Noble gases, adsorption

Interaction of the noble gas atom with condensed matter is considerably more complicated and is usually approximate simply by summing or integrating potentials pairwise. Such treatments are necessarily crude nevertheless, they allow an appraisal of the general features of an interaction and often provide realistic numerical values as well. Young and Crowell (1962), for example, review theoretical treatments of noble gas adsorption along these lines predicted potentials for adsorption on various forms of carbon, to consider one example, range from a few hundred calories per mole for He to a few kilocalories per mole for Xe, in reasonable agreement with observed heats of adsorption. 

One additional isotherm which has been used in parameterizing noble gas adsorption on natural materials (Figure 2.3) is the Freundlich isotherm ... 

The data in Table 2.2 are for natural samples. It is notoriously difficult to prepare and maintain a clean solid surface, since any freshly created surface quickly becomes contaminated with adsorbed species even with a rather good laboratory vacuum. Any naturally occurring solid material must be considered to have a surface extensively populated by adsorbed atoms and molecules rather than a pristine surface. Such surfaces are what are examined in most laboratory experiments (e.g., those reported in Table 2.2), and, of coruse, just such surfaces are geochemically relevant for noble gas adsorption. It is interesting to note, however, that in other situations, noble gas adsorption can be rather a stronger effect. Thus, for example, Bernatowicz et al. (1983) examined Xe adsorption on a vacuum-crushed lunar rock and concluded that a small part of the freshly created surface had an adsorption potential as high as 14 kcal/mole but that in a few days at 10 8torr this surface was rendered inaccessible to Xe by other chemical species that were better competitors for the sorbent surfaces. 

Figure 5.2 Noble gas abundance (normalized to 36Ar) relative to air abundance. Data are from Table 5.1. The noble gas adsorption curve on pulverized Allende meteorite (Fanale Cannon, 1972) is also shown for comparison.

Type VI isotherms are typical of adsorbents having a very uniform nonporous surface. Each step represents an adsorbed monolayer (i.e., noble gas adsorption on graphitized carbon blacks). 

Noble-gas adsorption is often assumed to be the least complicated form of physisorption. However, on clean solid surfaces the molecular area may depend on the formation of ordered structures of the adsorbate in registry with the adsorbent lattice. 

After 30 years of continuing investigation, the adsorption properties of the noble ses on metal and semiconductor surfaces have recently attracted renewed interest. On the one hand, some fundamental aspects have come within the reach of modem experimental and theoretical techniques, sueh as the very nature of physisorption and the noble gas - substrate interaction, the possibility to study growth and surface kinetics at the atomic scale, and the recent interest in nanoscale surface friction and related tribological issues, where noble gas adlayers serve as model systems [99P]. On the other hand, noble gas adsorption is being used as a non-destmctive and quantitative surface analytical tool as, for instance, in photoemission of adsorbed xenon (PAX) [97W] and for titration analysis of heterogeneous surfaees based on the site specificity of the interaction strength [96S, 98W]. 

Early woik has shown that upon noble gas adsorption the surface potential (woik function) of the substrate is lowered by as much as 1 eV (Table 9). These large values can be ascribed to the large polarizabihty a of the noble gas atoms (in particular Xe) and possible chemical contributions to the surface bond. The woik function change has also been used to measure the surface coverage, e.g., in isothermal adsorption experiments. An analytic relation, accounting for dipole-dipole interactions and mutual depolarization effects, is given by the Topping formula ... 

The oxidation of CO at low temperatures was the first reaction discovered as an example of the highly active catalysis by gold [1]. Carbon monoxide is a very toxic gas and its concentration in indoor air is regulated to 10-50 ppm depending on the conditions [61]. An important point is that CO is the only gas that cannot be removed from indoor air by gas adsorption with activated carbon. On the other hand, metal oxides or noble metal catalysts can oxidize CO at room temperature. 

The highly fractionated nature of the and Th series nuclides is illustrated by the measured activities in some representative waters in Figure 1. The highest activities are typically observed for Rn, reflecting the lack of reactivity of this noble gas. Groundwater Rn activities are controlled only by rapid in situ decay (Table 1) and supply from host rocks, without the complications of removal by adsorption or precipitation. The actinide U, which is soluble in oxidizing waters, is present in intermediate activities that are moderated by removal onto aquifer rocks. The long-lived... 

Effect of adsorption of common gases on electron ejection by noble gas ions. 

Janssens et al. [38, 40] used photoemission of adsorbed noble gases to measure the electrostatic surface potential on the potassium-promoted (111) surface of rhodium, to estimate the range that is influenced by the promoter. As explained in Chapter 3, UPS of adsorbed Xe measures the local work function, or, equivalently, the electrostatic potential of adsorption sites. The idea of using Kr and Ar in addition to Xe was that by using probe atoms of different sizes one could vary the distance between the potassium and the noble gas atom. Provided the interpretation in terms of Expression (3-13) is permitted, and this is a point the authors checked [38], one thus obtains information about the variation of the electrostatic potential around potassium promoter atoms. 

To pump out larger vessels, several adsorption pumps are used in parallel or in series. First, the pressure is reduced from atmospheric pressure to a few millibars by the first stage in order to capture many noble gas molecules of helium and neon. After the pumps of this stage have been saturated, the valves to these pumps are closed and a previously closed valve to a further adsorption pump still containing clean adsorbent is opened so that this pump may pump down the vacuum chamber to the next lower pressure level. This procedure can be continued until the ultimate pressure cannot be further improved by adding further clean adsorption pumps. 

