Solids outgassing

For instance9 as shown in Table I, when 0,6 ml of Si(0C2H ) is mixed with 10 ml of water and treated at 150 C under the corresponding water pressure the precipitated solids outgassed at 100 C have specific surface areas between 52 and 578 m /g according to the initial pH conditions ... 

The results obtained for the adsorption of butane on a ball-milled caldte" are also of interest. When the solid was outgassed at 150°C to remove physically adsorbed water, the butane isotherm was ofType II with c = 26 (Fig. 5.5, curve (ii)) but outgassing at 25°, which would leave at least a monolayer of molecular water on the surface, resulted in a Type 111 isotherm (Fig. 5.5, curve (i)). Though butane is nonpolar its polarizability is... 

Fig. 5.5 Adsorption isotherms of butane at 0°C on Iceland Spar ground for 1000 h. Curve (i), the solid was outgassed at 25°C. Curve (ii), the solid was outgassed at 1S0°C. O, adsorption p Q, desorption.

Fig. 5.20 Adsorption isotherms for water vapour on x-Fe,Oj at 15°C for various outgassing temperatures. Solid points indicate second isotherm after 25°C evacuation of physically adsorbed water. (Courtesy Zettlemoyer.) Outgassing temperature,<, 25°C , I00°C O, 250°C ...

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. 

Rapp, D.B. and J.E. Shelby, Photo-induced hydrogen outgassing of glass.. Non-Crystalline Solids, 349, 254-259,2004. 

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, ... 

On immersion of even well outgassed charcoal in water we must imagine that many of the micropores are not filled with water and thus an apparent diminution of the density of the solid occurs. These micropores cannot of course be empty for otherwise there is no reason why the liquid wetting the sides of the tubes should not fill them. If the micropores still contain small traces of gases not removed by the process of outgassing these will be compressed into pockets at the ends of the blind tubes, the pressure in the case of a micropore of 100 A. diameter would be about sixteen atmospheres. 

Chemical composition of fresh HTs was determined in a Perkin Elmer Mod. OPTIMA 3200 Dual Vision by inductively coupled plasma atomic emission spectrometry (ICP-AES). The crystalline structure of the solids was studied by X-ray diffraction (XRD) using a Siemens D-500 diffractometer equipped with a CuKa radiation source. The average crystal sizes were calculated from the (003) and (110) reflections employing the Debye-Scherrer equation. Textural properties of calcined HTs (at 500°C/4h) were analyzed by N2 adsorption-desorption isotherms on an AUTOSORB-I, prior to analysis the samples were outgassed in vacuum (10 Torr) at 300°C for 5 h. The specific surface areas were calculated by using the Brunauer-... 

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...

FIGURE 9. Changes in the absorption spectrum of 5,5 -dichloro-3,3, 9-triethylthiacarbocyanine (dye 3) adsorbed on Lippmann emulsion grains. Solid curve, measured in air dashed curve, measured after 16-hr vacuum outgassing dotted curve, measured after readmission of room air (266). 

Usually, the solid adsorbent under investigation must first be freed, as far as possible from previously adsorbed gases and vapours. Evacuation to c. 10 4 Torr (outgassing) for several hours will usually remove physically adsorbed gas. It is difficult, and often impossible, to remove chemisorbed gas completely unless the solid is heated to a high temperature (c. 100-400°C). Such treatment might cause sintering and an alteration of the sorptive capacity of the solid. 

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. 

The amount of adsorbate (n) may be expressed in different units (moles, grams, and cubic centimeters at STP or liquid volume) however, recommendations have been made to express it in terms of moles per gram of the outgassed solid [7],... 

The IUPAC/SCI/NPL programme on surface area standards examined a number of carbon blacks, activated charcoals, and silicas, and in the resulting publication [33] the results obtained in a number of laboratories were compared. As a result, two carbon blacks and two silicas lacking microporosity were accepted as standards. A major conclusion of this work, namely that outgassing conditions determine results obtained with high-area solids, was reinforced by the unsuccessful attempts made by the European Union s Community Bureau of Reference to obtain reproducible results with silica gels intended as reference standards [8e]. 

Fig. 13. Reflectance spectrum of MgO after outgassing at various temperatures and exposure to 133 nm2 (1 Torr) oxygen at room temperature to quench fluorescence solid curve, outgassed at 773 K dotted curve, outgassed at 923 K dashed curve, outgassed at 1073 K. [Figure according to Zecchina et at. (146).]...

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