Nitrogen adsorption/desoiption

Characterisation of silica low-density xerogels in presence of additives by image analysis and nitrogen adsorption-desoiption... 

Fig. 1. Nitrogen adsorption/desoiption isotherms for CexSBA samples.

To determine the surface area and pore size distribution by gas adsorption/desoiption measurements, nitrogen is most commonly used as the adsorbate, since liquid nitrogen is readily available and inexpensive. Total smface area can be determined from the isotherm between 0.05 and 0.3 relative partial pressme, at which point a complete monolayer has been formed on the sample surface. At higher partial pressures, the pores start to be filled by capillary condensation. From an analysis of the shape of the full isotherm, the distribution of pore sizes and total pore volume can be determined. The adsorption process is generally taken as completely reversible, but imder some conditions the isotherm may exhibit a different shape in desorption, as compared to absorption. This is called hysteresis. Sometimes, hysteresis data can be used to determine the structure and size of pores in the absorbent. Determinations of pore size distribution using adsorption/desorption isotherms are described in previous Porosity measurement chapter. 

As an example, Fig. 6 shows the experimental isotherms of the adsorption and desoiption of hydrogen on carbon fiber Busofit-M8 and wood-based activated carbon WAC 3-00 at the nitrogen temperature. Absence of an appreciable hysteresis confirms that reversible physisorption exclusively takes place with all investigated materials. 

Fig. 3.1 Adsorption of nitrogen on activated clay catalyst. Open circles indicate adsorption solid circles desoiption. Total BET surface. Sy, =339 m3 g-1 monomolecular volume V = 78.0 cm3 at STP.

Fig. 3.16 Adsorption-desorption isotherm of nitrogen on silica gel empty circles, adsorption black circles, desoiption.

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