Standard Isotherm Research Articles

Microcalorimetric Study of the Influence of Surface Chemistry on the Adsorption of Water by High Surface Area Carbons

Microcalorimetric techniques have been employed to investigate the adsorption of water on chemically modified, high surface area carbons. A matrix of treatments and carbons were selected to test some hypotheses. Heats of adsorption of water indicate that adsorption is a strong function of surface chemistry. Three mechanisms of water adsorption are delineated according to measured differential heats of adsorption (Hads): (i) chemical adsorption with Hads > 12 kcal/mol, (ii) condensation with Hads approximately 10 kcal/mol, and (iii) physical adsorption with Hads < 10 kcal/mol. The absolute and relative amounts of water adsorption arising from each mechanism are a function of surface chemistry. Adsorption of water on carbons dried at 175 °C in N2 generates typical Type V adsorption isotherms. Heat of adsorption data for water adsorbed on dried carbon indicates that condensation accounts for the sharp rise in adsorption at a relative humidity of approximately 0.5. Treating carbons with N2 at 950 °C generates surfaces that initially adsorb water through a mechanism of chemical adsorption, followed by condensation, and finally physical adsorption. On high-temperature N2-treated carbons heats of adsorption exceed 100 kcal/mol, suggesting chemisorption of water at unsaturated carbon surface sites produced during the high-temperature reduction of oxygen species. Carbons reduced with H2 at 950 °C are hydrophobic, and microcalorimetric data reveals that the small amount of adsorption observed arises from either chemical or physical adsorption rather than condensation. Hydrophobic carbon surfaces subsequently oxygenated at 150 °C showed significant increases in the amount of water adsorbed through physical adsorption. These results demonstrate that microcalorimetric techniques complement standard isotherm measurements in describing the nature of water adsorption on carbon surfaces.

Estimating cloud top height and spatial displacement from scan‐synchronous GOES images using simplified IR‐based stereoscopic analysis

An efficient method for estimating cloud top heights and correcting cloud image spatial displacements was developed. The method applies stereoscopic analysis to a pair of scan‐synchronous infrared cloud images received from two GOES satellites using a piecewise linear approximation of the relationship between height and infrared brightness temperature of top of the cloud element. The algorithm solves for cloud top heights and subsequently calculates the spatial displacements of cloud images. Optimal parameterization of the piecewise linear approximation is achieved using the shuffled complex evolution (SCE) algorithm. Because the proposed method simplifies the stereoscopic analysis, it allows for an easy implementation of stereoscopic technique on desktop computers. When compared to the standard isotherm matching approaches, the proposed method yielded higher correlation between GOES 8 and GOES 9 scan‐simultaneous images after the parallax adjustment. The validity of the linear approximation was tested against temperature profiles obtained from the multiple ground sounding measurements from the Tropical Rainfall Measuring Mission/Texas and Florida Underflights (TRMM/TEFLUN) experiments. The results of this comparison demonstrated good fit, particularly within the troposphere, between the optimized relationship and atmospheric sounding measurements. The data produced by this method, including cloud top temperatures and heights, atmospheric temperature profiles for cloudy sky areas, and spatial displacement‐adjusted cloud images, can be useful for weather/climate and atmospheric studies. In particular, the displacement‐adjusted cloud images can be critical to develop high‐resolution satellite rainfall estimates, which are urgently needed by mesoscale atmospheric modeling and studies, severe weather monitoring, and heavy precipitation and flash flood forecasting. Limitations of the proposed method are also identified and discussed.

Fractal analysis methods to characterize porous solids using nitrogen adsorption data

The use of the fractal concept to characterize the morphological nature of complex materials, such as porous solids, is considered. The most popular approaches, which are based on analysis of the shape of the adsorption isotherms of a substance vapor (in this case nitrogen at 77 K), are outlined and applied to characterizing a series of commercial carbon blacks. The most recent developments in this domain are given. The results obtained show that some methods, although derived using different arguments, are equivalent in some conditions, and the concept of a standard isotherm can be used in the fractal approach PACS Nos.: 61.43 H Fractals -68.45 -v

Simple Evaluation Scheme of Adsorbate-Solid Interaction for Nano-Pore Characterization Studied with Monte Carlo Simulation.

