Effect of adsorption of vapours on the electrical conductivity of some nitroaromatic semiconductors: adsorption and desorption kinetics

On adsorption of various vapours, the electrical conductivity of nitroaromatic semiconductors 9-nitroanthracene, 1,4-dinitronaphthalene and 1,3,5-trinitrobenzene changes appreciably. This phenomenon has been used as a probe to study the adsorption and desorption processes in these nitroaromatic polycrystals. The adsorption and desorption kinetics have been found to follow the modified Roginsky-Zeldovich relation. Two adsorption processes have been identified. One, a two-stage process, the first of which gives a Lennard-Jones potential energy curve and is followed by a rate-determining transition over a potential energy barrier to the second stage of adsorption forming weakly bound complexes between the vapour molecules and the nitroaromatic crystallites. In the second process, in addition to the two stages of the first process, a third stage with a comparatively deep potential energy surface is reached by activation over a second potential barrier and strongly bound complexes are finally formed. On leave from Government Arts and Science College, Durg 491 002, India     

Simultaneous determination of heats,equilibrium and kinetics of adsorption: 1- Ethoxy-2-propanol vapours

The heat, equilibrium, and kinetics of adsorption of 1-ethoxy-2-propanol vapours on granulated activated carbon were determined simultaneously by a reaction calorimeter SETARAM C80 D at T=298.15 K at various relative vapour pressures (0.1< p/p_s<0.8). The adsorption isotherm was correlated by the Freundlich equation. It was observed that the enthalpies of adsorption decrease slightly with increasing of the relative vapour pressure of the adsorptive. The rate of adsorption were calculated from analysis of the heat flux signals and it was found that the mass-transfer coefficient for 1-ethoxy-2-propanol vapours in granulated activated carbon increased with increasing relative vapour pressure of the adsorptive.This revised version was published online in November 2005 with corrections to the Cover Date.     

Entropy of adsorption of carbon monoxide on energetically heterogeneous surfaces

Standard entropies of adsorption (Δs ⁰) of CO on different materials (Cu catalysts, Au catalysts, ZnO and to TiO₂) are obtained from static adsorption microcalorimetry, adsorption isobars and temperature-programmed desorption, based on the thermodynamics of adsorption on energetically heterogeneous surfaces. Vibrational entropies of the surfaces s _vib^α are normally between the rotational and the standard translational entropy of CO in gas phase, and decrease with increasing adsorption energy, which agrees with the explanation of statistical thermodynamics. Δs ⁰ reflects both the mobility of adsorbates and the specific adsorbate-adsorbent interaction. Limits for reasonable values of the entropy of adsorption are proposed.     

Use of x-ray microtomography to visualise dynamic adsorption of organic vapour and water vapour on activated carbon

X-ray microtomography coupled with image analysis was used to quantify the adsorption of vapours on activated carbon beds. This technique was tested using three different challenges: CCl₄, water vapour and a mixture of water- and organic vapour. It is shown that the used technique allows determining the adsorption front progress in the case of organic vapour and mixture of water and organic vapour whereas the existence of this front was not so obvious in the case of water vapour. Experimental results obtained for organic vapours were interpreted on the basis of the Wheeler-Jonas equation: a good agreement was found between experimental and theoretical breakthrough times.     

Microcalorimetric methods for studying vapour adsorption and wetting of powders

The interaction of a powder with vapours or their wettability by liquids are involved in many industrial processes and then needs to be studied by simple and reproducible methods. Two microcalorimetric methods, one for the simultaneous measurement of adsorption isotherm and enthalpy and the other for the determination of wetting or immersion energy, are described. A few examples are given for the adsorption and wetting of powders by water and organic vapours. This revised version was published online in August 2006 with corrections to the Cover Date.     

Temperature effect on the adsorption and desorption processes in retinoic acid crystals: An electrical conductivity study

Using electrical conductivity as a probe, the effect of crystal temperature on the process of adsorption and desorption of some chemical vapours on retinoic acid semiconductor has been investigated. It has been observed that a three-stage adsorption process in a sample at low temperature changes over to a two-stage process at higher temperature. Measures of activation energies of adsorption and desorption at different stages have been estimated from kinetic data analysis by using a modified Roginsky-Zeldovich equation.     

