The single point BET method

The BET theory requires that a plot of [W Pq/P — 1)] versus P/Pq be linear with a finite intercept [cf. equation (4.38) and Fig. 4.1]. By reducing the experimental requirement to only one data point, the single-point method offers the advantages of simplicity and speed often with little loss in accuracy. 

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The specific surface areas of the samples were measured by the single point BET method (p/pQ=0.3). 

Measured using the single-point BET method with 20 % nitrogen in helium. 

As before, surface areas were measured by the single point BET method using a Micromeritics Flowsorb II 2300 analyser and a mixture of 30% N2 in He as the adsorbate gas. The observed values are 16 m2g 1 and 5 m2g 1... 

The supported catalysts were characterized for their basicity distribution by temperature programmed desorption (TPD) of CO2 from 50 to 950 C at a linear heating rate of 20°C.min in a flow of helium(40cm. min ). The surface area of the catalysts was determined by the single-point BET method, using a Monosorb surface-area analyser (Quanta Chrome Corp. USA). 

A fixed relative pressure of either 0.2 or 0.3 is often used with the single point BET method. The resulting errors are presented in Table 2.1 and it can be seen that this method should not be used for materials having c values less than 100. 

The Cr203 content of each catalyst was determined by atomic absorption spectroscopy (Varian/Spectr AA-20 plus) on acid-digested samples. Total surface areas were determined by a single point BET method (nitrogen adsorption-desorption at 77.5 K) using a mixture of 29.7% N2 in helium. Samples were wet-loaded into the flow tube and dried at 423 K in a hydrogen flow for 15 minutes and then for another 30 minutes at 513 K before cooling in helium. 

The GMA-EGDM copolymer was synthesised from a GMA EGDM molar feed ratio of 31.5 68.5. The crosslinking density of the copolymer beads was 128%. Particle size of copolymer was in the range of 250-420 pm and the surface area estimated by single point BET method was 152 M / g. The pore volume estimated by mercury porosimetry was 0.74 cm /g. The polymer bound 2-picolyl amine was generated by the reaction of 2-picolyl amine... 

The theory for single-point BET measurements is similar however, an assumption is made that the intercept of the PIV(P0-P) versus PIPQ plot equals zero. This assumption is not always valid and can result in surface area values that differ from multipoint values. When the assumption is valid, the single-point method is simpler and faster, and it results in accurate surface area values. 

Figure 2 shows a typical multipoint BET plot for bendroflumethiazide, and Table 2 summarizes the parameters and calculations used to obtain the surface area for this compound. The single-point surface area calculated from a PIP0 of 0.3108 is also presented in Table 2. Good agreement is evident between the two methods, considering the C constant given in Table 2 for this sample. 

The analysis immediately above discloses that, when the BET C value is 100, the single-point method using a relative pressure more than three... 

As indicated in Fig. 5.1, curve Y which is the BET curve for an arbitrary C value, approaches curve X, the single-point curve, as the value of C increases. In the limiting case of C equal to infinity the BET curve is coincident with the single-point curve. For all other C values the single point curve lies above the BET curve and their difference vanishes as the relative pressure approaches unity. Thus, as the value of C increases the knee of the isotherm becomes sharper and moves toward lower relative pressures. For lower C values, curves X and Y diverge and higher relative pressures must be used to make single-point surface areas conform to those obtained by the multipoint method. 

The assumption of a zero intercept reduces the BET equation to equation (5.3). This assumption is, of course, not realizable since it would require a BET C value of infinity. Nevertheless, many samples possess sufficiently high C values to make the error associated with the single-point method acceptably small (see Chapter 5 and Table 5.1). 

BET surface area measurements were performed using the single point method on a Micromeritics Flow Sorb II 2300 equipment. Vanadium and aluminum contents in the different prepared catalysts were determined by ICP technique. The total nitrogen content in the oxynitride samples was determined using the method described by Guyader et al [9]. Particle size distribution of the samples was determined using a Coulter LSI30 equipment. 

BET surface area was measured with the single point method, using a Micromeritics Flow Sorb II2300. XRD patterns were recorded using a Siemens D-5000 powder diffractometer equipped with a Ni-filtered Cu Ka source. XPS spectra were collected on a SSX-100 Model 206 Surface Science Instrument... 

Surface area and pore structure The BET specific surface areas (m g l) of the nanoparticles left for digestion at various times (0.5h, 8h, 24h and 25d) and dried thereafter at 110 C, 200 C, 300 C, 40QOC, 500°C and 700°C, were determined by the single-point method of N2 adsorption at T=77K using a Carlo Erba 1750 Sorpty apparatus. The results are shown in Figure 3. 

Synthesized powders were subjected to the following battery or characterization methods X-ray and electron diffraction, He-pycnometry, single point BET gas absorption, TEM, XPS analysis based on a 101.3 eV silicon 2p binding energy for stoichiometric SigN powders neutron activation, and wet chemical analysis. The activity of SijN powders, both exposed and unexposed, was determined by measuring the surface area of powders annealed in nitrogen as a function of temperature. 

ITie BET method is the most widely used procedure for determining the surface area of porous materials. In this chapter, BET results were obtained from single point measurements using a Micromeritics Flowsorb II 2300 surface area analyzer. A mixture of nitrogen in helium (30 70 mole percentage) was used. Although this simple method is not quantitative for the microporous materials studied in section 5, it still allows qualitative comparisons to be made. 

The surface area of graphite is determined by nitrogen adsorption using a BET single point method (equipment is available from Quatachrome Instruments, Boynton Beach, FL, USA). 

Sbet - BET specific surface area V, - single-point total pore volume w - pore width at the maximum of the pore size distribution calculated using the BJH method with the corrected form of the Kelvin equation [34]. 

Adsorption/desorption isotherms of nitrogen at 77 K were measured with an automated apparatus ASAP 2010 (Micromeritics, USA). The specific surface areas, Sbet, were calculated from the linear form of the BET equation, taking the cross-sectional area of the nitrogen molecule to be 16.210 m. Pore size distributions were calculated in the standard maimer by using BJH method [6]. The total pore volumes, Vp, for the samples under study were determined from a single point adsorption at a relative pressure of 0.98 by converting the value of the adsorbed gas to the volume of the liquid adsorbate. 

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