Desorption Of Xe Research Articles

Electric-field Controlled Switchable and Efficient Separation of Radioactive Xe/Kr on Borophene: A Theoretical Study.

The efficient and reversible separation of radioactive Xe/Kr during spent fuel reprocessing is important and challenging for the rapid development of nuclear energy. In this study, we firstly report a strategy of applying an electric field on the solid adsorbent borophene to realize efficient and switchable Xe/Kr separation via a density functional theory (DFT) investigation. Based on the calculational results, the adsorption energies for Xe and Kr on borophene without an electric field are -0.25 eV and -0.18 eV, respectively, indicating that Xe and Kr can only form weak adsorption on borophene. However, by applying an electric field (0.006 a.u.) to the systems, the adsorption energies for Xe and Kr on borophene are -0.98 eV and -0.47 eV, respectively, which shows that the interaction between Xe and borophene has increased dramatically compared with that of Kr, so Xe can be separated from radioactive Xe/Kr mixtures. What's more, when the electric field is removed, desorption of Xe from the surface of borophene is exothermic without an energy barrier. The adsorbent is recyclable. In summary, this theoretical study provides novel information for experimental researches, the highly efficient Xe/Kr separation can be controlled by turning on/off the applied electric field.

Evaluation of porous structure by Kr and Xe desorption halftimes with X-Ray Fluorescence analysis

X-Ray Fluorescence analysis based on excitation with 109 Cd source and registration of characteristic X-rays with Si(Li) detector was used to record the kinetics of desorption of Kr and Xe from carbonaceous samples of 1–100 mg after their treatment with the inert gases at temperatures from 20 to 700 °C. Activated coals with a known pore distribution were used to demonstrate that the Kr desorption half-life is inversely proportional to the square of the pore diameter in the range from 0.7 nm to 50 nm. Although the absorption of Xe was 2–10 times higher in the same pores than the absorption of Kr, the desorption of Xe was about 2.83 times slower than the desorption of Kr. The later fact speaks in favor of using Kr and Ar as a marker for evaluating porous structures rather than Xe. • Simultaneous ED-XRF control of Kr, Xe, Fe, Cu, Zn, Pb, and H in carbon samples. • Evaluation of pore diameters by halftimes of Kr and Xe desorption. • Clarification of correlations between metallic admixtures and pore distribution. • In the same pores, Xe desorption is by 2.83 times slower than that of Kr.

The effect of Cenozoic basin inversion on coal-bed methane in Liupanshui Coalfield, Southern China

Coalbed methane (CBM) is an important energy source globally, thus understanding its origin and evolution is essential to the assessment of reserves and developing exploration strategies. This study aims to identify the origin of CBM in Liupanshui Coalfield (LPC) in southern China and evaluate the influence of basin exhumation on gas storage in different rank coals. Ten CBM samples were collected from four blocks that have different thermal histories and coal rank ranges. Based on the C1/(C2 + C3) ratio (16–971), δ13CCH4 (−41.6 to −34.9‰) and δDCH4 (−185 to −140‰), the methane is mainly thermogenic in origin. Systematic compositional differences between the four blocks are consistent with coal maturity. Noble gases in LPC CBM are a mixture of air-derived gas dissolved in local groundwaters and radiogenic gases that were generated in the coals and diffused in from deeper crust. The radiogenic 4He/40Ar⁎ and 21Ne⁎/40Ar⁎ are lowest in CBM from Qingshan block, strong evidence of diffusive loss of light noble gases. 20Ne/36Ar (0.23–0.70) in all gases are higher than local air-saturated water (0.16) and are most easily explained by re-dissolution of noble gases that exsolved from groundwater during basin inversion. 84Kr/36Ar and 132Xe/36Ar ratios are higher than predicted from this process and likely reflect desorption of Xe and Kr that has been trapped on the coal matrix at the time of coal formation. The noble gases are consistent with gas exsolution and loss from the reservoir during Mesozoic-Cenozoic basin inversion. Modelling the loss of both free and adsorbed gas during basin inversion we find that the Qingshan block has lost the highest proportion of free gas (77%), yet the total gas loss (free and adsorbed) is the lowest (25%) due to the strong adsorption capacity of the mature coals. The volume of groundwater that has interacted with the gas phase during the basin inversion is estimated according to the re-dissolution process model to evaluate the role of groundwater in gas preservation and gas extraction. Combining gas production data for the four blocks, high gas content and efficient gas extraction in LPC are expected for the high maturity coals that have seen low groundwater during basin inversion but more at shallow depth.