Structure of copper and nickel amine complexes with polyoxometallates and their adsorption properties relative to lower alcohols

The rates of the adsorption and desorption of methanol and ethanol vapor to and from compounds formed by cationic copper(II) and nickel(II) amine complexes and polyoxometallate anions can be described in the linear driving force transfer model as one or two parallel processes. It was proposed that the differences observed should be attributed to the different crystal structures of the adsorbents. The compound consisting of a nickel tetramine complex and [HSiW₁₂O₄₀]³⁻ anion displays high selectivity toward ethanol relative to methanol. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 5, pp. 291–298, September–October, 2007.     

Propylene/propane mixture adsorption on faujasite sorbents

The adsorption of propylene and propane on zeolite NaX with and without a saturated (36 wt%) amount of CuCl have been investigated. The single component adsorption isotherms could be well described with a Dual-Site Langmuir model. The dispersion of CuCl results in a decrease of the maximum adsorption capacity of the zeolite for both components. For propylene a strong adsorption via π-complexation with CuCl is present, increasing the adsorption selectivity of the zeolite. The binary mixture (50:50) adsorption was determined via breakthrough/desorption experiments at 318, 358 and 408 K with a partial pressure of the two components between 0.8–54 kPa. For NaX the mixture loading could be well described with the IAS-theory and the single component isotherms, both qualitatively and quantitatively. A transition from an enthalpy controlled adsorption at lower loadings to an entropy affected adsorption at higher loadings was observed. The IAS-theory could only qualitatively describe the trends in the observed mixture adsorption for the CuCl/NaX adsorbent. The dispersion of CuCl in NaX results in a modest improvement of the adsorption selectivity for propylene over propane (from 3–7 to 15–30) but at the expense of a reduced capacity.     

The effect of surface ion-induced defects on CO adsorption on polycrystalline Ni

CO adsorption on polycrystalline nickel was investigated by dynamic secondary ion mass-spectroscopy at 10⁻⁵–10⁻³ Pa and 300–500 K. An increase of secondary ion currents NiCO⁺/Ni⁺ ratio was found in the range from 300 to 350 K, while at T>350 K it decreased sharply. These data were explained by a kinetic model, in which adsorption and desorption of tightly bound CO goes through weakly bound CO formed due to ion-induced defects.     

Dubinin"s theory and its contribution to adsorption science

Dubinin"s theory for the volume filling of micropores (TVFM), originally developed for the adsorption of single vapours by microporous solids such as activated carbons and zeolites, has gradually been extended to other areas. They include immersion calorimetry, the adsorption of water vapour and of mixtures, as well as adsorption from aqueous solutions. Recent studies in the field of adsorption from aqueous solutions, by activated carbons, suggest that the principle of temperature invariance is fulfilled and in the case of phenolic compounds a modified DRK equation can be used to predict the adsorption equilibrium over a certain range of temperatures. Computer modelling of CO₂ adsorption by carbons at 273 K leads to micropore distributions, which are in good agreement with those derived from other techniques. It also appears that the model isotherms in single slit-shaped micropores can be fitted to the Hill-de Boer isotherm, in agreement with mathematical studies of the origin of the Dubinin—Astakhov equation.     

Description of NaX zeolite deformation during adsorption of xenon

The equation describing the experimental data of NaX zeolite deformation during adsorption of xenon at pressures of 0.001–7 MPa and temperatures of 252–333 K was obtained on the basis of the generalized Lennard-Jones pair interaction potential. The temperature dependences of the constants in the equation were determined. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1887–1890, October, 1999.     

