Surface area determination by nitrogen adsorption

Various organic wastes, such as waste wood chip, sake lees, used tealeaves and so on, were carbonized with the super-heated water vapor at 623 K. By the 30 - 90 minutes treatment, the Oiganic wastes lost about SO - 90 % of their original weight. The capability of gas adsorption has been evaluated. The surface areas determined by nitrogen adsorption for the carbonized materials were much smaller than that of the activated carbon Granular Shirasagi. The surftce area determined by carbon dioxide adsorption, on the other hand, of the carbonated materials were almost the same order of m itude to that of the activated carbons. These results show that the carbonized materials have micro-pores whose diameter is less than SO nm. 

It can be seen in fig. 4 that the presence of the doping element increase the surface area of the zirconia compound as well as its textural stability. Specific surface areas above 200 m /g can be obtained., twice higher than those from conventional preparations under similar heat treatments (500°C). The values of surface areas determined by nitrogen adsorption are close to those calculated ffom the crystallite sizes of XRD patterns and according to the assumptions made in ref [7]. Under thermal treatment crystallite growth is not intense but inter-crystallite sintering occurs in the case of V, Nb and Cr dopants and consequently a strong decrease in the specific surface area is observed. 

In all these powders the surface area determined by nitrogen adsorption is in approximate agreement with the area calculated from the distribution of particle sizes bbserved in electron micrographs, assuming a density of-2.2 g cm (66). From this it can be concluded that particles of this type have a porosity to nitrogen of less than 5-10 vol. %. 

The surface area can be calculated from particle size measured with transmission electron microscope (TEM) (ASTM D3849). Generally, for rubber grade, the surface areas determined by TEM are in reasonable agreement with surface areas determined by nitrogen adsorption measurements. However, for those carbon blacks that have highly developed micropores such as special pigment blacks and blacks used for electrical conductivity, the surface areas calculated from their particle diameters are smaller than those calculated from gas absorption, as the internal surface area in the micropore is excluded. 

The numerical values of and a, for a particular sample, which will depend on the kind of linear dimension chosen, cannot be calculated a priori except in the very simplest of cases. In practice one nearly always has to be satisfied with an approximate estimate of their values. For this purpose X is best taken as the mean projected diameter d, i.e. the diameter of a circle having the same area as the projected image of the particle, when viewed in a direction normal to the plane of greatest stability is determined microscopically, and it includes no contributions from the thickness of the particle, i.e. from the dimension normal to the plane of greatest stability. For perfect cubes and spheres, the value of the ratio x,/a ( = K, say) is of course equal to 6. For sand. Fair and Hatch found, with rounded particles 6T, with worn particles 6-4, and with sharp particles 7-7. For crushed quartz, Cartwright reports values of K ranging from 14 to 18, but since the specific surface was determined by nitrogen adsorption (p. 61) some internal surface was probably included. f... 

The distillation fractions were also analysed for their caibon and hydrogen contents using a Leco CHN Determinator which was also used for similar analysis of die used catalysts. The hydrocracked liquid and the used catalysts were analysed for their sulphur contents using a Leco Suli iur Determinator. Some specific surface area analysis by nitrogen adsorption was carried out on the used catalysts using a Micromeritics instrument... 

Crystal structures of the bismuth molybdate and of the mixed iron and cobalt solid solution molybdate samples were controlled by X-ray diffraction (10). The chemical compositions of the samples were determined by atomic absorption and their surface areas measured by nitrogen adsorption using the BET method. 

More recentfy Mohlin and Gray (9J) determined adsorption isotherms on cellulose fibers for a variety of adsorbates (solutes). From the experimental type II isotherms specific surface areas of the fibers were computed, for eadi solute, with the results en in Table 11. The agreement observed between the different solutes is quite remarkable considering that the area of the solute molecule on the potymer surface must be known or estimated. Hie sirface area determined by nitrogen adsorption measurements at —196° was included for the purpose of comparison. The sli t di arity could possibly indicate that the area available to the smaller nitrogen molecule may be somewhat larger (1.9 compared to 1.6 m g" ). 

When a sol of such porous particles was heated at 90°C, pH 9.8, the area determined by nitrogen adsorption decreased to 22 m g, indicating that the diameter of the internal pores had decreased so that nitrogen molecules could not penetrate and thus could be adsorbed only on the outer surface. However, microporosity still existed because the area determined by OH" ion adsorption was still 707 m g". By heating the sol for a longer time at pH 10, the pores could be further closed and at least the surface became completely impervious. Some water and alkali were no doubt trapped within. 

Table 2.4 (a). Determination of adsorbed volume of nitrogen (b). surface area determination by gas adsorption... 

Comparison of surface areas determined by mercury porosimetry and by nitrogen adsorption ... 