The Henry constant J Cis a function of T but not P. (In some theoretical treatments, the Henry constant is the ratio of fugacity to quantity adsorbed, i.e., the inverse of the sense used here.) It is generally expected that adsorption will be governed by Henry s law at sufficiently low pressures. It is possible to construct theoretical models for adsorption in which an isotherm does not reduce to Henry s law, Equation (2.3), even in the limit P —> 0, but it is not clear that such situations obtain in practice and doubtful that they are important in noble gas geochemistry. 

The Langmuir model also provides a convenient basis for estimating when Henry s law or saturation effects can be expected. If an individual attachment site has an area a 2 x 10 15 cm2, the order of atomic cross-sectional area (cf. Table 2.1), then Ns 5 x 1014 atoms/cm2 = 2 x 10 5cm3STP/cm2. Surface concentrations approaching this order of magnitude can be expected to exhibit saturation behavior. Conversely, much lower concentrations indicate 9 1 and lead to the expectation of Henry s law behavior. Possible adsorption effects important in noble gas geochemistry always involve much lower concentration than this illustrative value, which is one reason why Henry s law violation is not expected. 

Adsorption is sometimes invoked not only as a factor in geochemistry but also as a laboratory nuisance. Analyzed samples are often found to have a superficial or loosely bound component ascribed to air contamination, which is frequently described as adsorbed on the sample. Without further qualification, this makes little sense. An air contamination effect certainly exists, as can be inferred clearly when an intrinsic sample gas is isotopically distinct (cf. Section 2.4), but whether adsorption is responsible or even involved is questionable. In all such noble gas analyses, a necessary step is storage in laboratory vacuum before gas extraction. By definition, adsorbed gas is desorbed under vacuum. The relevant factor is the timescale required... 

There is a wealth of both theoretical treatment and empirical data for the phenomenon of solution, including empirical data for noble gas solution. As with adsorption, however, data for solution in geochemically important materials are sparse. A prominent exception, the only one, is water, extensive data for which are presented in Chapter 4. Most of the available data for other materials of principal geochemical interest are summarized in Table 2.3. 

Noble gas incorporation is made at noble gas partial pressures in air except for Adsorption. b Table 2.3. c Table 2.2. 

Let us discuss the first issue, that is, how terrestrial noble gases, in extremely small but not negligible amounts, were captured by the Earth. Adsorption of noble gases by Earth-accreted dust grains had once been favored as a noble gas capture... 

Depending on the type of interaction between an adsorbed particle and a solid state surface there are cases, where adsorption enthalpies can be calculated using empirical and semi-empirical relations. In the case of atoms with a noble-gas like ground-state configuration and of symmetrical molecules the binding energy (EB) to a solid surface can be calculated as a function of the polarizability (a), the ionization potential (IP), the distance (R) between the adsorbed atom or molecule and the surface, and the relative dielectric constants (e) (Method 9) [58-61] ... 

If the elements 112 and 114 have a noble-gas like character [62], then, in a fictitious solid state, they would form non conducting colorless crystals. A physisorptive type of adsorption may occur and their adsorption properties, for example on quartz, can be calculated with this method [61], see Table 3. For physisorbed noble gas atoms a roughly uniform distance to different surfaces of about 2.47 0.2 A was deduced from experimental results [63]. 

Due to the expected high volatility of elements with atomic numbers 112 to 118 in the elemental state [104], see also Chapters 2 and 6, gas phase chemical studies will play an important role in investigating the chemical properties of the newly discovered superheavy elements. An interesting question is, if e.g. elements 112 and 114 are indeed relatively inert gases (similar to a noble gas) [105] due to closed s2 and p /22 shells, respectively, or if they retain some metallic character and are thus adsorbed quite well on certain metal surfaces, see Chapter 6, Part II, Section 3.2. Extrapolations by B. Eichler et al. [106] point to Pd or Cu as ideal surfaces for the adsorption of superheavy elements. 

The bulk properties of macroscopic crystals cannot be affected drastically by the difference which exists between the structure of the interior and that of a surface film which is approximately 10,000 atoms deep. However, even for macroscopic crystals, rate phenomena such as modification changes which are initiated within the surface are likely to be influenced by the environment, which would include molecules which are conventionally described as physically adsorbed. Apparently it is not generally understood that even the presence of a noble gas can affect the chemical reactivity of solids. Brunauer (3) explained that in principle physical adsorption of molecules should affect the solid in the same manner as chemisorption. As action and reaction are equal, chemisorption may have a stronger effect on both the solid and the adsorbed molecule. 

Pre-admitting of a noble gas influence the adsorption, while admitting a noble gas subsequently to butane adsorption has no influence at all. This is demonstrated in figure 6 for n-butane on MFI at 383 K. 

As there is no circulation pump present in the experimental set-up, part of the described phenomenas could be due to poor mixing. Therefore we performed a test in a microbalance. A BEA sample was pre-treated in a helium flow at 573 K. The sample was cooled down to 383 K in a helium flow. After cooling down the helium flow was switched to an n-butane/helium mixture. Comparison with the experiment described before revealed that the rate of adsorption was faster, but still much slower than without any noble gas present and that the amount adsorbed was even less than in the described experiments with a noble gas present. 

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