Krypton, adsorption isotherms

Gulbransen and Andrew (33) show that the C + 02 reaction at 500° C. causes an increase in surface area as determined by krypton adsorption isotherms. 

Duval, X. and Thorny, A. (1975). Interpretation of krypton adsorption-isotherms on exfohated graphite. Carbon, 13, 242—3. 

Following the pioneer work of Beebe in 1945, the adsorption of krypton at 77 K has come into widespread use for the determination of relatively small surface areas because its saturation vapour pressure is rather low (p° 2Torr). Consequently the dead space correction for unadsorbed gas is small enough to permit the measurement of quite small adsorption with reasonable precision. Estimates of specific surface as low as 10 cm g" have been reported. Unfortunately, however, there are some complications in the interpretation of the adsorption isotherm. 

Fig. 2.23 Adsorption isotherms on graphitized carbon black at 77 K. (A) argon (B) krypton. (Courtesy Dash.)...

Fig. 4.25 Adsorption isotherms showing low-pressure hysteresis, (a) Carbon tetrachloride at 20°C on unactivated polyacrylonitrile carbon Curves A and B are the desorption branches of the isotherms of the sample after heat treatment at 900°C and 2700°C respectively Curve C is the common adsorption branch (b) water at 22°C on stannic oxide gel heated to SOO C (c) krypton at 77-4 K on exfoliated graphite (d) ethyl chloride at 6°C on porous glass. (Redrawn from the diagrams in the original papers, with omission of experimental points.)...

FIG. 9.6 Adsorption isotherms and surface phases, (a) schematic illustration of ir versus a isotherms in the vicinity of a two-dimensional critical temperature, (b) experimental data for the adsorption of krypton on exfoliated graphite showing similar features. (Data from A. Thorny and X. Duval, J. Chem. Phys., 67, 1101 (1970).)... 

As an example, Fig. 9.7 shows adsorption isotherms of krypton on the (0001) face of graphite. The dashed lines were fitted using Eq. (9.35) with / = 4.5. In reality the coverage increases steeply and the two-phase region can be identified. Figure 9.7 shows another typical feature of adsorption The amount adsorbed decreases with increasing temperature. 

Figure 9.7 Left Frumkin-Fowler-Guggenheim (FFG) adsorption isotherms (coverage 0 versus the pressure in units of Kj 1). The curves were calculated using Eq. (9.35) with ft 0,2,4,6. For ft 6 the physically correct adsorption curve is plotted as a continuous curve while the one calculated with Eq. (9.35) is plotted as a dotted curve. Right Adsorption isotherms for krypton adsorbing to the (0001) plane of graphite at two different temperatures. The dotted curves were fitted using Eq. (9.35) with ft = 4.5. Experimental results were taken from Ref. [377],...

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. 

Inverse gas chromatography (IGC) is another technique that can be used to measure the specific surface area of a particulate material, as well as to measure a number of surface thermodynamic properties of powders. Such instrumentation operates on a different principle than traditional nitrogen/krypton adsorption using the BET isotherm. 

Amphlett, C. B., Greenfield, B. F., Krypton and Xenon Adsorption Isotherms on Charcoal Irradiated with 1 Mev Electrons, At. Energy Establishment, Ukaea Research Group, AERE C/R 2632 (July 1958). 

W have previously reported (3, 5) studies of the adsorption of argon at 77° and 90° K on muscovite mica which had been treated to replace the exchangeable surface potassium ions with other cations. The adsorption isotherms and thermodynamic functions evaluated from them showed significant differences among the various ion exchanged forms of mica. We have now obtained data for the adsorption of krypton on these substrates, and wish to discuss the differences in the behavior of the argon-mica and krypton-mica systems. 

In the systematic investigations undertaken by Thorny and Duval (1970 Thorny et al., 1972) of krypton adsorption on exfoliated graphite, a series of isotherms was determined over the temperature range 77 to 100 K. Stepwise multilayer character... 

Figure 4.1. Complete adsorption isotherm of krypton on exfoliated graphite at 77.3 K (courtesy Thorny et al., 1972).

For operational reasons, it becomes more difficult to measure nitrogen adsorption isotherms on low-area adsorbents (if a<5m2g 1). To overcome this problem, krypton is widely used. As a consequence of its low p° at 77 K ( 2 mbar), the dead space correction for unadsorbed gas is relatively small and it becomes possible to measure low uptakes of gas with acceptable accuracy. 

A systematic study of krypton adsorption on exfoliated graphite was subsequently undertaken by Thorny and co-workers (Thorny and Duval, 1969 Thorny et al., 1972). Their stepwise isotherm, determined at 77.3 K, is shown in Figure 4.1. The layer-by-layer nature of the physisorption process is clearly evident - at least up to four molecular layers. This isotherm shape is remarkably similar to that of the krypton isotherm on graphitized carbon black reported by Amberg et al., (1955). 

The adsorption isotherm of methane at 77 K on exfoliated graphite is shown in Figure 9.9 the stepwise character is clearly very similar to that of krypton at 77 K... 

Figure 4.12 Adsorption isotherms for krypton on gold obtained using TSM devices in a vacuum chamber. Curve A was at 90°K before baking while Curve B was at 96°K after baking the gold film at 3S0°C for 12 hours. (Reprinted with petmUsion. See Ref. [106].)...

The stepwise isotherms of type VI are only observed under a number of idealized conditions for uniform non-porous surfaces. Ideally, the step-height represents monolayer capacity In the simplest case, it remains constant for two or three layers. Argon or krypton adsorption on graphltized carbon blacks at liquid nitrogen temperature are amongst the best examples. 

The results for nitrogen, argon and krypton adsorption on pristine MCM-48 materials can be summarized as follows (i) Argon sorption isotherms at 87 K (T/Tc = 0.58, where Tc is the critical temperature of the bulk fluid) reveal for all MCM-48 silica phases used in this study pore condensation but no hysteresis at relative pressures p/po < 0.4. With increasing pore size... 

The carbon fiber surface areas were previously determined by BET krypton adsorption to be 0.62 0.01 m g-1 and 0.74 0.01 n g-1 for T-300 and P-55, respectively. The molecular area of krypton was taken as 0.195 nm2. Prior to these measurements, the fibers were degassed at 300°C for 15 h. The elution of a characteristic point method of finite concentration IGC was used to determine the Isotherms for a series of n-alkanes. Approximately 15 to 20 Injections were used for each Isotherm. The hand-drawn curve through the peak maxima was digitized for Integration and subsequent data handling. 

Similar conditioning treatments for the HT fibers were used prior to IGC and krypton adsorption measurements. Adsorption Isotherms for n-nonane on P-55HT at 70°C are shown In Figure 2. Corresponding BET parameters and the surface areas, calculated using Groszek s molecular area for n-nonane, are given In Table II. 

The upward displacement of the adsorption Isotherms after heat treatment, and the Increase In q and C values may Indicate the exposure of micropores after removal of adsorbed contaminants. The surface areas of these samples (0.61 m g-1 for T-300HT and 0.74 m g 1 for P-55HT) are In excellent agreement with the values from krypton adsorption. 

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