D2 Desorption Research Articles

An empirical dependence of frequency in the oscillatory sorption of H2 and D2 in Pd on the first ionization potential of noble gases

Oscillatory heat evolution in sorption of H2 and D2 in Pd can be induced by an admixture of ∼10 vol.% of an inert gas (He, Ne, Ar, Kr, or N2) to either isotope prior to its contact with palladium powder. The oscillations are represented in the form of a calorimetric time series, recorded using a gas flow-through microcalorimeter at the temperatures of 75 °C for D2 and 106 °C for H2. For both D2 and H2, the oscillation parameters change as a function of the kind of inert gas used: the amplitude increases and the frequency decreases in passing from He to Kr. An empirical dependence of the oscillation frequencies observed for various admixtures and normalized with respect to Kr has been found. Accordingly, the frequency is a function of a product of the first ionization potential and the square root of atomic mass of the inert gas (He, Ne, Ar, Kr, or N2). On the other hand, invariance of the thermal effects of sorption is evident from the integrated areas under the calorimetric time series yielding the molar heats of sorption conserved, irrespective of the inert gas admixture. A novel calibration procedure has been devised in order to deal with an instability of calibration factor arising in desorption of H2 and D2 from Pd. A method of dynamic calibration factor made it possible to obtain a good agreement between the heats of sorption and desorption of both H2 and D2 within individual sorption-desorption cycles for all inert gas admixtures.

Experimental clarification of the desorption of H2, D2 and He mixtures from cryosorption pumps

The behavior of dynamic desorption of He, H2 and D2 from a cryosorption pump is experimentally investigated for simplified rough separation of D-T fuel exhaust from impurities. As a fundamental study to separate the unburned D-T fuel and impurities dynamically, the discharge rates of H2 and D2 (as a representative of T2), He (as a major impurity) are determined as a function of time and temperature, when the cryosorption pump is regenerated from 10K to the room temperature of 285–305K. The release behavior of H2 and D2 is compared, and the desorption isotherms of H2, D2 and He for activated carbon used in the cryosorption pump are determined from their respective discharge curves. The present result may lead to provide a simplified technique to decrease the throughput of the following fuel purification system and to decrease the tritium inventory by rapid separation of the D-T fuel from impurities.

Dissipative dynamics within the electronic friction approach: the femtosecond laser desorption of H2/D2 from Ru(0001)

An electronic friction approach based on Langevin dynamics is used to describe the multidimensional (six-dimensional) dynamics of femtosecond laser induced desorption of H(2) and D(2) from a H(D)-covered Ru(0001) surface. The paper extends previous reduced-dimensional models, using a similar approach. In the present treatment forces and frictional coefficients are calculated from periodic density functional theory (DFT) and essentially parameter-free, while the action of femtosecond laser pulses on the metal surface is treated by using the two-temperature model. Our calculations shed light on the performance and validity of various adiabatic, non-adiabatic, and Arrhenius/Kramers type kinetic models to describe hot-electron mediated photoreactions at metal surfaces. The multidimensional frictional dynamics are able to reproduce and explain known experimental facts, such as strong isotope effects, scaling of properties with laser fluence, and non-equipartitioning of vibrational, rotational, and translational energies of desorbing species. Further, detailed predictions regarding translations are made, and the question for the controllability of photoreactions at surfaces with the help of vibrational preexcitation is addressed.

Hydrogen Retention Properties of SiC/SiC Composites as Plasma Facing Material of Fusion Reactor

In order to examine the deuterium retention properties of SiC/SiC compositess, the deuterium ion irradiation was conducted followed by a thermal desorption spectroscopymeasurement. The deuterium retained was desorbed in forms of HD, D2, HDO, CD4 and C2D4. The ratio of the amount of desorbed hydrocarbons to the total amount of retained deuterium was approximately 6%, which is much smaller, compared with the case of carbon material. Namely, we confirmed that the chemical erosion of SiC/SiC composites isquite small. In the D2 desorption spectra, two desorption peaks appeared. The lower and higher temperatures peaks were regarded to be due to detrappings of Si-D and C-D bonds, respectively. The ratio of deuterium amount desorbed at highertemperature decreased with increase of the deuterium fluence.This behavior could be explained that the carbon content at thesurface is reduced by the selective sputtering due to deuteriumions.

