Dissociation Of Molecular Hydrogen Research Articles

Reactivity of Transition Metals (Pd, Pt, Cu, Ag, Au) toward Molecular Hydrogen Dissociation: Extended Surfaces versus Particles Supported on TiC(001) or Small Is Not Always Better and Large Is Not Always Bad

The reactivity of Pd4, Pt4, Cu4, Ag4, and Au4 clusters supported on TiC(001) toward molecular hydrogen dissociation has been studied by means of density functional based theory and periodic models and compared to that of the (111) and (001) surfaces. Pd4 and Pt4 interact rather strongly with the TiC(001) substrate, but the interaction of molecular hydrogen with the Pd4/TiC and Pt4/TiC systems is also very strong. As a consequence of the substantial admetal↔carbide interactions, the adsorbed H2 molecule becomes more difficult to dissociate than on the corresponding extended (111) and (001) surfaces. Here, having a small supported particle does not lead to an enhanced chemical activity. On the contrary, for the Cu4/TiC, Ag4/TiC, and Au4/TiC systems the combination of the small size of the particle and the polarization induced by the underlying carbide facilitates the dissociation of the hydrogen molecule with respect to the case of the extended surfaces. Here, the reduced size effectively enhances the activ...

Contribution of Pt layer to hydrogen mediation in ZnCoO

AbstractWe succeeded in injecting large amounts of hydrogen into Co‐doped ZnO (ZnCoO) thin films by coating the films with a Pt layer, which facilitated the dissociation of molecular hydrogen into hydrogen ions. Because of the hydrogen mediation effect in ZnCoO, the ferromagnetic spin ordering between the Co ions was proportional to the injected thin film. Hydrogenation of the ZnCoO films was carried out by a post plasma treatment process. The thin Pt layer fabricated on the ZnCoO films helped in enhancing the adsorption rate of hydrogen near the film surface up to 50 nm of depth. The Pt layer evaporated during the plasma process because of the weak adhesion between Pt and ZnCoO, and therefore, we could not optimize the hydrogen injection efficiency. To increase the number of applications based on the hydrogen mediation effect, it is necessary to gain a thorough understanding of the behavior of hydrogen ions in transition metal‐doped ZnO films and the role of the Pt layer in hydrogen mediation.

On the dissociation of molecular hydrogen by Au supported on transition metal carbides: choice of the most active support

A systematic density functional study of the adsorption and dissociation of H(2) on the clean (001) surface of various transition metal carbides (TMCs; TM = Ti, Zr, V, Mo) and on Au(4) nanoclusters supported on these TMCs is presented. It is found that the H(2) dissociation on the bare clean TMCs strongly depends on the chemical nature of the support. Thus, the H(2) molecule interacts rather strongly with TiC(001) and ZrC(001) but very weakly with VC(001) and δ-MoC(001). For the supported Au(4) cluster, two different types of molecular mechanisms are found. For Au(4)/TiC(001) and Au(4)/ZrC(001), H(2) dissociation leads to a H atom directly interacting with the Au(4) cluster while the second H atom is transferred to the support. In contrast, for Au(4)/VC(001) and Au(4)/δ-MoC(001), both H atoms interact with the Au(4) cluster. Overall, the present study suggests that, among the systems studied, Au/ZrC is the best substrate for H(2) dissociation.

Development of an In Vitro Compartmentalization Screen for High-Throughput Directed Evolution of [FeFe] Hydrogenases

