Characterization porous samples

Characterization of Porous Samples. Adsorption isotherms are a useful analytical method for characterizing porous samples. This measurement consists of monitoring the amount of a molecular species taken up by the sample as a function of the relative saturation of the species in the gas phase over the sample. This relative saturation is defined as p/p, where p is the partial pressure of the test species and p is its saturation vapor pressure at the temperature of the run. In a typical measurement, p/p is increased from zero to a value near 1 (onset of bulk condensation) and then decreased back down to zero. For many porous samples, hysteresis is observed between the adsorption and desorption branches of the isotherm obtained in this manner (14). 

Porous surfaces, such as wood, plaster, and concrete, are also often sampled with wipes. Surface sampling of porous materials, however, does not provide any information on the contaminants that may be trapped inside the pores, cracks, and seams of the sampled surface. A better way to characterize porous materials is to collect and analyze chip samples. 

In conclusion, TPM has proved to be a very efficient tool for the characterization of soft materials. The intense development of porous samples with very well controlled pore size distribution, as carried out in our group, allows determination of calibration curves for numerous solvents. In particular, calibrations for solvents able to swell polymeric materials are now available, making TPM a very attractive technique. The simplicity and the low cost associated with TPM are further arguments for extended use of this technique. 

Modeling physical adsorption in confined spaces by Monte Carlo simulation or non-local density functional theory (DFT) has enjoyed increasing popularity as the basis for methods of characterizing porous solids. These methods proceed by first modeling the adsorption behavior of a gas/solid system for a distributed parameter, which may be pore size or adsorptive potential. These models are then used to determine the parameter distribution of a sample by inversion of the integral equation of adsorption, Eq. (1). 

A pore size distribution (PSD) of a sample is a measure of the cumulative or differential pore volume as a function of pore diameter. PSDs can be calculated from adsorption isotherms based on an analysis which accounts for capillary condensation into pores. This analysis (14.16) uses a model of the pore structure combined with the Kelvin equation (12) to relate the pore size to the value of p/Po at which pore "filling" occurs. Due to limitations in this technique, only pores with diameters from about 3 to 50 nm, called mesopores (14), can be characterized. This pore size range, however, is typical of many porous samples of interest. For samples with pores smaller or larger than this range, alternative techniques, such as mercury intrusion for large pores (14.16), are typically more suitable. 

The results obtained with these films indicate that this SAW-based technique is a powerful tool for characterizing porous thin films. It is based on a standard industry practice involving the use of N2 adsorption isotherms. The main advantage of the technique is that the SAW device decreases the detection limit for the amount of adsorbed Nj by several orders of magnitude over conventional techniques used for this type of characterization (1 ). This increased sensitivity is critical for thin films, due to the small total surface area present. For example, conventional instruments typically require a total sample surface area on the order of 10,000 cm ... 

Helium pycnometry Characterization method permitting to determine the skeletal density and specific volume of a porous sample by measuring the pressure change of helium in a calibrated volume HEMA Hydroxyethyl methacrylate... 

For convenience, the following discussions will assume a nitrogen adsorbate and liquid nitrogen as the temperature controlling fluid. The sample will be referred to as a powder which is not a requirement. Contiguous, open porous samples are also characterized by the techniques. 

Afterward, the porous scaffolds were physically and chemically characterized using FTIR, SEM and XRD. Their porosity and mechanical strength were also evaluated. The relative density of the porous scaffold was measured by the Archimedes method. The porosity of all porous samples was determined from its actual and relative densities. The relative density was calculated by taking the theoretical density of HA as 3.156 g cm. ... 

Increasing the surface-to-bulk ratio of the sample to be studied. This is easily done in the case of highly porous materials, and has been exploited for the characterization of supported catalysts, zeolites, sol-gels and porous silicon, to mention a few. 

A microscopic description characterizes the structure of the pores. The objective of a pore-structure analysis is to provide a description that relates to the macroscopic or bulk flow properties. The major bulk properties that need to be correlated with pore description or characterization are the four basic parameters porosity, permeability, tortuosity and connectivity. In studying different samples of the same medium, it becomes apparent that the number of pore sizes, shapes, orientations and interconnections are enormous. Due to this complexity, pore-structure description is most often a statistical distribution of apparent pore sizes. This distribution is apparent because to convert measurements to pore sizes one must resort to models that provide average or model pore sizes. A common approach to defining a characteristic pore size distribution is to model the porous medium as a bundle of straight cylindrical or rectangular capillaries (refer to Figure 2). The diameters of the model capillaries are defined on the basis of a convenient distribution function. 

When a dilute solution of a polymer (c << c ) is equilibrated with a porous medium, some polymer chains are partitioned to the pore channels. The partition coefficient K, defined as the ratio of the polymer concentration in the pore to the one in the exterior solution, decreases with increasing MW of the polymer (7). This size exclusion principle has been used successfully in SEC to characterize the MW distribution of polymer samples (8). 

Extensive experimental techniques have been developed for porous material characterization [1], including direct imaging [2-5] and bulk measurement techniques for the statistical properties of the pore space. NMR is one such bulk measurement that is both non-destructive and compatible with large samples. 

In the NO-SCR by NH3, we note the highest reduction activity and selectivity on catalyst containing both vanadium and molybdenum than catalysts issued containing Mo or V, only. Furthermore, it should be underlined that a higher efficiency is obtained with ZSM-5 as host structure than samples issued from USY and MOR. Where a higher loss of porous volume were observed. On the basis of characterization data it has been suggested that the observed synergism in the SCR reaction is related to the existence of electronic interaction between the V and Mo species. In particular, it has been proposed that the presence of such electronic interactions modifies the catalysts redox properties, which have been claimed an essential property in the NO-SCR by NH3 reaction. 

X-ray photoelectron spectroscopy is indeed quite informative, but requires the use of expensive instrumentation. Also, the detection of photoelectrons requires the use of ultrahigh vacuum, and therefore can mostly be used for ex situ characterization of catalytic samples (although new designs are now available for in situ studies [146,147]). Finally, XPS probes the upper 10 to 100 A of the solid sample, and is only sensitive to the outer surfaces of the catalysts. This may yield misleading results when analyzing porous materials. 

Porous texture characterization of all the samples was performed by physical adsorption of N2 at 77K. and CO2 at 273K, using an automatic adsorption system (Autosorb-6, Quantachrome). The micropore volume, Vpp (N2), was determined by application of Dubinin-Radushkevich equation to the N2 adsorption isotherm at 77K up to P/Po< 0.1. The volume of narrow micropores, Vnpp (DR,C02>, (mean pore size lower than 0.7 nm) was calculated from CO2 adsorption at 273 K. 

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