Adsorption Of H2 Molecules Research Articles

Adsorption and dissociation of hydrogen on Li3N surface: A first principles study

The adsorption and dissociation of hydrogen on Li3N surface are studied by density functional theory calculations and experiment. The surface energies of four low index surfaces (001), (010), (100), and (010) are calculated. Results show that (001) has the lowest surface energy of 1.28J∕m2 compared to other three surfaces; therefore, (001) surface is selected for H2 adsorption and dissociation study. For H2 molecule adsorption, results show that the most stable adsorption site is on the N top with adsorption energy of 0.14eV. For the dissociated hydrogen atoms, the most stable adsorption site is on two N tops, with adsorption energy of 5.12eV per H atom. The calculated energy barrier for H2 dissociation is about 0.79eV, which is smaller than that of experimental value of 1.22eV.

First-Principle Study of Adsorption of Hydrogen on Ti-Doped Mg(0001) Surface

Ab initio density functional theory (DFT) calculations are performed to study the adsorption of H2 molecules on a Ti-doped Mg(0001) surface. We find that two hydrogen molecules are able to dissociate on top of the Ti atom with very small activation barriers (0.103 and 0.145 eV for the first and second H2 molecules, respectively). Additionally, a molecular adsorption state of H2 above the Ti atom is observed for the first time and is attributed to the polarization of the H2 molecule by the Ti cation. Our results parallel recent findings for H2 adsorption on Ti-doped carbon nanotubes or fullerenes. They provide new insight into the preliminary stages of hydrogen adsorption onto Ti-incorporated Mg surfaces.

Membranes for Hydrogen Purification: An Important Step toward a Hydrogen-Based Economy

AbstractProduction of pure molecular hydrogen is essential to the realization of the proposed “hydrogen economy” that could ultimately provide hydrogen as a clean, renewable source of energy; eliminate the industrialized world's dependence on petroleum; and reduce the generation of greenhouse gases linked to global warming. A crucial step in obtaining pure hydrogen is separating it from other gaseous compounds—mainly CO2—that often accompany hydrogen in industrial chemical reactions. Advanced membrane technology may prove to be the key to the successful, economical production of molecular hydrogen.Size-sieving glassy polymer membranes can separate H2 on the basis of its small size. Alternatively, reverse-selective rubbery polymers can expedite the passage and, hence, removal of CO2 due to its relatively high solubility in such membranes alone or in conjunction with dissociative chemical reactions. Transition-metal membranes and their alloys can adsorb H2 molecules, dissociate the molecules into H atoms for transport through interstitial sites, and subsequently recombine the H atoms to form molecular H2 again on the opposite membrane side. Microporous amorphous silica and zeolite membranes comprising thin films on a multilayer porous support exhibit good sorption selectivity and high diffusion mobilities for H2, leading to high H2 fluxes. Finally, carbon-based membranes, including carbon nanotubes, may be viable for H2 separation on the basis of selective surface flow and molecular sieving. A wide variety of materials challenges exist in hydrogen purification, and the objective of this issue of MRS Bulletin is to address those challenges and their potential solutions from basic principles.

Efficient H2 Adsorption by Nanopores of High-Purity Double-Walled Carbon Nanotubes

DWNT buckypaper adsorbed much more hydrogen than did a SWNT bundle. XRD measurements and GCMC simulation results suggested that the DWNT bundle is loosely packed into an hexagonal array with interstitial pores which can efficiently adsorb H2 molecules.

A Density Functional Theory Study of Molecular and Dissociative Adsorption of H2 on Active Sites in Mordenite

Adsorption and chemisorption of H2 in mordenite is studied using ab initio density functional theory (DFT) calculations. The geometries of the adsorption complex, the adsorption energies, stretching frequencies, and the capacity to dissociate the adsorbed molecule are compared for different active sites. The active centers include a Brønsted acid site, a three-coordinated surface Al site, and Lewis sites formed by extraframework cations: Na+, Cu+, Ag+, Zn2+, Cu2+, Ga3+, and Al3+. Adsorption properties of cations are compared for a location of the cation in the five-membered ring. This location differs from the location in the six-membered ring observed for hydrated cations. The five-membered ring, however, represents a stable location of the bare cation. In this position any cation exhibits higher reactivity compared with the location in the six-membered ring and is well accessible by molecules adsorbed in the main channel of the zeolite. Calculated adsorption energies range from 4 to 87 kJ/mol, depending on electronegativity and ionic radius of the cation and the stability of the cation-zeolite complex. The largest adsorption energy is observed for Cu+ and the lowest for Al3+ integrated into the interstitial site of the zeolite framework. A linear dependence is observed between the stretching frequency and the bond length of the adsorbed H2 molecule. The capacity of the metal-exchanged zeolite to dissociate the H2 molecule does not correlate with the adsorption energy. Dissociation is not possible on single Cu+ cation. The best performance is observed for the Ga3+, Zn2+, and Al3+ extraframework cations, in good agreement with experimental data.