Background[FeFe] hydrogenase enzymes catalyze the formation and dissociation of molecular hydrogen with the help of a complex prosthetic group composed of common elements. The development of energy conversion technologies based on these renewable catalysts has been hindered by their extreme oxygen sensitivity. Attempts to improve the enzymes by directed evolution have failed for want of a screening platform capable of throughputs high enough to adequately sample heavily mutated DNA libraries. In vitro compartmentalization (IVC) is a powerful method capable of screening for multiple-turnover enzymatic activity at very high throughputs. Recent advances have allowed [FeFe] hydrogenases to be expressed and activated in the cell-free protein synthesis reactions on which IVC is based; however, IVC is a demanding technique with which many enzymes have proven incompatible.Methodology/Principal FindingsHere we describe an extremely high-throughput IVC screen for oxygen-tolerant [FeFe] hydrogenases. We demonstrate that the [FeFe] hydrogenase CpI can be expressed and activated within emulsion droplets, and identify a fluorogenic substrate that links activity after oxygen exposure to the generation of a fluorescent signal. We present a screening protocol in which attachment of mutant genes and the proteins they encode to the surfaces of microbeads is followed by three separate emulsion steps for amplification, expression, and evaluation of hydrogenase mutants. We show that beads displaying active hydrogenase can be isolated by fluorescence-activated cell-sorting, and we use the method to enrich such beads from a mock library.Conclusions/Significance[FeFe] hydrogenases are the most complex enzymes to be produced by cell-free protein synthesis, and the most challenging targets to which IVC has yet been applied. The technique described here is an enabling step towards the development of biocatalysts for a biological hydrogen economy.

An ONIOM study of H 2O interacting with the C-terminated surface of silicon carbide

A combined first principle and molecular mechanics research on the adsorption and surface reactions of H 2O molecule with the C-terminated silicon carbide surface was carried out. The three-layered ONIOM(B3PW91/6-31g(d):AM1:UFF) approach on the (SiC) 192H 2O model was examined. Three distinct adsorption styles (molecular adsorption, molecular hydrogen dissociation adsorption and the H-OH bond dissociative adsorption) and eight possible reaction channels were found. The results show that there are three competitive reactions over the eight channels and the H 2O molecule is in favor of dissociating the H-OH bond by the absorptions on the adjacent surface atoms.

Comparative analysis of the hydriding kinetics of LaNi5, La0.8Nd0.2Ni5 and La0.7Ce0.3Ni5 compounds

In two-phase domains, the plateau pressure of hydride forming materials (such as intermetallic compounds or IMCs) depends markedly on the operating temperature (Van’t Hoff relationships). Therefore, for practical applications, it is necessary to select hydrogen storage materials by considering the thermal environment of the hydride tank. The thermodynamic properties (absorption and desorption pressure plateaux) of IMCs can be adjusted to some extend by chemical alloying with foreign metals and substitution on different crystallographic sites. In this paper, we report on the hydriding kinetics of substituted AB5 compounds. Isotherms have been measured at different temperatures on LaxNd1−xNi5 (x ≈ 0.2) and LaxCe1−xNi5 (x ≈ 0.3) compounds. Pneumato-chemical impedance spectroscopy has been used to analyze the hydriding kinetics and to determine microscopic rate parameters associated with surface dissociation of molecular hydrogen, diffusion-controlled transport of atomic hydrogen to bulk regions and hydride formation. Results have been compared to those measured on LaNi5 and the interest of using such substituted compounds for application in auxiliary power units is discussed.

DISENTANGLING THE CIRCUMNUCLEAR ENVIRONS OF CENTAURUS A. II. ON THE NATURE OF THE BROAD ABSORPTION LINE

We report on atomic gas (HI) and molecular gas (as traced by CO(2-1)) redshifted absorption features toward the nuclear regions of the closest powerful radio galaxy, Centaurus A (NGC 5128). Our HI observations using the Very Long Baseline Array allow us to discern with unprecedented sub-parsec resolution HI absorption profiles toward different positions along the 21 cm continuum jet emission in the inner 0."3 (or 5.4 pc). In addition, our CO(2-1) data obtained with the Submillimeter Array probe the bulk of the absorbing molecular gas with little contamination by emission, not possible with previous CO single-dish observations. We shed light with these data on the physical properties of the gas in the line of sight, emphasizing the still open debate about the nature of the gas that produces the broad absorption line (~55 km/s). First, the broad H I line is more prominent toward the central and brightest 21 cm continuum component than toward a region along the jet at a distance ~ 20 mas (or 0.4 pc) further from it. This suggests that the broad absorption line arises from gas located close to the nucleus, rather than from diffuse and more distant gas. Second, the different velocity components detected in the CO(2-1) absorption spectrum match well other molecular lines, such as those of HCO+(1-0), except the broad absorption line that is detected in HCO+(1-0) (and most likely related to that of the H I). Dissociation of molecular hydrogen due to the AGN seems to be efficient at distances <= 10 pc, which might contribute to the depth of the broad H I and molecular lines.

