Adsorption, nanoporous materials porous material characterization

In the past two decades, 129Xe NMR has been employed as a useful technique for the characterization of the internal void space of nanoporous materials. In particular, the xenon chemical shift has been demonstrated to be very sensitive to the local environment of the nuclei and to depend strongly on the pore size and also on the pressure [4—6], Assuming a macroscopic inhomogeneity resulting from a distribution of adsorption site concentrations, 129Xe NMR spectra of xenon in zeolites have been calculated, and properties such as line widths, shapes as well as their dependence on xenon pressure can be reproduced qualitatively. A fully quantitative analysis, however, remains difficult due to the different contributions to the xenon line shift. (See Chapter 5.3 for a more detailed description of Xe spectroscopy for the characterization of porous media.)... 

The majority of adsorbents applied in industry has porous sizes in the nanometer region. In this pore-size territory, adsorption is an important method for the characterization of porous materials. To be precise, gas adsorption provides information concerning the microporous volume, the mes-opore area, the volume and size of the pores, and the energetics of adsorption. Also, gas adsorption is an important unitary operation for the industrial and sustainable energy and pollution abatement applications of nanoporous materials. 

Industrial applications of nanoporous carbons are based on both their porosity and surface properties, and consequently, their characterization is of great importance. The results presented here demonsfrate a great usefulness of gas adsorption measurements for the characterization of nanoporous carbons. Low-pressure measurements provide an opportunity to study the microporous structure and surface proptaties of these materials and to monitor changes in these properties that result fiom structure and surface modification. High-pressure adsorption data allow for a detailed characterization of mesoporous structures of carbonaceous porous materials, providing their surface areas and pore size distributions. 

The use of nonlocal density functional theory (NLDFT) for modeling adsorption isotherms of Lennard-Jones (LJ) fluids in porous materials is now well-established [1-5], and is central to modem characterization of nanoporous carbons as well as a variety of other adsorbent materials [1-3]. The principal concept here is that in confined spaces the potential energy is related to the size of the pore [6], thereby permitting a pore size distribution (PSD) to be extracted by fitting adsorption isotherm data. For carbons the slit pore model is now well established, and known to be applicable to a variety of nanoporous carbon forms, where the underlying micro structure comprises a disordered aggregate of crystallites. Such slit width distributions are then useful in predicting the equilibrium [1-5] and transport behavior [7,8] of other fluids in the same carbon. 

Nitrogen adsorption at low temperature is a routine characterization technique of nanoporous materials. For instance, the specific surface of porous materials is usually assessed from adsorption experiments (prior to capillary condensation of the fluid) on the basis of the Brunauer, Emmett, and Teller (BET) method. The BET model corresponding to the N2 adsorption isotherm at 77 K in the atomistic model of MCM-41 materials fits very well the simulated data with a correlation coefficient = 0.999 (see [39] for the comparison). We found Sbet 1000 m /g (the latter value is obtained by considering as the surface area occupied by an adsorbed N2 molecule, A 2 = 0.162 nm ) and C = 100. The value obtained for C... 

Gas adsorption is an important method for characterization of nanoporous carbons because it allows for evaluation of the specific surface area, pore volume, pore size, pore size distribution and surface properties of these materials [1, 10-12]. Although various techniques for measurement of gas adsorption data and methods of their analysis pear to be well established, an accurate and reliable evaluation of adsorption properties is still a difficult task. This can be attributed to the inherent features of many porous carbonaceous materials, namely, to their strong surface and structural heterogeneity. The effects of structural and surface heterogeneity in adsorption on nanoporous carbons are often difficult to separate. 

To interpret the pore size distribution of porous carbons, an intersecting capillaries model (ICM) is often used to approximate the microstructure of nanoporous carbons. The pores are assumed to be a poly-disperse ensemble of slit pores with smooth graphite walls, which are connected in some way. Despite its simple geometry, this ICM has been used successfiilly to characterize carbon adsorbents and to predict adsorption in these materials and thus will likely remain the dominant approach to characterization by adsorption [1]. 

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