Adsorption Of Noble Gases Research Articles

Adsorption of noble gases on metal surfaces and the scanning tunneling microscope

Physisorption on metal surfaces, and the tunneling currents through the adsorbed species, are calculated using a unified formalism that presents both problems on the same footing. Our method is based on a self-consistent LCAO approach whereby the different interaction parameters defining the bonds, and the tunneling currents, are calculated using the atomic properties of the atomic species forming the interface. Green function methods and the Keldish formalism are used to calculate the different physical properties. We present results for xenon adsorbed on aluminum.

Stability of helium and hydrogen films

We use the variational theorem to evaluate the domain of parameter space such thatsome film exists on a given surface. This is accomplished by determining the binding energy variationally for a monolayer film at T=0. The He results are for the most part consistent with those of Cheng, et al., for the domain of wetting films. New results for H2 are compared with calculated parameters of noble gas adsorption potentials.

Adsorption and Desorption of Noble Gases on Activated Charcoal

Detailed desorption studies using petroleum-based activated charcoals were conducted in monolayers and packed beds. Less extensive studies were conducted on several other types of charcoal. Kinetic studies, using 133Xe, demonstrated the existence of a micropore volume with entrance capillaries that together determined the response characteristics of charcoal to external concentration gradients of tracer gases. This new two-phase model, composed of micropores and entrance capillaries, describes the desorption dynamics of an adsorbed gas in the presence of water vapor. Condensed water vapor in the entrance capillaries of the charcoal reduced the effective pore radius and increased the diffusion half-time. Water could also adversely affect the integrating capability of the charcoal dramatically if the adsorbed water completely blocked the entrance capillaries. The amount of adsorbed water required to block the capillaries varied with the charcoal type and was termed here as the "break-point." The desorption parameters measured in this work can be used to design an improved passive Rn monitor to effectively integrate during a 3-7 d exposure period by eliminating the adverse effects of water vapor. The improved canister design would provide more accurate and reproducible measurements of indoor Rn concentrations than are currently available.

Adsorption and Desorption of Noble Gases on Activated Charcoal

The adsorptive and desorptive characteristics of canisters containing a petroleum-based charcoal were investigated under controlled conditions of temperature, relative humidity, and Rn concentration. Charcoals exposed in a monolayer and packed bed during exposure intervals of 1-7 d demonstrate that Rn adsorption and desorption are dependent on bed depth and the amount of water adsorbed. Changes in the adsorptive and desorptive properties of the charcoal occurred near the break-point where the pores became occluded by water vapor that condenses in the entrance capillaries. Radon-222 adsorption is decreased by an order of magnitude as the amount of adsorbed water exceeds the break-point of the charcoal. The reduction in pore surface due to adsorbed water results in a marked increase in the rate of Rn loss from exposed canisters, accounting for reduced adsorption. The apparent desorption time-constant for a 2-cm bed of loose Witco 6 x 10 mesh charcoal containing 0.220-0.365 kg H2O kg-1 is typically between 2-8 h. The apparent desorption time-constant for an equivalent packed bed containing a water vapor content of 0.026-0.060 kg H2O kg-1, which is below the break-point of the charcoal, is about 15-30 h. Conventional charcoal canisters, if exposed in the fully-opened configuration, can achieve the break-point in less than 4 d at 70% humidity. The use of a diffusion barrier would allow for longer exposure times until the break-point of the charcoal is achieved.

Adsorption and isotopic fractionation of Xe

We give a theoretical description of the mechanisms of isotopic fractionation arising during adsorption of noble gases in a Henry's Law pressure regime. We also present experimental data on the isotopic composition of Xe adsorbed on activated charcoal in the temperature range 220°K to 350°K. Both theoretical considerations and the experimental data indicate that equilibrium adsorption does not significantly alter the isotopic structure of adsorbed noble gases. Therefore, if adsorption is responsible for the elemental noble gas pattern in meteorites and the earth, the heavy noble gas isotopic fractionation between them must have been produced prior to and by a different process than equilibrium adsorption.

Sulfur dioxide adsorption and reaction with atomic oxygen on the Ag(110) surface

The adsorption of SO 2 on Ag(110) and the reaction of SO 2 with oxygen adatoms have been studied under ultrahigh vacuum conditions using low energy electron diffraction, temperature programmed reaction spectroscopy and photoelectron spectroscopy. Below 300 K, SO 2 adsorbs molecularly giving p(1×2) and c(4×2) LEED patterns at coverages of one half and three quarter monolayers. respectively. At intermediate coverages, streaked diffraction patterns, similar to those reported for noble gas and alkali metal adsorption on the (110) face of face-centered cubic metals were observed, indicating adsorption out of registry with the surface. A feature at low binding energy in the ultraviolet photoemission spectrum appeared which was assigned to a weak chemisorption bond to the surface via the sulfur, analogous to bonding observed in SO 2-amine charge transfer complexes and in transition metal complexes. SO 2 exhibited three binding states on Ag(110) with binding energies of 41, 53 and 64 kJ mol −1; no decomposition on clean Ag(110) was observed. On oxygen pretreated Ag(110), SO 2 reacted with oxygen adatoms to form SO 3(a), as determined by X-ray photoelectron spectroscopy. Reacting preadsorbed atomic oxygen in a p(2 × 1) structure with SO 2 resulted in a c(6 × 2) pattern for SO 3(a). The adsorbed SO 3(a) decomposed and disproportionated upon heating to 500 K to yield SO 2( g) , SO 4(a) and subsurface oxygen.