Adsorption and dissociation of molecular hydrogen on Pt/CeO 2 catalyst in the hydrogen spillover process: A quantum chemical molecular dynamics study

Ultra accelerated quantum chemical molecular dynamics method (UA-QCMD) was used to study the dynamics of the hydrogen spillover process on Pt/CeO 2 catalyst surface for the first time. The direct observation of dissociative adsorption of hydrogen on Pt/CeO 2 catalyst surface as well as the diffusion of dissociative hydrogen from the Pt/CeO 2 catalyst surface was simulated. The diffusion of the hydrogen atom in the gas phase explains the high reactivity observed in the hydrogen spillover process. Chemical changes, change of adsorption states and structural changes were investigated. It was observed that parallel adsorption of hydrogen facilitates the dissociative adsorption leading to hydrogen desorption. Impact with perpendicular adsorption of hydrogen causes the molecular adsorption on the surface, which decelerates the hydrogen spillover. The present study also indicates that the CeO 2 support has strong interaction with Pt catalyst, which may cause an increase in Pt activity as well as enhancement of the metal catalyst dispersions and hence increasing the rate of hydrogen spillover reaction.

Effect of palladium addition on the electrochemical properties of amorphous 2Mg + 3d alloys doped by nickel atoms (3d = Fe, Ni)

AbstractAmorphous 2Mg+ 3d/x wt% Ni materials were prepared by mechanical alloying (MA) of Mg and 3d elemental powders (3d = Fe, Ni x =0, 100 and 200 wt.%) under high purity argon atmosphere in SPEX 8000 Mixer Mill. The effect of the Ni addition on the electrochemical properties of the synthesized nanostructured alloys was investigated in details. The discharge capacity of amorphous 2Mg+Fe (Ni) high energy ball milled with Ni was improved. With increasing nickel content in the studied 2Mg+Fe (Ni) materials, at first cycle, the discharge capacity increases first and then decreases, and for example, for x = 100 in 2Mg+Fe reaches a maximum value of 155 mAh g–1. Additionally, when coated with palladium, the discharge capacity of an amorphous 2Mg+Fe (Ni) powders was increased as well. The catalytic elements (Ni, Pd) were distributed on the surface of ball milled alloy particles homogenously and role of these particles is to catalyze the dissociation of molecular hydrogen on the surface of studied alloy. Mechanical coating with palladium effectively reduced the degradation rate of the studied electrode materials. Compared to that of the uncoated powders, the degradation of the coated was suppressed. The amorphous Mg‐based hydrides offer a breakthrough in prospects for practical applications (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Suprathermal hydrogen produced by the dissociation of molecular hydrogen in the extended atmosphere of exoplanet HD 209458b

The ionization and dissociation of molecular hydrogen by the ultraviolet (UV) radiation of the parent star lead to the formation of hydrogen atoms with an excess of kinetic energy and, thus, are an important source of suprathermal hydrogen atoms in the upper atmosphere of exoplanet HD 209458b. Contemporary aeronomical models did not investigate these processes because they assumed the fast local thermalization of the hot atoms of hydrogen by elastic collisions. However, the kinetics and transfer of these atoms were not calculated in detail, because they require the solving of the Boltzmann equation for a nonthermal atom population. This work estimates the effect of the UV radiation of the parent star and the accompanying photocleacton flux on the production of the suprathermal fraction of atomic hydrogen in the H2 → H transition region. We also consider the formation of the escaping flux of Hatoms created by this effect in the upper atmosphere of HD 209458b. We calculate the production rate and energy spectrum of the hydrogen atoms with excess kinetic energy during the dissociation of H2. Using the numerical stochastic model created by Shematovich (2004) for a hot planetary corona, we investigate the molecular-scale kinetics and transfer of suprathermal hydrogen atoms in the upper atmosphere and the emergent flux of atoms evaporating from the atmosphere. The latter is estimated as 3.4 × 1012 cm−2 s−1 for a moderate stellar activity level of UV radiation, which leads to a planetary atmosphere evaporation rate of 3.4 × 109 g s−1 due to the process of the dissociation of H2. This estimate is close to the observational value of ∼1010 g s−1 for the rate of atmospheric loss of HD 209458b.

