Nitrogen adsorption surface area

Brunauer and Emmett 120), in their extensive studies on synthetic ammonia catalysts have concluded, by a comparison of the CO uptakes and nitrogen adsorption surface area measurements, that on pure iron at temperatures between —78 and — 183°C CO chemisorbs up to one molecule per surface atom. Beebe and Stevens 121) from measurements of differential heats of adsorption confirmed that chemisorption rather than physical adsorption was occurring in this system. 

Oxide products from thorium oxalate decomposition are normally characterized by their behavior as slurries. In addition, they have been characterized by means of electron micrograph pictures, their nitrogen adsorption surface areas, particulate properties as measured by sedimenta-... 

The evaluation of pore size distribution by application of the Kelvin equation to Type IV isotherms has hitherto been almost entirely restricted to nitrogen as adsorptive. This is largely a reflection of the widespread use of nitrogen for surface area determination, which has meant that both the pore size distribution and the specific surface can be derived from the same isotherm. 

Physico-chemical Characterization Surface area, pore volume and pore size distribution of alumina samples were determined by adsorption- desorption isotherm of nitrogen at 77K using Sorptomatic 1900 (Carlo Erba Instruments, Italy). The sample was degassed at 200°C for 2-3 hr. under vacuum ( lO mm Hg) prior to N2 adsorption. Surface area was calculated using B.E.T. isotherm. Pore size distribution was determined from nitrogen desorption data at p/p° = 0.03 and above, using the method of Barret, Joyner and Halenda (3). 

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. 

Adsorption of nitrogen and surface area determination by BET method is probably the most widely used method for surface area characterization of reinforcing fillers (Kraus and Jansen, 1978 Wampler, 1997 Magee, 1995a,b). 

ASAP2010M [Micromeritics, USA] is used—a fully automatic computer controlled setup for physical adsorption of gases [nitrogen, argon, carbon dioxide, etc] for pressures down to 1 mPa. The measurement is based on the adsorption and desorption of an inert gas on the whole surface of the sample at the same temperature. The adsorption and desorption isotherms are usually measured in the range of p/Po 0.001 to 0.999 in liquid nitrogen. The surface area... 

Physical adsorption isotherms involve measuring the volume of an inert gas adsorbed on a material s surface as a function of pressure at a constant temperature (an isotherm). Using nitrogen as the inert gas, at a temperature close to its boiling point (near 77K), such isotherms are used to determine the amount of the inert gas needed to form a physisorbed monolayer on a chemically unreactive surface, through use of the Brunauer, Emmett, and Teller equation (BET). If the area occupied by each physisorbed N2 molecule is known (16.2A ), the surface area can then be determined. For reactive clean metals, the area can be determined using chemisorption of H2 at room temperature. Most clean metals adsorb one H atom per surface metal atom at room temperature (except Pd, which forms a bulk hydride), so if the volume of H2 required for chemisorption is measured, the surface area of the metal can be determined if the atomic spacings for the metal is known. The main use of physical adsorption surface area measurement is to determine the surface areas of finely divided solids, such as oxide catalyst supports or carbon black. The main use of chemisorption surface area measurement is to determine the particle sizes of metal powders and supported metals in catalysts. 

This study does not make a note of the problematic status of the BET equation and the way it is used to interpret isotherm shape (as explained in Chapter 4) particularly for carbons with mixed porosities of micro and mesopores. The interpretation problems are minimized for Type-I isotherms (adsorption in micropores only) but are maximized for carbons with mesopores of various shapes. Should carbons, of a microporous nature, also possess porosity of dimensions <1.0 nm, then activated diffusion effects (Chapter 4) prevent entry of nitrogen as an adsorbate at 77 K into these smallest of micropores. It is always advisable to check out the validity of the nitrogen BET surface area by comparing with the adsorption of carbon dioxide at 273 K. 

Sur ce area and pore size characterizations were performed using a Micromeritics ASAP2000 gas adsorption surface area analyzer. The specific surface area of the samples was determined from the nitrogen isotherms at 77K and by using the BET equation. Micropore volume was determined using the DR equation and the total volume of pores was calculate at a relative pressure (p/p ) of 0.97. Table 1 shows the activation conditions and the structural characterization. More details are given elsewhere [3]. 

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

TPDE profile of SO2 from ACF sdter the adsorption of SO2 and O2 exhibited two adsorbed species. More SO2 was found over modified ACF than those of as-received or benzene treated ACFs. Pyridine derived grain accelerates the adsorption and oxidation activity against SO2. The larger amount of deposited carbon provides an increased number of nitrogen atoms, however the surface area tends to decrease. Thus, the balance between amount of nitrogen and surface area must be optimized for the highest activity. 

N2-adsorption-desorption isotherms were determined using a Digisorb 2600 surface-area, pore-volume analyzer (Micromeritics Instrument Corporation). The samples were first outgassed at 200 C for 3 h, and the isotherms were recorded at liquid nitrogen temperature. Surface areas were calculated using the BET equation. 

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 adsorption of a gas by a solid can, in principle, be made to yield valuable information as to the surface area and pore structure of the solid. In practice the range of suitable adsorptives is quite narrow, by far the most commonly used one being nitrogen at its boiling point, 77 K. 

A Type II isotherm indicates that the solid is non-porous, whilst the Type IV isotherm is characteristic of a mesoporous solid. From both types of isotherm it is possible, provided certain complications are absent, to calculate the specific surface of the solid, as is explained in Chapter 2. Indeed, the method most widely used at the present time for the determination of the surface area of finely divided solids is based on the adsorption of nitrogen at its boiling point. From the Type IV isotherm the pore size distribution may also be evaluated, using procedures outlined in Chapter 3. 

Specific surface area of carbon blacks before and after graphitization, determined by electron microscopy (A,) end by nitrogen adsorption (A ]t... 

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