Desorption Of Hydrogen Molecules Research Articles

Insight into efficient inhibitory of La3+ adsorbate on hydrogen permeation into steel

Addition of trace lanthanum salt in acid solution was found to be enhanced inhibitory effect on hydrogen permeation to X70 pipeline steel. The hydrogen permeation curves were performed to show that the addition of La 3+ inhibits hydrogen permeation by accelerating the recombination of the adsorbed hydrogen atoms and the desorption of hydrogen molecule, which decrease the subsurface hydrogen concentration. The inhibitory effect of La 3+ involves the film formation process. Moreover, electrochemical impedance spectroscopy analysis (EIS) depict that the inhibitive effect is caused by the catalysis of the recombination-desorption-evolution of adsorbed hydrogen atoms by La(OH) 3 /La 2 O 3 . This provides a novel solution for the prevention of hydrogen-induced cracking of high-strength steel, which often causes catastrophic accidents.

Effect of monovacancy on stability and hydrogen storage property of Sc/Ti/V-decorated graphene

With the depletion of fossil fuels and the environmental problems, the development and utilization of new energy resources is imminent. Hydrogen energy is one of the main new energy sources in the 21st century. Finding stable and efficient hydrogen storage materials is the key to achieving the hydrogen economy. Transition metal (TM)-decorated graphenes have been widely studied as hydrogen storage materials theoretically, but they suffer metal agglomeration and H<sub>2</sub> dissociation. Our calculations show that the reconstruction energy of Sc, Ti, V decorated pristine graphenes in the process of adsorption and desorption of hydrogen molecules are only 0.00, 0.12 and 0.08 eV, respectively. The adsorption energy values of the first H<sub>2</sub> dissociation adsorption on the Sc, Ti, V decorated pristine graphenes are –1.34, –1.34, and –1.16 eV, respectively. So, some hydrogen molecules are difficult to desorb at room temperature and medium pressure. In this paper, the stability and hydrogen storage properties of Sc, Ti, V decorated monovacancy graphene are also investigated based on density functional theory. The results show that the binding energy values between Sc, Ti, V and themonovacancy graphene are –6.93, –8.82, –9.30 eV, respectively, which indicate monovacancy can effectively avoid metal aggregation. The Sc, Ti and V atoms decorated on the monovacancy graphene would transfer more electrons to the carbon material with charge of +1.24|<i>e</i>|–+1.37|<i>e</i>|. They can adsorb 7, 3 and 4 hydrogen molecules through electrostatic interaction. When a monovacancy is introduced, all of the hydrogen molecules are adsorbed in molecular form. The average adsorption energy values of H<sub>2</sub> are –0.13, –0.20 and –0.18 eV, respectively, which are in the best energy range for the adsorption/desorption process at room temperature and medium pressure. The most important thing is that their deformations in the adsorption/desorption process are very small, which is conducive to the rapid hydrogen adsorption/desorption. The calculated results show that the monovacancy introduction can effectively solve the two major problems, i.e. metal agglomeration and hydrogen molecular dissociation during hydrogen storage on Sc, Ti, V decorated pristine graphenes. The research in this paper will be helpful to further understand the hydrogen storage mechanism of 3d TM-decorated carbon nanomaterials.

Long-Time Non-Debye Kinetics of Molecular Desorption from Substrates with Frozen Disorder.

The experiments on the kinetics of molecular desorption from structurally disordered adsorbents clearly demonstrate its non-Debye behavior at “long” times. In due time, when analyzing the desorption of hydrogen molecules from crystalline adsorbents, attempts were made to associate this behavior with the manifestation of second-order effects, when the rate of desorption is limited by the rate of surface diffusion of hydrogen atoms with their subsequent association into molecules. However, the estimates made in the present work show that the dominance of second-order effects should be expected in the region of times significantly exceeding those where the kinetics of H2 desorption have long acquired a non-Debye character. To explain the observed regularities, an approach has been developed according to which frozen fluctuations in the activation energy of desorption play a crucial role in the non-Debye kinetics of the process. The obtained closed expression for the desorption rate has a transparent physical meaning and allows us to give a quantitative interpretation of a number of experiments on the desorption kinetics of molecules not only from crystalline (containing frozen defects) but also from amorphous adsorbents. The ways of further development of the proposed theory and its experimental verification are outlined.