BLACK HOLE FORMATION IN PRIMORDIAL GALAXIES: CHEMICAL AND RADIATIVE CONDITIONS

In massive primordial galaxies, the gas may directly collapse and form a single central massive object if cooling is suppressed. Line cooling by molecular hydrogen can be suppressed in the presence of a strong soft-ultraviolet radiation field, but the role played by other cooling mechanisms is less clear. In optically thin gas, Lyman-Alpha cooling can be very effective, maintaining the gas temperature below 10^4 K over many orders of magnitude in density. However, the large neutral hydrogen column densities present in primordial galaxies render them highly optically thick to Lyman-Alpha photons. In this letter, we examine in detail the effects of the trapping of these Lyman-Alpha photons on the thermal and chemical evolution of the gas. We show that despite the high optical depth in the Lyman series lines, cooling is not strongly suppressed, and proceeds via other atomic hydrogen transitions, in particular the 2s-1s and the 3-2 transitions. At densities larger than 10^9 cm^{-3}, collisional dissociation of molecular hydrogen becomes the dominant cooling process and decreases the gas temperature to about 5000 K. The gas temperature evolves with density as $T \propto \rho^{\gamma_{\rm eff} - 1}$, with $\gamma_{\rm eff} = 0.97-0.98$. The evolution is thus very close to isothermal, and so fragmentation is possible, but unlikely to occur during the initial collapse. However, after the formation of a massive central object, we expect that later-infalling, higher angular momentum material will form an accretion disk that may be unstable to fragmentation, which may give rise to star formation with a top-heavy IMF.

Hydrogen on graphene under stress: Molecular dissociation and gap opening

Density-functional calculations are employed to study the molecular dissociation of hydrogen on graphene, the diffusion of chemisorbed atomic species, and the electronic properties of the resulting hydrogen on graphene system. Our results show that applying stress to the graphene substrate can lower the barrier to dissociation of molecular hydrogen by a factor of 6 and change the process from endothermic to exothermic. These values for the barrier and the heat of reaction, unlike the zero stress values, are compatible with the time scales observed in experiments. Diffusion, on the other hand, is not greatly modified by stress. We analyze the electronic structure for configurations relevant to molecular dissociation and adsorption of atomic hydrogen on a graphene single layer. An absolute band gap of 0.5 eV is found for the equilibrium optimum configuration for a narrow range of coverages $(\ensuremath{\theta}\ensuremath{\approx}0.25)$. This value is in good agreement with experiment [D. Elias et al., Science 323, 610 (2009)].

Pd Nanoparticles Embedded into a Metal-Organic Framework: Synthesis, Structural Characteristics, and Hydrogen Sorption Properties

The metal-organic framework MIL-100(Al) has been used as a host to synthesize Pd nanoparticles (around 2.0 nm) embedded within the pores of the MIL, showing one of the highest metal contents (10 wt %) without degradation of the porous host. Textural properties of MIL-100(Al) are strongly modified by Pd insertion, leading to significant changes in gas sorption properties. The loss of excess hydrogen storage at low temperature can be correlated with the decrease of the specific surface area and pore volume after Pd impregnation. At room temperature, the hydrogen uptake in the composite MIL-100(Al)/Pd is almost twice that of the pristine material. This can be only partially accounted by Pd hydride formation, and a "spillover" mechanism is expected to take place promoting the dissociation of molecular hydrogen at the surface of the metal nanoparticles and the diffusion of monatomic hydrogen into the porosity of the host metal-organic framework.