The influence of step geometry on the desorption characteristics of O2, D2, and H2O from stepped Pt surfaces

We have compared the desorption characteristics of O(2), D(2), and H(2)O from the Pt(533) surface to the Pt(553) surface using temperature programmed desorption. Both surfaces consist of four atom wide (111) terraces interrupted by monoatomic steps of the different step geometries: (100) versus (110), respectively. We find that desorption is influenced significantly by the presence of step sites and the geometry of those sites. In general, molecules and atoms are thought to be bound more strongly to step sites than to terrace sites. Our D(2) desorption data from Pt(553) provide an anomalous counterexample to this common belief since D atoms on this surface appear to be bound stronger by terrace sites. We also show that it is not possible to say a priori which step geometry will bind atoms or molecules stronger: recombinatively desorbing O atoms are bound stronger to (100) sites, whereas H(2)O molecules are bound stronger to (110) sites. Furthermore, the amount of adatoms or molecules that are affected by the presence of steps varies for the different species, as is evident from the various step: terrace ratios of approximately 1:1.3 for O(2) (O), approximately 1:3 for D(2) (D), and approximately 1:1 for H(2)O. This indicates that, in contrast to deuterium, more oxygen atoms and water molecules are affected by the presence of steps than would be expected on geometrical arguments alone.

Chemistry of Silicon Nanocrystal Surfaces Exposed to Ammonia

Silicon nanocrystals (NCs) 8−10 nm in diameter are grown on SiO2 surfaces in an ultrahigh-vacuum chamber using hot wire chemical vapor deposition. These NCs are subjected to varying exposures of deuterated ammonia (ND3). The surface chemistry of Si NCs is studied using X-ray photoelectron (XP) spectroscopy and temperature-programmed desorption (TPD). The dissociative adsorption of ND3 on Si NCs results in the formation of ND2 species prior to TPD, and Si3N (nitride) formation is observed after TPD. D2 desorption is observed only from the monodeuteride species at 780 K. In separate experiments, a hot tungsten filament is used to predissociate ND3 before adsorption on the NC surface. XP spectra reveal that ND2 species form initially, and as the dose is increased, ND species dominate. After TPD, a SixNy species is observed. D2 desorption is observed from the mono-, di-, and trideuteride species when ND3 is predissociated. Irrespective of the technique used for dosing ammonia, TPD spectra do not contain any N...

Thermal degradation analysis of deuterium-ion-implanted V25Cr40Ti35 using synchrotron radiation photoelectron spectroscopy

The thermal degradation of a native oxide layer on the hydrogen storage alloy V25Cr40Ti35 and the thermal desorption property of deuterium molecules were studied by synchrotron radiation soft X-ray photoelectron spectroscopy and thermal desorption mass spectrometry. Photoelectron spectra of O-1s, C-1s, V-2p, Cr-2p, and Ti-2p were observed for an as-received sample and a deuterium-ion-implanted one. Although the oxide layer changed dramatically by thermal annealing between 373 K and 473 K for the un-implanted sample, the change of the deuterium-ion-implanted sample was lying between 473 K and 573 K corresponding to D2 desorption. The implantation of deuterium resulted in the stabilization of the surface oxide layer by approximately 100 deg.

