Xe Adsorption Research Articles

Stress-Based Model for the Breathing of Metal-Organic Frameworks.

Gas adsorption in pores of flexible metal-organic frameworks (MOF) induces elastic deformation and structural transitions associated with stepwise expansion and contraction of the material, known as breathing transitions between large pore (lp) and narrow pore (np) phases. We present here a simple yet instructive model for the physical mechanism of this enigmatic phenomenon considering the adsorption-induced stress exerted on the material as a stimulus that triggers breathing transitions. The proposed model implies that the structural transitions in MOFs occur when the stress reaches a certain critical threshold. We showcase this model by drawing on the example of Xe adsorption in MIL-53 (Al) at 220 K, which exhibits two consecutive hysteretic breathing transitions between lp and np phases. We also propose an explanation for the experimentally observed coexistence of np and lp phases in MIL-53 materials.

Nanoporous materials and their adsorption properties

Principal regularities of forming microporous adsorbents with different surface composition, in particular, active coals, zeolites, silica gels, alumina gels, fullerenes, and carbon nanotubes are considered. The data on the structure-energetecal characteristics of the adsorbents and systems are correlated. Principal characteristic features of the microporous adsorbents deformation during CO2,CH4, Ar, N2, and Xe adsorption in a 177.65–393 K temperature range at the pressures of up to 7.0 MPa are studied by example of the AUK microporous carbonaceous adsorbent and the NaX zeolite. An analysis of the adsorption deformation of the microporous adsorbents as a function of the adsorption amount, temperature, and physicochemical properties of adsorbed gas is presented. Based on the analysis of benzene adsorption isotherms for the C60 and C70 fullerenes, a suggestion is advanced on the formation of fullerene-benzene adsorption complexes comprising on average three fullerene and two benzene molecules; their characteristic energy comes to Eo ∼ 25 kJ/mol. It is shown, by using the adsorption and SEM data that the cuminol adsorption at the nanotubes yields supramolecular complexes in which the cumene molecules act as coordinating ones. By using the micropore bulk filling theory, the hydrogen adsorption is calculated for model supramolecular systems in which the carbon nanotubes are bundled up, with leaving gaps between the bundles.

The growth process of first water layer and crystalline ice on the Rh(111) surface

The adsorption states and growth process of the first layer and multilayer of water (D(2)O) on Rh(111) above 135 K were investigated using infrared reflection absorption spectroscopy (IRAS), temperature programed desorption, spot-profile-analysis low-energy electron diffraction, and scanning tunneling microscopy (STM). At the initial stage, water molecules form commensurate ( radical3x radical3)R30 degrees islands, whose size is limited for several hexagonal units; the average diameter is approximately 2.5 nm. This two-dimensional (2D) island includes D-down species, and free OD species exist at the island edge. With increasing coverage, the D-up species starts to appear in IRAS. At higher coverages, the 2D islands are connected in STM images. By the titration of Xe adsorption we estimated that the D-down domain occupies about 55% on Rh(111) at the saturation coverage. Further adsorption of water molecules forms three-dimensional ice crystallites on the first water layer; thus, the growth mode of crystalline water layers on Rh(111) is a Stranski-Krastanov type. We have found that an ice crystallite starts to grow on D-down domains and the D-down species do not reorient upon the formation of a crystalline ice.

Effect of rare-gas adsorption on the spin-orbit split bands of a surface alloy: Xe on Ag(111)-(3×3)R30°−Bi

We have investigated by angle-resolved photoemission spectroscopy changes induced by Xe adsorption on the spin-orbit-split bands of the epitaxial Ag(111) $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}\text{\ensuremath{-}}\mathrm{Bi}$ surface alloy. We observe an extensive backfolding of the alloy band structure due to the formation of a $9\ifmmode\times\else\texttimes\fi{}9$ commensurate or quasicommensurate Xe overlayer and the opening of a small hybridization gap at the intersection of two sets of bands of (primarily) $\mathrm{Bi}\phantom{\rule{0.2em}{0ex}}s{p}_{z}$ and ${p}_{xy}$ characters. The large spin-orbit splitting of the substrate bands is essentially unaffected by the Xe overlayer, in sharp contrast to the enhancement reported for the clean Au(111) surface. This points to different mechanisms at the origin of the splitting in the clean surfaces and in the surface alloy.