The Role of Preadsorbed Atomic Hydrogen in the NO Dissociation on a Zigzag Stepped Gold Surface: A DFT Study

NO dissociation with and without the presence of hydrogen on the Au(321) surface was investigated using density functional theory and a periodic supercell approach. The role of hydrogen in the reaction of NO dissociation is studied by comparing four different routes (i.e. direct dissociation of NO on the clean surface and paths involving reaction with hydrogen adatoms prior to N−O bond cleavage). In the latter situation, two routes via a NOH intermediate were considered, producing N* + OH* or N* + H2O*, and another route via a NHOH intermediate was also considered, yielding NH* and OH* species. The calculations predict that the kinetically most favorable route is that producing nitrogen adatoms and water, i.e., NO* + 2H* → N* + H2O* via the NOH* intermediate. This reaction is exothermic (1.1 eV) and the energy barriers for the separate NO* + H* → NOH* and NOH* + H* → N* + H2O* reactions are ∼0.5 eV. An identical energy barrier was calculated for the reaction of the dissociation of molecular hydrogen on th...

First‐principles study of molecular hydrogen dissociation on doped Al12X (X = B, Al, C, Si, P, Mg, and Ca) clusters

Inspired by the concept of superatom via substitutionally doping an Al13 magic cluster, we investigated the H2 molecule dissociation on the doped icosahedral Al12X (X = B, Al, C, Si, P, Mg, and Ca) clusters by means of density functional theory. The computed reaction energies and activation barriers show that the concept of superatom is still valid for the catalysis behavior of doped metal clusters. The hydrogen dissociation behavior on metal clusters characterized by the activation barrier and reaction energy can be tuned by controllable doping. Thus, doped Al12X clusters might serve as highly efficient and low-cost catalysts for hydrogen dissociation.

Hydrogen absorption and desorption in metallic glass and nanocrystalline Zr 52.5Cu 17.9Ni 14.6Ti 5Al 10 alloy

To illuminate the intrinsic surface activity of Zr 52.5Cu 17.9Ni 14.6Ti 5Al 10 alloy in its glass and nanocrystalline states, hydrogen absorption and desorption in both states was investigated by gas chromatographic analysis. The results show that the Zr 52.5Cu 17.9Ni 14.6Ti 5Al 10 alloy in the nanocrystalline state can absorb a larger amount of hydrogen than that in glass state at room temperature after activation. According to the desorption process and surface state, the significant change in absorption induced by crystallization is proposed to result from that the glassy alloy has a higher desorption energy, which can adsorb gas physically and nonselectively, and is difficult to activate, while the nanocrystalline alloy can absorb much hydrogen due to the inter-atomic or intra-atomic electron transfer, which accelerates the kinetics of the catalytic reaction for the dissociation of molecular hydrogen into atomic hydrogen.

Adsorption and dissociation of hydrogen on MgO surface: A first-principles study

Periodic density functional theory (DFT) has been used to investigate the adsorption and dissociation of hydrogen on MgO surface. Weak molecular adsorptions were observed and the bridge site with the vertical mode was the most favorable molecular adsorption site. The reaction barrier along with dissociation of the hydrogen molecule on the MgO(0 0 1) surface has also been studied. Our results show the most favorable dissociation channel needs activation energy of around 2.2 eV at the bridge site which is in agreement with the previous results (2.34–2.94 eV). The minimum energy pathway for surface diffusion of atomic H indicates the existence of small classical barriers with a value of about 0.37 eV. And our calculations show that H atom can diffuse from the surface sites to subsurface and further into the bulk sites. The most stable absorption configurations are the interstitial octahedral sites in diffusion movements. The largest barrier in these processes is up to 1.52 eV in the diffusion step of surface to subsurface. Once atomic H penetrates the subsurface layers, the corresponding diffusion barriers decrease to 1.02 eV (from subsurface-1 (SB1) to subsurface-2 (SB2) site) and 1.2 eV (from SB2 to subsurface-3 (SB3) site), respectively. Our calculations indicate that dissociation of molecular hydrogen is strongly inhibited on the magnesium oxide surface during the hydrogenation on magnesium films.