D2 desorption kinetics on amorphous solid water: from compact to porous ice films

The desorption kinetics of D(2) from amorphous solid water (ASW) films have been studied by the temperature-programmed desorption (TPD) technique in the 10-30 K temperature range. Compact (and nonporous) films were grown at 120 K over a copper substrate. Ultra-thin porous films were additionally grown at 10 K over the compact base. The TPD spectra from compact and from up to 20 monolayers (ML) porous films were compared. The simulation of the TPD experimental traces provides the corresponding D(2) binding-energy distributions. As compared to the compact case, the binding-energy distribution found for the 10 ML porous film clearly extends to higher energies. To study the transition from compact to porous ice, porous films of intermediate thicknesses (<10 ML), including ultra-thin films (<1 ML), were grown over the compact substrate. The thermal D(2) desorption peak was found to shift to higher temperatures as the porous ice network was progressively formed. This behavior can be explained by the formation of more energetic binding sites related to porous films. TPD spectra were also modelled by using a combination of the two energy distributions, one associated to a bare compact ice and the other associated to a 10 ML porous ice film. This analysis reveals a very fast evolution of the binding-energy distribution towards that of porous ice. Our results show that few ML of additional porous film are sufficient to produce a sample for which the D(2) adsorption can be described by the energy distribution found for the 10 ML porous film. These experiments then provide evidence that the binding energy of D(2) on ASW ice is primarily governed by the topological and morphological disorder of the surface at molecular scale.

Interaction of D2 with Ti-Adsorbed Polyaniline and Implication for Hydrogen Storage

Interaction of D2 with Ti-adsorbed polyaniline (PANI) and TiO2 at low Ti coverages has been investigated by temperature programmed desorption, Auger electron spectroscopy, and H2-D2 exchange in a high pressure cell. In contrast to recent DFT calculations (Lee et al. Phys. Rev. Lett. 2006, 97, 56104-56107) that have shown a multiple number of molecular hydrogen chemisorption on Ti-decorated PANI, only 0.52 D2 molecules per Ti atomically adsorb on Ti-deposited PANI at 87 K at a nominal Ti coverage of 2 ML. Broad TPD spectra of D2, HD, and H2 were observed, which showed a common peak at 250 K and another isotope-dependent peak at higher temperature. Ti deposited on TiO2 forms clusters with a size distribution at 87 K, on which D2 atomically adsorbs. As the Ti coverage increases, the D2 desorption peak gradually shifts from 200 to 260 K and the number of D2 molecules adsorbed per Ti also increases from 0.26 at 0.1 ML to 0.87 at 2 ML. We attribute this to the fact that D2 adsorbs on larger clusters with a greater adsorption energy. TPD spectra of D2 desorbing from Ti (2 ML)/PANI and Ti (1 ML)/TiO2 look very similar to each other. This led us to conclude that Ti also adsorbs on PANI in clusters. We suggest that no molecular D2 adsorption on Ti/PANI at 87 K is due to a reduced electron backdonation ability of Ti upon clustering. The small number of adsorbed D2 per Ti on both substrates was ascribed to irreversible D2 adsorption only on relatively large Ti clusters, while D2 adsorbs reversibly on small clusters. This was indirectly confirmed by the observation that continuous H2-D2 isotope exchange occurs to produce HD on Ti/PANI as well as on Ti/TiO2 at 106 K in a high pressure cell at 5.7 mTorr. Desorption mechanisms involving H atom abstraction from PANI by Ti clusters are proposed to account for the complex isotope-dependent TPD spectra. Implication of the present results for hydrogen storage based on Ti-dispersed polymers is briefly addressed.