Influence of quantum effects on the mechanism of adsorption and phase diagram of rare gases in carbon nanotubes

We present results of grand canonical Monte Carlo simulations of rare gases (He, Ne, Ar, Kr and Xe) adsorption in carbon nanotubes. The interaction model includes both quantum effects (via effective Feymann-Hibbs potential) and the atomic roughness of the tube. We show that the quantum contribution to interactions does not suppress the energetic corrugation of carbon nanotube but decreases only its average strength. In the case of Ne, the phase diagram and, in particular, the melting temperature for layers adsorbed on and within an individual tube does not depend on tube chirality. However, the structure of layers adsorbed on outer surface of the tube is strongly related to the atomic structure of the underlying tube.

Xe adsorption on aC60monolayer on Ag(111)

Low-energy electron diffraction (LEED) experiments and grand canonical Monte Carlo simulations were carried out to study the adsorption of Xe on a substrate composed of a monolayer of ${\mathrm{C}}_{60}$ molecules on a Ag(111) surface. LEED adsorption isobars indicated that the adsorption occurs in steps, with the Xe initially adopting a structure having the same unit cell as the ${\mathrm{C}}_{60}$. Isosteric heats corresponding to the first two steps were measured to be $234\ifmmode\pm\else\textpm\fi{}8$ and $204\ifmmode\pm\else\textpm\fi{}14\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, respectively. For the simulations, the interaction potential of Xe with the composite substrate was modeled as the sum of two parts: the Xe-Ag part was computed using an ab initio van der Waals potential that varies as an inverse-distance cubed and the $\mathrm{Xe}\text{\ensuremath{-}}{\mathrm{C}}_{60}$ part was computed using a spherically averaged ${\mathrm{C}}_{60}$ potential [E. S. Hernandez et al., J. Low Temp. Phys. 134, 309 (2004)]. The resulting adsorption potential is highly corrugated, with the most attractive sites located in the threefold hollows between the ${\mathrm{C}}_{60}$ molecules, forming a honeycomb array. The simulations (at temperatures ranging from $55\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}90\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) show that these attractive sites are filled first, followed by adsorption in two types of secondary sites, where a competition exists due to steric hindrance. The thermodynamic properties of film growth obtained in the simulation are in good agreement with the experiment.

Analysis of the Temperature and Pressure Dependence of the 129Xe NMR Chemical Shift and Signal Intensity for the Derivation of Basic Parameters of Adsorption as Applied to Zeolite ZSM-5

Temperature and pressure dependences of the 129Xe NMR chemical shift and the signal intensity have been investigated using ZSM-5 as an adsorbent under routine conditions without using any high-pressure or especially high-temperature facilities. The use of a rigorously shielded system and a calibration sample for the signal intensity was found to be valuable to obtain reliable data about the chemical shift and the signal intensity. The 129Xe NMR data obtained between 0.05 and 1.5 atm and from 24 to 80 degrees C were analyzed based on the Dubinin-Radushkevich equation as well as the Langmuir type equation. In both analyses, chemical shift data succeeded only partially in providing the profile of adsorption, such as energetic aspects, surface area, saturated amount of Xe adsorption and specific parameters of 129Xe chemical shift. It was shown that the reliable total analysis was achieved when the chemical shift data were used together with the intensity data. Such an analysis of the chemical shift data, aided by the intensity data, will be useful in performing nano-material analysis on 129Xe NMR without invoking the traditional methodology of gravimetric or volumetric adsorption experiments.

