Xe Adsorption Research Articles

Stepwise heating of lunar anorthosites 60025, 60215, 65315 possibly reveals an indigenous noble gas component on the Moon

Despite extensive effort during the last four decades, no clear signature of a lunar indigenous noble gas component has been found. In order to further investigate the possible occurrence of indigenous volatiles in the Moon, we have re-analyzed the noble gas and nitrogen isotopic compositions in three anorthosite samples. Lunar anorthosites 60025, 60215 and 65315 have the lowest exposure duration (∼2Ma) among Apollo samples and consequently contain only limited cosmogenic (e.g. 124,126Xe) and solar wind (SW) noble gases. Furthermore, anorthosites have negligible contributions of fissiogenic Xe isotopes because of their very low Pu and U contents. As observed in previous studies (Lightner and Marti, 1974; Leich and Niemeyer, 1975), lunar anorthosite Xe presents an isotopic composition very close to that of terrestrial atmospheric Xe, previously attributed to “anomalous adsorption” of terrestrial Xe after sample return. The presumed atmospheric Xe contamination can only be removed by heating the samples at medium to high temperatures under vacuum, and is therefore different from common adsorption. To test this hypothesis, we monitored the adsorption of Xe onto lunar anorthositic powder using infrared reflectance spectroscopy. A clear shift in the anorthosite IR absorbance peaks is detected when comparing the IR absorbance spectra of the lunar anorthositic powder before and after exposure to a neutral Xe-rich atmosphere. This observation accounts for the chemical bonding (chemisorption) of Xe onto anorthosite, which is stronger than the common physical bonding (physisorption) and could account for the anomalous adsorption of Xe onto lunar samples.Our high precision Xe isotope analyses show slight mass fractionation patterns across 128–136Xe isotopes with systematic deficits in the heavy Xe isotopes (mostly 136Xe and marginally 134Xe) that have not previously been observed. This composition could be the result of mixing between an irreversibly adsorbed terrestrial contaminant that is mostly released at high temperature and an additional signature. Solar Wind (SW) Xe contents, estimated from SW-Ne and SW-Ar concentrations and SW-Ne/Ar/Xe elemental ratios, do not support SW as the additional contribution. Using a χ2 test, the latter is best accounted for by cometary Xe as measured in the coma of Comet 67P/Churyumov-Gerasimenko (Marty et al., 2017) or by the primordial U-Xe composition inferred to be the precursor of atmospheric Xe (Pepin, 1994; Avice et al., 2017). It could have been contributed to the lunar budget by volatile-rich bodies that participated to the building of the terrestrial atmosphere inventory (Marty et al., 2017).

Dynamic adsorption of Xe on a fixed-bed adsorber at 77 K

In designing a fixed-bed adsorber, it is vital to understand dynamic adsorption properties of the unit. Temperature is an important effect on adsorbent performance, as the dynamic adsorption coefficients tend to increase with decreasing temperature. To minimize the volume of the fixed-bed adsorber, the dynamic adsorption characteristics of Xe were studied at 77 K by employing a variety of adsorbents under different operational conditions. The carbon molecular sieve performed better than that of activated carbon. Both operational conditions and the presence of gaseous impurities were found to affect the adsorption properties.

Atomic simulation of Xe and Kr separation in silica-templated amorphous mesoporous carbons CMK-3 and CMK-5

We perform a molecular simulation study on adsorption and separation of the noble gases Xe and Kr in silica-templated amorphous mesoporous carbons (CMK) materials. We generate the atomic models of CMK-3 and CMK-5 materials by adsorbing carbon in a model MCM-41 pore. Our carbon structures can capture the surface roughness and the disordered nature of the carbon rods and carbon pipes as reported in the experiment. The adsorption isotherms and isosteric heats of pure gases have been examined further. We find that the existence of the carbon interconnections between nanorods for CMK-3 and between nanopipes for CMK-5 will reduce the excess uptakes of the noble gases, whereas the isosteric heats are favoured in the materials with interconnections. The carbon interconnections are not advantageous to the adsorption storage of pure gases, but they can improve the separation ability of Xe for gas-mixture adsorption. The effects of temperature and concentration on the Xe separation are investigated and it is shown that the selectivities of Xe in the CMK-5 materials are insensitive to the two factors. We also find that both gas storage and separation of CMK materials are comparable to IRMOF-1 and UMCM-1 metal-organic frameworks.

Efficiently mapping structure-property relationships of gas adsorption in porous materials: application to Xe adsorption.