Thermally Triggered Dedeuteration of Cesium-doped Deuterofullerene Films

We have probed the electronic structure and thermal stability of Cs-doped deuterofullerene thin films, Csx(C60Dn), as generated under ultrahigh vacuum conditions, using ultraviolet photoionization (UP) spectroscopy and thermal desorption spectroscopy. Mass spectra taken during sublimation indicate that deuterofullerene (C60Dn) and cesium fulleride regions (CsxC60) can coexist at elevated temperatures. The yield of Cs fulleride phases obtained by heating increases with the amount of Cs atoms initially deposited/intercalated into the deuterofullerene thin films. At highest initial Cs doses, a CsxC60 phase with doping degree of 4 ≤ x ≤ 6 is eventually generated. UP spectra of the as-prepared films taken before heating suggest that C60−D−d−Cs+d complexes are initially formed. Thermally activated conversion of these complexes into Cs fullerides can be followed by recording UP features characteristic for (increasing) C60-LUMO state occupation. The transition from cesium intercalated C60Dn into CsxC60 sets in at a temperature of 610 K and is completed by 670 K. This narrow transition temperature range is also characterized by synchronous desorption of Cs and D2. We conclude that Cs intercalation can significantly weaken >C−D bond strengths relative to C60Dn, thus facilitating thermally activated dedeuteration.

Experimental Verification of Hydrogen Isotope Separation by Pressure Swing Adsorption

Focusing on synthetic zeolites that adsorb hydrogen isotopes at liquid N2 temperature with priority in the order of T2, DT, D2, HT, HD and H2, we have been developing a pressure swing adsorption process system for hydrogen isotope separation. For this purpose, we carried out fundamental experiments of adsorption and desorption of a tracer D2 in bulk H2 with zeolite packedbed columns. In this paper, the results are reported that D2 is enriched in the adsorbed phase at separation factors near 2.0, flowing through zeolite 5A and 13X packed-beds at 77.4 K. These are in agreement with values predicted from the multi-component equilibrium characteristics. In the gas samples recovered by evacuating the packed-beds, however, D2 was detected at a relative concentration of 1.20 or 1.32 to that in the feed gas. This lower range results from the isotopic mass effect in kinetic process. That suggests a highly D2-enriched residual left during evacuation. This is verified with an unusually high enrichment factor of 6.68 or 9.21 for zeolite 5A or 13X measured in the residual sample desorbed from the packed-bed by heating up to room temperature.

Phosphorus incorporation during Si(001):P gas-source molecular beam epitaxy: Effects on growth kinetics and surface morphology

The effects of P doping on growth kinetics and surface morphological evolution during Si(001):P gas-source molecular beam epitaxy from Si2H6 and PH3 at temperatures Ts=500–900°C have been investigated. With increasing PH3∕Si2H6 flux ratio JP∕Si at constant Ts, we observe a decrease in the film growth rate R and an increase in the incorporated P concentration CP, both of which tend toward saturation at high flux ratios, which is accompanied by increased surface roughening and pit formation. At constant JP∕Si, R increases with increasing Ts, while CP initially increases, reaches a maximum at Ts=700°C, and then decreases at higher growth temperatures. We use in situ isotopically tagged D2 temperature programed desorption (TPD) to follow changes in film surface composition and dangling bond density θdb as a function of JP∕Si and Ts. Measurements are carried out on both as-deposited Si(001):P layers and P-adsorbed Si(001) surfaces revealing β1 and β2 peaks due to D2 desorption from Si monohydride and dihydride species, respectively, as well as the formation of a third peak β3 corresponding to D2 desorption from mixed SiP dimers. Dissociative PH3 adsorption on Si(001) results in a decrease in θdb and an initial increase in P surface coverage θP with increasing Ts. Saturation θP values reach a maximum of ∼1 ML at Ts=550°C, and decrease with Ts&amp;gt;600°C due to the onset of P2 desorption. Comparison of θP(Ts) results obtained during film growth with postdeposition CP(Ts) results reveals the presence of strong P surface segregation. From measurements of θP versus CP in Si(001):P layers grown as a function of Ts, we obtain a P segregation enthalpy ΔHs=−0.86eV. By using the combined set of results, we develop a predictive model for CP versus Ts and, JP∕Si incorporating the dependence of the PH3 reactive sticking probability SPH3 on θP, which provides an excellent fit to the experimental data.