Influence of albumin adsorption on physico-chemical properties of alumina surfaces

The effect of albumin adsorption on neutral active aluminium oxide was investigated in the presence of polar and non-polar liquids. The adsorbed values were highest near the isoelectric point of albumin and varied in the range 5–10 and 3–11 mg g^–1 with phosphate buffer and potassium chloride respectively after 2 and 24 h. In the case of aluminium oxide the effect of albumin adsorption on total heterogeneity of adsorbents is not explicit. On the one hand, the modified samples showed decreasing surface area with increase of surface coverage with albumin. On the other hand, modifications under the same conditions but without albumin caused similar changes. These effects suggest the strong influence of medium pH on surface properties (due to surface polarization) and competitive co-adsorption of ions on the process. The volumetric fractal dimensions of the studied materials change in the range 2.25–2.32 for pure aluminium oxide and BSA modified from the phosphate solution. E _d,max values (desorption energy in the maximum of distribution function) diminish (in the range 40–45 kJ mol^–1) compared with pure aluminium oxide (E _d,max=52 kJ mol^–1) for water thermodesorption at modified surfaces to the increase of a number of active centers of hydrophobic character, and weakening of the adsorbent–adsorbate increases.     

Adsorption and desorption of macromolecules on solid surfaces studied by on-line size exclusion chromatography 2. Preferential adsorption and exchange processes

Preferential and exchange adsorption of polymers differing in molar mass and/or chemical nature under dynamic conditions were investigated using on-line size-exclusion chromatography (SEC). The sample investigated dissolved in an appropriate solvent was injected into a small adsorption–desorption column packed with nonporous silica. A nonadsorbed or desorbed fraction of the polymer was directed into an SEC column for determination of both the amount and the molecular characteristics. This approach is in many aspects superior to other techniques for studies of polymer adsorption onto solid surfaces due to its low sample and time consumption. At a low degree of surface coverage, adsorption and desorption of macromolecules were rapid and were affected by the rate of supply of macromolecules to the adsorbent surface. The exchange between macromolecules at the stage of surface saturation was found to depend on the mean molar masses of preadsorbed and displacing polymer species and possibly also on the chain flexibility of the macromolecules. It was shown that the preferential adsorption driven by the chain-length difference upon saturation of the adsorbent surface was more noticeable if the preadsorbed macromolecules were smaller. Received: 7 April 1999 Accepted in revised form: 21 July 1999     

Borate adsorption at Pt(111) in acidic medium

The adsorption/desorption process of borate was studied at Pt(111) in acidic solution by cyclic voltammetry. A so-called butterfly wave in the cyclic voltammogram of Pt(111) in HClO₄ shifted to negative direction upon the addition of boric acid with the disappearance of its sharp spikes. The shift in potential was found to be −57 mV with a tenfold increase of boric acid concentration. This illustrates that this anomalous wave is due to borate adsorption/desorption by a one-electron transfer process. The borate adsorption/desorption wave was observed to shift by −63 mV/pH. At pH>3, the anomalous wave splits forming two separate waves, depending on the pH and the scan rate. The appearance of two waves is assigned to the change in the adsorption mode of borate or the participation of OH in the adsorption process.     

Study of adsorption kinetics for fluorescent whitening agent on fiber surfaces

Adsorption kinetics for a stilbene derivative type fluorescent whitening agent (FWA) on fiber surfaces was studied based on a real-time spectroscopic measurement system. Results showed that the time-dependent behavior of FWA agrees to that of a mono-molecular adsorption layer on fiber surfaces,as characterized in Langmuir-type expression. The adsorption has two distinguishable stages includ-ing initial fast phase with the primary constant of 1.51×1014 and the later near-equilibrium phase with the secondary constant of 4.96×10-4. The maximum amount of FWA adsorbed on fiber surfaces is 1.67×10-4g (per dry weight based fiber) in the initial phase. A mathematical model of adsorption kinetics was therefore established and evaluated. This model is important for the optimization of FWA applica-tion in papermaking.     

Competitive adsorption

A review is presented on the multicomponent competitive adsorption, static equilibrium adsorption, and dynamic nonequilibrium adsorption from gaseous and liquid media onto the surface of porous and disperse solids. The state of the theory of multicomponent adsorption is analyzed. Not only traditional methods were used for investigations in this field but also NMR spectroscopy (difference in the spectra of adsorbed and free compounds), FTIR spectroscopy (difference in the spectra of surface groups and bonds in adsorbates), calorimetry, thermally stimulated depolarization, photon correlation spectroscopy, electrophoresis, and cryoporometry. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 3, pp. 133–169, May–June, 2007.     

