Adsorption-desorption isotherm technique

The adsorption/desorption isotherms measured by NMR (equivalent to conventionally measured isotherms), extracted from two different regions of the imaging field of view corresponding to the two ceramics, are shown in Figure 3.5.9. Once these local isotherms are extracted, they are simply the local adsorption for that point in space contained within the material, measured non-invasively and non-destructively. Conventional analysis techniques for adsorption isotherms (such as BET theory) can therefore be applied to the data, to determine the microstructural properties corresponding to that isotherm curve. 

Surface Acoustic Waves (SA Ws). The basic idea of this technique is to use the dependence of the frequency and propagation of surface acoustic waves on mass loading in a film. The porous film has to be deposited on a piezoelectric substrate (quartz), which is then placed into a physisorption setup to condense nitrogen at 77 K. Adsorption and condensation of N2 result in a shift of the oscillation frequency, and thus measurements of the oscillation frequency as a function of N2 partial pressure provide an adsorption-desorption isotherm.30 Although the technique has proven to provide a concise characterization of porous films,29,30 the requirement for the deposition directly onto the SAW piezoelectric substrate represents a certain restriction. 

The BET surface areas and pore diameters were determined by nitrogen adsorption/ desorption isotherms at 77 K using a static volumetric technique (Quantachrome Autosorb 1). Before the physisorption measurements the samples were outgassed at 100° C for 15 hours under vacuum. 

The porous properties of mesoporous layers can be characterized conveniently using classical methods based on the N > adsorption-desorption isotherm at 77 K. However, it is important to use additional techniques based on transmembrane transport properties in order to characterize the passing-through pores. Moreover, these dynamic methods will easily detect the... 

For description of textural properties of carbonaceous adsorbents, adsorption/desorption isotherms of vapours and gases in static conditions as well as mercury porosimetry are used. The latter method often leads to destruction of porous structure of investigated materials while the usage of the former one is affected by the specific properties of molecular sieves described above. Taking into account these limitations, in this work the authors have made an attempt of determination of porous structure of carbon molecular sieves with the used of the pycnometric technique. 

Mesoporous melamine-formaldehyde and phenolic-formaldehyde resins were synthesized in the process of polymerization in the presence of fumed silica as an inorganic template. The surface and structural characteristics of the obtained sorbents were investigated using XPS technique and sorption from gas phase. The parameters characterizing porous structure of the synthesized resins in a dry state were determined from nitrogen adsorption/desorption isotherms. The sorption processes of benzene and water vapor accompanied by simultaneous swelling of both polymers were also studied. 

The textural properties of all calcined samples were determined by nitrogen isotherms at liquid N2 temperature, using a Micromeritics ASAP 2010 apparatus (static volumetric technique). Before determination of adsorption-desorption isotherms the samples ( 0.2 g) were outgassed for 16 h at 350 °C under vacuum. 

The interpretation of adsorption-desorption isotherms provides a wealth of information on the texture of the adsorbent. The main parameters that can be assessed arc specific surface area, pore distribution, specific porous volume and information on the structure (pore shape, interconnection, etc.). The technique is highly suitable for the study of samples where the pore size is between approximately 2 and 50 nm, which corresponds to the mesoporous domain for which the adsorbed gas has liquid phase properties well described by thermodynamic models. 

The bulk densities were calculated from weight and volume measurements. Skeletal densities were measured by He pycnometry N2 adsorption-desorption isotherms were determined at 77 K on a Carlo Erba Sorptomatic 1900 and their analysis was done using a set of well-known techniques [5], Mercury porosimetry up to a pressure of 200 MPa is performed on a Carlo Erba Porosimeter 2000. Samples were examined using a transmission electron microscope to obtain particle and aggregate sizes [2]. 

