Dissociation Of Molecular Hydrogen Research Articles

A novel surface modification approach for synthesizing supported nickel phosphide catalysts with high activity for hydrodeoxygenation of benzofuran

A novel and simple surface modification approach for preparing a highly active nickel phosphide hydrodeoxygenation (HDO) catalyst is proposed, in which the original nickel phosphide catalyst is modified with oxygen at 373K under a flow of air for 1h instead of being passivated with an O2/N2 mixture. In this way, no pre-treatment of the catalyst prior to reaction is needed. The effect of surface modification with oxygen on catalytic performance in HDO of benzofuran was investigated. This surface modification apparently promotes the formation of smaller Ni2P particles in catalyst, favoring more dissociation of molecular hydrogen. The Ni2P preparation step is simplified and therefore energy-efficient, and the obtained catalyst exhibits a much higher O-free products yield (88%) than that with the unmodified catalyst (38%).

Effect of Nuclear Mass in Strong-Field Ionization of Hydrogen Molecules and Dissociation of Hydrogen Molecular Ions

SynopsisWe measure isotope effect in strong-field ionization and dissociation of molecular hydrogen using a mixed-gas target in a Reaction Microscope (REMI). We compare experimental results with theoretical predictions.

A density functional theory analysis of the molecular hydrogen dissociation on Al n Pt (n = 1-12) clusters

Bimetallic alloys are considered to be a promising type of catalysts with improved activity and selectivity that are distinct from those of the corresponding pure nanoclusters [1-4]. Using first principles density functional calculations, we study the structures and energies of AlnPt bimetallic clusters up to 13 atoms. If platinum, nickel, and other transition metal catalysts are particularly important in the catalysis of hydrogen, hydrogen adsorption on a metal surface is an important step in the catalytic reaction. Because of large exothermic energy changes and relatively small activation energies, Al7Pt and Al12Pt could serve as highly efficient and low-cost catalysts for the hydrogen dissociation. To clarify this assumption and achieve a good understanding, the H2 adsorption and dissociation over bimetallic AlPt clusters are systematically investigated in our work.

Hot-wire assisted ALD of tungsten films:In-situstudy of the interplay between CVD, etching, and ALD modes

In this work, we investigated an approach of hot-wire assisted ALD (HWALD), utilizing a hot (up to 2000 °C) tungsten (W) wire. Tungsten films were deposited by this method using alternating pulses of WF6 gas and atomic hydrogen (at-H). The latter was generated by catalytic dissociation of molecular hydrogen (H2) upon the hot-wire. The W films were grown on a 100-nm thick thermal SiO2. The growth process was monitored in real time by an in-situ spectroscopic ellipsometer (SE). The real-time SE monitoring revealed the coexistence of three processes: CVD, etching, and ALD of the W film. WF6 could back-stream diffuse to the hot-wire, resulting in WF6 decomposition and generation of a flux of fluorine (F). The latter caused etching of the grown W film and the filament, and provided extra tungsten supply, which might cause CVD. Higher pressure and higher carrier gas flow rate were found to largely suppress the back-stream diffusion of WF6, which efficiently limited CVD. By controlling the dose of WF6 and process pressure, the etching had also been minimized. X-ray photoelectron spectroscopy of optimized HWALD grown W revealed 99 at% of W; concentrations of oxygen and fluorine were lower than 1%, below the detection limit.

GTC OSIRIS transiting exoplanet atmospheric survey: detection of potassium in HAT-P-1b from narrow-band spectrophotometry★