129Xe NMR Spectroscopy Study of Porous Cyanometallates

Zinc and cadmium hexacyanocobaltates(III) were prepared, and their porous networks were explored using 129Xe spectroscopy. The crystal structures of these two compounds are representative of porous hexacyanometallates, cubic (Fm-3m) for cadmium and rhombohedral (R-3c) for zinc. In the cubic structure, the porosity is related to systematic vacancies created from the elemental building block (i.e., the hexacyanometallate anion), whereas the rhombohedral (R-3c) structure is free of vacant sites but has tetrahedral coordination for the zinc atom, which leads to relatively large ellipsoidal pores communicated by elliptical windows. According to the Xe adsorption isotherms, these porous frameworks were found to be accessible to the Xe atom. The structure of the higher electric field gradient at the pore surface (Fm-3m) appears and is accompanied by a stronger guest-host interaction for the Xe atoms and a higher capacity for Xe sorption. For cadmium, the 129Xe NMR signal is typical of isotropic movement for the Xe atom, indicating that it remains trapped within a spherical cavity. From spectra recorded for different amounts of adsorbed Xe, the cavity diameter was estimated. For the zinc complex, 129Xe NMR spectra are asymmetric because of the Xe atom movement within an elongated cavity. The line-shape asymmetry changes when the Xe loading within the porous framework increases, which was ascribed to Xe-Xe interactions through the cavity windows. The Xe adsorption revealed additional structural information for the studied materials.

Study of Xe and Kr adsorption on open single-walled carbon nanotubes using molecular dynamics simulations

We have used molecular dynamics (MD) simulation to study the adsorption isotherms of noble gases on open ended single-walled carbon nanotubes (SWNTs). Adsorption isotherms of xenon at several temperatures between 95 and 130 K, and of krypton between 75 and 95 K on (10,10) tubes have been measured. Adsorption coverage, isosteric heat, and binding energy were calculated at various temperatures and pressures. It was shown that adsorption occur both inside and outside an open ended SWNTs. The interior coverage of 0.06 Xe–C and 0.08 Kr–C were reported at saturation conditions. The monolayer coverage on the exterior surface is 0.23 Xe–C and 0.25 Kr–C. These values are in good agreement with experimental measurements and theoretical predictions.

Ordering of rare gas films on a decagonal Al–Ni–Co quasicrystal

This paper reviews recent progress in the study of rare gas films on quasicrystalline surfaces. The adsorption of Xe on the 10-fold surface of decagonal Al–Ni–Co was studied using low-energy electron diffraction (LEED). The results of these studies prompted the development of a theoretical model, which successfully reproduced the thermodynamic parameters found in the experiment. Grand canonical Monte Carlo (GCMC) simulations for Xe-produced structures that agreed with the experimental observations of the adsorption structures and provided a deeper insight into the nature of the ordering. A first-order commensurate–incommensurate transition, which involves a transition from a quasicrystalline five-fold structure to a periodic hexagonal structure, was discovered and characterized for the Xe monolayer. The five rotational domains of the hexagonal structure observed in the LEED study were shown in the GCMC study to be mediated by pentagonal defects, which are entropic in nature, and not by substrate defects. The GCMC study found an absence of any such transition for Kr, Ar and Ne on the same surface. A detailed analysis of this transition led to the conclusion that the formation of the hexagonal layer depends on matching the gas and substrate characteristic lengths.

Density Functional Theory Model of Adsorption Deformation

Molecules adsorbed in pores cause elastic deformations of the solid matrix leading to either contraction or swelling of the material. Although experimental manifestation of adsorption-induced deformation in clays, coals, carbons, silicas, and other materials has been known for a long time, a rigorous theoretical description of this phenomenon is lacking. We report the nonlocal density functional theory (NLDFT) calculations that reproduce almost quantitatively the adsorption and strain isotherms of Kr and Xe on zeolite X. This system exhibits characteristic contraction at low vapor pressures and swelling at high vapor pressures. We show that the experimentally observed changes in the adsorbent volume are proportional to the solvation (disjoining) pressure caused by the adsorption stress exerted on the pore walls. The proposed NLDFT model can be used for the interpretation of adsorption measurements in micro- and mesoporous materials and for the characterization of their mechanical properties.

