Krypton, adsorption

Alloy films are commonly sintered during preparation by deposition on substrates heated to, say, 400°C or by subsequent annealing at such temperatures, and, consequently, rather small surface areas have to be measured, perhaps in vessels of substantial volume. Krypton adsorption at liquid nitrogen temperature was used with induction-evaporated Cu-Ni, Fe-Ni, and Pd-Ni films, and BET surface areas of 1000-2000 cm2 were recorded (48), after correction for bare glass. The total area of Cu-Ni films was measured by the physical adsorption of xenon at — 196°C (70) in addition, the chemisorption of hydrogen on the same samples enabled the quantity a to be determined where... 

In one of the earliest reports of the use of clean evaporated alloy films in surface studies, Stephens described the preparation and characterization of Pd-Au films and presented some results for the adsorption of oxygen on them 46). Films of pure Pd and 60% Au were evaporated directly from wires, while films of 80% Au and pure Au were evaporated from a pre-outgassed tungsten support wire. The films were evaporated in a UHV system and the pressure was kept below PC8 Torr during evaporation. After evaporation, the films were stabilized by cycling between —195° and 30°C four times. They w ere characterized by X-ray diffraction and chemical analysis surface areas were measured by the BET method using krypton adsorption. 

Simply calculating specific surface areas from the values in Tables 3-5 leads to apparent specific surface areas of approximately 400-500 m2/g [49,51], Specific surface areas obtained from similar analyses of nonpolar gas (nitrogen or krypton) adsorption studies, however, are typically in the range of 1 m2/g, independent of sample pretreatment. 

Chiu, C. Y. Chiang, A. S. T. Chao, K. J. 2006. Mesoporous silica powders and films—Pore size characterization by krypton adsorption. Microporous Mesoporous Mater. 91 244-253. 

To measure the low pressures associated with krypton adsorption a thermocouple or thermistor can be used after calibration against a McLeod gauge. 

In the pressure range useful for krypton adsorption the mean free path is approximately equal to the diameter of the stem of the sample cell. Equation (14.16) is not applicable in this range and an empirical equation devised by Rosenberg must be used. His equation is... 

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. 

Krypton adsorption at 77 K is often used for the determination of relatively low solid-surface areas. At this temperature the vapour pressure of krypton (and so the dead-space correction) is small, and a reasonable precision is attainable. 

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. 

A BET plot of all the data at 77° is linear over a wide range, and least-squares analysis of all the experimental values in the monolayer region (0.025 < p/p0 < 0.20) yields values of vm = 0.336 cc. per gram and c = 131. While the c value is very close to that previously observed for krypton adsorption on this mica (5), the surface area is appreciably smaller. This decrease in vm has occurred progressively with time for all our ground mica samples, and at present we have no... 

From our krypton adsorption data, we must conclude that the adsorption energetics are the same within experimental error on all three mica samples for the range 0.2 <0< 0.8 (with the possible exception of the barium muscovite up to 0 0.4, as noted above). We have calculated differential heats and entropies... 

The sonic nozzle is capable of providing a constant gas flow irrespective of the change of downstream pressure. Its successful application is dependent on the maintenance of a high upstream pressure. In theory, the pressure ratio should be at least 2, but in practice it is advisable to keep the pressure ratio above 6. At present, the sonic nozzle technique is the only reliable method for controlling low gas flow (e.g. -0.01 cm3 h 1, which is required for krypton adsorption measurements on powders of low surface area). 

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... 

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 best value for the effective molecular cross-sectional area, o(Kr), of krypton in the BET monolayer at 77 K has been under discussion for many years. In their original work on krypton adsorption, Beebe et al. (1945) recommended the value 0.195 nm2 for o(Kr) and this empirical value is still used by many investigators. For the adsorption of krypton on graphitized carbon, Ismail (1990, 1992) gives preference to the value molecular area calculated from the liquid density and determined by X-ray scattering. This, of course, implies that Kr and N2 molecules undergo localized adsorption on the same sites. For ungraphitized carbons, Ismail (1992) recommends cr(Kr) = 0.214 nm2. 

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. 

Figure 18, for instance, represents the krypton adsorption curve for a sample of Gonal F75 UI from Serono and we can see that it gives a pretty accurate reading despite the fact that the absolute surface measured is only 0.07 m. ... 

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... 

Specific surface area (SSA) by gas adsorption Specific surface areas (expressed in m /g) were measured by Krypton adsorption at 77K (Micromeritics ASAP2400) and calculated using BET equation [3] Prior to measurements, powder specimens were outgassed under vacuum (5 millitorrs) at room temperature. Krypton was chosen as adsorbate because of the low SSA of some samples (SSAphysisorption method allows for measuring the overall surface developed by the powder particles including interparticular porosity (except sealed pores). 

BET surface area mcasurcmcnr were realized with a Micromcritics ASAP-2000 instrument using krypton adsorption at -196°C. 

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. 

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

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