We present the detection of potassium in the atmosphere of HAT-P-1b using optical transit narrowband photometry. The results are obtained using the 10.4 m Gran Telescopio Canarias (GTC) together with the OSIRIS instrument in tunable filter imaging mode. We observed four transits, two at continuum wavelengths outside the potassium feature, at 6792 {\AA} and 8844 {\AA}, and two probing the potassium feature in the line wing at 7582.0 {\AA} and the line core at 7664.9 {\AA} using a 12 {\AA} filter width (R~650). The planet-to-star radius ratios in the continuum are found to be $R_{\rm{pl}}/R_{\star}$ = 0.1176 $\pm$ 0.0013 at 6792 {\AA} and $R_{\rm{pl}}/R_{\star}$ = 0.1168 $\pm$ 0.0022 at 8844 {\AA}, significantly lower than the two observations in the potassium line: $R_{\rm{pl}}/R_{\star}$ = 0.1248 $\pm$ 0.0014 in the line wing at 7582.0 {\AA} and $R_{\rm{pl}}/R_{\star}$ = 0.1268 $\pm$ 0.0012 in the line core at 7664.9 {\AA}. With a weighted mean of the observations outside the potassium feature $R_{\rm{pl}}/R_{\star}$ = 0.1174 $\pm$ 0.0010, the potassium is detected as an increase in the radius ratio of {\Delta}$R_{\rm{pl}}/R_{\star}$ = 0.0073 $\pm$ 0.0017 at 7582.0 {\AA} and {\Delta}$R_{\rm{pl}}/R_{\star}$ = 0.0094 $\pm$ 0.0016 at 7664.9 {\AA} (a significance of 4.3 and 6.1 $\sigma$ respectively). We hypothesise that the strong detection of potassium is caused by a large scale height, which can be explained by a high-temperature at the base of the upper atmosphere. A lower mean molecular mass caused by the dissociation of molecular hydrogen into atomic hydrogen by the EUV flux from the host star may also partly explain the amplitude of our detection.

MINIMUM CORE MASSES FOR GIANT PLANET FORMATION WITH REALISTIC EQUATIONS OF STATE AND OPACITIES

Giant planet formation by core accretion requires a core that is sufficiently massive to trigger runaway gas accretion in less that the typical lifetime of protoplanetary disks. We explore how the minimum required core mass, M_crit, depends on a non-ideal equation of state and on opacity changes due to grain growth, across a range of stellocentric distances from 5-100 AU. This minimum M_crit applies when planetesimal accretion does not substantially heat the atmosphere. Compared to an ideal gas polytrope, the inclusion of molecular hydrogen (H_2) dissociation and variable occupation of H_2 rotational states increases M_crit. Specifically, M_crit increases by a factor of ~2 if the H_2 spin isomers, ortho- and parahydrogen, are in thermal equilibrium, and by a factor of ~2-4 if the ortho-to-para ratio is fixed at 3:1. Lower opacities due to grain growth reduce M_crit. For a standard disk model around a Solar mass star, we calculate M_crit ~ 8 M_Earth at 5 AU, decreasing to ~5 M_Earth at 100 AU, for a realistic EOS with an equilibrium ortho-to-para ratio and for grain growth to cm-sizes. If grain coagulation is taken into account, M_crit may further reduce by up to one order of magnitude. These results for the minimum critical core mass are useful for the interpretation of surveys that find exoplanets at a range of orbital distances.

Hydrogen activation, diffusion, and clustering on CeO₂(111): a DFT+U study.

We present a comprehensive density functional theory+U study of the mechanisms underlying the dissociation of molecular hydrogen, and diffusion and clustering of the resulting atomic species on the CeO2(111) surface. Contrary to a widely held view based solely on a previous theoretical prediction, our results show conclusively that H2 dissociation is an activated process with a large energy barrier ~1.0 eV that is not significantly affected by coverage or the presence of surface oxygen vacancies. The reaction proceeds through a local energy minimum--where the molecule is located close to one of the surface oxygen atoms and the H-H bond has been substantially weaken by the interaction with the substrate--, and a transition state where one H atom is attached to a surface O atom and the other H atom sits on-top of a Ce(4+) ion. In addition, we have explored how several factors, including H coverage, the location of Ce(3+) ions as well as the U value, may affect the chemisorption energy and the relative stability of isolated OH groups versus pair and trimer structures. The trimer stability at low H coverages and the larger upward relaxation of the surface O atoms within the OH groups are consistent with the assignment of the frequent experimental observation by non-contact atomic force and scanning tunneling microscopies of bright protrusions on three neighboring surface O atoms to a triple OH group. The diffusion path of isolated H atoms on the surface goes through the adsorption on-top of an oxygen in the third atomic layer with a large energy barrier of ~1.8 eV. Overall, the large energy barriers for both, molecular dissociation and atomic diffusion, are consistent with the high activity and selectivity found recently in the partial hydrogenation of acetylene catalyzed by ceria at high H2/C2H2 ratios.

