BET area

Comparison of BET areas from nitrogen isotherms with independent values... 

The most direct test is to compare the BET area with the geometrical area of the solid. Unfortunately, comparisons of this kind are relatively rare on account of experimental difficulties. The choices are to work with, say, single crystals having a well defined surface, when techniques of quite extraordinary sensitivity will be needed for measurement of the adsorption or, to obtain a larger surface area by use of thin sheets, narrow rods or small spheres, and run the risk that the surface will not be truly smooth so that the actual area will exceed the geometrical area. 

Most tests of the validity of the BET area have been carried out with finely divided solids, where independent evaluation of the surface area can be made from optical microscopic or, more often, electron microscopic observations of particle size, provided the size distribution is fairly narrow. As already explained (Section 1.10) the specific surface obtained in this way is related to the mean projected diameter through the equation... 

Various other substances have also been used in tests. Robens, for example, with glass spheres 20 to 60 pm in diameter, obtained a BET area (taking = 16-2 A ) which was only 5 per cent higher than the... 

Similar discrepancies were found for other vapours by Harris and Emmett, who quoted their results as the ratio fi of the BET area calculated from the isotherm of the particular adsorbate to the BET nitrogen area (Table 2.7). The value of fi varied, again sometimes widely, for any one gas on different adsorbents, so that the divergences could not be removed by use of a single revised value of a for a given vapour. 

Striking confirmation of the conclusion that the BET area derived from a Type IV isotherm is indeed equal to the specific surface is afforded by a recent study of a mesoporous silica, Gasil I, undertaken by Havard and Wilson. This material, having been extensively characterized, had already been adopted as a standard adsorbent for surface area determination (cf. Section 2.12). The nitrogen isotherm was of Type IV with a well defined hysteresis loop, which closed at a point below saturation (cf. F, in Fig. 3.1). The BET area calculated from it was 290 5 0 9 m g , in excellent agreement with the value 291 m g obtained from the slope of the initial region of the plot (based on silica TK800 as reference cf. p. 93). 

The discrepancy between the pore area or the core area on the one hand and the BET area on the other is proportionately larger with silica than with alumina, particularly at the higher degrees of compaction. The fact that silica is a softer material than alumina, and the marked reduction In the BET area of the compact as compared with that of the loose material, indicates a considerable distortion of the particles, with consequent departure of the pore shape from the ideal of interstices between spheres. The factor R for cylinders (p. 171), used in the conversion to pore area in the absence of a better alternative, is therefore at best a crude approximation. 

One is obliged to conclude that this method, like those which derive the cumulative surface area from pore size calculations, can be regarded as no more than ancillary to the BET or Point B methods. The few cases where reasonable agreement with the BET area is obtained are probably to be explained by compensation of opposing effects. 

Core wall area from "Pore wall area from BET area... 

The method has been applied by Rootare and Prenziow" to the determination of the surface area of twenty different powders having BET areas in the range 0 1 to 110 g where the pores would be mainly or entirely in the form of the interstices between the particles. The value... 

Usually the plaques produced by either method are coined (compressed) in those areas where subsequent welded tabs are coimected or where no active material is desired, eg, at the edges. The uncoined areas usually have a Bmnauer-Emmet-TeUer (BET) area in the range of 0.25—0.5 m /g and a pore volume >80%. The pores of the sintered plaque must be of suitable size and intercoimected. The mean pore diameter for good electrochemical efficiency is 6—12 p.m, deterrnined by the mercury-intmsion method. 

Specimen Bum-off (%) BET Area (mVg) Pore volume [t-method] (cmVg) DA Pore radius (nm)... 

Specimen ID Pre-activation. density (g/cm ) % DKDX fibers Bum-off (%) BET area (mVg) DR pore vol. (cmVg)... 

Porous polymers with large 77 K nitrogen surface areas also have infenor methane uptakes. For example Dow XV 43546, a porous polystyrene with a 77 K nitrogen BET area of 1600 mVg has a methane uptake of 67 mg/g at 25°C, 3.4 MPa, whereas Takcda HGK 971 carbon of similai" 77 K nitrogen surface area has an uptake of 116 mg methane per g. Rohm and Haas Amberlite porous polymers give similarly lower than expected methane uptakes. 

In addition to actual synthesis tests, fresh and used catalysts were investigated extensively in order to determine the effect of steam on catalyst activity and catalyst stability. This was done by measurement of surface areas. Whereas the Brunauer-Emmett-Teller (BET) area (4) is a measure of the total surface area, the volume of chemisorbed hydrogen is a measure only of the exposed metallic nickel area and therefore should be a truer measure of the catalytically active area. The H2 chemisorption measurement data are summarized in Table III. For fresh reduced catalyst, activity was equivalent to 11.2 ml/g. When this reduced catalyst was treated with a mixture of hydrogen and steam, it lost 27% of its activity. This activity loss is definitely caused by steam since a... 

The benefits of the method are appreciated when the textural parameters are compared. Data derived from N2-physisorption isotherms show that Fenton detemplation leads to improved textural parameters, with BET areas around 945 m g for a pore volume of 1.33 cm g , while calcination leads to reduced textural parameters (667m g 0.96cm g ). T-plot analysis, strictly speaking, is not apphcable for these bi-modal materials but it gives a good estimate. It shows that the micropore volume is doubled, which corresponds to an increase in the calculated micropore area from about... 

Samples CO uptake / pmol g BET area 4,6-DMDBT HDS conversion / %... 

Sample Au loading (wt.%) Ti02 crystal size (nm) BET area Pre-treatment Au particle size before (after) reaction (nm)... 

The BET area of a catalyst or a catalyst support is one of the first properties one wants to know in catalyst development. All industrial laboratories and many academic laboratories possess equipment for measuring this property. 

What is the difference between this area and the BET area ... 

The Ru metal area was determined by volumetric H2 chemisorption in the quartz U-tube of an Autosorb 1-C set-up (Quantachrome) following the procedure described in ref. [16]. Prior to chemisorption, the catalysts were activated by passing 80 Nml/min high-purity synthesis gas (Pnj / Phj -1/3) from a connected feed system through the U-tube and heating to 673 K for alkali-promoted catalysts or to 773 K for alkali-free catalysts with a heating rate of 1 K/min. The BET area was measured by static N2 physisorption in the same set-up. 

As expected, 7-AI2O3 (BET area 110 m /g) turned out to be the more stable support with a higher surface area than MgO (BET area 52 m /g). The BET area of RU/AI2O3 was found to be 104 m /g after NH3 synthesis at 773 K which decreased significantly to 70 m /g as a result of cesium impregnation. After NH3 synthesis at 773 K, the specific area of Ru/MgO was observed to be 25 m /g compared with 52 m /g found for the MgO support. Cesium impregnation caused a further decrease in specific area to 23 m /g. 

Catalyst BET area Ni content Average Ni particle size... 

BET area Kg-1) H2 temperature-programmed reduction (25-500° C) p.molH2g1 N2 (N20) formed (mmolg-1) ... 

The electrocatalysts for oxygen reduction were prepared as follows. These complex compounds were inoculated onto the carbon (AG-3, BET area near 800 m2/g) by means of adsorption from dimethylformamide solutions. The portion of complex compound weighed so as to achieve 3% of Co content was mixed with the carbon, then 5 ml of dimethylformamide per 1 g of the carbon were added and the mixture was cured at room temperature for 24 hours. Series of samples obtained were thermally treated (pyrolyzed), and the resulting grafted carbons were tested as electrode materials in the reaction of molecular oxygen reduction. 

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