Designing better porous materials for gas storage or separations applications frequently leverages known structure-property relationships. Reliable structure-property relationships, however, only reveal themselves when adsorption data on many porous materials are aggregated and compared. Gathering enough data experimentally is prohibitively time consuming, and even approaches based on large-scale computer simulations face challenges. Brute force computational screening approaches that do not efficiently sample the space of porous materials may be ineffective when the number of possible materials is too large. Here we describe a general and efficient computational method for mapping structure-property spaces of porous materials that can be useful for adsorption related applications. We describe an algorithm that generates random porous "pseudomaterials", for which we calculate structural characteristics (e.g., surface area, pore size and void fraction) and also gas adsorption properties via molecular simulations. Here we chose to focus on void fraction and Xe adsorption at 1 bar, 5 bar, and 10 bar. The algorithm then identifies pseudomaterials with rare combinations of void fraction and Xe adsorption and mutates them to generate new pseudomaterials, thereby selectively adding data only to those parts of the structure-property map that are the least explored. Use of this method can help guide the design of new porous materials for gas storage and separations applications in the future.

Xe adsorption and separation properties of a series of microporous metal–organic frameworks (MOFs) with V-shaped linkers

A series of microporous metal–organic frameworks (MOFs) constructed by using a V-shaped linker, 4,4′-sulfonyldibenzoic acid, were evaluated for their Xe gas adsorption properties.

Discriminating between Metallic and Semiconducting Single-Walled Carbon Nanotubes Using Physisorbed Adsorbates: Role of Wavelike Charge-Density Fluctuations.

Discriminating between metallic (M) and semiconducting (S) single-walled carbon nanotubes (SWNTs) remains a fundamental challenge in the field of nanotechnology. We address this issue by studying the adsorption of the isotropic atoms Xe, Kr, and a highly anisotropic molecule n heptane on M- and S-SWNTs with density functional theory that includes many-body dispersion forces. We find that the distinct polarizabilities of M- and S-SWNTs exhibit significantly different physisorption properties, which are also strongly controlled by the SWNT's diameter, adsorption site, adsorbate coverage, and the adsorbate's anisotropy. These findings stem from the wavelike nature of charge-density fluctuations in SWNTs. Particularly, these results allow us to rationalize the unusual sqrt[3]×sqrt[3]R30^{0} phase of Kr atoms on small gap M-SWNTs and the double desorption peak temperatures of n heptane on M-SWNTs in experiments, and also propose the n heptane as an effective sensor for experimentally discriminating M- and S-SWNTs.

Polarizability-Induced Vibrational Shifts for Infrared Spectroscopy of Molecular Surfaces: Xe as a Surface Probe of Hexadecanethiol Self-Assembled Monolayers and Hexane Films on Au(111).

Adsorption of atomic and molecular species on several model film surfaces is shown to lead to systematic and reproducible spectral shifts for infrared absorption peaks associated with vibrations at the film/adsorbate interface. These shifts to lower frequencies are demonstrated to be versatile spectral signatures of this exposed interface. While this is found to occur for a wide variety of adsorbates, this work focuses on the adsorption of Xe onto molecular surfaces. By using the pre-Xe film reflectivity as a reference in calculating the spectra, the changes induced by Xe adsorption due to van der Waals forces are isolated from the conventional film spectrum which contains both surface and subsurface spectral contributions. A model-free algorithm containing two user-defined fitting parameters is described to reconstruct the spectrum of the surface from which these shifted features originate. The high surface sensitivity of the technique is exhibited for the case of a hexadecanethiol self-assembled monolayer (SAM), where the reconstruction algorithm successfully reproduces all peaks attributed to the methyl tail groups with minimal spectral contributions from the methylene groups composing the subsurface chain backbone; such methylene spectral features dominate the conventional surface infrared spectra. The algorithm is applied to films of hexane to demonstrate its application to disordered multilayer films as well. The technique and reconstruction algorithm are shown to extract spectroscopic information from the uppermost ∼0.3 nm of the film, representing the fundamental limit of vibrational surface sensitivity. Rudimentary modeling of a CO-Xe3 system using Morse/Lennard-Jones interactions demonstrates the expected distance dependence of the van der Waals interaction which gives rise to the observed vibrational spectral shifts.