Pore dimensions for all samples studied were calculated from the adsorption data. Adsorption/desorption isotherms were recorded using a conventional volumetric technique with nitrogen adsorbate at 77 K. The adsorption isotherms were measured with ASAP 2010 (Micromeritics) automatic gas adsorption apparatus. The specific surface areas were determined from the nitrogen adsorption isotherms using the BET method, assuming the cross section ofN2 molecule as 0.162 nm. ... 

There are other techniques that can be used to characterize the thin film. For powdered materials, nitrogen adsorption-desorption isotherms are typically used to determine the pore volume and surface area. However, this technique is rather difficult for films, as the... 

As a conclusion, one can say that the thermogravimetric technique is a usefull method in the investigations of the porosity of solids. Our analysis, based on the Kelvin equation, of the thermogravimetric curves for silica gel wetted with liquid n-butanol and carbon tetrachloride leads to core/pore size distributions curves which are similar, but not identical in shape to the pore size distribution curves derived by standard procedure from low temperature nitrogen adsorption/desorption isotherms. The linear heating mode is... 

Before pyrolysis, specific surface areas (Sbet) vary from 280 to 510 m /g, and pass through a maximum when the pH increases. The microporous volume slightly increases with the pH and seems to reach a step (0.20 to 0.25 cm /g). The total porous volume (Vp) decreases when pH increases. This pore volume varies from 0.25 to more than 1.30 cm /g. This last value is very high for materials dried by simple evaporation. Note that the total porous volume of samples containing macropores (i.e.. Cl, Nil, Pdl and Pd2) is underestimated by N2 adsorption-desorption isotherm analysis because of the technique... 

The aim of this work is to explore the applicability of the sol-gel method for the preparation of Ag/Si02 and Cu/Si02 catalysts and to see whether such a method can yield silver and copper species stabilized by the carrier. Characterization of the catalyst structure by several physical and chemical techniques, including N2 adsorption-desorption isotherms, mercury porosimetry measurements, X-ray diffraction and transmission electron microscopy, has been used to correlate the microstructure of Ag/Si02 and Cu/Si02 catalysts with their catalytic performance. 

Gas adsorption is by far the most common technique used to gain information about the porous structure from the adsorption/desorption isotherms. As adsorbent, nitrogen is the most commonly used. The treatment of results here is based on a cylindrical pore model. 

Powder X-ray diffraction patterns were collected on a Sbimadzu XD-3A diffractometer operated with Cu Ka radiation (X=1.541sA). Infrared spectra were recorded on a Nicolet 510P FT-IR instrument using KBr wafer techniques. The surface area and pore size distribution of the sample were measured on a Micromeritics ASAP 2000 instrument. The sample was first degassed at SSO C, and followed by the measurement of Nj adsorption-desorption isotherm at liquid nitrogen temperature. 

The gas adsorption-desorption technique relates to the adsorption of nitrogen (or, less commonly, carbon dioxide, argon, xenon, and krypton), at cryogenic temperatures, via adsorption and capillary condensation from the gas phase, with subsequent desorption occurring after complete pore filling. An adsorption-desorption isotherm is constructed based upon the relationship between the pressure of the adsorbate gas and the volume of gas adsorbed/desorbed. Computational analysis of the isotherms based on the BET (Brunauer-Emmett-Teller) (Brunauer et al. 1938) and/or BJH (Barrett-Joyner-Halenda) (Barrett et al. 1951) methods, underpinned by the classical Kelvin equation, facilitates the calculation of surface area, pore volume, average pore size, and pore size distribution. 

Figure 24-1. Nitrogen adsorption/desorption isotherms of a mesoporous film with cubic structure functionalized with -CNgroups after surfactant removal by solvent extraction were measured by a surface acoustic wave (SAW) technique andyieldeda type IVisotherm with a very narrow hysteresis loop that is typicalfor mesoporous materials. Inset is pore size distribution calculated from adsorption isotherm. (Liu, N., Assink, R. A. and Brinker, C. J. Chem. Commun. 2003 370-371, Reproduced by permission of The Royal Society of Chemistry)...

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