We propose a simple method for identifying interaction strength between an adsorbate molecule and a solid surface. The interaction strength is necessary for precise evaluation of pore sizes of the nanometer order, in which the conventional Kelvin model fails to describe the condensation phenomena, and for which we proposed an alternative model taking account of the effect of attractive potential from pore walls on the condensation: The model contains a constant to express the interaction strength between an adsorbate and a solid surface, and it should be determined from a standard isotherm. For investigation and verification of the method, standard adsorption isotherms were prepared by the grand canonical Monte Carlo simulations for the system of a Lennard-Jones particle on a homogeneous solid surface. The Frenkel theory was basically applied for this purpose because of its explicit inclusion of the interaction parameter within the equation. The theory shows good agreement with simulation results in a limited range of relative pressures from 0.005 to 0.25, while certain deviations exist in other portions of the isotherm. The result is that only the portion of the standard isotherm with good agreement should be used to evaluate the interaction strength employing the Frenkel theory. Through examinations in systems with various interaction strengths, it is found that the interaction strength is able to be evaluated correctly if the equation is applied to the range of surface coverage from 0.8 to 1.8 in all cases. The interaction constant calculated by this method is proved to have sufficient accuracy for use in characterization of nano-scale pores.

Pore size distributions of active carbons assessed by different techniques

(3n 21) n 3 CM (Fig. 1) the frequency of the micropore widths f(L) 5 3W L a exp[2aL ] /G(n) (4) 0 observed by STM on the surface of the solid, DN /DL, is This function, like the distribution based on the DS slightly different from the volumic distribution DW /DL, equation [1,7–9], has a single maximum and it is not since the STM analysis does not take into account the suited for the description of bimodal micropore distribuactual depth of the micropores. However, the two distions, provided that they exist. In the case of carbon CM tributions are related and STM, like TEM, confirms that the pore size distribution Eq. (4) shown in Fig. 1(b), has the micropores of active carbons are locally slit-shaped. been obtained from the adsorption isotherms of CH (253, 4 This, in turn, provides the basic model for computer 273, 308 K) and of CO (253, 273, 298 K) fitted to Eq. 2 simulations. (1). This distribution is in good agreement with the In the case of microporous carbons, it has also been histogram of Fig. 1(a), derived from liquids with molecular possible to derive PSD from adsorption data within the sizes between 0.4 and 1.5 nm. As confirmed independently framework of Dubinin’s theory for the volume filling of by STM, the experimental distribution has no secondary micropores [5,6]. A possible relation is the so-called maximum around 1.5 nm. Dubinin–Stoeckli (DS) equation [1,7,8], which applies to Computer modeling of adsorption and the determination strongly activated carbons, with relatively wide distribuof pore size distributions based on standard isotherms has tions. As reported by Daley at al. [9], good agreement is become increasingly popular and it has been discussed found between the PSD derived from the DS equation and recently [10]. At the present time, modeling is frequently the experimental distribution observed by STM on strongly based on the adsorption of N [11–13], CO [14,15] and 2 2 activated carbons. CH [16–19], but a systematic comparison with pore size 4 Recently [1], a modified Dubinin isotherm has also been distributions obtained by independent techniques is still suggested to obtain PSD in the micropore range, lacking. For example, to our knowledge, no direct com-

On the use of standard DRK isotherms in Dubinin’s t/F method

It is shown that the adsorption of benzene, carbon tetrachloride, dichloromethane and nitrogen by a typical non-porous carbon black follows the Dubinin–Radushkevich–Kaganer equation. The requirement for temperature invariance is fulfilled, with an average characteristic energy Eo=10.8 kJ mol−1. This expression is compared with the standard isotherms for benzene and carbon tetrachloride at 293 K proposed by Dubinin and used as a reference in the so-called t/F method, which leads to the non-porous surface area of active carbons. It appears that Dubinin’s isotherm contains inconsistencies, which are compensated for internally. Alternative DRK expressions, applicable to different vapours, are therefore proposed. The present study also shows the limits of Dubinin’s method with respect to comparison plots at higher relative pressures.