A no-go theorem for direct collapse black holes without a strong ultraviolet background

Abstract Explaining the existence of supermassive black holes larger than ∼109 M⊙ at redshifts z ≳ 6 remains an open theoretical question. One possibility is that gas collapsing rapidly in pristine atomic cooling haloes (Tvir ≳ 104 K) produces 104–106 M⊙ black holes. Previous studies have shown that the formation of such a black hole requires a strong UV background to prevent molecular hydrogen cooling and gas fragmentation. Recently, it has been proposed that a high UV background may not be required for haloes that accrete material extremely rapidly or for haloes where gas cooling is delayed due to a high baryon-dark matter streaming velocity. In this work, we point out that building up a halo with Tvir ≳ 104 K before molecular cooling becomes efficient is not sufficient for forming a direct collapse black hole (DCBH). Though molecular hydrogen formation may be delayed, it will eventually form at high densities leading to efficient cooling and fragmentation. The only obvious way that molecular cooling could be avoided in the absence of strong UV radiation, is for gas to reach high enough density to cause collisional dissociation of molecular hydrogen (∼104 cm−3) before cooling occurs. However, we argue that the minimum core entropy, set by the entropy of the intergalactic medium when it decouples from the cosmic microwave background, prevents this from occurring for realistic halo masses. This is confirmed by hydrodynamical cosmological simulations without radiative cooling. We explain the maximum density versus halo mass in these simulations with simple entropy arguments. The low densities found suggest that DCBH formation indeed requires a strong UV background.

Significant Quantum Effects in Hydrogen Activation

Dissociation of molecular hydrogen is an important step in a wide variety of chemical, biological, and physical processes. Due to the light mass of hydrogen, it is recognized that quantum effects are often important to its reactivity. However, understanding how quantum effects impact the reactivity of hydrogen is still in its infancy. Here, we examine this issue using a well-defined Pd/Cu(111) alloy that allows the activation of hydrogen and deuterium molecules to be examined at individual Pd atom surface sites over a wide range of temperatures. Experiments comparing the uptake of hydrogen and deuterium as a function of temperature reveal completely different behavior of the two species. The rate of hydrogen activation increases at lower sample temperature, whereas deuterium activation slows as the temperature is lowered. Density functional theory simulations in which quantum nuclear effects are accounted for reveal that tunneling through the dissociation barrier is prevalent for H2 up to ∼190 K and for D2 up to ∼140 K. Kinetic Monte Carlo simulations indicate that the effective barrier to H2 dissociation is so low that hydrogen uptake on the surface is limited merely by thermodynamics, whereas the D2 dissociation process is controlled by kinetics. These data illustrate the complexity and inherent quantum nature of this ubiquitous and seemingly simple chemical process. Examining these effects in other systems with a similar range of approaches may uncover temperature regimes where quantum effects can be harnessed, yielding greater control of bond-breaking processes at surfaces and uncovering useful chemistries such as selective bond activation or isotope separation.

Phenylacetylene hydrogenation on Fe@C and Ni@C core–shell nanoparticles: About intrinsic activity of graphene-like carbon layer in H2 activation

Me@C nanocomposites were prepared by evaporation of overheated liquid drop of Me in the flow of inert gas containing a hydrocarbon. The resulting carbon-coated nickel and iron nanoparticles contain metal cores of about 5nm in size that are wrapped in a few layers of graphene-like carbon. Experimental data and theoretical results give evidence of the ability of carbon coating in nanocomposites Fe@C and Ni@C to H2 activation by dissociative adsorption due to the presence of space and structure defects and/or the presence of transition metal in subsurface layer. Since molecular hydrogen dissociation is the key step of hydrogenation reactions, both Ni@C and Fe@C provide high conversion of phenylacetylene (PA) (about 100%) during hydrogenation at the temperatures above 150 and 300°C, respectively. Fe@C provides excellent styrene (ST) selectivity: 86% at 99% PA conversion at 300°C. ST selectivity is moderate on Ni@C (about 60%) in the temperature range of 100–150°С and low at higher temperatures.