Interaction of Deuterium with Well-Defined PtxRu1−x/Ru(0001) Surface Alloys

The adsorption and desorption of deuterium on structurally well-characterized, pseudomorphic PtxRu1−x surface alloys on a Ru(0001) substrate was investigated by temperature programmed desorption. The interaction of deuterium with these surface alloys, which, based on previous high resolution scanning tunneling microscopy measurements, have a nearly random distribution of the surface atoms, is markedly altered compared to that with the pure components, Ru(0001) and Pt(111). With increasing Pt surface content, the onset of D2 desorption shifts to lower temperature, leading to a significant broadening of the spectra and finally to a pronounced low-temperature peak at 145 K. The low-temperature state becomes dominant at Pt concentrations close to 100% and is assigned to deuterium desorption from a Ru(0001) supported Pt monolayer film. The characteristic changes in the deuterium desorption behavior induced by Pt surface atoms are discussed in a picture involving geometric ensemble effects as well as electronic...

Time-of-flight studies on catalytic model reactions

Time-of-flight spectroscopy (TOF) and REMPI-TOF (resonance enhanced multi-photon ionization-TOF) were applied to measure the angular and translational energy distribution, as well as the internal state resolved energy distribution of desorption and reaction products on some model systems. Desorption of hydrogen and deuterium from clean and modified Pd(111) surfaces was studied, where the palladium sample was part of a permeation source. Water formation by reaction of oxygen with hydrogen on palladium was investigated by using different types of hydrogen supply: molecular H2 exposure and atomic H exposure from the gas phase, as well as H exposure by permeating hydrogen. Vanadium oxide nanostructures on Pd(111) were prepared and the influence on D2 desorption and D2O production was investigated with the permeation technique. Additionally, deuterium desorption from sulfur and oxygen covered V(111) and V(100) surfaces was studied by TOF and REMPI-TOF spectroscopy. From the TOF spectra information concerning the reaction and desorption dynamics (activation barriers) could be gained.

In situ diagnostic for monitoring of deuterium and tritium in re-deposited carbon layers by laser induced desorption

Tritium retention in the plasma facing components and co-deposition in re-deposited layers is a key issue for ITER, since the operation of the machine will be restricted to a maximum inventory of tritium. Development of methods to detect in real time the amount of retained fuel in the in vessel components is urgently required. A promising method for this is the pulsed laser induced desorption, by which the imbedded gas in such layers is released into the plasma where the injected atoms and molecules are excited and ionised and the characteristic line emission is detected spectroscopically. In this paper the release process is investigated in detail for amorphous hydrocarbon layers in a laboratory experiment, as a prerequisite for the later application and determination of conversion factors between line radiation and desorbed flux in TEXTOR plasmas. A Nd:YAG laser has been used with nearly constant output power leading to a smooth controllable increase of layer temperature, resulting in a complete desorption of D2 and CD4 as well as preventing carbon sublimation.

Kinetic Isotope Effect for H2 and D2 Quantum Molecular Sieving in Adsorption/Desorption on Porous Carbon Materials

Adsorption and desorption of H(2) and D(2) from porous carbon materials, such as activated carbon at 77 K, are usually fully reversible with very rapid adsorption/desorption kinetics. The adsorption and desorption of H(2) and D(2) at 77 K on a carbon molecular sieve (Takeda 3A), where the kinetic selectivity was incorporated by carbon deposition, and a carbon, where the pore structure was modified by thermal annealing to give similar pore structure characteristics to the carbon molecular sieve substrate, were studied. The D(2) adsorption and desorption kinetics were significantly faster (up to x1.9) than the corresponding H(2) kinetics for specific pressure increments/decrements. This represents the first experimental observation of kinetic isotope quantum molecular sieving in porous materials due to the larger zero-point energy for the lighter H(2), resulting in slower adsorption/desorption kinetics compared with the heavier D(2). The results are discussed in terms of the adsorption mechanism.