DFT Cluster Modeling of Molecular and Dissociative Hydrogen Adsorption on Zn2+Ions with Distant Placing of Aluminum in the Framework of High-Silica Zeolites

The problem of various cationic positions in zeolites with high Si/Al ratio in the framework is discussed. The statistical distribution of aluminum in the lattice of pentasils makes probable appearance of the structures with distant placing of two aluminum atoms. Cations, localized at such sites, should be very strong Lewis acids that are highly active in different chemical reactions. An example of such a site is considered for two Zn2+ ions stabilized in the zeolite fragment represented by two adjacent five-membered rings sharing the common edge. DFT calculations of molecular and dissociative hydrogen adsorption by such sites are in agreement with experimental results. Adsorption of dihydrogen by zinc ion at such sites results in an unusually large low-frequency shift of H–H stretching vibrations indicating essential activation of adsorbed H2 molecule. The calculated path of heterolytic dissociative adsorption of dihydrogen and of the proton migration to the distantly placed basic oxygen of such acid-base pair are in agreement with the previously published DRIFT experimental data.

Dissociative hydrogen adsorption on palladium requires aggregates of three or more vacancies.

During reaction, a catalyst surface usually interacts with a constantly fluctuating mix of reactants, products, 'spectators' that do not participate in the reaction, and species that either promote or inhibit the activity of the catalyst. How molecules adsorb and dissociate under such dynamic conditions is often poorly understood. For example, the dissociative adsorption of the diatomic molecule H2--a central step in many industrially important catalytic processes--is generally assumed to require at least two adjacent and empty atomic adsorption sites (or vacancies). The creation of active sites for H2 dissociation will thus involve the formation of individual vacancies and their subsequent diffusion and aggregation, with the coupling between these events determining the activity of the catalyst surface. But even though active sites are the central component of most reaction models, the processes controlling their formation, and hence the activity of a catalyst surface, have never been captured experimentally. Here we report scanning tunnelling microscopy observations of the transient formation of active sites for the dissociative adsorption of H2 molecules on a palladium (111) surface. We find, contrary to conventional thinking, that two-vacancy sites seem inactive, and that aggregates of three or more hydrogen vacancies are required for efficient H2 dissociation.

Theoretical Study of the MoS2 (100) Surface: A Chemical Potential Analysis of Sulfur and Hydrogen Coverage. 2. Effect of the Total Pressure on Surface Stability

We investigate theoretically, by quantum DFT calculations, the adsorption of H2 molecules on the [100] MoS2 surfaces, considering various edge sulfur stoichiometries. Depending on the nature of the...

Analysis of H2 dissociation dynamics on the Pd(111) surface

We perform a detailed analysis of the dynamics of the dissociative adsorption of H2 molecules on a Pd(111) surface using ab initio data for the molecule–surface interaction and classical trajectory methods. We show that the reaction probability is completely determined by the molecule–surface interaction in the approach toward the surface before it reaches a critical distance of 1.5 Å. The corresponding dynamics can be reduced to a 2D one, involving only the translational and rotational degrees of freedom, except in the lower energy range where an important role is played by dynamic trapping. We establish the relation between the dissociation probability and the shape of 2D cuts of the potential energy surface using a simple model of the evolution of orientational forces as the molecule approaches the surface. Whereas above 1.5 Å the molecule evolves “as a whole,” below 1.5 Å the dynamics has the character of independent atom–surface interactions which explains why it dissociates with a probability close to one once it has reached the critical distance of 1.5 Å.

First-principles Study on the Structure and Hydrogen Storage Properties of Na decorated BN sheet

The Na decorated BN sheets have a stable sandwich structure and can be regarded as an excellent hydrogen storage material. With respect to first-principles calculation, the electronic structure, geometric structure, partial density of states, mulliken population and ability of absorbing hydrogen molecules of the Na decorated BN sheet have been investigated. The results show that: (1) The most stable structure is Na atom adsorption on the top of N atom, it has the greatest binding energy. (2) Na decorated BN sheet can adsorb twelve H2 molecules and the average adsorption energy is 0.530 eV/H2. (3) The hydrogen storage rate of Na decorated BN sheet is about 8.943wt%. (4) The adsorbed H2 molecules have polarization phenomenon.