Kinetics of C, N, and Xe release during the quasi-isothermal pyrolysis and subsequent oxidation of nanodiamond from the Orgueil CI meteorite

Analysis of the C, N, and Xe release kinetics of intermediate-sized nanodiamond fraction from the Orgueil CI meteorite during isothermal pyrolysis conducted for the first time and subsequent oxidation indicates that (a) the rate of C, N, and Xe release at pyrolysis at a constant temperature decreases with time; (b) the relative amount of released Xe, which mostly has a normal isotopic composition (Xe-P3) at various pyrolysis time up to 800°C, is controlled, first of all, by the heating temperature, whereas the amount of N is controlled by both the temperature and heating time; and (c) prolonged pyrolysis notably modifies the distribution of nitrogen of normal (δ15N = 0) and anomalous (δ15N= −350‰) isotopic composition in diamond grains. The identified features of the C and N release kinetics are explained by differences in the binding energy of chemically adsorbed O with C atoms and the accommodation of the main amounts of N in extended defects of the crystal structure of nanodiamond. The major factors of the decrease in the Xe-P3 release rate during the isothermal pyrolysis of nanodiamond are either the differences between the Xe desorption parameters of the traps in graphite-like phases containing Xe-P3 or the differences between the radiation-induced defectiveness of grains of the population containing implanted Xe-P3. Our results led us to conclude that (1) meteoritic nanodiamond contains relatively low amounts of a phases carrying the P3 component of noble gases, regardless of the nature of this component, and (2) the population of nanodiamond grains containing most of isotopically anomalous nitrogen was produced at a high rate to preserve this nitrogen, first of all, at extended defects in the diamond crystal structure.

Photoinduced Desorption of Xe from Porous Si following Ultraviolet Irradiation: Evidence for a Selective and Highly Effective Optical Activity

Photoinduced desorption (PID) of Xe from porous silicon (PSi) following UV irradiation has been studied. A nonthermal, morphology, and wavelength dependent phenomenon with more than 3 orders of magnitude enhancement of Xe PID within pores over atoms adsorbed on top of flat surfaces has been recorded, displaying extraordinary large cross sections up to σ(Xe/PSi)=2×10(-15) cm(2). A long-lived, photoinduced, charge separated silicon-xenon complex is proposed as the precursor for this remarkable photodesorption process.

Photostimulated desorption of Xe from Au(001) surfaces via transient Xe−formation

Photo-stimulated desorption (PSD) of Xe atoms from the Au(001) surface in thermal and nonthermal regimes was investigated by the time-of-flight measurement at photon energies of 6.4 and 2.3 eV. Xe was desorbed in a thermal way at high laser fluence, which was in good agreement with theoretical simulations. At a low laser fluence, on the other hand, desorption was induced only at a photon energy of 6.4 eV by a non-thermal one-photon process. We argue that the nonthermal PSD occurs via transient formation of Xe- on Au(001). The lifetime of Xe- is estimated to be ~15 fs with a classical model calculation. Whereas the electron affinity of Xe is negative in the isolated state, it is stabilized by the metal proximity effect.

ChemInform Abstract: Molecular Dynamics for Reactions of Heterogeneous Catalysis

An overview is given of Molecular Dynamics, and numerical integration techniques, system initialization, boundary conditions, force representation, statistics, system size, and simulations duration are discussed. Examples from surface science are used to illustrate the pros and cons of the method. Two new methods are presented with which it is possible to compute reaction rates and reaction mechanisms in spite of activation barriers that are much higher than thermal energies, and results are shown for Xe desorption from Pd(100).