The effects of the electronic structure of micropores on the small angle scattering of X-rays and neutrons

The effects of the electronic structure of micropores on the small angle scattering of X-rays and neutrons

Simulation study on the relationship between a high resolution αs-plot and the pore size distribution for activated carbon

The effects of the micropore structure of activated carbons on the high resolution α s-plot for nitrogen adsorption isotherms were examined with the grand canonical Monte Carlo simulation. Adsorption isotherms of nitrogen were simulated in graphitic slit pores at 77 K as a function of the slit width ( w). As no pore effect was observed below P/ P 0=0.6 for w=3.5 nm, α s-plots for the simulated adsorption isotherms were constructed using the standard isotherm simulated for w=3.5 nm. The simulated α s-plots had filling and cooperative swings which were experimentally shown in the previous works, and the shape of the simulated α s-plot varied with the micropore structure. As the subtracting pore effect (SPE) method for the specific surface area (SSA) determination using the α s-plot was proposed in the previous experimental works, the theoretical ground for the SPE method was discussed. The best evaluation method of SSA using the α s-plot was shown, almost agreeing with the SPE method. This simulation study showed clearly that the SPE method is available for pore systems of w≥0.7 nm, whereas even the SPE method underestimates the SSA of the pores of w≤0.6 nm. The observed swings of the α s-plot were simulated using the different micropore size distribution. The bimodal micropore size distribution lead to both of filling and cooperative swings, while a single pore size distribution <0.9 nm gave only the filling swing. Thus, four representative types of the α s-plot for activated carbons were proposed and it was shown how to understand the micropore size distribution through the α s-plot.

Modified Langmuir Equation for S-Shaped and Multisite Isotherm Plots

Standard isotherm equations do not estimate capacity (Qmax) and distribution coefficient (Kd) for complex or non-Langmuir-shaped isotherm plots. In this study, two mycotoxins, that is, aflatoxin B1 (AfB1) and cyclopiazonic acid (CPA), were mixed with kaolinite and a naturally acidic montmorillonite clay (LPHM) at 25 °C, respectively. Isotherm data gave S-type plots. The data were fitted to the models of Langmuir (LM) and multi-Langmuir (MLM); however, these models did not provide a good fit for data that displayed multisite adsorption or multiple plateaus. While a published modification of the Langmuir equation (QKLM), which defines an effective partition coefficient as a function of the surface coverage, was able to fit simple isotherm plots from all categories (H, L, S, C), it did not fit complex or multisite isotherm plots. Importantly, an equation that enables the estimation of Qmax and Kd for both S-shaped and multisite isotherm plots has not yet been reported. Since the LM, MLM, and QKLM did not pro...

Applicability of the Fractal FHH Equation

The current work deals with the applicability of the fractal version of the Frenkel−Halsey−Hill (FHH) theory of multilayer adsorption. The concept of the standard isotherm is introduced to examine the weakness of this equation when the solid−gas potential controls the adsorption process of gas on a given solid. The results of the determination of the fractal dimension of typical samples of carbon blacks, activated carbon, and aerogels (mixed SiO2−ZrO2 and pure silica) are given. They show that the fractal dimension depends critically of the coverage exponent s for a flat surface of the same chemical nature.

