Dissociative Chemisorption Of Molecular Hydrogen Research Articles

Physisorption and dissociative chemisorption of H2 on sub-nanosized Al $_{13}^{-}$ anion cluster: ab initio study

We have studied the interaction of Al $_{13}^{-}$ anion cluster with H2. Both the long range interaction and dissociative adsorption have been examined using the established correlated ab initio methods, MP2 and CCSD(T), in conjunction with the augmented correlation consistent basis sets up to aug-cc-pVTZ. The formation of the weakly bound (physisorbed) end-on anion complex Al $_{13}^{-}$ ...H2 is predicted for the interacting Al...H distances of 3.95 A with the H-H axis pointing towards the ‘hollow’ site of Al $_{13}^{-}$ and binding energy ( $D_{e})$ of 0.7 kcal/mol at the estimated complete basis set (CBS) limit of CCSD(T). The barrier height for H2 dissociation on Al $_{13}^{-}$ of 41.6 (42.9) kcal/mol calculated at the ZPVE-corrected CCSD(T)/aug-cc-pVTZ (estimated CCSD(T)/CBS) level is at least twice as large as that evaluated by us for a dissociative adsorption of H2 on an open-shell Al13 neutral cluster. To our knowledge, this report presents the first “benchmark” quality study of the physisorption and dissociative chemisorption of molecular hydrogen on Al $_{13}^{-}$ anion cluster.

Energetics and dynamics of hydrogen adsorption, desorption and migration on a carbon-supported palladium cluster

The functionality of metal-doped carbon materials in catalytic and hydrogen storage applications is governed by the characteristics of their interaction with hydrogen. The dynamics of hydrogen chemisorption, its migration and desorption are investigated by first-principles molecular dynamics simulations of a model Pd4 cluster supported on a coronene molecule. Longer time scale events are accelerated using the metadynamics technique. This article is a comprehensive report of the energetics and dynamics of (i) dissociative chemisorption of molecular hydrogen on a carbon-supported Pd cluster, (ii) transport of atomic hydrogen on the cluster, towards the carbon support and (iii) the associative desorption of atomic hydrogens in a molecular form. It is found that the dissociative chemisorption of hydrogen on the cluster involves a negligible energy barrier and takes place readily at room temperature. The migration of atomic hydrogen from the tip of the tetrahedral Pd4 cluster to its sides, on the other hand, involves an energy barrier of ∼6 KJ/mole. This energy barrier, however, is reduced when the cluster is partially saturated with pre-existing hydrogens adsorbed on its sides. The energy barrier for the subsequent migration of atomic hydrogen, from the sides of the cluster towards the carbon support, is however not affected significantly by the presence of pre-existing hydrogens. Overall, the transport of atomic hydrogen from the tip of the cluster towards the carbon support is energetically favourable and only involves low energy barriers. The associative desorption of two hydrogen atoms from the cluster to form molecular hydrogen is an endothermic process and also involves a small energy barrier. This work sheds light onto the mechanism of the hydrogen interaction with Pd-doped-carbon-supported catalytic materials and, more generally, onto the mechanism of hydrogen spillover on such materials.

Dissociative Adsorption of Hydrogen Molecule on Aluminum Clusters: Effect of Charge and Doping

The dissociative chemisorption of molecular hydrogen on charged and neutral aluminum clusters Al12X (X = Mg, Al, Si) was investigated using DFT and a modified G3(MP2)-RAD procedure. Reaction barriers and enthalpies were determined for both neutral and singly charged clusters. The lowest barrier for dissociative adsorption of H2 on a neutral cluster was found for the Al12Mg cluster, whereas the highest barrier was found to be on the closed-shell Al12Si. The interaction of H2 with Al13(+) is found to proceed via an association complex that is 0.07 eV lower in energy than the isolated species and from which the barrier to H2 dissociative adsorption is only 0.16 eV. The most exothermic reaction of H2 with Al12X occurs for the Al13(+)/H2 system. In comparison, reactions with the closed-shell Al13(-) and Al12Si clusters are found to be endothermic. The barriers for H2 desorption from the dihydrogenated clusters are generally quite substantial.

