Nitrogen desorption method

The surface area was calculated using the BET equation,36 while the total pore volume and the average pore size were calculated from the nitrogen desorption branch applying the Barrett-Joyner-Halenda (BJH) method.37 BET and BJH adsorption measurements were carried out with a Micromeritics Tri-Star system on both the supports and the calcined catalysts. Prior to measurements, the samples were evacuated at 433 K to approximately 50 mTorr for 4 h. 

While for macroporous structures the inner surface can be calculated from the geometry, meso and micro PS layers require other methods of measurement First evidence that some PS structures do approach the microporous size regime was provided by gas absorption techniques (Brunauer-Emmet-Teller gas desorption method, BET). Nitrogen desorption isotherms showed the smallest pore diameters and the largest internal surface to be present in PS grown on low doped p-type substrates. Depending on formation conditions, pore diameters close to, or in, the microporous regime are reported, while the internal surface was found to... 

For membranes with pore diameters smaller than 3.5 nm, the nitrogen adsorption/desorption method based on the widely used BET theory ean be employed. This measurement technique, however, is good only for pore diameters ranging from 1.5 nm to 100 nm ( = 0.1 micron). Typical data from this method are split into two portions adsorption and desorption. The nitrogen desorption curve is usually used to describe the pore size distribution and corresponds better to the mercury intrusion curve. Given in Figure... 

Nitrogen adsorption/desorption. One of the most common techniques used for analyzing size distributions of mesopores is the nitrogen adsorption/desorption method. The method is capable of describing pore diameters in the 1.5 nm to 100 nm (or 0.1 micron) range. Pore size distribution can be deteimined from either the adsorption or the desorption isotherm based on the Kelvin equation ... 

Figure 4.11 Differential pore size distribution of a Y-AI2O3 membrane by the nitrogen adsorption/desorption method [Anderson etal., 1988]...

This method has been applied to ceramic membranes (e.g., gamma-alumina membranes) and compared to other methods such as nitrogen adsorption/desorption and thermoporometry (to be discussed next) in Figure 4.13. It can be seen that the mean pore diameter measured by the three methods agrees quite well. The pore size distribution by permporometry, however, appears to be narrower than those by the other two techniques. Similar conclusions have been drawn regarding the comparison between permporometry and nitrogen adsorption/desorption methods applied to porous alumina membranes [Cao et al., 1993]. The broader pore size distribution obtained from nitrogen adsorption/desorption is attributable to the notion that the method includes the contribution of passive pores as well as active pores. Permporometry only accounts for active pores. 

Two mesoporous silica molecular sieves synthesized by using n-octadecyl-ammonium bromide and n-dodecylammonium bromide as a templates were characterized for their pore size distribution by temperature programmed desorption method and low temperature nitrogen adsorption method. The pore size distributions and total pore volumes determined by the two methods agree quite well and are within experimental error. 

Figures 6 and 7 show PSDs calculated from nitrogen adsorption method and-thermo-desorption method.

The spectrum of pore dimensions. Pores of radius 10-300 A. are studied by analyzing, according to the C. Pierce method, the nitrogen desorption isotherms at the temperature of liquid nitrogen 21). For pores of radius between 300 A. and 6m, the mercury porosimetry under high pressure 22, 23) is used. 

In both cases the pore distributions are deformed and the location of the pore size distribution peaks differs considerably from those obtained from the nitrogen adsorption method. If is too slow, desorption will be too slow, so that air can mix with the vapour. When p < 1 atm the evapouration from the pores at each measuring temperature is possi-... 

Broens, Bargeman and Smolders( ) reported on the use of nitrogen sorption/desorption method for studying pore volume distributions in ultrafiltration membranes. The pore volume distributions were calculated for a cylindrical capillary model. More recent results from the same laboratory are published in this volume ( ). In our view, applicability of cylindrical pore models for asymmetric membranes should be verified, rather than assumed. This can be done, for example, by analysis of both branches of the sorption isotherm. For a reasonable model choice, the two pore volume distributions should be in substantial agreement. 

It is likely, therefore, that pores analyzed by the nitrogen sorption/desorption method are Interstices in the subsurface strata. In the membranes presented here, these strata consisted of agglomerated spherical nodules. It is worth noting that curve maxima in Figures 4b,d are in the unexpected order A>B>C. 

The total surface area of the catalyst was determined using a Carlo Erba Sorpty 1750 by applying BET method. The pores volume and size distribution were established with a Sorptomatic instrument 1900 Carl Erba, using the nitrogen desorption isotherm of the sample, assuming cylindrical pores and physical adsorption of the gas (theory of Wheeler). 

Physico-chemical Characterization Surface area, pore volume and pore size distribution of alumina samples were determined by adsorption- desorption isotherm of nitrogen at 77K using Sorptomatic 1900 (Carlo Erba Instruments, Italy). The sample was degassed at 200°C for 2-3 hr. under vacuum ( lO mm Hg) prior to N2 adsorption. Surface area was calculated using B.E.T. isotherm. Pore size distribution was determined from nitrogen desorption data at p/p° = 0.03 and above, using the method of Barret, Joyner and Halenda (3). 

The BET specific surface area measurements were carried out on a Micromeritics ASAP 2000 analyser, using nitrogen or krypton at -196°C. Samples were previously outgassed under vacuum at 150°C. The pore size distribution and total pore volume were determined by using the BJH (desorption) method. 

Of all existing GC techniques for determining the specific area of a solid, the heat desorption method is the one most often used. This method was developed by Nelson and Eggertsen and modified by a number of workers. In principle, the heat desorption method is based on the traditional Brunauer, Emmett, Teller (BET) technique in which the quantity of adsorbed gas (usually nitrogen) at a temperature near its boiling point is determined. By determining the adsorption at various pressures, it is possible, using the BET equation, to calculate the amount of adsorbate required for the formation of a monolayer. 

Choudhary, V.R. Mayadevi, S., and Kamble, K..R., Adsorption of oxygen and nitrogen on AIPO4-5 and SaPO-5 at moderate pressures using a novel adsorption/desorption method, Ind. Eng. Chem. Res., 33(5), 1319-1323 (1994),... 

The adsorption-desorption method is a popular and commonly used method for characterization of surface and structural properties of porous materials, allowing the determination of their surface area, pore-size distribution, pore volume and adsorption energy distribution. Nitrogen is often used for the adsorbent gas but other adsorbent gases such as argon can also be used. According to this method, adsorption-isotherm (amount of adsorbed gas versus relative pressure [pressure/saturation vapor pressure of the adsorbent]) is drawn and the data are analyzed by assuming capillary condensation. 

To determine the distribution of pores with diameters smaller than 20 nm, a nitrogen desorption technique is employed which utilizes the Kelvin equation to relate the pore radius to the ambient pressure. The porous material is exposed to high pressures of N2 such that P/Po 1 and the void space is assumed to be filled with condensed N2, then the pressure is lowered in increments to obtain a desorption isotherm. The vapor pressure of a liquid in a capillary depends on the radius of curvature, but in pores larger than 20 nm in diameter the radius of curvature has little effect on the vapor pressure however, this is of little importance because this region is overlapped by the Hg penetration method. 

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