Specific surface areas of the catalysts used were determined by nitrogen adsorption (77.4 K) employing BET method via Sorptomatic 1900 (Carlo-Erba). X-ray difiraction (XRD) patterns of powdered catalysts were carried out on a Siemens D500 (0 / 20) dififactometer with Cu K monochromatic radiation. For the temperature-programmed desorption (TPD) experiments the catalyst (0.3 g) was pre-treated at diflferent temperatures (100-700 °C) under helium flow (5-20 Nml min ) in a micro-catalytic tubular reactor for 3 hours. The treated sample was exposed to methanol vapor (0.01-0.10 kPa) for 2 hours at 260 °C. The system was cooled at room temperature under helium for 30 minutes and then heated at the rate of 4 °C min . Effluents were continuously analyzed using a quadruple mass spectrometer (type QMG420, Balzers AG). 

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). 

The structure of the catalysts was characterized by X-ray diffraction, IR-spectroscopy and transmission electron microscopy, their thermal stability was followed by thermal analytical method. The specific surface area and pore size distribution of the samples were determined by nitrogen adsorption isotherms. 

The NH4-Beta-300 (Zeolyst International, number denote Si02/Al203 molar ratio) was transformed to corresponding proton form using a step calcination procedure at 500 °C. H-Beta-300 was partially modified with Fe by repeated ion-exchange method (Fe(III)nitrate). The surface areas as well as acidities (Bronsted and Lewis acid sites) of Fe-Beta (iron content - 0.1 wt %) were determined by nitrogen adsorption and pyridine desorption at 250, 350 and 450 °C using FTIR spectroscopy [6]. 

The BET surface areas and pore diameters were determined by nitrogen adsorption/ desorption isotherms at 77 K using a static volumetric technique (Quantachrome Autosorb 1). Before the physisorption measurements the samples were outgassed at 100° C for 15 hours under vacuum. 

The detailed synthesis procedure and textural properties (surface area, Sggy in m2 g-1 pore volume, V in ml g"1 and main pore diameter, d in nm), determined by nitrogen adsorption from 8.E.T. method have been published elsewhere (refs. 13-18) and are summarized in Table 1, where the surface acidity and basicity of supports are also collected. These values were determined by a spectro-photometric method described elsewhere (ref. 19), that allows titration of the amount (in tunol g 1) of irreversibly adsorbed benzoic acid (BA, pKa> 4.19), pyridine (PY, pka= 5.25) or 2,6-diterbutyl-4-methylpyridine (DTMPY, pKa 7.5) employed as titrant agents of basic and acid sites, respectively. Furthermore, the apparent rate constant values of different supports in the gas-phase skeletal isomerization of cyclohexene (CHSI), in Mmol atm"1 g"1 s-1, at 673 K, are also collected in Table 1, because these values are another way of measuring the stronger acid sites of supports (ref. 19). 

There are various kinds of polytetrafluoroethylene. One is granular polymer consisting of spongy, white particles having a median size of about 600/l The specific surface of this polymer is on the order of 2 m2/g (determined by nitrogen adsorption and calculations by the method of Brunauer, Emmett, and Teller). Since this specific surface area is about 1700 times the observed outer surface of the particles, these measurements confirm the porous, spongelike structure that can be seen in the photomicrograph of a cross section of several particles in Fig. la. 

All of the hydrotaleite-derived magnesia supports were prepared by first coprecipitating magnesium aluminum hydroxycarbonate in the presence of Mg and Al nitrates, KOH, and K2C03 according to procedures already described (8,9). Hydrotaleites were then decomposed by calcination at 873 K for 12-15 h to yield the binary oxide to be used for a catalyst support. The specific surface area of the hydrotaleites determined by nitrogen adsorption was typically about 220 m2g 1 after calcination. X-ray powder diffraction patterns of the materials were recorded on a Scintag X-ray diffractometer. 

The specific surface area, determined by adsorption and desorption of nitrogen for titania, goethite and silica samples were 13.5, 42.0, and 261.7 m2/g, respectively. 

Fumed silica aggregates are obviously linear and branched particle structures with a mean size of about 100 to 200 nm. By TEM we derive the size of the partially fused primary particles of about 10 run. This very small particle size correlates well with the high surfaces area of fumed silica which usually is larger than 100 m g as determined by nitrogen adsorption at 78 K according to BET [5]. Adsorption techniques and electron microscopy provide very close values of surface areas. This indicates that fumed silica exhibits a smooth particle surface in the range of nanometers, apparently its surface is free of micropores. 

Leica Cambridge Stereoscan 360). The SEM micrographs of H-ZSM-5, Zn-H-ZSM-5 and Ga-H-ZSM-5 exhibited the typical crystal form of ZSM-5 zeolite and the average crystal size was 2 pm-4 pm. The quantitative analysis of the Ga and Zn content in the ZSM-5 zeolite was performed by an X-ray fluorescence analyzer X-MET 880 (Outokumpu). The specific surface area of the synthesized zeolite catalysts was determined by nitrogen adsorption using a Sorptomatic 1900 (Carlo Erba Instmments). The specific surface area calculated by the Dubinin method was found to be 427, 445 and 372 m /g for Zn-H-ZSM-5, Ga-H-ZSM-5 and H-ZSM-5, respectively. It was observed from different characterization techniques that the introduction of Ga and Zn by this method did not destroy the structure of the ZSM-5 zeolite. 

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