Nanoparticle aggregation due to dewetting of sandwiched buffer layers

The deposition of Au onto thin condensed volatile buffer layers produces small clusters. Sublimation of the buffer converts these clusters into compact or ramified structures, depending on the thickness of the buffer, in a process called buffer-layer-assisted growth. We have used bilayer structures of Xe on CO 2 or Xe on H 2O on amorphous carbon substrates to investigate effects of second layer dewetting and the impact of the initial particle size on aggregation. Compact particles formed by Xe desorption aggregate during removal of the CO 2 or H 2O layer but little aggregation occurs for ramified particles produced on Xe layers thicker than ∼100 ML. Instead, the large structures tend to break up on the CO 2 film, producing smaller, more compact particles. CO 2 and H 2O impurities in the Xe film significantly reduce particle coalescence and accelerate Xe dewetting.

Direct Observation of Atoms Entering and Exiting l-Alanyl-l-valine Nanotubes by Hyperpolarized Xenon-129 NMR

The kinetics of gas --> channel and channel --> gas exchange of Xe in self-assembled L-alanyl-L-valine (AV) nanotubes was facilitated by continuous flow hyperpolarized xenon-129 two-dimensional exchange NMR spectroscopy. Cross-peaks due to gas atoms entering or exiting were observed at Xe pressures of 92, 1320, and 3300 mbar, corresponding to Xe fractional occupancies ranging from 0.047 to 0.64. At each pressure, the rate of desorption from the channels was determined by fitting the mixing time dependence of the cross-peak and diagonal-peak signals to a magnetization exchange model, assuming steady-state Langmuir adsorption under hyperpolarized gas flow conditions. The observed rate constant for desorption of Xe from AV nanotubes decreased from 4.5 s-1 to 2.0 s-1 over the occupancy range studied, a finding that is consistent with a decrease in the diffusivity in the channels.

Sublimative desorption of xenon from Ru(100)

Sublimative desorption is a process in which desorption of multilayers of an adsorbate precedes melting and surface diffusion. Here we report on the desorption kinetics of Xe atoms from multilayer coverage studied using temperature programmed desorption and optical diffraction methods. It is found that decay of the diffraction peak intensities from multilayer coverage grating during surface heating cannot be explained as one-dimensional diffusion process. Instead, the diffraction signal follows Xe desorption, as deduced from simultaneous linear diffraction and desorption measurements. This observation suggests that no macroscopic two-dimensional melting and diffusion occur in the case of multilayers of Xe before the onset for desorption. It is concluded that Xe atoms undergo sublimative desorption from the topmost layers. Similar results were obtained in the case of water multilayers on Rus100d. These results suggest that on solid surfaces the desorption of multilayers is thermodynamically favorable over surface melting or diffusion.

Oblique-incidence Reflectivity Difference (OI-RD) and Leed Studies of Adsorption and Growth of Xe on Nb(110)

AbstractWe studied adsorption, growth and desorption of Xe on Nb(110) using an in-situ obliqueincidence reflectivity difference (OI-RD) technique and low energy electron diffraction (LEED) from 32 K to 100 K. The results show that Xe grows a (111)-oriented film after a transition layer is formed on Nb(110). The transition layer consists of three layers. The first two layers are disordered with Xe-Xe separation significantly larger than the bulk value. The third monolayer forms a close packed (111) structure on top of the tensile-strained double layer and serves as a template for subsequent homoepitaxy. The adsorption of the first and the second layers are zeroth order with sticking coefficient close to one. Growth of the Xe(111) film on the transition layer proceeds in a step flow mode from 54K to 40K. At 40K, an incomplete layer-by-layer growth is observed while below 35K the growth proceeds in a multilayer mode.