Xe films on a decagonal Al-Ni-Co quasicrystalline surface

The grand canonical Monte Carlo method is employed to study the adsorption of Xe on a quasicrystalline Al-Ni-Co surface. The calculation uses a semiempirical gas-surface interaction, based on conventional combining rules and the usual Lennard-Jones Xe-Xe interaction. The resulting adsorption isotherms and calculated structures are consistent with the results of LEED experimental data. In this paper we focus on five features not discussed earlier (Phys. Rev. Lett. 95, 136104 (2005)): the range of the average density of the adsorbate, the order of the transition, the orientational degeneracy of the ground state, the isosteric heat of adsorption of the system, and the effect of the vertical cell dimension.

Xenon as a probe for minority sites on solid surfaces

Atomic-level microscopies have proved useful to map solid-surface sites directly, but, because of their lack of chemical specificity, they are less adept at identifying unique chemical activity on those sites. Here we present a dual-titration approach developed to probe minority sites on solid surfaces with unique chemical properties of potential relevance to heterogeneous catalysis. Our methodology involves the initial dosing of a chemical probe such as carbon monoxide or ammonia to drive its selective adsorption onto specific sites with particular chemical activity, and the subsequent adsorption of xenon to help identify the nature of those sites. A combination of photoelectron and temperature-programmed desorption spectroscopies are used to characterize the Xe adsorption. The chemistry of oxygen-modified Ni(110) single crystals was probed to test this technique. It was observed that whereas CO does not discriminate among the various sites present on those surfaces, ammonia binds preferentially to the end of -Ni-O rows and modifies their local electrostatic potential. In addition, it was determined that adsorbed CO aids in a reversible surface reconstruction involving the coalescence of fragmented surface -Ni-O rows at high (>350 K) temperatures.

Role of van der Waals interactions in adsorption of Xe on Cu(111) and Pt(111)

We consider some recently developed schemes for treating van der Waals interactions within the density functional theory (DFT) on the widely discussed example of adsorption of Xe on Cu(111) and Pt(111) surfaces. Consistent with the overall weakness of the Xe surface and Xe-Xe interactions we assess the performance of the schemes that are appropriate to systems consisting of nearly isolated fragments in which the coefficients of the van der Waals expansion are deduced from DFT calculations. Such generalized DFT calculations of potential energy surfaces yield the structure of Xe adlayers in good agreement with experiments and retrieve the dilation of commensurate monolayer phase in which the intralayer Xe-Xe radial force constants are strongly reduced. This provides a first principles interpretation of the observed vibrational properties of adlayers, in general, and the much debated dispersion of in-plane polarized vibrations, in particular.

Adsorption of Simple Gases in MCM-41 Materials: The Role of Surface Roughness

This paper reports the development and testing of atomistic models of silica MCM-41 pores. Model A is a regular cylindrical pore having a constant section. Model B has a surface disorder that reproduces the morphological features of a pore obtained from an on-lattice simulation that mimics the synthesis process of MCM-41 materials. Both models are generated using a similar procedure, which consists of carving the pore out of an atomistic silica block. The differences between the two models are analyzed in terms of small angle neutron scattering spectra as well as adsorption isotherms and isosteric heat curves for Ar at 87 K and Xe at 195 K. As expected for capillary condensation in regular nanopores, the Ar and Xe adsorption/desorption cycles for model A exhibit a large hysteresis loop having a symmetrical shape, i.e., with parallel adsorption and desorption branches. The features of the adsorption isotherms for model B strongly depart from those observed for model A. Both the Ar and Xe adsorption branches for model B correspond to a quasicontinuous pore filling that involves coexistence within the pore of liquid bridges and gas nanobubbles. As in the case of model A, the Ar adsorption isotherm for model B exhibits a significant hysteresis loop; however, the shape of the loop is asymmetrical with a desorption branch much steeper than the adsorption branch. In contrast, the adsorption/desorption cycle for Xe in model B is quasicontinuous and quasireversible. Comparison with adsorption and neutron scattering experiments suggests that model B is too rough at the molecular scale but reproduces reasonably the surface disorder of real MCM-41 at larger length scales. In contrast, model A is smooth at small length scales in agreement with experiments but seems to be too ordered at larger length scales.

