Nitrogen adsorbed, area

The reported surface area is the combined surface area of zeolite and matrix. In zeolite manufacturing, the measurement of the zeolite surface area is one of the procedures used by catalyst suppliers to control quality. The surface area is commonly determined by the amount of nitrogen adsorbed by the catalyst. 

Data taken from the adsorption leg of the isotherm of Figure 17.11 are listed in the first two columns of the following table. Test the applicability of the following equilibrium theories (a) Langmuir (b) infinite BET and (c) Harkins and Jura. From (a) and (b) obtain estimates of the surface area of the adsorbent and compare the values with that obtained by the point B method. One molecule of nitrogen adsorbed on alumina occupies 0.162 nm2. 

The usual method of measuring the statistical thickness is to divide the liquid volume of nitrogen adsorbed, by the BET surface area... 

The results obtained by Loebenstein and Deitz agreed with vacuum volumetric measurements on a large variety of samples with a wide range of surface areas. They were also able to establish that the quantities of nitrogen adsorbed were independent of the presence of helium. 

From 500°C to 600°C, the samples maintain pratically their high surface area (more than 600 m2/g), even if from a general point of view the value decreases slowly. For both MCM-41 and MCM-48 materials, the value of specific surface area decreases dramatically after calcination at 600°C. The maximum volume of nitrogen adsorbed by samples decreases from 1200 cm3/g for calcination at 500°C to 180 cm3/g for calcination at 700°C for the MCM-48 materials and from 1130 cm3/g to 200 cm3/g for the MCM-41 one. These observations indicate that a calcination temperature superior to 600°C can destroy almost completely the structure of MCM-41 and MCM-48 materials. 

The sharp increase in the adsorbed volume of nitrogen due to capillary condensation for the sample obtained with decane is relatively vertical, reflecting the homogeneity of the sample which has a high specific surface area of 750 m2/g. For other compounds, the capillary condensation is less pronounced, meaning that only a part of material is well crystallized. This is confirmed by the low value of nitrogen adsorbed and low specific surface areas (500 m2/g for heptane, 405 m2/g for undecane). 

From the quantity of nitrogen adsorbed at the three nitrogen flowrates which, in turn, correspond to three relative pressures, a plot of the BET equation is obtained. The surface area is then calculated according to Equation 11.1. 

Vapors in equilibrium with liquid in fine capillaries or pores will have depressed vapor pressure as a result of the Kelvin effect. In fact, if the pores are adequately small, the vapor will condense at pressures far below normal. By measuring the volume of nitrogen adsorbed at a relative pressure, i.e., p/po, of 0.99 and with prior knowledge of the surface area, the average pore diameter can be calculated. 

There are two values of surface area and volume of nitrogen adsorbed (BJH method), obtained with the parent H-Y zeolite and the H-Y/TFA sample (Table 1) the first corresponds to the zeolite-type micropores and the other, to the mesopores. Figure 1 shows the pore size distribution of the H-Y/TFA catalyst there is a sharp peak (not shown here) in the micropore region and another peak at 4nm in the mesopore region. Such a bimodal pore size distribution was also observed with the parent zeolite. 

Volume Adsorbed by Particles—The volume of gas adsorbed by particles is a function of the surface-area. Most soils at ordinary temperatures and pressures adsorb from 5 to 8 cc of nitrogen per g. Colloid fractions of these soils averaging less than 0.05 u adsorb more than three times these amounts. It is interesting to point out that approximately 0.8 of the nitrogen is adsorbed in forming a monomolecular layer. With regard to activated charcoal and silica gel, the amounts of nitrogen adsorbed may be more than 20 times those mentioned above for soils. 

The surface of a certain soil is measured by means of the Emmett-Brunauer method using nitrogen. The amount of nitrogen adsorbed at STP is 6 cu cm per g. Calculate the surface-area of the particles per cu cm and their average diameter (assumed spheres). 

An early normalizing procedure, proposed by Kiselev (1957) to compare adsorption isotherms of hydrocarbons, water vapour, etc. on a series of different adsorbents, was simply to plot the surface excess concentration F (=n/A), obtained from a knowledge of the BET-nitrogen surface area, A (BET), versus p/p°. It is also possible to plot, instead of f, the reduced adsorption , n/nm, which still relies on the BET method to determine the monolayer capacity nm but does not require knowledge of the molecular cross-sectional area a. 