Adsorption Properties of Xe on Ag/Cu(111) System: Real-Time Photoemission Investigation

We discuss here the adsorption process of rare gas atoms on a heterogeneous surface, Xe on Ag submonolayer partially covering Cu(111) surface. As a function of the Xe dosing, we have monitored in real-time the evolution of the Xe 4d core levels and of the LEED patterns, as well as the modification of the Shockley state band. We confirmed that the reduction in the density of states near the Fermi level strongly promotes the adsorption of Xe atoms on the surface. More interesting, it seems that the Xe overlayer on the Ag monolayer exhibits a disordered structure, which differs substantially from those observed for Xe on most of the single-crystal metal surfaces, where the Xe atoms form generally an epitaxial layer with hexagonally close-packed structure. Additionally, the commensurate (9 × 9) reconstruction characterizing the 1 ML Ag/Cu(111) is preserved after adsorption of Xe, and consequently leads to a band back-folding of shifted interface state on the 1 ML Ag islands covered by Xe. This finding is esse...

Selective dynamic separation of Xe and Kr in Co-MOF-74 through strong binding strength between Xe atom and unsaturated Co2+ site

From pure experimental isotherms and ideal adsorbed solution theory (IAST), we confirmed a recent report that Co-MOF-74 provides the highest Xe uptake as well as the highest Xe/Kr selectivity among the three M-MOF-74 series (M = Co, Mg, and Zn). From breakthrough experiments, we then showed the first demonstration of the potential of Co-MOF-74 for Xe/Kr separations under mixture flow conditions. Remarkably, the experimental breakthrough curves for three consecutive cycles are essentially unchanged even if the column was regenerated under helium flows at room temperature between each cycle. Isosteric heat of adsorption (Qst) for Xe, adsorbed Xe molecules per metal, and binding strengths and electronic density of states (DOS) analyses from first principles calculations all indicate that unsaturated Co2+ sites attract Xe more strongly than do unsaturated Mg2+ and Zn2+ sites. The DOS analyses show that the d orbital of the Co2+ is the main contributors for the strong interaction. These results suggest that Co-MOF-74 is a promising adsorbent for separations of Xe and Kr.

Krypton‐xenon separation properties of SAPO‐34 zeolite materials and membranes

Separation of the radioisotope 85Kr from 136Xe is an important target during used nuclear fuel recycling. We report a detailed study on the Kr and Xe adsorption, diffusion, and membrane permeation properties of the silicoaluminophosphate zeolite SAPO‐34. Adsorption and diffusion measurements on SAPO‐34 crystals indicate their potential for use in Kr‐Xe separation membranes, but also highlight competing effects of adsorption and diffusion selectivity. SAPO‐34 membranes are synthesized on α−alumina disk and tubular substrates via steam assisted conversion seeding and hydrothermal growth, and are characterized in detail. Membrane transport measurements reveal that SAPO‐34 membranes can separate Kr from Xe by molecular sieving, with Kr permeabilities around 50 Barrer and mixture selectivity of 25–30 for Kr at ambient or slight sub‐ambient conditions. The membrane transport characteristics are modeled by the Maxwell‐Stefan equations, whose predictions are in very good agreement with experiment and confirm the minimal competing effects of adsorption and diffusion. © 2016 American Institute of Chemical Engineers AIChE J, 63: 761–769, 2017

Computational Modeling of Adsorption of Xe and Kr in M-MOF-74 Metal Organic Frame Works with Different Metal Atoms

Management of radioactive noble gases, i.e., Xe and Kr, needs special attention in the context of nuclear fuel reprocessing and postulated severe accident scenarios. Entrapment of these species in suitable matrix is generally difficult since Xe and Kr do not form bonds with substrates (physisorbed by van der Waals interactions) due to their chemical inertness. In recent years metal–organic frameworks (MOFs) are being actively discussed for adsorption/separation of various gases including noble gases. MOFs are considered superior in such applications owing to their high intake capacity, better selectivity and also the possibility of tuning the chemical properties by varying organic linkers or metal atoms. Theoretical modeling of MOFs and tailoring their properties appropriately for noble gas adsorption/separation are potentially useful to identify suitable candidates. In this regard, we investigate the binding energies of Xe and Kr in the well-known M-MOF-74 employing computer simulations. To understand th...