On the Sequential Hydrogen Dissociative Chemisorption on Small Platinum Clusters: A Density Functional Theory Study

We present a simple cluster model to understand the dissociative chemisorption of molecular hydrogen and the desorption of atomic hydrogen on platinum small clusters using the gradient-corrected density functional theory. Successive H2 decomposition and sequential H desorption on the selected Ptn (n = 2−5, 7−9) clusters were systematically studied, and the H2 dissociative chemisorption energies and the H desorption energies at the full H saturation were identified. The reaction processes are driven by charge transfer from Pt atoms to H atoms assisted by strong orbital overlaps between Pt 5d orbitals and H 1s orbital, which leads to electron delocalization in large clusters of metal hydrides. It was found that the number of H atoms chemisorbed on the small Pt clusters increases almost linearly with the size of the selected Pt cluster.

Atomic Hydrogen Diffusion in Novel Magnesium Nanostructures: The Impact of Incorporated Subsurface Carbon Atoms

Ab initio Density Functional Theory (DFT) calculations are performed to study the diffusion of atomic hydrogen on a Mg(0001) surface and their migration into the subsurface layers. A carbon atom located initially on a Mg(0001) surface can migrate into the sub-surface layer and occupy a fcc site, with charge transfer to the C atom from neighboring Mg atoms. The cluster of postively charged Mg atoms surrounding a sub-surface C is then shown to facilitate the dissociative chemisorption of molecular hydrogen on the Mg(0001) surface, and the surface migration and subsequent diffusion into the subsurface of atomic hydrogen. This helps rationalize the experimentally-observed improvement in absorption kinetics of H2 when graphite or single walled carbon nanotubes (SWCNT) are introduced into the Mg powder during ball milling.

DRIFTS study of the nature and chemical reactivity of gallium ions in Ga/ZSM-5: II. Oxidation of reduced Ga species in ZSM-5 by nitrous oxide or water

The oxidation of reduced Ga species in ZSM-5 zeolite modified via anchoring of trimethyl gallium or by incipient wetness impregnation with a solution of gallium nitrate was studied. Reduction of such materials in hydrogen and evacuation at high temperature resulted in quantitative replacement of the Brønsted acidic protons by Ga + ions. However, the extent of dealumination of the material prepared by incipient wetness impregnation was higher. Oxidation of univalent gallium species by nitrous oxide at 673 K by either preparation method led to the quantitative formation of charge-compensating [Ga 3+(O 2−)] + fragments. Ga + ions can also be oxidized at 573 K by water vapor, whereupon approximately 25% of the Ga + ions are transformed into [GaO] + with concomitant release of molecular hydrogen. Dissociative chemisorption of molecular hydrogen via oxidative addition of hydrogen atoms to Ga + cations at 773 K resulted in the formation of gallium dihydride species, which are very stable and are decomposed only in vacuum above 673 K. Oxidation of such [Ga 3+(H −) 2] + cations by N 2O at 573 K gave positively charged [Ga 3+(H −)(OH) −] + species. At higher temperature (673 K), the latter are further oxidized by nitrous oxide, giving neutral GaOOH species and Brønsted acidic protons. The similarity of infrared spectra of ethane adsorbed on Ga/ZSM-5 and HZSM-5 at 298 K indicates weak polarization of ethane molecules by Ga + cations. However, oxidative addition of ethane at 523 K led to the formation of [Ga 3+(H −)(C 2 H 5 − )] + species. Above 573 K the grafted ethyl fragments decomposed to gallium hydride species and ethylene. Indications were also found for subsequent oligomerization and aromatization of the resulting olefin.

A theoretical study of the chemisorption of molecular hydrogen on a seven atom gold cluster

The dissociative chemisorption of molecular hydrogen on the gold surface, modeled by a seven atom cluster, has been studied using explicitly correlated wave functions. The spin-orbit effect on the barrier was calculated using the microscopic spin-orbit Hamiltonian. The reaction is found to be endothermic by about 33kcal/mol with a barrier to dissociative chemisorption of about 45kcal/mol. The barrier to associative desorption is estimated to be of the order of 12kcal/mol. The spin-orbit effect on the barrier was 1.2kcal/mol and estimated to be insignificant for the infinite surface.