CO-induced morphology modification in buffer-layer-assisted growth of Pd nanostructures

The effects of chemisorbed CO on the aggregation and coalescence of Pd nanometer-sized clusters during Xe buffer-layer-assisted growth (BLAG) have been studied with transmission electron microscopy. The Pd clusters were either grown on Xe and then exposed to CO or they were grown on solid CO condensed on Xe at 20 K. In the former case, the adsorbed CO presented no barrier to aggregation when clusters came into contact during Xe desorption, and they retained their diffusion-limited cluster–cluster kinetics. It did, however, significantly impede their coalescence, thus producing with branched islands with thinner branches and lower profile than those produced without CO. The critical radius for crossover from compact to branched growth was ∼2 nm for CO-clad Pd compared to ∼4 nm for bare Pd clusters. In the second case when Pd clusters were grown on CO on Xe, their rate of growth during Xe desorption was slower, possibly because of the presence of mixed Pd–CO moieties among them. We also show that BLAG and desorption-assisted coalescence observed with rare gas buffers also occur with CO, with differences that reflect adatom and cluster interactions with the CO.

Theoretical study on the photo-stimulated desorption of Xe from an oxidized Si(0 0 1) surface

Photo-stimulated desorption (PSD) of Xe from an oxidized Si(0 0 1) surface has been investigated theoretically in connection with a recent experiment by Watanabe and Matsumoto (Faraday Discuss. 117 (2000) 203). We have performed ab initio calculations for the potential energy curves (PECs) of the ground and excited states of Xe/oxidized-Si(0 0 1) and have found that Xe is strongly attracted to the surface in the excited state for a model cluster that takes into account backbond oxidation. The kinetic energy of the desorbed Xe, obtained by classical trajectory calculations using the ab initio PECs, is qualitatively in good agreement with the experimental value.

Dynamic displacement of N2 from Ru(0001) by incident D and H atoms

Exposing a N2 covered Ru(0001) surface to a D or H atom beam leads to desorption of the N2 molecules. This displacement is kinetically prompt at all N2 coverages and the process is identified as dynamic displacement. By showing that the cross section for displacement by D atoms is roughly twice that for H atoms, we suggest that the mechanism for this dynamic displacement is some phonon mediated process rather than an electronically nonadiabatic one suggested earlier. As a contrast, the displacement of Xe adsorbed on Pt(111) induced by CO adsorption has also been measured. In this case, the displacement is not prompt and there is a total coverage on the surface that is necessary to induce desorption of Xe. This seems well described by a thermodynamic displacement mechanism.

Excitation mechanisms and photochemistry of adsorbates with spherical symmetry.

By comparing the photo-stimulated desorption of Xe from an oxidized Si(100) surface with the photochemistry of methane on metal surfaces, we try to deduce a common concept in the excited state and the excitation mechanism responsible for the photo-induced processes. Xe atoms are desorbed from the oxidized Si(100) surface by the irradiation of photons in the range 1.16-6.43 eV. Two velocity components with average kinetic energy 0.85 and 0.25 eV are observed in the time-of-flight distributions. The fast component appears only if the photon energy exceeds approximately 3 eV, but the slow component is present over the entire photon energy range. By analogy with the photochemistry of methane on metal surfaces, the excitation mechanism responsible for the fast component is postulated to be a transition from the 5p state of Xe to the excited state originating from strong hybridization between the 6s state of Xe and the dangling bond at a surface silicon atom bonded to oxygen inserted in the dimer bond. In this scheme an excited electron is transferred from the adsorbate to the substrate, which is the reverse direction to the substrate-mediated excitation frequently assumed in surface photochemistry.

Self-assembly of multilayer arrays from Ag nanoclusters delivered to Ag(111) by soft landing

Three-dimensional Ag nanostructures containing 150–600 000 atoms were created on Xe multilayers at 50 K. They were delivered to Ag(111), strained Ag(111), and Ag/Si(111)-( 3 × 3 )R30° following Xe desorption, and they were imaged with a scanning tunneling microscope after warming to room temperature. On Ag(111), they wetted the surface and produced hexagonal multilayer epitaxial islands that decayed via step–step contact while maintaining a constant mean terrace width of ≈11 Å. On strained Ag(111), there was wetting and an irregular hexagonal footprint but strain stabilized the multilayers by changing the energetics of atom exchange and step fluctuation. On Ag/Si(111)-( 3 × 3 )R30°, contacts were established but decay was not observed.