Interaction of D2 with H2O amorphous ice studied by temperature-programed desorption experiments

The gas-surface interaction of molecular hydrogen D2 with a thin film of porous amorphous solid water (ASW) grown at 10 K by slow vapor deposition has been studied by temperature-programmed-desorption (TPD) experiments. Molecular hydrogen diffuses rapidly into the porous network of the ice. The D2 desorption occurring between 10 and 30 K is considered here as a good probe of the effective surface of ASW interacting with the gas. The desorption kinetics have been systematically measured at various coverages. A careful analysis based on the Arrhenius plot method has provided the D2 binding energies as a function of the coverage. Asymmetric and broad distributions of binding energies were found, with a maximum population peaking at low energy. We propose a model for the desorption kinetics that assumes a complete thermal equilibrium of the molecules with the ice film. The sample is characterized by a distribution of adsorption sites that are filled according to a Fermi-Dirac statistic law. The TPD curves can be simulated and fitted to provide the parameters describing the distribution of the molecules as a function of their binding energy. This approach contributes to a correct description of the interaction of molecular hydrogen with the surface of possibly porous grain mantles in the interstellar medium.

Influence of surface morphology on D2 desorption kinetics from amorphous solid water

The influence of surface morphology/porosity on the desorption kinetics of weakly bound species was investigated by depositing D2 on amorphous solid water (ASW) films grown by low temperature vapor deposition under various conditions and with differing thermal histories. A broad distribution of binding energies of the D2 monolayer on nonporous and porous ASW was measured experimentally and correlated by theoretical calculations to differences in the degree of coordination of the adsorbed H2 (D2) to H2O molecules in the ASW depending on the nature of the adsorption site, i.e., surface valleys vs surface peaks in a nanoscale rough film surface. For porous films, the effect of porosity on the desorption kinetics was observed to be a reduction in the desorption rate with film thickness and a change in peak shape. This can be partly explained by fast diffusion into the ASW pore structure via a simple one-dimensional diffusion model and by a change in binding energy statistics with increasing total effective surface area. Furthermore, the D2 desorption kinetics on thermally annealed ASW films were investigated. The main effect was seen to be a reduction in porosity and in the number of highly coordinated binding sites with anneal temperature due to ASW restructuring and pore collapse. These results contribute to the understanding of desorption from porous materials and to the development of correct models for desorption from and catalytic processes on dust grain surfaces in the interstellar medium.

Hydrogen Adsorption/Desorption Property of Activated Carbon Loaded with Platinum

Abstract The hydrogen adsorption isotherms of activated carbon fiber loaded with platinum (Pt–ACF) in the 303–673 K range indicated that a temperature above 573 K was required to give rise to an increase in the amount of adsorbed hydrogen due to the spillover phenomenon. Two peaks at 800 and 1200 K in the D2 desorption profile of Pt–ACF clarified that two kinds of sites for adsorption or bonding of spiltover deuterium exist on the carbon surface.

Reactivity of TiO2 with hydrogen and deuterium

The reactivity of rutile phase TiO2 (110) and polycrystalline anatase surfaces with molecular and ionized deuterium and hydrogen was investigated. Thermal Desorption Spectroscopy (TDS), following exposure to more than 100kL of molecular deuterium, showed a D2 desorption peak at ∼440K on both single crystal rutile and polycrystalline anatase surfaces. The desorption peak was observed following exposure only at surface temperatures between 140–270K. Ionized D2 is significantly more reactive with the titania surface and two desorption peaks at 380K and ∼550K were observed together with a small D2O peak observed at ∼440K. Dosing the surfaces with hydrogen and deuterium either in succession or as a mixture showed HD desorption with no change in the desorption peaks, consistent with dissociative adsorption of both ionized and molecular species. The experimental data was compared to Density Functional Theory calculations and modeled as a two-step process of hydrogen dissociation at oxygen vacancy sites on TiO2.