Transport of phosphorus through soil in an effluent-irrigated tree plantation

A young plantation of Pinus radiata in south-eastern Australia was irrigated with secondary-treated sewerage effluent for 48 months, when evapotranspiration exceeded rainfall. Concentrations of phosphorus (P) in saturation pastes of soil were measured after 30, 37, 42, and 48 months to monitor vertical transport of P, and results were compared with depth of transport predicted from P sorption isotherms. Standard laboratory isotherms greatly underestimated the capacity of soil to retain P. Thirty months after plantation establishment, 165 kg/ha of P had been applied in effluent, and P in soil solution was predicted from sorption isotherms to be at a concentration of 5 · 7 mg/L to a depth of 0 · 29 m. In comparison, the concentration of P in saturation paste at this time decreased exponentially from 3 · 5 mg/L at 0 · 025 m to 0 · 43 mg/L at 0 · 30 m. Similarly, 48 months after plantation establishment, 318 kg/ha of P had been applied, and P was predicted to be in equilibrium to a depth of 0 · 51 m. At this time, concentration of P in saturation paste was 4 · 7 mg/L at 0 · 25 m but decreased to a concentration of 0 · 11 mg/L at 0 · 45 m. The concentration of P in saturation paste in the bore-water control was low (&lt;0 · 36 mg/L) thoughout the profile. The discrepancy between predicted and observed results was found from subsequent experiments to be due largely to the short (17 h) equilibration time and to the wide solution : soil ratio (10 : 1) used during construction of standard sorption isotherms. In the 0-0 · 1 m layer of soil, P-retention capacity determined in saturation paste and after 21 days incubation was about 9-fold greater than when the standard method was used. Leaching of dissolved P is unlikely to threaten the sustainable life-time of the effluent-irrigated plantation studied.

The Development and Characterisation of Sorbents from Irish Sphagnum Moss Peat

AbstractA range of carbon chars was produced from sphagnum moss peat by heating in a furnace with a reduced oxygen atmosphere. A series of heating times and temperatures were selected for the preparation of the chars. Comparative porosity and surface area studies, based on nitrogen sorption isotherms and mercury porosimetry data, were carried out to characterise the chars. Further characterisation studies were condućted by electron microscopy techniques. Optimum charring conditions, yielding maximum surface area for N2 adsorption, were observed at a charring temperature of 800°C and charring time 30 minutes. Using the optimum charring conditions, a further series of chars was produced. Prior to charring the raw peat was chemically pre‐treated with a range of metal salt solutions of varying percentage concentration. The resulting chars were characterised as above. The adsorptive capacity of these chars for N2 was investigated on the Sorptomatic 1900 (Fisons Instruments). The adsorption isotherms were compared to the standard isotherms of Langmuir. Adsorbate uptake was seen to vary with surface area availability, pore volume, pore type and the nature of the adsorbate.

Verification and Application of a New Adsorption Model for Fractal Surfaces

ABSTRACTA recently proposed isotherm model for adsorption on fractal surfaces is fitted to a wide range of isotherm data obtained from the literature. All curve fitting attempts produced good fits and the model parameters obtained are shown to be physically reasonable. Also, all significant model parametrs can be obtained from a single isotherm. As expected, when surface roughness is taken into account, the specific surface area determined from monolayer volume may be significantly higher than that obtained using the standard smooth surface isotherm models.

Numerical modeling of polymolecular adsorption of argon on a model amorphous oxide

Using the Monte Carlo method for the grand canonical ensemble, we have calculated the Ar adsorption isotherms in the range 77–107 K on an inhomogeneous surface with random close packing of spheres (the Bemal model), representing oxygen ions in a model oxide. The isotherm at 77 K is very close to the standard Ar adsorption isotherm on nonporous silica. The difference involves the weak step character of the calculated isotherm. The adsorbate on the model oxide at 77 K has a layered structure, reminiscent of the structure of Ar on the basal face of graphite, but significantly more extended. The atomic area of Ar depends on the method for determination of the capacity of the monolayer and lies within the range 0.1–0.13 nm2.

Surface area and pore size studies of alumina gels

Alumina gels were prepared using acid and base catalysts. Surface area and porosity of the gel-derived powders were studied with variations in catalyst type, catalyst-to-alkoxide molar ratio and heat-treatment temperature. The surface area decreased with increasing ratio of catalyst to alkoxide. Base-catalysed samples showed a higher surface area than acid-catalysed samples which were free from micropores and possessed only mesopores. Pore sizes and pore volumes were estimated using the Kelvin equation. Microporosity was determined using a standard isotherm concept. The specific surface area was found to decrease with increasing heat-treatment temperature.

