Brunauer-Emmett-Teller nitrogen

EXHAUST CONTROL, AUTOMOTIVE] (Vol 9) Brunauer-Emmett-Teller nitrogen adsorption... 

Nitrogen adsorption was performed at -196 °C in a Micromeritics ASAP 2010 volumetric instrument. The samples were outgassed at 80 °C prior to the adsorption measurement until a 3.10 3 Torr static vacuum was reached. The surface area was calculated by the Brunauer-Emmett-Teller (BET) method. Micropore volume and external surface area were evaluated by the alpha-S method using a standard isotherm measured on Aerosil 200 fumed silica [8]. Powder X-ray diffraction (XRD) patterns of samples dried at 80 °C were collected at room temperature on a Broker AXS D-8 diffractometer with Cu Ka radiation. Thermogravimetric analysis was carried out in air flow with heating rate 10 °C min"1 up to 900 °C in a Netzsch TG 209 C thermal balance. SEM micrographs were recorded on a Hitachi S4500 microscope. 

EA = elemental analysis IR = infrared spectroscopy PXRD = powder X-ray diffraction BET = Brunauer-Emmett-Teller method (specific BET surface area) and BJH = Barrett-)oyner-Halenda method (determination of pore volume and diameter), both determined by nitrogen physisorption ... 

Measurement of Surface Area. The Teachability determined by these methods is usually reported as g/cm day. The total surface area of particulate material can be assessed 1) by assuming a particle shape e.g.spherical) and estimating the number of particles, or 2) by measurements using the Brunauer-Emmett-Teller (BET) nitrogen adsorption technique ( ). Unfortunately, the BET method measures the area of surfaces to which nitrogen has access this is not necessarily the same as the area to which a solution has access. Access by solutions requires much larger pore areas. 

Brunauer-Emmett-Teller (BET) analysis was carried out on the ceramic aggregate with both nitrogen and krypton as adsorbates. The sample was crushed and dry sieved, and the fraction between 74 and 589 micrometer was retained for analysis. Results were repeatable, and consistent for the two gases. Over the range of firing and proportioning conditions studied, specific surface area of the ceramic aggregate was between 0.3 and 6 m Vg, which can be compared, for example, to a non-porous... 

The most common method used for the determination of surface area and pore size distribution is physical gas adsorption (also see 1.4.1). Nitrogen, krypton, and argon are some of the typically used adsorptives. The amount of gas adsorbed is generally determined by a volumetric technique. A gravimetric technique may be used if changes in the mass of the adsorbent itself need to be measured at the same time. The nature of the adsorption process and the shape of the equilibrium adsorption isotherm depend on the nature of the solid and its internal structure. The Brunauer-Emmett-Teller (BET) method is generally used for the analysis of the surface area based on monolayer coverage, and the Kelvin equation is used for calculation of pore size distribution. 

Surface area Nitrogen adsorption multipoint Brunauer-Emmett-Teller method... 

The heats of adsorption of nitrogen on evaporated metal films of nickel and iron have been reported to decrease from 10 to 5 kcal./mole as the surface coverage increased from about 0.1 to 1.0 monolayer. Beeck (150) states that nitrogen is unsuitable for the evaluation of surface areas of evaporated iron and nickel films by the Brunauer-Emmett-Teller method because of its high heat of adsorption at 78°K., which would yield a value for monolayer adsorption too high by 50%. The author feels that this objection does not apply to the nitrogen adsorption on reduced electropolished planar copper plates for the following reasons. 

The Brunauer-Emmett-Teller (BET) nitrogen adsorption isotherms for EG and DG are shown in Fig. 5. Both the samples exhibit Type I + Type II behavior and their surface areas are 925 and 520 m2 g 1 respectively. The pore size distributions of the samples are shown as insets in Fig. 5, and reveal that EG is... 

The area is an important surface parameter for catalytic studies. It is needed to evaluate the rate constant of the surface reaction from the kinetics as well as to allow a fair comparison to be made of the effectiveness of different catalysts. Areas are commonly determined by nitrogen or krypton gas adsorption interpreted by the Brunauer-Emmett Teller (BET) isotherm [30, 32], A number of other methods has been proposed and utilised including microscopy, isotopic exchange, chromatography, gas permeability, adsorption from solution, and negative adsorption (desorption) of co-ions [30, 33]. 

The sigmoidal shape of the nitrogen isotherm of Figure 1 is quite amenable to analyses by the Brunauer-Emmett-Teller (BET) multilayer theory (8). The BET surface area calculated is 2.8 mVg. This value is quite consistent with the predominant l-to-10- xm size distribution observed on this sample with an optical microscope. Assuming cubic habits and density of about 2.3 g/cm, we find the range of specific surface area to be 0.26 to 2.6 m /g. Apparently the sorbed N2 does penetrate into the internal portion of the particles only to a limited extent. 

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. 

The specific surface area of solid materials is usually determined by applying the Brunauer-Emmett-Teller (BET) equation to nitrogen adsorption data between relative pressures (P/Pq) approximately 0.05 and 0.3 [51]. However, there are many shortcomings of the BET model. For example, it does not consider adsorption in pores. It is well known that the BET method seriously overestimates the specific surface area for many porous materials. For carbons, the theoretically highest possible specific surface area is approximately 2630 m /g... 

Example 8-2 From the Brunauer-Emmett-Teller plot in Fig. 8-4 estimate the surface area per gram of the silica gel. Use the data for adsorption of nitrogen at — 195.8°C. 

The nitrogen adsorption-desorption isotherms for specific surface area and porosity assessment were recorded at -196 C in a Gemini instrument from Micromeritics. The specific surface areas were determined by the Brunauer-Emmett-Teller (BET) method. The pore size distributions were obtained from the desorption branch, and the micropore volume was determined by the t-plot method, using literature software [14]. 

Brinker and Scherer (8) pointed out that the area of a surface is defined largely by the method of surface area measurement. Many of the measurements of surface areas in work reported before the 1980s were based on the method of determining monolayer capacity of an adsorbent molecule of known cross-sectional area. In the Brunauer-Emmett-Teller (BET) method (45) the apparent surface area is determined from nitrogen adsorption. However, because the nitrogen molecule surface area is 16.2 A2, this definition of the surface excludes microporosity that is accessible, for example, to water molecules. 

In their pioneering studies of silica sols, Alexander and Iler (4) employed low-temperature nitrogen adsorption to determine the surface areas of the colloidal particles after removal of the aqueous medium. The Brunauer-Emmett-Teller (BET) areas were found to be only slightly larger than the values obtained from the particle size distributions as determined by light scattering and electron microscopy. These remarkable measurements indicated little change in the particle size or shape after the stabilized silica sols were carefully dried. 

---