A surface thermodynamics approach to modelling single-file adsorption in ultramicroporous materials

A new thermodynamic approach is proposed to interpret adsorption equilibrium in ultramicropores with single-file adsorption. By considering the adsorbed phase as a one-dimensional fluid, phase equilibria thermodynamics can be used to derive a one-dimensional analogue of the monocomponent Gibbs adsorption isotherm. Equations such as Langmuir, Volmer, Fowler–Guggenheim or Hill–de Boer can thus be used as representations of phenomenological models of the one-dimensional adsorption system, rather than just as mathematical correlations. The bidirectional relation between the equations of state characterising the adsorbed phase and the adsorption isotherm equations allow great insight to be had simply by adsorption isotherm determination. In order to show the potential and limitations of this approach, the adsorption isotherms of Xe and CO2 in four crystalline hydrophobic dipeptides of the VA-class are analysed with this approach. It was possible to assess that adsorbate–adsorbate interactions are relevant for adsorption of both species, being attractive for Xe and repulsive for CO2; Xe adsorption is mainly distributed while CO2 adsorption is mainly localised.

ZIF-Derived Nitrogen-Doped Porous Carbons for Xe Adsorption and Separation.

Currently, finding high capacity adsorbents with large selectivity to capture Xe is still a great challenge. In this work, nitrogen-doped porous carbons were prepared by programmable temperature carbonization of zeolitic imidazolate framework-8 (ZIF-8) and ZIF-8/xylitol composite precursors and the resultant samples are marked as Carbon-Z and Carbon-ZX, respectively. Further adsorption measurements indicate that ZIF-derived nitrogen-doped Carbon-ZX exhibits extremely high Xe capacity of 4.42 mmol g−1 at 298 K and 1 bar, which is higher than almost all other pristine MOFs such as CuBTC, Ni/DOBDC, MOF-5 and Al-MIL-53, and even more than three times of the matrix ZIF-8 at similar conditions. Moreover, Carbon-ZX also shows the highest Xe/N2 selectivity about ~120, which is much larger than all other reported MOFs. These remarkable features illustrate that ZIF-derived nitrogen-doped porous carbon is an excellent adsorbent for Xe adsorption and separation at room temperature.

Density-functional theory with screened van der Waals interactions applied to atomic and molecular adsorbates on close-packed and non-close-packed surfaces

Modeling the adsorption of atoms and molecules on surfaces requires efficient electronic-structure methods that are able to capture both covalent and noncovalent interactions in a reliable manner. In order to tackle this problem, we have developed a method within density-functional theory (DFT) to model screened van der Waals interactions (vdW) for atoms and molecules on surfaces (the so-called DFT+${\text{vdW}}^{\mathrm{surf}}$ method). The relatively high accuracy of the DFT+${\text{vdW}}^{\mathrm{surf}}$ method in the calculation of both adsorption distances and energies, as well as the high degree of its reliability across a wide range of adsorbates, indicates the importance of the collective electronic effects within the extended substrate for the calculation of the vdW energy tail. We examine in detail the theoretical background of the method and assess its performance for adsorption phenomena including the physisorption of Xe on selected close-packed transition metal surfaces and 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) on Au(111). We also address the performance of DFT+${\text{vdW}}^{\mathrm{surf}}$ in the case of non-close-packed surfaces by studying the adsorption of Xe on Cu(110) and the interfaces formed by the adsorption of a PTCDA monolayer on the Ag(111), Ag(100), and Ag(110) surfaces. We conclude by discussing outstanding challenges in the modeling of vdW interactions for studying atomic and molecular adsorbates on inorganic substrates.

The adsorption and fast transport of Xe in single walled carbon nanotubes

Combined GCMC and MD simulations have been used to investigate the adsorption and diffusion of Xe gases in carbon nanotubes (CNTs) at different conditions.

Xe adsorption site distributions on Pt(111), Pt(221) and Pt(531)

The ideal structures of the Pt(111), Pt(221) and Pt(531) surfaces expose adsorption sites that can be qualitatively described as terrace sites on Pt(111), both step and terrace sites on Pt(221), and kink sites on Pt(531). The real surface structures of these surfaces can be complicated by imperfections such as misorientation, reconstruction and thermal roughening, all of which will influence their distributions of adsorption sites. Xe adsorption sites on the Pt(111), Pt(221) and Pt(531) surfaces have been probed using both photoemission of adsorbed Xe (PAX) and temperature programmed desorption (TPD) of Xe. Both PAX and Xe TPD are sensitive to the adsorption sites of the Xe and serve as complementary means of assessing the distributions of adsorption sites on these three Pt surfaces. The adsorption of Xe is sufficiently sensitive to detect the presence of residual steps on the Pt(111) surface at a density of ~1.5% step atoms per Pt atom. On the Pt(221) surface, PAX and Xe TPD reveal adsorption at both terrace and step sites simultaneously. Although the ideal structure of the Pt(531) surface has no well-defined steps or terraces, Xe adsorption indicates that its adsorption sites are best described as a distribution of both step and kink sites with roughly twice as many steps sites as kinks.

