically favorable.

Influence of the carbonaceous conductive network on the electrochemical performance of ZnFe2O4 nanoparticles

Herein, the influence of the carbon precursor used for the realization of carbonaceous percolating network incorporating ZnFe2O4 is investigated. Three different precursors, namely sucrose (Suc), citric acid (CA), and oleic acid (OLEA) were used for the realization of such carbonaceous matrix. The composite materials were characterized by means of thermogravimetric analysis (TGA), gas adsorption using the BET isotherm, Raman spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Electrodes based on the resulting composite materials were investigated, performing electrochemical impedance spectroscopy and galvanostatic cycling. The results showed that the homogeneity of the carbon coating rather than the absolute amount of carbon enhanced the electrochemical performance of ZnFe2O4 nanoparticles. The most advantageous coating properties were obtained with sucrose as carbon precursor, obtaining a highly stable specific capacity higher than 1000 mAh g−1 for more than 60 cycles.