An Apparatus for Studying Phosphor Luminescence upon Excitation by Atomic–Molecular Beams

The scheme, design, and hardware of a new laboratory apparatus for studying the interaction of beams of free atoms and molecules with the surfaces of phosphors are described. Methods for measuring the efficiency of electronic radiative processes on surfaces for studying the mechanisms of energy transfer and surface changes based on the spectral–kinetic characteristics of heterogeneous chemiluminescence are presented. Luminescent methods for studying the heterogeneous recombination of hydrogen atoms on the surfaces of solids allow one to explicitly select the impact (Rideal–Eley) and diffusion (Langmuir–Hinshelwood) recombination mechanisms and estimate the fraction of contributions of these mechanisms to the total recombination rate of atoms depending on the flow density of free atoms and the sample temperature. Examples of studying the spectra of the heterogeneous chemiluminescence (HCL) and photoluminescence (PL) at different temperatures of an AlN : Eu3+ phosphor under the excitation with hydrogen atoms and a mercury lamp are given. During processing of the kinetic curves of the HCL buildup and decay for ZnS : Tm3+ phosphor upon switching the Н + Н2 atomic–molecular beam on and off, an example of obtaining the parameters of atomic–molecular processes on the surface (adsorption, impact and diffusion recombination of atoms, and desorption of hydrogen molecules) is presented.

Molecular and dissociated adsorption of hydrogen on TiC6H6

The dissociation of H2 molecule on metal-decorated carbon-based materials is a widespread phenomenon. And it is controversial that the dissociation of H2 molecule had significant effects on the hydrogen storage capacity. In this paper, comprehensive and detailed researches on the adsorption, dissociation and desorption of H2 molecules on the complex TiC6H6 were performed using Coupled-Cluster theory.We found that the dissociation of the H2 on TiC6H6 almost has no effect on the maximum number of adsorbed H2 but weakens stepwise adsorption energies of the later H2 molecules. The adsorption pathway is complicated due to the transformations of adsorption structures. The adsorption pathway along TiC6H6→1a→2a→3a pathway will be more favorable thermodynamically at room temperature. The most favorable desorption pathway is 3b→2b→1b→1a→TiC6H6, and its rate-determining step is the transition from 1b to 1a. These results indicate that the adsorption and desorption of hydrogen molecules on TiC6H6 follow different pathways due to dissociation of H2 molecules. Once the three H2 are completely liberated, the TiC6H6 system is ready to restart the absorption/desorption process again. In addition, by using CCSD(T) calculations as benchmark, the predictions using the B3LYP functional is valid for hydrogen sorbent materials including TiC6H6.

Thermodynamics Study of Dehydriding Reaction on the PuO2(110) Surface from First Principles

Using the first-principles atomistic thermodynamics method in combination with the DFT + U theory, we calculate the pressure–temperature phase diagrams of the dehydriding reactions on the hydrogenated PuO2(110) surface and determine the thermodynamic boundaries that control the feasibility of reaction. Effects of hydrogen coverage on the surface dehydriding processes are investigated. Compared to the dehydriding in the form of water molecule accompanied by oxygen vacancy on the surface, desorption of hydrogen molecule is more energetically favorable. However, the relative hydrogen and water vapor pressure could modify the feasibility of two types of dehydriding processes, which may lead to water desorption superior to hydrogen one, even at 300 K.

Two dimetallocenes with vanadium and chromium: Electronic structures and their promising application in hydrogen storage

The hydrogen storage capacities of the two sandwich-type Cp2TM2 [Cp = cyclopentadienyl (C5H5); TM = V or Cr)] dimetallocenes were studied using first-principles calculations. According to the B3PW91 method in density functional theory (DFT), Cp2V2 and Cp2Cr2 could adsorb up to seven and six hydrogen molecules, respectively. The predicted hydrogen storage densities of Cp2V2 and Cp2Cr2 were 5.73 and 4.91 wt%, respectively. Additionally, because hydrogen binding energies bound by Kubas model were 0.39 and 0.43 eV/H2, respectively, both the Cp2V2 and Cp2Cr2 dimetallocenes were proven to be favourable for reversible adsorption and desorption of hydrogen molecules under ambient conditions.