Reactions of gas-phase atomic hydrogen and deuterium with chemically modified Ir(111) surfaces

The reactions of gas-phase atomic hydrogen (H(g)) and deuterium (D(g)) with the chemically modified Ir(111)–p(1×2)-O, and deuterium (D(a)) and hydrogen (H(a)) adatom precovered Ir(111) surfaces at 100 K have been studied using thermal desorption mass spectrometry. Although the Ir(111)–p(1×2)-O surface at a temperature of 100 K is passivated with respect to adsorption of gas-phase H2 and D2, the interaction of H(g) and D(g) with this surface at 100 K results in the subsequent desorption of water in thermal desorption spectra. These results suggest that while the dissociative chemisorption of molecular hydrogen on Ir(111) is precluded by the presence of the (1×2)-O oxygen overlayer, energetically “hot” H(g) reacts readily with this oxygen-modified surface. Moreover, a strong kinetic isotope effect has been observed in the interaction of H(g) and D(g) with D(a) and H(a) precovered Ir(111) surfaces at 100 K. We find that H(a) is more readily replaced by D(g) (abstraction cross section of σD(g)=4.7±0.4×10−16 cm2) than is D(a) by H(g) (σH(g)=2.6±0.2×10−16 cm2). These calculated cross sections assume a unity probability for reaction of H(g) and D(g) with the Ir(111)–p(1×2)-O surface. This observed isotopic difference in abstraction cross sections is consistent with the differences associated with the expected zero point energy of the transition state for the abstraction reaction and differences associated with the zero point energy between H and D adatoms.

Re(de)construction-induced friction signatures of polished polycrystalline diamond films in vacuum and hydrogen

Pin-on-flat SEM tribometry was performed with polished, mostly C(100)-textured and acid-cleaned polycrystalline CVD diamond films heated to 950°C then cooled to room temperature. Testing in~1.33×10-3 Pa = 1× 10-5 Torr vacuum was followed by similar experimentation in 13 to 40 Pa (0.1to 0.3 Torr) partial pressures of 99.999%-pure H2. In vacuum, all tests showed the characteristic step function-with-trough coefficient of friction (COF) signatures previously hypothesized as footprints of wear- and thermal desorption-induced generation, re(de)construction and passivation of the danglingσ bonds on the interacting surfaces. In hydrogen, all wear tracks exhibited stepfunction-like COF curves caused by adsorbate de(re)sorption on heating and cooling. A distinct re(de)construction COF trough obtained at the highest temperatures could be duplicated during repeated sliding in the same track on a large number (but not all) of the wear paths. The repeatable, incremental reduction in COF at the onset of heating and its substantial reduction on final cooling are attributed to tribocatalytically enhanced dissociative chemisorption of molecular hydrogen. The wear rates of the polished diamondon the pin tip, as controlled by the progressively reduced unit stresses caused by the enlargement of the wear scar, are between3.9×10-16 and 2.6×10-16m3/(N m) in PH 2, in good agreement with previous data.

Thermal desorption studies of high-coverage hydrogen overlayers on Ru(001) created with gas-phase atomic hydrogen

The adsorption of gas-phase atomic hydrogen on the Ru(001) surface results in a saturation coverage of 1.42 hydrogen adatoms per primitive surface unit cell, which is significantly greater than the saturation coverage of one hydrogen adatom per primitive unit cell in the case of dissociative chemisorption of molecular hydrogen. Thermal desorption spectra of these high-coverage hydrogen overlayers are characterized by a peak temperature of ∼320 K which shifts downward slightly with increasing coverage beyond one monolayer. An isosteric analysis of the thermal desorption spectra yields a desorption energy that decreases from 29.5 kcal/mol at zero coverage to 22 kcal/mol at a coverage of 1.42.