Application of a reference material to the characterization of porous carbons

A recently published standard isotherm for a non-porous carbon 1 has been used to analyse the N 2 (77 K) adsorption isotherms on three series of porous carbons of different origins. The results were compared with those obtained from the Dubinin-Radushkevich (DR) equation. For carbons with isotherms that most strongly resemble the type I isotherm the α and DR methods yield similar micropore volumes (provided the latter can be applied in the relative pressure range 0.05-0.3 for carbons with wide microporosity-high burnoff). For carbons with type I + II composite isotherms, the application of both methods is not as straightforward and the results may differ considerably.

３パラメター吸着式の誘導とその適応性 ＩＶ 微小表面積粉体へのＮ２吸着等温線に対する３パラメター吸着式の適応性

Nitrogen adsorption isotherms at 77K on non-porous powders, such as γ-Fe2O3, A1100 and SM and silicon carbide, SiC(P) have been measured. The application of experimental isotherms to the 3-parametric adsorption equation was compared with that of the BET equation. The applicable relative pressure range of the 3-parametric adsorption equation was compared with that of the BET equation. The applicable relative pressure range of the 3-parametric adsorption equation was wider, from 0.05 to 0.65, than that of the BET equation which was in the range from 0.05 to 0.35. The surface areas, ∑T obtained from the 3-parametric adsorption equation showed an agreement within 10% with that from the BET equation, ∑B. Since the straight line in αs-plots of N2 adsorption isotherms were found to pass through the origin of the coordinate, the powders were confirmed to have no micropore. The outer plane surface areas, ∑Tαs and ∑Bαs, were determined from the slope of the straight line in αs-plots of the standard isotherm of N2 on non-porous hydroxylated silica, on the basis of the surface areas by application of the standard isotherm to the 3-parametric adsorption equation and BET equation, respectively. There is a reasonable agreement of ∑T and ∑B with ∑Tαs, and ∑Bαs, respectively, and therefore it will be difficult to determine which surface areas were the most reasonable. However, the surface areas obtained from the measurement of the particle diameter in use of a photograph of a scanning electron microscope showed better and reasonable agreement with the surface areas from the 3-parametric adsorption equation than that from BET equation. Therefore, the 3-parametric adsorption equation seemed to be more reasonable than the BET equation.

Pore structure and chemical state of the surface of carbon adsorbents Communication 1. Selection of the comparative isotherm of adsorption of nitrogen vapors on the surface of carbon adsorbents

1. A standard adsorption isotherm (CAI) was plotted from the isotherms of adsorption of N2 on nonporous and mesoporous carbon adsorbents for analyzing the pore structure of carbon adsorbents. 2. The analysis of the pore structure of active carbon with a relatively large area of mesopores and predominantly microporous active carbon shows that the CAI obtained adequately describes the adsorption of N2 on the surface of the mesopores of these samples.

Study of the texture of monodisperse silica sphere samples in the nanometer size range

The texture of monodisperse silica spheres in the diameter range 8–200 nm, and in the form of powders and slabs is examined. The analysis of nitrogen adsorption—desorption isotherms shows the respective contributions of the voids between particles and the internal microporosity, as well as the influence of pelletization on texture. The importance of the adsorbent—adsorbate interactions on the selection of the standard isotherms to obtain a self-consistent picture of the solid texture is also emphasized.

Sixty years in the physical adsorption of gases

The paper gives an account, necessarily highly selective and compressed, of the development of the subject, from the point of view of one whose working life has been spent in and around the study of the physical adsorption of gases. Much of the paper is arranged under the headings of the isotherm types I to VI. Particular reference is made to standard isotherms, α s-plots, the validity of pore size calculations, step-like isotherms and microporosity and activated entry. Finally it is emphasized that adsorbents are solids almost invariably in a state far removed from equilibrium.