First-principles predictions of potential hydrogen storage materials: Novel sandwich-type ethylene dimetallocene complexes

The hydrogen storage capacities of a sandwich-type ethylene dimetallocene complex (Cp2Ti2C2H4) are studied using first-principles calculations. It is found that the TiC2H4Ti molecule can intercalate into the two cyclopentadienyl (Cp) rings and form a stable sandwich-type complex. Each Ti atom can adsorb a maximum of three H2 molecules, which corresponds to a gravimetric storage capacity of 4.73 wt%. This hydrogen storage capacity is close to the 2015 target of 5.5% set by the US Department of Energy (DOE) in 2009. Furthermore, the Cp2Ti2C2H4 molecule proposed in this paper is favorable for both adsorption and desorption of hydrogen molecules at room temperature and ambient pressure because its average binding energy of 0.34 eV/H2.

Palladium Clusters Anchored on Graphene Vacancies and Their Effect on the Reversible Adsorption of Hydrogen

The hydrogen storage capacity of nanoporous carbons can be enhanced through metal doping, for instance doping with palladium. However, there are two problems that may limit the positive effect of metal doping on the reversible storage capacity. First, clustering of the metal atoms decreases its effectiveness, which is largest for maximum dispersion. A second problem that is often overlooked is that the desorption of metal–hydrogen complexes may compete favorably with the desorption of hydrogen molecules. Desorption of complexes would spoil the reversible storage of hydrogen in the material. Both problems can be avoided by firmly anchoring the metal atoms and clusters to defects of the carbon substrate, for instance vacancies. With this goal in mind, we have performed density functional calculations to investigate the desorption of hydrogen and of Pd–hydrogen complexes from Pd atoms and clusters supported on pristine graphene and on graphene layers with vacancies. We show that palladium atoms bind much stronger to graphene vacancies, binding energy Eb = 5.13 eV, than to pristine graphene, Eb = 1.09 eV. The Pd atoms tend to nucleate and form clusters around the vacancies and the small Pdn clusters (n = 2–6) also bind strongly, Eb ≈ 5 eV, to the vacancies. However, the Pd–Pd interaction is much smaller than the Pd–vacancy interaction, and therefore, the vacancies favor the dispersion of palladium on the graphene layer. For hydrogen adsobed on Pd atoms and clusters supported on pristine graphene, desorption of Pd–hydrogen complexes competes with desorption of molecular hydrogen. However, for hydrogen adsobed on a Pd atom anchored on a graphene vacancy, the desorption of the PdH2 complex costs 4.2 eV, and therefore, it does not compete with the desorption of molecular hydrogen, which takes place with an energy cost of only 0.2 eV. This shows the beneficial effect that anchoring Pd atoms and clusters to graphene vacancies has on the reversible adsorption/desorption of hydrogen.

A study on hydrogen adsorption behaviors of open-tip carbon nanocones

Hydrogen adsorption behaviors of single-walled open-tip (tip-truncated) carbon nanocones (CNCs) with apex angles of 19.2° at temperatures of 77 and 300 K were investigated by the molecular dynamics simulations. Four nanomaterials (including three CNCs with different dimensions and a reference CNT) were analyzed to examine the hydrogen adsorption behaviors and influences of cone sharpness on the behaviors of the CNCs. Physisorption of hydrogen molecules could be observed from the distribution pattern of the hydrogen molecules adsorbed on the nanomaterials. Because of the cone geometry effect, the open-tip CNCs could have larger storage weight percentage and less desorption of hydrogen molecules (caused by the temperature growth) on their outer surfaces, as compared with those of the reference CNT. The hydrogen molecules inside the CNCs and the reference CNT, however, were noted to have similar desorption behaviors owing to the confinement effects from the structures of the nanomaterials. In addition, the sharper CNC could have higher storage weight percentage but the cone sharpness does not have evident enhancement in the average adsorption energy of the CNC. Combination of confinement and repulsion effects existing near the tip region of the CNC would be responsible for the non-enhancement feature.