Nanolayers on nanochannels for hydrogen purification

The purification of hydrogen rich gases is of great technological importance in the “hydrogen economy” and is achieved by selective membranes made of organic or inorganic materials. In this field, a strong challenge is the synthesis of defect-free ultrathin Pd-based selective membranes. We present a study on the synthesis and performances of a bilayer structure consisting of 100 nm nanoporous silica coated with a 150 nm Pd–Ag layer. An alumina disk having periodic microsieves structure was used as support for the bilayer. The hydrogen transport through this nanocomposite membrane is controlled by the dissociation of molecular hydrogen at the surface of the Pd–Ag functional layer. When operating at 573 K, the membrane exhibits high H2/N2 selectivity (a factor as high as 600–900), high H2 permeance (∼10−6 mol m−2 s−1 Pa−1), and operative stability on long-term operations.

Nanocomposite Hydride LaNi&lt;sub&gt;5&lt;/sub&gt;/A- and Mg&lt;sub&gt;2&lt;/sub&gt;Ni/A-Type Materials (A=C, Cu, Pd)

In this work, we have synthesized LaNi5/A and Mg2Ni/A (A = graphite, copper or palladium) nanocomposites. The A elements were distributed on the surface of ball milled alloy particles homogenously and role of these particles is to catalyze the dissociation of molecular hydrogen on the surface of studied alloy. Mechanical coating with graphite or palladium effectively reduced the degradation rate of the studied electrode materials. Results showed a significant broadening of the valence bands of studied nanocomposites compared to those obtained by theoretical band calculations. The reasons responsible for the band broadening of the nanocrystalline LaNi5- and Mg2Ni-type alloys are probably associated with a strong deformation of the nanocrystals in the mechanically alloyed (MA) samples. Normally the interior of the nanocrystal is constrained and the distances between atoms located at the grain boundaries expanded. The valence band spectra of the MA samples could be also broadened due to an additional disorder introduced during formation of the nanocrystalline structure.

Adsorption and dissociation of molecular hydrogen on the (0001) surface of double hexagonal close packed americium

Hydrogen molecule adsorption on the (0001) surface of double hexagonal packed americium has been studied in detail within the framework of density functional theory using a full-potential all-electron linearized augmented plane wave plus local orbitals method (FP-L/APW+lo). Weak molecular hydrogen adsorptions were observed. Adsorption energies were optimized with respect to the distance of the adsorbates from the surface for three approach positions at three adsorption sites, namely t1 (one-fold top), b2 (two-fold bridge), and h3 (three-fold hollow) sites. Adsorption energies were computed at the scalar-relativistic level (no spin-orbit coupling NSOC) and at the fully relativistic level (with spin-orbit coupling SOC). The most stable configuration corresponds to a horizontal adsorption with the molecular approach being perpendicular to a lattice vector. The surface coverage is equivalent to one-fourth of a monolayer (ML), with the adsorption energies at the NSOC and SOC theoretical levels being 0.0997 eV and 0.1022 eV, respectively. The respective distance of the hydrogen molecule from the surface and hydrogen-hydrogen distance was found to be 2.645 A and 0.789 A, respectively. The work functions decreased and the net magnetic moments remained almost unchanged in all cases compared with the corresponding quantities of bare dhcp Am (0001) surface. The adsorbate-substrate interactions have been analyzed in detail using the partial charges inside the muffin-tin spheres, difference charge density distributions, and the local density of states. The effects of adsorption on the Am 5f electron localization-delocalization characteristics have been discussed. Reaction barrier for the dissociation of hydrogen molecule has been presented.