Barrett-Joyner-Halenda method

TPRS = temperature-programmed reaction spectroscopy XRD = X-ray diffraction BET = Brunauer-Emmett-Teller method (specific BET surface area) and BJH = Barrett-Joyner-Halenda method (determination of pore volume and diameter), both determined by nitrogen physisorption NMR= characterization by solid-state NMR. 

With respect to the values of the apparent specific surfiice area, we have to note here that modem instmmentation permitting the measurement of nitrogen adsorption isotherms is usually provided with software permitting surface area calculation according to several mathematical models, including the BJH (corresponding to the names of Barrett, Joyner, Halenda) method, Langmuir adsorption isotherms, and the BET theory these methods may provide for the same sample of a porous material, values that may differ by as much as 30%. 

Physical properties of calcined catalysts were investigated by N2 adsorption at 77 K with an AUTOSORB-l-C analyzer (Quantachrome Instruments). Before the measurements, the samples were degassed at 523 K for 5 h. Specific surface areas (,S BEX) of the samples were calculated by multiplot BET method. Total pore volume (Vtot) was calculated by the Barrett-Joyner-Halenda (BJH) method from the desorption isotherm. The average pore diameter (Dave) was then calculated by assuming cylindrical pore structure. Nonlocal density functional theory (NL-DFT) analysis was also carried out to evaluate the distribution of micro- and mesopores. 

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. 

The nitrogen adsorption-desorption isotherms were obtained at 77K by AutoSorb-1 -C (Quantachrome). Prior to measurement, the samples were outgassed at 300°C for 3 h. The specific surface areas of the samples were determined from the linear portion of the BET plots. Pore size distribution was calculated from the desorption branch of N2 desorption isotherm using the conventional Barrett-Joyner-Halenda (BJH) method, as suggested by Tanev and Vlaev [15], because the desorption branch can provide more information about the degree of blocking than the adsorption branch. 

Figure 1. Plots of differential pore volume against pore diameter calculated from the N2 gas adsorption isotherms obtained from meso/macroporous carbon specimens I (-0-), II (- -), and III (-A-) using Barrett-Joyner-Halenda (BJH) method. Reprinted with permission from G. -J. Lee and S. -I. Pyun, Carbon, 43 (2005) 1804. Copyright 2005, with permission from Elsevier.

However, the characterization of composite isotherms, i.e those indicating the presence of supermicropores and/or mesopores, requires the combined application of more than one method to deduce the PSD extending over the micro- and mesopore range e g. the Barrett-Joyner-Halenda (BJH) method [6] to evaluate the mesopore range and one of the DR-DA-DS, HK, BP (i.e. the method of Mikhail et al. [7]) for the evaluation of microporosity. Micropore volume and surface area estimates are practically obtained by using the empirical a,5-plot method [1]. 

Important trends in N2 isotherm when the PS beads are used as a physical template are shown in Table 1 and Fig. 2. In Table 1, PI is the alumina prepared without any templates, P2 is prepared without ]4iysical template (PS bead), P3 is prepared without chemical template (stearic acid), and P4 is prepared with all templates. For above 10 nm of pore size and spherical pore system, the Barrett-Joyner-Halenda (BJH) method underestimates the characteristics for spherical pores, while the Broekhoff-de Boer-Frenkel-Halsey-Hill (BdB-FHH) model is more accurate than the BJH model at the range 10-100 nm [13]. Therefore, the pore size distribution between 1 and 10 nm and between 10 and 100 nm obtained from the BJH model and BdB-FHH model on the desorption branch of nitrogen isotherm, respectively. N2 isotherm of P2 has typical type IV and hysteresis loop, while that of P3 shows reduced hysteresis loop at P/Po ca. 0.5 and sharp lifting-up hysteresis loop at P/Po > 0.8. This sharp inflection implies a change in the texture, namely, textural macro-porosity [4,14]. It should be noted that P3 shows only macropore due to the PS bead-free from alumina framework. 

Bulk Si/Al ratios were determined by AAS. Surface areas and pore volumes were determined by N2 absorption isotherms measured at liquid nitrogen temperature using a Micromeritics ASAP 2000M (Table 1). The zeolites were degassed under vacuum at 150°C for the as-s)mthesised and 450°C for the modified zeolites for at least 3 hours. The total surface area was derived using the BET equation [12], the micropore volume and the external surface area (ESA) were estimated by means of the t-plot method of Lippens et al [13] and the total and mesopore volumes were calculated by Barrett-Joyner-Halenda anaylsis of the desorption branch of the N2 isotherm [14]. 

Such more realistic models of porous materials can also be used to rigorously test existing characterization methods. The model material is precisely characterized (we know the location of every atom in the material, hence the pore sizes, surface area and so on). By simulating adsorption of simple molecules in the model material and then inverting the isotherm, we can obtain a pore size distribution for any particular theory or method. Such a test for porous glasses is shown in Figure 8, where the exactly known (geometric) PSD is compared to that predicted by the Barrett-Joyner-Halenda (BJH) method, which is based on the modified Kelvin equation. 

DFT). In addition, the Barrett-Joyner-Halenda (BJH) method is generally available for the computation of mesopore size distribution. 

Pore size distribution (PSD) was calculated by using the desorption data, following the method by Barrett-Joyner-Halenda [6] with correction for the surface film thickness. 

The most relevant characteristic of porous materials is the disposal of a high effective surface/volume relationship, usually expressed in terms of their specific surface area (area per mass unit), which can be determined from nitrogen adsorption/desorption data. Different methods are available for determining the specific surface area (Brunauer-Emmett-Teller, Langmuir, and Kaganer), micropore volume (f-plot, ttj, and Dubinin-Astakhov), and mesopore diameter (Barrett-Joyner-Halenda Leroux et al., 2006). Table 1.1 summarizes the values of specific surface area for selected porous materials. 

For a qualitative determination of the mesopore size distribution, mathematical models have to be used. Of these, the Barrett, Joyner, Halenda (BJH) method [30] is widely used for OMCs [14, 31, 32] and other carbon materials. However, for OMCs this model has some important shortcomings. As already mentioned above, the OMC mesopores might be as narrow as 2 nm. For such mesopores, the BJH method seriously underestimates the pore width [33]. Thus, improved data treatment methods have been proposed [33, 34]. As an example, the mesopore size distributions for an OMC of the CMK-1 type calculated with... 

Figure l8.6 Mesopore size distribution of an ordered mesoporous carbon (OMC) (sample CMK-IF(A) of Ref. [13]) calculated with the Barrett, Joyner, Halenda (BJH) [30] and a modified BJH method [34], respectively, using desorption data. 

The solids were analyzed by X-ray diffraction (XRD) using Cu-Ka radiation in the range between 0.5 to 20 °2Q. The data obtained from the nitrogen adsorption/desorption isotherms were used to estimate the specific surface area SBE-d [6], the pore size distribution by the Barrett-Joyner-Halenda (BJH) method [7] and to establish the micropore volume (F icro) by the t-plot analysis [8]. 

Nitrogen adsorption-desorption isotherms at 77K were measured with a Quantachrome Autosorb 1 instrument. The sample surface area was calculated using the Brunaner-Emmett-Teller (BET) model [12], whereas the pore size distribution was obtained using the Barrett-Joyner-Halenda(BJH) model [13]. The l-plot method was used to check the microporosity [14]. 

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