Nitrogen desorption

The narrow pore size distribution of TUD-1 is illustrated in Figure 41.5 by the single peak derived from the nitrogen desorption isotherm. Moreover, an important feature of the material is the easy tunability of the pore sizes over a wide range while maintaining a narrow pore size distribution. 

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... 

Figure 3.7. Pore size distribution by nitrogen desorption of an unsupported experimental alumina membrane film.

Typical pore size distribution data are shown in Fig. 5, where the integral penetration of mercury into the pores is plotted as a function of applied pressure. The calculated pore diameters in angstroms are shown across the top. The integral curve clearly shows a bimodal pore distribution with mean pore diameters at 20,000 and 50 A. The latter is at the lower limit of the technique. A nitrogen desorption isotherm is required to obtain an accurate measure in the region below 100 A. 

Figure 6. Carbon Norit R 0.8 Extra PSD determined by nitrogen desorption data (N2), and PSDs of pores filled by water unfrozen in different environments at T < 273 K.

The nitrogen desorption isotherm was treated according to the model developed by Delon and Dellyes for parallel plate pores in phyllosilicates. The mesopore liquid volume obtained using this treatment was relatively small,... 

Figure 1 Apparent pore volume distributions of particulate silicas Gl-G to G5-G according to BJH from the nitrogen desorption isotherms.

Figure 1. Apparent pore size distributions obtained for (a) whole and (b) fragmented samples from batch E2. ( , mercury intrusion and Kloubek correlation o, mercury extrusion and Kloubek correlation , mercury extrusion and new correlation , nitrogen adsorption , nitrogen desorption).

The results of simulations demonstrating the effect of various factors on nitrogen desorption from porous solids are presented in Figs. 16-18. To describe the desorption process, we use Eqs. (24) and (30). The adsorption branch of the isotherm is described by Eq. (18). The size distributions of necks and voids are assumed to be lognormal ... 

By applying a vacuum to the system after adsorption is complete and measuring the rate of nitrogen desorption it is possible to obtain the pore volume of the material as well as the size distribution of the pores (under 50 nm) of the material. ... 

As with experimental sorption data, it is possible to obtain the ratio Ff as a function of pressure (and thus pore size via the Kelvin equation) for simulations of the nitrogen sorption experiment on the model grids derived from NMR images. In order to determine the effect, if any, of the macroscopic heterogeneities (non-randomness) in the spatial distribution of voidage and pore size on the nitrogen desorption isotherm it is... 

As is seen PSDs calculated from the data obtained by using different techniques are close together. It should be noted that in the case of the nitrogen desorption data corrections with respect to the surface film thickness were introduced. 

Assessment of the total pore volume liberated shows that thermal treatment of the mesophase up to 120°C does not liberate any porosity. Heating up to 150°C liberates up to 25% of the total porosity whereas only around 50% of the total porosity is liberated when the mesophase was heated up to 300°C. The maximum pore volume is obtained after thermal treatment up to 760°C. When comparing these results with those obtained by SCTA, it would seem that pore blocking occurs. Indeed, after heating to 150°C, with around 25% of the porosity liberated, around 45% of the surfactant is removed. After heating to 300°C, with around 50% of the porosity liberated, up to 70% of the surfactant is removed. We have observed that it is only after treatment to 500°C, with the loss of almost 80% of the total surfactant, that an equivalent amount of pore volume becomes accessible. This pore blocking effect may also explain why the nitrogen desorption isotherms at 77K do not rejoin the adsorption branches at relative pressures below 0.2. 

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. 

The nonlocal density functional theory (NLDFT) has been used to characterize mesocaged structures.[164] NLDFT analysis gives accurate information about the cage size, the total meso- and micropore volumes and surface area, and the pore-wall thickness in combination with XRD measurements. Argon- and nitrogen-desorption data on FDU-1 provided evidence that there are two major populations of pore entrances. Argon desorption was superior in providing information about pore connectivity in FDU-1 samples. 

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