Effect of hydrogen gas impurities on the hydrogen dissociation on iron surface

A small addition of oxygen to hydrogen gas is known to mitigate the hydrogen embrittlement (HE) of steels. As atomic hydrogen dissolution in steels is responsible for embrittlement, catalysis of molecular hydrogen dissociation by the steel surface is an essential step in the embrittlement process. The most probable role of oxygen in mitigating HE is to inhibit the reactions between molecular hydrogen and the steel surface. To elucidate the mechanism of such surface reaction of hydrogen with the steel in the presence of oxygen, hydrogen, and oxygen adsorption, dissociation, and coadsorption on the Fe(100) surface were investigated using density functional theory. The results show that traces of O2 would successfully compete with H2 for surface adsorption sites due to the grater attractive force acting on the O2 molecule compared to H2. The H2 dissociation would be hindered on iron surfaces with predissociated oxygen. Prompted by the notable results for H2 + O2, other practical systems were considered, that is, H2 + CO and CH4. Calculations were performed for the CO chemisorption and H2 dissociation on iron surface with predissociated CO, as well as, CH4 surface dissociation. The results indicate that CO inhibition of H2 dissociation proceeds via similar mechanism to O2 induced inhibition, whereas CH4 traces in the H2 gas have no effect on H2 dissociation. © 2014 Wiley Periodicals, Inc.

Theoretical investigation of molecular hydrogen adsorption and dissociation on AlnV(n = 1–13) clusters

We present density functional calculations of H2 adsorption and dissociation on small-sized AlnV clusters for n = 1–13. The growth pattern for AlnV (n = 2–4, 8, 10–12) clusters is V atom occupying a peripheral position of Aln clusters. And the growth pattern for AlnV (n = 6, 9, and 13) clusters is V-substituted Aln+1 clusters. It is found that the V atom substituted the surface atom of Aln+1 cluster and occupies a peripheral position. H2 is easily physically absorbed on the top V atom of AlnV (n = 1–13) clusters with a side-on orientation rather than an end-on orientation because of the more effective orbital overlap in the side-on orientation. The reaction of AlnV with H2 would produce AlnVH2 because of large exothermic energy changes and relatively small activation energies especially for AlV and Al7V, which might serve as highly efficient and low-cost catalysts for hydrogen dissociation.

Hot-Wire Generated Atomic Hydrogen and its Impact on Thermal ALD in TiCl4/NH3 System

We present the generation of atomic hydrogen made by the dissociation of molecular hydrogen upon collision with a tungsten (W) filament kept at a high temperature (T ≈ 1600–1900°C). We demonstrate the ability to create atomic hydrogen and to introduce it in short pulses in experiments on etching of tellurium (Te) films. We further utilize the generated atomic hydrogen (H) to explore its impact on surface reactions in the TiCl4/NH3 precursor system. Atomic hydrogen is introduced in pulses additionally to TiCl4 and NH3 with different pulse sequences. For the TiCl4/NH3/H sequence, there is no influence on the process compared to the ALD without H-pulses. The growth rate remains at 0.02 nm/cycle and the oxygen (residual gas) content - at 3–5 at%. For the TiCl4/H/NH3 pulse sequence, the growth rate decreases to 0.01 nm/cycle and the oxygen content increases to 30–35 at%. Only TiCl4/H pulses result in no growth after the formation of approximately one monolayer. Similar effect occurs after introducing NH3 via the hot filament, pointing to the decomposition of NH3 and the formation of atomic hydrogen.