Direct Observation of Xe and Kr Adsorption in a Xe-Selective Microporous Metal-Organic Framework.

The cryogenic separation of noble gases is energy-intensive and expensive, especially when low concentrations are involved. Metal-organic frameworks (MOFs) containing polarizing groups within their pore spaces are predicted to be efficient Xe/Kr solid-state adsorbents, but no experimental insights into the nature of the Xe-network interaction are available to date. Here we report a new microporous MOF (designated SBMOF-2) that is selective toward Xe over Kr under ambient conditions, with a Xe/Kr selectivity of about 10 and a Xe capacity of 27.07 wt % at 298 K. Single-crystal diffraction results show that the Xe selectivity may be attributed to the specific geometry of the pores, forming cages built with phenyl rings and enriched with polar -OH groups, both of which serve as strong adsorption sites for polarizable Xe gas. The Xe/Kr separation in SBMOF-2 was investigated with experimental and computational breakthrough methods. These experiments showed that Kr broke through the column first, followed by Xe, which confirmed that SBMOF-2 has a real practical potential for separating Xe from Kr. Calculations showed that the capacity and adsorption selectivity of SBMOF-2 are comparable to those of the best-performing unmodified MOFs such as NiMOF-74 or Co formate.

Understanding the Adsorption Mechanism of Xe and Kr in a Metal-Organic Framework from X-ray Structural Analysis and First-Principles Calculations.

Enhancement of adsorption capacity and separation of radioactive Xe/Kr at room temperature and above is a challenging problem. Here, we report a detailed structural refinement and analysis of the synchrotron X-ray powder diffraction data of Ni-DODBC metal organic framework with in situ Xe and Kr adsorption at room temperature and above. Our results reveal that Xe and Kr adsorb at the open metal sites, with adsorption geometries well reproduced by DFT calculations. The measured temperature-dependent adsorption capacity of Xe is substantially larger than that for Kr, indicating the selectivity of Xe over Kr and is consistent with the more negative adsorption energy (dominated by van der Waals dispersion interactions) predicted from DFT. Our results reveal critical structural and energetic information about host-guest interactions that dictate the selective adsorption mechanism of these two inert gases, providing guidance for the design and synthesis of new MOF materials for the separation of environmentally hazardous gases from nuclear reprocessing applications.

Strengthening gold–gold bonds by complexing gold clusters with noble gases

We report an unexpectedly strong and complex chemical bonding of rare-gas atoms to neutral gold clusters. The bonding features are consistently reproduced at different levels of approximation within density-functional theory and beyond: from GGA, through hybrid and double-hybrid functionals, up to renormalized second-order perturbation theory. The main finding is that the adsorption of Ar, Kr, and Xe reduces electron–electron repulsion within gold dimer, causing strengthening of the AuAu bond. Differently from the dimer, the rare-gas adsorption effects on the gold trimer's geometry and vibrational frequencies are mainly due to electron occupation of the trimer's lowest unoccupied molecular orbital. For the trimer, the theoretical results are also consistent with far-infrared multiple photon dissociation experiments.

Characteristics of flexibility in metal-organic framework solid solutions of composition [Zn2(BME-bdc)x(DB-bdc)2−xdabco]n: In situ powder X-ray diffraction, in situ NMR spectroscopy, and molecular dynamics simulations

Porosity switching in the crystalline solid state is a unique phenomenon observed only in a limited number of materials. The switching behavior of two metal-organic frameworks as well as their respective solid solutions of composition [Zn2(BME-bdc)x(DB-bdc)2−xdabco]n (x = 2; 1.5; 1.0; 0.5; 0) is studied in situ during the adsorption of CO2 and Xe using X-ray diffraction and NMR techniques. The diffraction data, measured during the adsorption suggest a direct one-step phase transition (switching) from the narrow pore phase to the large pore phase beyond the transition pressure. An intermediate phase was found only in one compound within a narrow pressure range around the phase transition pressure region. In situ high-pressure 13C NMR spectroscopy of adsorbed CO2 also allowed following the gating behavior of the studied materials by monitoring the signal of adsorbed CO2. The 13C NMR spectra exhibit a pronounced broadening indicating a certain degree of order for the adsorbed molecules inside the pores. This ordering effect and the resulting line broadening depend on the linker functionalization as could be confirmed by corresponding molecular dynamics (MD) simulations.