Structure Sensitive Reaction Channels of Molecular Hydrogen on Silicon Surfaces.

The ability of the Si(001) surface to adsorb H2 molecules dissociatively increases by orders of magnitude when appropriate surface dangling bonds are terminated by H atoms. Through molecular beam techniques the energy dependent sticking probability at different adsorption sites on H-precovered and stepped surfaces is measured to obtain information about the barriers to adsorption, which decrease systematically with an increase in coadsorbed H atoms. With the help of density functional calculations for interdimer adsorption pathways, this effect is traced back to the electronic structure of the different adsorption sites and its interplay with local lattice distortions.

Molecular Clusters in Interstellar Clouds

Cluster ions of the type H+3 (H2)p, H3O+ (H2O)q, and mixed clusters of the type H3O+ (H2O)q(H2)p may be formed by gas-phase chemistry or by cosmic-ray-induced desorption from dust grains in dense interstellar clouds. An analysis of formation mechanisms leads to the prediction of an equilibrium abundance of H+3 (H2)p clusters, where p = 3-4, of ~10-10 n. The initial stage in the gas-phase formation of these cluster ions would be via radiative association of H+3 and H2 at a rate ~10-16 cm3 s-1. Desorption from H2 monolayers by H+3 or He+ collisions with grains leads to a similar production rate for H+3 (H2)p clusters. Such cluster ions have been observed in laboratory experiments on charged particle impact with solid H2 layers. Cosmic-ray sputtering of adsorbed layers on dust can form cluster ions via the creation of energetic ions such H+3 and H3O+. An equilibrium abundance of H+3 (H2)p clusters, independent of cloud density, of ~10-8 cm-3 is predicted due to cosmic-ray sputtering of adsorbed H2 molecules. Sputtering of ice layers by cosmic rays should produce a range of large cluster ions H3O+ (H2O)q in interstellar clouds. Laboratory data on sputtering of H2O with keV He+ ions shows that clusters with q 50 are possible. The fragmentation of such clusters on electron-ion recombination is likely to lead to a range of neutral clusters. The abundance of such clusters, which may be considered to be a population of very small grains, is predicted to be comparable to that of dust grains. These clusters can accrete other atomic and molecular species and may constitute a gas-phase route toward grain formation in dense interstellar clouds.

STRUCTURES AND STABILITIES OF ${\rm H}_3^+ ({\rm H}_2)_n$ CLUSTERS (n=1–11)

Geometries and energies for [Formula: see text] clusters (n=0,…, 11) have been calculated using standard ab-initio methods. Up to the clusters with n=6, three different Pople basis sets (DZ, TZ, TZP) have been used in the calculations. For larger cluster sizes, the calculations have been carried out with one basis set (DZ) using the HF/CISD method. We discuss here only the nice counterplay of polarization effects between the central [Formula: see text] ion and the adsorbed H2 molecules, which naturally governs both the geometric structure and energy of the clusters.

MAGNETO-OPTIC AND MAGNETO-CATALYTIC EFFECT

Anticrossing and Resonance Spectroscopy provide an important amount of information on the internal processes active in chemical reactions. We investigate theoretically a simple magneto-catalytic experiment, namely ortho-para hydrogen conversion on a paramagnetic surface. In particular, we stress the importance of non-diagonal exchange interactions between the substrate and the adsorbate, in inducing optical transitions within the adsorbed H2 molecules. The state-mixing is thereafter insured by the molecular hyperfine interactions and corresponds to a double Singlet-Triplet, electronic and nuclear, mixing. The observed magnetic field effects are discussed.

Adsorption and Diffusion of Hydrogen on Nickel

The surface diffusion and adsorption of hydrogen on Ni field emitters has been investigated by a technique previously used for similar studies of H and O diffusion on W. It was found that diffusion occurs with an activation energy of 7±1 kcal in the chemisorbed layer, without the precipitation boundaries observed on W. This is believed to result from the inherent homogeneity of f.c.c. substrates, on which most adsorption sites seem equivalent. This is supported by the marked lack of emission anisotropy of hydrogen-covered Ni field emitters. A field dependent transformation in the adsorbed film was found between 2° and 4°K and interpreted as a shift in the equilibrium between adsorbed H2 molecules and the terminal fraction of adsorbed H atoms. The implications of this result for the mechanism of H2–D2 exchange are discussed. The activation energy of desorption at very low coverage is determined to be 46±4 kcal from the temperature dependence of desorption rates.