The heat of adsorption of water on nonporous hydrophilic carbon adsorbents in the model of two-stage adsorption

The behavior of integral and differential heats of adsorption of water in the region of transition from the first stage of adsorption (formation of clusters) to the second stage (formation of a stretched liquid film) was considered. The curve of integral heat of adsorption has an inflection at the transition point, and the differential heat of adsorption changes jumpwise. The values of these effects were estimated by the simplest model of formation of one and two hydrogen bonds between a water molecule and an adsorption center on the surface of the carbon adsorbent. Curves of differential heat of adsorption with transition points for real systems are presented. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 8, pp. 1479–1483, August, 1999.     

新型多级微/介孔固态胺吸附剂的制备及其CO2吸附性能研究

将HZSM-5与MCM-41按不同质量比混合得到复合分子筛载体,以四乙烯五胺(TEPA)为改性剂,采用浸渍法将其负载到复合分子筛上,制备了一系列新型的具有多级微/介孔结构的固态胺吸附剂。采用N₂吸脱附、傅里叶变换红外光谱(FT-IR)、热重分析(TGA)等手段对吸附剂进行表征。在固定床反应器中考察了HZSM-5和MCM-41的质量比、TEPA负载量、吸附温度、进气流量和CO₂分压等因素对CO₂吸附性能的影响。结果表明,当HZSM-5与MCM-41的质量比为1:1、TEPA负载量为30%、吸附温度为55℃、进气流量为30 mL/min时,平衡吸附量高达3.57 mmol/g,且经10次吸脱附循环后,吸附量仅下降8.1%。HZSM-5/MCM-41-30%TEPA对CO₂的吸附过程包括快速的穿透吸附和相对缓慢的逐渐平衡阶段,且穿透吸附量接近于平衡吸附量的80%。HZSM-5/MCM-41-30%TEPA对CO₂的吸附过程符合Avrami动力学模型,表明CO₂吸附是物理吸附和化学吸附的结果。     

Reversible adsorption of spherical particles from binary mixtures: long-time behaviour

Formation of monolayers of spherical particles in processes with reversible adsorption from mixtures of large and small particles was simulated in computer experiments. Computer program was based on an algorithm that took into account random sequential adsorption, desorption and lateral diffusion of adsorbed particles (RSA–DLD model). Computer experiments were performed for systems with rate constants of particle adsorption at least 10³ times higher than rate constants of desorption. In processes with very fast adsorption and slow desorption, formation of monolayer can be divided into two stages. During the first stage, the total surface coverage (the coverage with particles of both types) increases very fast and becomes very close to that at equilibrium. During the second stage, the total coverage changes very slowly and the system approaches equilibrium mainly by the replacement of large particles with the small ones. A simple kinetic model for evolution of the monolayer composition during the second stage has been proposed. Kinetic equations related to this model allow the determination of large particles’ desorption rate constants on the basis of changes in the surface concentrations of adsorbed large and small microspheres. The validity of the model has been tested comparing large particles’ desorption rate constants values that had been used for simulations with values of the corresponding rate constants determined using analytical equations, with a view to analysing the simulation results. To cite this article: S. Slomkowski et al., C. R. Chimie 6 (2003).     

Static and Kinetic Studies on the Adsorption Behavior of Sulfadiazene

To investigate the nature of interactive forces between sulfadiazene molecules and alumina surface the experiments were performed for the adsorption of sulfadiazene (SD) from its aqueous sulution onto the alumina surfaces at 25 ± 0.2°C and the influence of factors such as increasing concentration of SD (4.0–20.0 × 10^–3 mol cm^–3), the time required for adsorption equilibrium, pH (2.0–12.0) and temperature (5–45°C) of the adsorption medium, the presence of ions like Cl^–, SO^2– ₄ and PO^3– ₄ (0.01–0.30 M) and organic solvents (5% v/v) were observed on the course of adsorption of SD. Various adsorption and kinetic parameters such as adsorption coefficient, the rate constants for adsorption and desorption were also evaluated. The results of the above cited studies facilitated to formulate the mechanisms of interaction between SD and alumina surfaces. From application view point the present work may be a potential tool for an effective chromatographic separation of sulfa drugs from industrial effluents.