Adsorption of noble gases on the carbon adsorbents supersorbon HS and Jado

The isotherms, isosters and heats of adsorption of argon, krypton and xenon were measured on the active carbon Supersorbon HS and the carbon adsorbent Jado LD 1191 - in a temperature range 298-473 K, using gas chromatography. Differences were observed in the adsorption behaviour of both types of adsorbents - which showed especially on the nonlinearity of adsorption isosters of all gases studied with the Jado LD 1191 adsorbent. It was found that both types of adsorbents possess good adsorption properties, which can be - for Supersorb HS - utilized in the beds of the delaying lines of radioactive noble gases. The practical application of the Jado LD 1191 adsorbent for the same purposes is limited by the presence of inactive teflon support in the structure of this adsorbent.

A Low Cost Radioiodine Air Sampling System for Nuclear Incidents: Experience at Three Mile Island

An air mover was developed to draw 5 ± .5 ft3 min-1 through a combined HEPA filter and absorber. The air mover can be operated on 110V alternating current or 12V direct current available from standard automotive electrical systems. Air mover cost was a major design consideration. The third (last) prototype was used to estimate production cost of 100 air movers. The overall unit cost for an air mover and a filter-canister assembly was between $157 and $107 in 1978 dollars. A 4% silver loaded silica gel adsorber was developed that would trap organic iodine, hypoiodous acid, and elemental iodine with better than 90% efficiency under severe ambient conditions, for a 5 minute collection cycle. The principal advantage of the 4% silver loaded silica-gel adsorber was low noble gas adsorption efficiency. Xenon adsorption efficiency was less than 5 × 10-3% at 55° C and 4 × 10-2% at 7° C. The last figure is about 200 times better than charcoal at the same temperatures. High photon efficiency G. M. detectors were tested for improved filter-adsorber readout sensitivity. A specially filtered Victoreen 6306 tube provided a sensitivity improvement of more than 10 times the standard CDV 700 probe. The new Victoreen probe was designed to be about a factor of 2 less sensitive to 135Xe than 131I. The standard CDV 700 probe is more sensitive to 135Xe than 131I. Three Mile Island provided an operational test of the air sampling system. The following observations and conclusions are offered: a.

Effect of the form and the parameters of intermolecular interaction potential on calculated values. Henry constant for adsorption of noble gases on graphitized black and the specific surface area determined therefrom

The effect of the form and the parameters of the atom-atom potential of intermolecular interaction, as well as that of the approximations used in its summation over the atoms of a solid, on the values of the Henry constant K 1 for the adsorption of noble gases on the basal face of graphite has been examined. At fixed values of the depth and of the position of the atom-atom potential minimum, K 1 values are only slightly sensitive to the form of this potential, if its summation is performed taking into account the layer structure of graphite. Disregarding the laminated structure of the graphite lattice leads to highly underestimated values of K 1. The values of the parameter c 1 of the attractive forces of the potential (6, 8, exp) for the intermolecular interaction of the atoms of noble gases with the carbon atom of graphite have been determined, using experimental values of K 1 for the adsorption of these gases on graphitized thermal carbon blacks. The corresponding changes in internal energy, entropy and heat capacity have been calculated. The effect of the form and the parameters of the potential function φ of the intermolecular interaction of a monoatomic molecule with the basal face of graphite on the energy φ o in the minimum of the potential curve φ( z ), and on the product sz o of the specific surface area s , and the equilibrium distance between the molecule and the surface z o, has been studied. φ o and sz o were determined from experimental values of the Henry adsorption constant K 1 measured at different temperatures. From the experimental values of K 1 for the adsorption of noble gases, the value of s for graphitized thermal carbon black Sterling PT, 2700 (P-33), has been determined. This value of s does not essentially depend on the adsorbate and lies within the limits of error of the s value, determined for this carbon black by the BET method. When determining s from K 1, it is necessary to take into account the structure of the solid so as to choose the form of potential φ correctly. Values of z o should be determined from the atom-atom potential function of the adsorbate-adsorbent intermolecular interaction.