129Xe NMR study of Xe adsorption on multiwall carbon nanotubes

129Xe NMR spectroscopy has been used to study the adsorption of Xe on multi-wall carbon nanotubes (MWCNT). The results obtained have shown the 129Xe NMR ability to probe the intercrystalline (aggregate) and the inner porosity of CNT. In particular, the effects on porosity of tubes openings by hydrogen exposure and of ball milling were examined. Dramatic changes observed in the 129Xe NMR spectra after moderate ball milling of MWCNTs were attributed to the destruction of the initial intercrystalline pore structure and to the Xe access inside the nanotubes. To examine the exchange dynamics the mixture of as-made and milled MWCNTs was studied with one- and two-dimensional (1D and 2D) 129Xe NMR. The exchange between the interior of milled nanotubes and the aggregate pores of as-made MWCNTs was fast on the NMR acquisition time scale. The Xenon exchange between the interior of the as-made MWCNTs and the large aggregate pores occurred on a longer time scale of 10 ms, as was established by 2D 129Xe NMR exchange spectroscopy. Variable temperature 129Xe NMR data were also discussed and analyzed in terms of the fast exchange approximation.

Xe adsorption on metal surfaces: First-principles investigations

In the present paper we report in detail the results of our first-principles investigations of Xe/metal adsorption systems which were briefly presented in an earlier publication. We investigate Xe adsorption on the Mg(0001), Al(111), Ti(0001), Cu(111), Pd(111), and Pt(111) surfaces in the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ structure. Our calculations are based on density functional theory within the local-density approximation and generalized gradient approximation of Perdew, Burke, and Ernzerhof functions and employing the all-electron full-potential linearized augmented plane wave method. From analysis of the lateral and perpendicular potential-energy surfaces we find that Xe atoms adsorb in the on-top sites in all cases. We identify a noticeable site dependence of the Pauli repulsion, which largely actuates the adsorption site preference. We anticipate that the identified microscopic nature of the binding between Xe and metal atoms may also be valid for other rare-gas atom/metal systems.

XANES/EXAFS study and catalytic properties of the confined Cr carbonyl–MCM-41 system

X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) were performed to determine the structural stability and atomic local order of transition metal carbonyls Cr(CO) 6 confined into the channel network of MCM-41. These species interact with the surface siloxane groups of the MCM-41 walls as proved by XANES/EXAFS. These Cr species were previously characterized by FT-IR, Xe adsorption and 29Si NMR, which confirmed their monodispersity, chemical interaction and confinement into the pore network of MCM-41. In this study, XAS provides additional evidence on the high dispersion of the metal species and symmetry of the Cr sites. Also, the Cr O bond lengths, Cr coordination and oxidation state were determined after thermal treatments, thus confirming the interaction of the sub-carbonyl species with the siloxane groups. The change of the oxidation state from 0 to almost 3 might explain the unusual catalytic activity for ethylene polymerization of the Cr sub-carbonyl species confined into MCM-41.

Adsorption of Gases, Vapors and Liquids by Microporous Adsorbents

Adsorption of Xe, Kr, Ar, N2, O2, H2 CH4, CO2, He, and freons by PAU-10 and ACC microporous carbon adsorbents as well as by A and X zeolites was investigated over a wide range of pressures (0.1 Pa – 20 MPa) and temperatures (77, 120–600 K). The amount of gases, vapors and liquids adsorbed by microporous adsorbents increases steadily with increasing pressure and does not change dramatically if phase transitions occur in the adsorptive. Isosteres of adsorption constructed as a curve of ln P against f(1/T) a retain a linear form over a wide range of pressures and temperatures. The slope of isosteres does not vary on going through the critical temperature of the gaseous phase. At high pressures, due to non-ideality of the gaseous phase and non-inert behavior of the adsorbent the differential molar heat of adsorption is dependent on temperature. At high fillings of micropores the differential molar isosteric heat capacities of adsorption systems show maxima that indicate the occurrence of structural rearrangements in the adsorbate.

Adsorption Purification of Xenon to Remove Hexafluoroethane Impurities

We present the individual isotherms for Xe and C2F6 adsorption on activated carbon (T = 213 K), curves of C2F6 adsorption on activated carbon from a mixture of Xe and N2 (T = 160 K), and calculated output curves for Xe and C2F6 on activated carbon (T = 160 K).