Outgassing the adsorbent

Prior to determination of an isotherm, all physisorbed material has to be removed from the surface of the adsorbent. This is best achieved by exposure of the surface to high vacuum, the exact conditions required (temperature and residual pressure) being dependent on the particular gas-solid system. In routine determinations of surface area it is generally advisable not to remove any chemisorbed species which may be present thus, the hydroxylated oxides are usually outgassed at 1S0°C. Microporous adsorbents such as zeolites or active carbons however require higher temperatures (350-400 C, say) for complete removal of physisorbed material from their narrowest pores. An outgassing period of 6-10 hours (e.g. overnight) is usually sufficient to reduce the residual pressure to 10 Torr. 

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Outgassing the adsorbent sample 79 Aim of the outgassing 79 Conventional vacuum outgassing 80 Controlled vacuum outgassing by means of CRT A 81 Outgassing with a carrier gas 83... 

The BET method (Brunauer, Emmett and Teller, 1938) with N2 as the adsorbate, is by far the most common method of measuring the surface areas of Fe oxides. Various commerical instruments are available for these measurements. The method involves measuring the extent of adsorption of N2 (at the boiling temperature of liquid N2 - 77 K) on the outgassed solid as a function of the relative pressure, p/po. he. the adsorption isotherm p is the partial pressure of the adsorbate and po is its equilibrium vapour pressure. The following linear relationship exists between the amount adsorbed, v, (cm g ) and the relative vapour pressure, p/po, ... 

Figure 6.2,15 shows the adsorption isotherms of C02 on the CaHAP particles with different Ca/P ratios (67). Before the measurement of the first isotherms, shown by the open symbols, the samples were outgassed at 300°C for 2 h. The second set of isotherms, shown by the solid symbols, were measured on the samples outgassed at 25°C for 2 h after taking the first isotherms. The first and second isotherm sets are parallel, and the adsorbed amount of the second isotherm is less than that of the first one, which signifies that a part of the C02 adsorbed is irreversibly adsorbed. The amount of the irreversibly adsorbed C02 (noted as n,) was evaluated by subtracting the adsorbed amount at 500 torr in the second isotherm from that in the first one. The values are plotted against Ca/P ratios by the open symbols in Figure 6.2.16. Similar results were reported on SrCaHAP (29), SrHAP (68), and MgCaHAP...

The specific surface area of the fibers was determined using inert gas adsorption in a commercial volumetric adsorption system (Micromeritics Instrument Corp.). Krypton gas was used because of its sensitivity to the small specific surface areas of the glass fibers ( 0.2 mz/g). The fibers were degassed at 100°C to a pressure of 80mTorr before introducing the adsorbate gas into the sample chamber. Several samples were also outgassed at 80 and 200°C (to 80 mTorr) to confirm that outgassing was sufficiently complete under the standard test conditions. A standard five-point surface area determination was made for each inert gas adsorption experiment. 

The adsorption of a gas or vapour can be measured by admitting a known amount of the adsorbate into an evacuated, leak-free space containing the outgassed adsorbent. The extent of adsorption can then be determined either volumetrically or gravimetrically. 

When an outgassed solid (the adsorbent) is confined to a closed space and exposed to a gas or vapor (the adsorptive) at a given pressure and temperature, an adsorption process takes place. The adsorptive molecules are transferred to, and accumulate in, the interfacial layer, as a consequence of an attractive force between the surface of the solid and the adsorptive (the adsorptive actually adsorbed by the adsorbent is named adsorbate). After some time, the pressure becomes constant and the thermodynamic equilibrium of adsorption is achieved. 

Qzm has been treated to decrease the surface hydrophilicity. Three different hydrophobization procedures have been followed (i) the condensation of silanols into siloxane bridges by outgassing the powder at 1073 K, largely irreversible by re-exposure to water (ii) the functionalisation of silanols with tryethylsilyl groups and (iii) treatment with diluted HF. This last treatment dissolves the external disordered layers of particles containing impurities. In Figure 4, the amount of BSA adsorbed on the three treated quartz dusts is compared with untreated samples of pure quartz. All treatments decreased the... 

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