Adsorption in slit-like and cylindrical micropores in the henry's law region. A model for the microporosity of carbons

Calculations are presented of the potential energy profiles for atoms adsorbed in slit-like pores, and of the enhancement of the depth of the potential energy well compared with that for adsorption by a single surface. The vibrational frequency normal to the wall is calculated as a function of slit width and methods of analysing experimental data are suggested. A comparison is made with similar expressions for adsorption in cylindrical pores. Recently published data on the adsorption of noble gases by a series of microporous carbons is analysed and shown to be best represented in terms of slit-like pores having an average width (between the nuclei of carbon atoms) of about 0.77 nm. The width available to an adsorbed molecule is about 0.52 nm which is in quantitative agreement with the known molecular sieve properties of these carbons. A well-activated carbon has pores with a somewhat larger mean width. The calculated vibration frequencies lie in the range 5–10 × 1011 Hz. The calculated surface areas and micropore volumes are 3–4 times smaller than the “monolayer equivalent areas” and pore volumes derived from micropore adsorption of nitrogen and benzene. It is concluded, in agreement with Dubinin, that the micropore volume can be divided into ultra-micropores, consisting of 25–30 % of the total micropore volume having free widths of about 0.52 nm, together with supermicropores which are larger and are filled with adsorbed vapour only at higher relative pressures.

Determination of the specific surface area of anatase by physical adsorption of noble gases

Determination of the specific surface area of anatase by physical adsorption of noble gases

Advances in atom‐probe field ion microscopy*

SUMMARYThe ToF atom‐probe FIM is a microanalytical tool of ultimate sensitivity. Design and performance of the three presently operating instruments are described, including applications to metallurgical problems and contributions to basic effects in field ion microscopy. Up to four‐fold ionic charges of metals, field adsorption of noble gases, and the occurrence of metal‐helide ions have been surprising results. The mass resolution of present atom‐probes is marginal due to random ion energy deficits. A new energy deficit compensated atom‐probe offers a very satisfactory mass resolution of 1/1000.

Atom-Probe Field-Ion Microscopes and High-Field Surface Effects

Atom-probe FIMs are micro-analytical devices of ultimate sensitivity by combining the single atom resolution of the field-ion microscope with the single particle detection of a mass spectrometer. The original ToF version may achieve a resolution ΔM/M of 1/200 and is thus adequate for routine metallurgical applications. The resolution is limited by an inadequate subnanosecond pulse front and can be improved tenfold in an energy deficit compensating ToF atom-probe. A magnetic sector also offers high resolution capability. New surface effects discovered by the atom-probe and affecting image interpretation in field-ion microscopy are the occurrence of up to fourfold charged metal ions as products of field evaporation, the field-induced adsorption of noble gases, and the formation of metal-helium molecular ions.

Consequences of assuming a maxwellian energy distribution for adsorption sites

A general adsorption isotherm has been previously proposed by us and applied to zeolites and metals. The isotherm is based on a Maxwell–Boltzmann distribution function for the energy of the adsorption sites and on the Langmuir isotherm for each set of homotactic sites. The general isotherm is explored further, and is applied to the adsorption of noble gases on metal films in ultra-high vacuum conditions. The general picture of adsorption on metals and on zeolites as it emerges from such considerations is discussed and the predicted behaviour of the differential heats of adsorption is compared to the predictions of the Dubinin Radushkevich isotherm.

Noble gas adsorption characteristics of charcoal beds. Van Deemter's coefficients evaluation

Noble gas adsorption characteristics of charcoal beds. Van Deemter's coefficients evaluation

Change of superconducting transition temperature caused by adsorption of noble gases

Surface adsorption of Ne or Xe reduced the superconducting transition temperature of metal films. Shifts, ΔTcTc, for T1 and Sn were larger by a factor of 10 or more than those observed for Pb, Bi and Ga. Small accompanying increases in normal state conductivity were found for some of the metals and decreases for others.

Studies of the Mean Displacement of Surface Atoms in the (100) and (110) Faces of Silver Single Crystals at Low Temperatures

The intensities of back-diffracted low-energy (10-300-eV) electrons have been measured as a function of temperature for the (110) and (100) faces of single-crystal silver in the temperature range -195 to 85\ifmmode^\circ\else\textdegree\fi{}C. From these data, the rms displacements $〈{u}_{\ensuremath{\perp}}〉$ of surface atoms perpendicular to the surface planes have been calculated and the magnitudes of the parallel components $〈{u}_{\ensuremath{\parallel}}〉$ estimated. The perpendicular components of the surface rms displacements on silver (110) and (100) surfaces are 60 and 100% larger than those of bulk atoms. There is little difference in magnitude between the parallel and perpendicular components for these faces, as has also been reported for the (111) face. The parallel components of the rms displacements did not exhibit anisotropy within the accuracy of the measurements, and the physical adsorption of several noble gases had no effect on any of these measurements. The surface rms displacement for silver and those other fcc metals studied are larger than predicted by theory, which assumes bulk force constants for the surface atoms. This suggests that force constants for surface atoms are smaller than those of bulk atoms.

1052. Dynamic adsorption of fission-product noble gases on activated charcoal

1052. Dynamic adsorption of fission-product noble gases on activated charcoal

Physical Adsorption of Noble Gases on Pyrex Glass at Very Low Pressures

Physical Adsorption of Noble Gases on Pyrex Glass at Very Low Pressures

Gas chromatographic separation and adsorption of noble gases, oxygen and nitrogen on modified zeolites of type X

Gas chromatographic separation and adsorption of noble gases, oxygen and nitrogen on modified zeolites of type X