High-capacity hydrogen storage of magnesium-decorated boron fullerene

By theoretical analysis, we have explored the feasibility of functionalizing boron fullerene (B 80) by adsorbing Mg atoms for the application as hydrogen storage nanomaterials. Our results show that due to the charge transfer from Mg to B atoms Mg atoms reside above the pentagonal faces of the B 80 cage. The electric field induced around the positive charged Mg atoms polarizes H 2 molecules, and the resulting binding is strong enough to adsorb H 2 without dissociation. Further calculations indicated that the 12Mg-decorated-B 80 has a high hydrogen storage capacity storing up to 96 H 2 molecules with an ideal binding energy of 0.20 eV/H 2 according to the approximation of GGA and 0.5 eV/H 2 according to LDA, corresponding to a hydrogen uptake of 14.2%. This suggested a possible method of engineering new structure for high-capacity hydrogen storage materials with the reversible adsorption and desorption of hydrogen molecules.

Dynamic Monte-Carlo modeling of thermal desorption of H 2 from H + irradiated graphite

Thermal desorption of hydrogen molecules from H + irradiated graphite is studied using dynamic Monte Carlo simulation. The purpose of this study is to understand the experimentally observed phenomena that the thermal desorption of H 2 from the graphite exhibits sometimes single desorption peak, sometimes double peaks, and even three desorption peaks under certain circumstances. The study result reveals that the fluence of pre-implanted H +, the concentration of trap sites, porosity, and mean crystallite volume are important parameters in determining the number of desorption peaks. It is found that low implantation fluence and high concentration of trap sites easily lead to the occurrence of single desorption peak at around 1000 K, and high implantation fluence and low concentration of trap sites favor the occurrence of double desorption peaks, with a new desorption peak at around 820 K. It is also found that small porosity of graphite and large crystallite volume benefit the occurrence of single desorption peak at around 1000 K while large porosity of graphite and small crystallite volume facilitate the occurrence of double desorption peaks, respectively, at around 820 and 1000 K. In addition, experimentally observed third desorption peak at lower temperature is reproduced by simulation with assuming the graphite containing a small concentration of solute hydrogen atoms.

Structural transitions and stabilization of palladium nanoparticles upon hydrogenation

Using an explicit empirical potential and a combination of Monte Carlo and moleculardynamics simulations, we investigate the energetic, thermal and dynamical stability ofhydrogenated palladium nanoclusters containing a few hundred atoms. At zerotemperature, icosahedral clusters favour tetrahedral absorption sites, while cubic clusterspreferentially absorb at octahedral sites. As a consequence, icosahedra can absorb a largerquantity of hydrogen than cubic clusters. However, even a moderate amount of absorbedhydrogen is found to favour cubic structures. There are two distinct effects of temperature.First, thermal desorption of hydrogen atoms can take place spontaneously even at lowtemperatures. Second, hydrogen rich cubic particles undergo a structural transitiontoward icosahedra before melting occurs. A simplified diagram is proposed toaccount for the observed behaviour. Finally, the coalescence between two clusters isstudied dynamically for several clusters at various temperatures, both in the gasphase and for particles embedded in a simple model solvent. The heat producedfrom coalescence results in the desorption of hydrogen molecules. This leads to asignificant cooling of the nanoparticle, thereby contributing to its stabilization.

A new passivation method for porous silicon

In this paper, we show the enhancement and stabilization of the luminescence when depositing diamond-like carbon (DLC) thin films on top of porous silicon (PS) layers. DLC thin films reduce the influence of different ambients to PS, which can cause the desorption of hydrogen molecules from the Si–H x bonds leaving dangling bonds which operate as non-radiative recombination traps. So DLC thin films can lead to a more stable luminescence from PS layers. At the same time, hydrogenated carbon nitride films can further enhance the photoluminescence efficiency of PS because more dangling bonds are passivated by nitridation.

Isothermal desorption of hydrogen molecules from a W(110) surface at temperature ∼5 K

Isothermal desorption of hydrogen molecules from a W(110) surface at temperature ∼5 K

Silane plasma and surface processes in amorphous silicon deposition

In order to understand plasma physics and surface processes in amorphous silicon deposition, coherent anti-Stokes Raman spectroscopy as well as laser-induced fluorescence and plasma-optical-emission techniques are employed to measure species distribution in the bulk plasma as well as the space close to the substrate. Electron distribution and desorption of hydrogen molecules from the substrate surface are discussed.