The collapse of protoplanetary clumps formed through disc instability: 3D simulations of the pre-dissociation phase

We present 3D smoothed particle hydrodynamics simulations of the collapse of clumps formed through gravitational instability in the outer part of a protoplanetary disc. The initial conditions are taken directly from a global disc simulation, and a realistic equation of state is used to follow the clumps as they contract over several orders of magnitude in density, approaching the molecular hydrogen dissociation stage. The effects of clump rotation, asymmetries, and radiative cooling are studied. Rotation provides support against fast collapse, but non-axisymmetric modes develop and efficiently transport angular momentum outward, forming a circumplanetary disc. This transport helps the clump reach the dynamical collapse phase, resulting from molecular hydrogen dissociation, on a thousand-year timescale, which is smaller than timescales predicted by some previous spherical 1D collapse models. Extrapolation to the threshold of the runaway hydrogen dissociation indicates that the collapse timescales can be shorter than inward migration timescales, suggesting that clumps could survive tidal disruption and deliver a proto-gas giant to distances of even a few AU from the central star.

Adsorption and dissociation of molecular hydrogen on the edges of graphene nanoribbons

The dissociation and adsorption of molecular hydrogen on the edges of graphene nanoribbons of widths of 1.14 and 1.36 nm, is investigated within the density functional formalism. Here, graphene nanoribbons are used as models for the pore walls of some nanoporous carbons (carbide-derived carbons among others) which have been shown to be formed by one-atom thick graphene layers interconnected among them and exhibiting exposed edges (Lopez et al. in J Chem Phys 135:104706, 2011). The aim of this study is to shed some light on the contribution of the edges of the pore walls to the hydrogen storage capacity of nanoporous carbons. Nanoribbons with zigzag and armchair edge terminations have been considered. Molecular hydrogen dissociates and adsorbs atomically at the nanoribbon edges with no or small activation barrier. The adsorption energies per hydrogen molecule are quite large, 2.5 and 5.7 eV for armchair and zigzag edges, respectively. This indicates that the graphene edges are very reactive and will be saturated with hydrogen whenever available. However, under mild conditions of pressure and temperature hydrogen cannot be desorbed from the edges and, therefore, the edges do not contribute to the reversible storage capacity of the material. The magnetic properties of saturated and unsaturated ribbons are also discussed.

Adsorption and Dissociation of Molecular Hydrogen on Palladium Clusters Supported on Graphene

Doping of porous carbon materials with metallic atoms, clusters and nanoparticles is viewed as a way to enhance the hydrogen storage in those materials. Transition metals are dopants of interest. For this reason we present a theoretical study of the interaction of molecular hydrogen with small palladium clusters (Pdn, n = 1–6) supported on a graphene layer. The adsorption of H2 on those supported Pd clusters leads to two types of adsorption states. The simplest one is an activated state of the hydrogen molecule, with the H–H distance stretched and the H–H bond weakened, but not broken. Adsorption in the activated states occurs with no barriers and the binding energies are in a range of values of interest for achieving a favorable reversible hydrogen storage in the doped material. The second type is a dissociated and chemisorbed state, with the separated hydrogen atoms attached to the Pd cluster. The dissociative chemisorption states are more stable than the activated states. However, starting with Pd4, th...

THE IMPACT OF BARYON PHYSICS ON THE STRUCTURE OF HIGH-REDSHIFT GALAXIES

We study the detailed structure of galaxies at redshifts z > 2 using cosmological simulations with improved modeling of the interstellar medium and star formation. The simulations follow the formation and dissociation of molecular hydrogen, and include star formation only in cold molecular gas. The molecular gas is more concentrated towards the center of galaxies than the atomic gas, and as a consequence, the resulting stellar distribution is very compact. For halos with total mass above 10^{11} Mo, the median half-mass radius of the stellar disks is 0.8 kpc at z = 3. The vertical structure of the molecular disk is much thinner than that of the atomic neutral gas. Relative to the non-radiative run, the inner regions of the dark matter halo change shape from prolate to mildly oblate and align with the stellar disk. However, we do not find evidence for a significant dark disk of dark matter around the stellar disk. The outer halo regions retain the orientation acquired during accretion and mergers, and are significantly misaligned with the inner regions. The radial profile of the dark matter halo contracts in response to baryon dissipation, establishing an approximately isothermal profile throughout most of the halo. This effect can be accurately described by a modified model of halo contraction. The angular momentum of a fixed amount of inner dark matter is approximately conserved over time, while in the dissipationless case most of it is transferred outward during mergers. The conservation of the dark matter angular momentum provides supporting evidence for the validity of the halo contraction model in a hierarchical galaxy formation process.