An optical differential reflectance study of adsorption and desorption of xenon and deuterium on Ni(111)

We studied adsorption and desorption of Xe and deuterium on Ni(111) using an optical differential reflectance technique. The main findings are: (i) the differential reflectance varies almost linearly with the surface densities of deuterium and Xe adatoms, and the signals can be described well with a three-layer model and the known dielectric responses of the surface layers: (ii) the adsorption of deuterium atT = 120 K follows the Langmuir kinetics, while the adsorption of Xe atT = 38 K follows the zeroth-order kinetics; (iii) nearT = 70 K, the rate of Xe desorption is almost coverage-independent with an activation energy ofEdes = 4.4 ± 0.2 kcal/mol. Our analysis suggests that the Xe desorption is likely to be dominated by the escape rate from the corners of two-dimensional Xe islands.

Adsorption properties and formation of [formula omitted] surface alloys

The morphology of Pt Cu surface alloys is studied by CO and Xe adsorption/desorption after annealing Pt films on a Cu(111) substrate at temperatures above 500 K. The topmost layer consists of dispersed Pt atoms in a Cu matrix. The Xe desorption peak maximum proportionally shifts to lower temperatures with decreasing Pt surface concentration. Accordingly, Xe TDS can be used to determine the Pt concentration in the surface. The adsorption properties of CO on the thus prepared PtCu surfaces have been studied by thermal desorption spectroscopy (TDS), high resolution electron energy loss spectroscopy (HREELS), and work function change measurements (Δ∅). Pt, Cu, and mixed adsorption sites can be distinguished with TDS. HREELS shows that CO on top of Pt is the prevalent adsorbed species. Both HREELS and Δ∅ show that the amount of bridge bonded CO has a minimum at a surface concentration of 20% Pt.

Stochastic simulations of temperature programmed desorption kinetics

In principle, temperature programmed desorption (TPD) can yield a wealth of information on the rates of surface reactions. In practice, however, it is often difficult or impossible to extract such information from the experimental data. We describe an accurate stochastic modelling technique that facilitates the determination of mechanisms and rate constants from TPD measurements. The model for a particular reaction or process is cast in terms of a detailed mechanism, which can be set up in a variety of ways depending on the primary characteristics of the system under study. Starting from experimental rate constants obtained under limiting conditions, a series of simulations are used to estimate and refine unknown rate constants. Three types of examples are given to illustrate the methodology used. First, simulations of desorption of Xe from Pt(111) are used to show how a very simple system can be modelled. By comparing experimental data and calculations performed with several different mechanisms, we explore the kinetic consequences of various modes of desorption from defect sites. Next, we investigate the competitive desorption and dissociation of CO on W(110). In this case the model successfully reproduces experimental TPD and surface coverage data, allowing experimental rate constants to be optimized. Finally, we show that stochastic simulations can be used to study systems with strong repulsive lateral interactions using a model which explicitly tracks changes in the environment of a desorbing species with temperature and coverage. Previous Monte Carlo simulations of desorption from a square lattice are reproduced using this approach. It is also applied to desorption of CO from Ru(001), yielding simulated TPD spectra in excellent agreement with experiment and allowing the magnitude of repulsive interactions to be estimated. In both the square and hexagonal lattice systems TPD data are simulated with coverage-independent pre-exponential factors: all features of the spectra are attributable to variations in populations of surface species alone. The stochastic method is shown to be valuable as an integral part of an experimental study, allowing detailed models to be proposed and refined, and yielding microscopic data which are amenable to theoretical analysis.

Structural changes upon sorption and desorption of Xe from Cd-exchanged zeolite rho: a real-time synchrotron X-ray powder diffraction study

Xe has been trapped and controllably released from Cd-exchanged zeolite rho. The competitive sorption of H 2O into the Xe-loaded sample has been followed in real time using synchrotron X-ray powder diffraction and a position-sensitive detector. Rietveld refinement shows that upon exposure to the atmosphere, Xe is lost in two steps involving desorption firstly from the double 8-rings (D8R) and then single 6-rings (S6R). Essentially all the Xe is lost from the D8R in the first hour.