It is generally assumed that a nitrogen molecule occupies 16.4 on the polar silica surface. The adsorbent surface area is then calculated as a product of the total amount of nitrogen in the monolayer (n ) and the nitrogen molecular area (16.4 A ). 

Adsorbent surface area is calculated as the product of the monolayer capacity estimated from BET equation and nitrogen molecular area, con,- If the... 

The adsorption isotherms of nitrogen at -196 °C for the samples studied are shown in Fig. 4. All the isotherms are type II in the BDDT classification [10]. The BET apparent surface areas values are in Fig. 4. The increase in the temperature of treatment produces a decrease in the amount of nitrogen adsorbed, with an important decrease in the BET areas due to an ordering in the sample structure. The great differences in the BET surface areas of the samples make it possible to measure a wide range of aetive surface areas. 

As evidenced by curves on figures 4 and 5, changes in texture characteristics are quasi-linearly related to increasing carbon content. It also appears that calculated surface areas strongly depend on the physisorption conditions, while measured adsorbate volumes are less affected. This observation is in favor of erroneous assumptions on nitrogen molecule area for the calculations of specific surface area [10]. But, as mentioned above, nitrogen molecule could penetrate in smaller pores, increasing measured micropore volume and surface areas. 

Nitrogen isotherms were measured by using an ASAP (Micromeritics) at 77K. Prior to each analysis, the samples were outgassed at S73K for 10 - 12 h to obtain a residual pressure of less than 10 torr. The amount on nitrogen adsorbed was used to calculate specific surface area, and the micro pore volumes determined from the BET equation [14] and t-plot method [15], respectively. Also, the Horvath-Kawazoe model [16] was applied to the experimental nitrogen isotherms for pore size distribution. 

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. ... 

A measure of the area available for adsorption. The larger the surface area, the greater the adsorptive capacity. Measured by determining the amount of nitrogen adsorbed by the carbon and reported as m /g. 

It has to be stressed that the monolayer surface phase capacity is assumed to be constant over the whole bulk concentration region, i.e., n = const., for x (0,1). Under this assumption we can assess the specific surface areas of the solid adsorbents if the cross - sectional areas of adsorbed molecules are known. However, the following question arises here what molar areas to assign to the different kinds of molecules This problem is similar in the case of gas - solid adsorption and it may be sufficient to refer to the compilation by McLellan and Harnsberger [13]. It has been found that cross - sectional molar areas calculated by means of the molar volumes of the pure components are mostly in agreement with nitrogen surface area values [14]. 

Values for the specific surface area of OMC and other porous carbons can be obtained from the APD. As mentioned above, in this data treatment method, no assumptions about the pore geometry are made. Thus, a possible source for error is eliminated. However, the APD of the carbon sample should show a monolayer formation peak. Thus, the surface of the carbon material has to have a certain graphitic order. This is, for example, the case for graphitized and furnace CB. For these samples, the completion of the monolayer formation is clearly indicated by a minimum in the APD at the high adsorption potential end of the monolayer formation peak, located at adsorption potentials of 2.3 and 2.8 kj/mol, respectively (Fig. 18.7). The amount of nitrogen adsorbed corresponds to the monolayer, from which the specific surface area can be calculated. The surface areas calculated by the APD and the BET method differ by 7% and 15%, respectively (Table 18.2). For activated carbons, the difference between the surface areas obtained by APD and the DFT method is usually less than 10% [39]. 

The total surface area or specific surface area (area/unit weight) is determined by the nitrogen absorption method known as the BET (Brunauer, Emmett, and Teller) absorption isotherm of an inert gas. The principle of this technique is based on the monolayer adsorption of nitrogen at low temperature, which has a fixed spherical volume. Thus, the amount of nitrogen adsorbed is proportional to the total surface area of the sample. 

Surface areas of the supports were determined from desorption isotherms of nitrogen adsorbed at -198 °C after evacuation at 350 °C for 5 h on ASAP 2010, Micromeritics, USA. 

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