Direct reaction of gas-phase atomic hydrogen with chemisorbed hydrogen on Ru(001)

The adsorption of gas-phase atomic hydrogen on the Ru(001) surface results in a saturation coverage of 1.42 hydrogen adatoms per primitive surface unit cell, which may be compared with a saturation coverage of one hydrogen adatom per primitive surface unit cell in the case of dissociative chemisorption of molecular hydrogen. The observed saturation fractional coverage of 1.42 results from a steady-state balance of adsorption of gas-phase atomic hydrogen and reaction of gas-phase hydrogen with chemisorbed hydrogen adatoms, which produces molecular hydrogen that desorbs from the surface at a temperature at least 150 K below the temperature of recombinative desorption of two hydrogen adatoms. The cross section of this direct reaction of hydrogen was found to be remarkably large, approximately 40% of the cross section for chemisorption of the gas-phase atomic hydrogen. The reaction was found not to depend on surface temperature nor was there an observable kinetic isotope effect.

Kinetics of hydrogen absorption by chemically modified Pd(110)

The kinetics of hydrogen absorption by Pd(110) have been measured as a function of impurity sulfur coverage on the metal surface. In contrast to results obtained on the clean surface in which the absorption process was found to be rate-limited by the diffusion of atomic hydrogen into the bulk, submonolayer coverages of sulfur cause the kinetics to become surface dominated with the dissociative chemisorption of molecular hydrogen now the rate-limiting step. This change was also evident in the measured energetics with the activation energy lowered from a clean surface value of ∼ 5.4 to ∼ 3.4 kcal mol on a sulfur covered surface. This latter result may reflect an activation barrier for adsorption induced by the presence of the impurity sulfur, chemisorption being nonactivated on a clean surface. The involvement of a subsurface binding site for hydrogen may also explain the observed kinetics.

Dissociative chemisorption of molecular hydrogen on graphite: A mindo/3 study

The dissociative chemisorption of molecular hydrogen on graphite has been studied using a small cluster to represent the graphite surface. Calculations were carried out using the UHF and RHF (including configuration interaction) versions of the MINDO/3 program. Results show that geometry optimization is necessary and that the choice of the reaction coordinate is of major importance. Dissociative chemisorption is predicted and a potential barrier is found. Results are in agreement with the molecular beam experimental data in the literature.

Dissociative chemisorption of molecular hydrogen on graphite: A mindo /3 study

Dissociative chemisorption of molecular hydrogen on graphite: A mindo /3 study

The dissociative chemisorption of molecular hydrogen on lithium clusters

The interaction of a hydrogen molecule with a surface simulated by eight lithium atoms is examined by use of ab initio SCF-MO calculations in an STO-3G basis. Three surface configurations are employed, one in which all eight lithium atoms from a surface plane and the other two configurations contain six lithium atoms in one plane and the remaining two Li are above and below the plane, respectively. Two variables are examined in some detail, namely the distance of the molecule from the surface and the internuclear separation in the molecule. For a planar configuration an activation barrier for dissociative chemisorption of approximately 90 kcal/mole is found for the H 2 molecule approximately 1.4 Å from the surface.

An electron paramagnetic resonance study of the anodic oxidation of hydrogen on platinum activated carbon electrodes

The e.p.r. method is applied to a kinetic study of the hydrogen anodic oxidation. The absence of any paramagnetic intermediates (such as H or H2+) during the H2 adsorption or oxidation, and the enhancement of the electrode e.p.r. spectrum intensity with adsorption time suggest a completely dissociative chemisorption of molecular hydrogen. From this reaction path model, theoretical equations are derived for the current-voltage characteristics, and for the variation of the electrode spectrum intensity with time. The experimental verifications of these equations give the adsorption limiting rate and the overall reaction limiting current. From their proportionality, the diffusion-adsorption step is thought to be rate determining in the hydrogen oxidation reaction.

An electron paramagnetic resonance study of the anodic oxidation of hydrogen on platinum activated carbon electrodes

The e.p.r. method is applied to a kinetic study of the hydrogen anodic oxidation. The absence of any paramagnetic intermediates (such as H or H 2 + ) during the H 2 adsorption or oxidation, and the enhancement of the electrode e.p.r. spectrum intensity with adsorption time suggest a completely dissociative chemisorption of molecular hydrogen. From this reaction path model, theoretical equations are derived for the current-voltage characteristics, and for the variation of the electrode spectrum intensity with time. The experimental verifications of these equations give the adsorption limiting rate and the overall reaction limiting current. From their proportionality, the diffusion-adsorption step is thought to be rate determining in the hydrogen oxidation reaction.