A density functional theory study of hydrogen dissociation and diffusion at the perimeter sites of Au/TiO2

Reactivity of supported gold catalysts is a hot topic in catalysis for many years. This communication reports an investigation on the dissociation of molecular hydrogen at the perimeter sites of Au/TiO(2) and the spillover of hydrogen atoms from the gold to the support using density functional theory calculations. It is found that the heterolytic dissociation is favoured in comparison with homolytic dissociation of molecular hydrogen at the perimeter sites. However, the surface oxygen of the rutile TiO(2)(110) surface at these sites can be readily passivated by the formed OH, suggesting that further dissociation of molecular hydrogen may occur at pure gold sites.

Present-day star formation: From molecular cloud cores to protostars and protoplanetary disks

Essential physical processes in the formation of protostars and protoplanetary disks are described. Recent advances in non-ideal magnetohydrodynamics simulations, which cover a huge dynamic range from molecular cloud core density (104/cc) to stellar density (1022/cc) in a self-consistent manner, enable us to study the realistic evolution of the magnetic field and rotation of protostars and the dynamics of outflows and jets. First we emphasize the importance of radiative heating and cooling, and describe thermal evolution in a self-gravitationally collapsing cloud. The increased pressure at the center creates the first hydrostatic core, which consists of molecular gas. After the dissociation of molecular hydrogen triggers the second gravitational collapse at the center of the first core, a protostar is quickly formed and the first core gradually transforms into a circumstellar disk that eventually accretes onto the central protostar. The importance of the short-lived first core formed in the early collapsing phase is emphasized in the contexts of driving magnetohydrodynamical bipolar outflows and self-gravitational fragmentation into binary or multiple stars. When the central density becomes sufficiently high (1012/cc), ohmic dissipation largely removes the magnetic flux from a collapsing cloud core, and the strongly twisted magnetic field lines are straightened. The magnetic field lines are twisted and amplified again for much higher density (1016/cc) where the magnetic field is recoupled with warm gas (∼103 K). Finally, protostars at their formation epoch have magnetic fields of 0.1–1 kG, which is comparable to observed values of pre-main-sequence stars. A substantially reduced magnetic flux at the center results in passively wound-up magnetic field lines just after the formation of a protostar. This is followed by driving of a fast bipolar jet along the rotation axis by the resultant magnetic pressure due to excessive winding. Strong collimation of the jet is due to the hoop stress of piled-up toroidal field lines. The angular momentum in a collapsing cloud is removed by magnetohydrodynamical effects such as magnetic braking and driving of outflows and jets. The rotation velocity of the protostar tends to be on the order of break-up speed at its formation epoch, and thus, a further removal mechanism for the angular momentum, such as through the interactions between the protostar, disk, jets, and winds, should be important in its long-term evolution. The circumstellar disk is born in the “dead zone,” a region that is decoupled from the magnetic field. The outer radius of the disk increases with that of the dead zone during accretion from the envelope of the molecular cloud core. A rapid increase in the disk size occurs after depletion of the envelope. The circumstellar disks remain massive in their formation phase, and are subject to gravitational instability, even at 10 AU from the central stars. The further long-term evolution of massive disks is also described. This may provide an improved description for the realistic initial condition and environments for planet formation in gaseous protoplanetary disks.

Influence of hydrogen impurities on atomic and electronic structure of palladium nanowires and nanocontacts

We present an ab initio study of the influence of hydrogen impurities (atoms and molecules) on the atomic structure and electronic properties of Pd nanowires and atomic-size nanocontacts. Various atomic positions for hydrogen adsorption within palladium structures are considered. It is found that adsorbed hydrogen can penetrate into the atomic structure of Pd nanocontacts and nanowires and by that increase their stability close to and beyond their breaking points. The interaction between hydrogen and palladium atoms alters the interatomic bonding and hybridization of electronic states of palladium atoms. The interaction of palladium with hydrogen leads to the suppression of magnetic properties of Pd nanowires and nanocontacts. Additionally, the possibility of dissociation of molecular hydrogen at palladium nanocontacts is discussed, which is an important issue for catalytic applications.