Effect Of Hydrogen Chemisorption Research Articles

Effects of hydrogen chemisorption on the structure and deformation of single-walled carbon nanotubes

We report results of molecular-dynamics simulations for the effects of atomic hydrogen chemisorption on the structure and deformation of single-walled carbon nanotubes. Upon hydrogenation, the nanotubes expand and the degree of expansion depends on the hydrogen coverage. There is a critical hydrogen coverage, ∼25%–30%, that marks the onset of a structural transition associated with the sp2-to-sp3 bonding transition: at lower-than-critical coverage, sp2 C–C bonding dominates and nanotube swelling is negligible; at higher-than-critical coverage, however, sp3 C–C bonding dominates and radial and axial strains increase monotonically with coverage. This behavior is independent of nanotube chirality and diameter and of temperature.

The structure of the diamond (111) surface — a SLAB-MINDO study

In this paper we have investigated the reconstruction of the diamond (111) surface via the SLAB-MINDO method. Several π-bonded chain models which have been proposed in the literature have been examined. Since the diamond (111) surface actually shows a 2 × 2 LEED pattern, and the cleaved surface is hydrogen terminated, we have also studied the stability of the 2 × 2 surface unit cell, the triple bond cleavage surface and the effect of hydrogen chemisorption. For the single bond cleavage surface we predict that both a dimerized and non-dimerized π-bonded chain model topology will occur. For the triple bond cleavage surface we find that a non-dimerized single chain model with a small degree of buckling has the lowest energy. Calculations employing 2 × 2 surface unit cells to represent both single and triple bond cleavage surfaces always reverted to a 2 × 1 topology. Thus we have found no evidence for a 2 × 2 surface symmetry as an explanation of the 2 × 2 LEED pattern. The single and triple bond cleavage energies for the clean diamond (111) surface have been calculated and a single bond cleavage mechanism determined to be more energetically favored. When hydrogen chemisorption is included into the cleavage process, however, we find that triple bond cleavage of this particular surface may in fact occur.

The structure of the diamond (111) surface—a SLAB-MINDO study

Abstract In this paper we have investigated the reconstruction of the diamond (111) surface via the SLAB-MINDO method. Several π-bonded chain models which have been proposed in the literature have been examined. Since the diamond (111) surface actually shows a 2 × 2 LEED pattern, and the cleaved surface is hydrogen terminated, we have also studied the stability of the 2 × 2 surface unit cell, the triple bond cleavage surface and the effect of hydrogen chemisorption. For the single bond cleavage surface we predict that both a dimerized and non-dimerized π-bonded chain model topology will occur. For the triple bond cleavage surface we find that a non-dimerized single chain model with a small degree of buckling has the lowest energy. Calculations employing 2 × 2 surface unit cells to represent both single and triple bond cleavage surfaces always reverted to a 2 × 1 topology. Thus we have found no evidence for a 2 × 2 surface symmetry as an explanation of the 2 × 2 LEED pattern. The single and triple bond cleavage energies for the clean diamond (111) surface have been calculated and a single bond cleavage mechanism determined to be more energetically favored. When hydrogen chemisorption is included into the cleavage process, however, we find that triple bond cleavage of this particular surface may in fact occur.

The effect of hydrogen chemisorption on titanium surface bonding

The effect of hydrogen chemisorption on the strength of Ti-Ti bonds is studied byab initio configuration interaction techniques using an embedding theory to describe the electronic structure. A Ti adatom on Ti(0001) is modelled by a Ti20H cluster with boundary potentials determined from the embedding treatment. Hydrogen atom chemisorption is highly exothermic for adsorption atop the adatom, a three-fold site formed by the adatom and in the interstitial site below the adatom. Compared to the planar Ti(0001) surface the adatom region binds hydrogen much more strongly. Removal of Ti from the surface is energetically much more favorable if H remains on the surface as opposed to the removal of TiH. The exchange reaction Ti20+H→Ti19H+Ti is endothermic by 0.3 eV. These results suggest high reactivity of the adatom region on Ti(0001) but not such that the surface is more easily fragmented by removal of Ti or TiH.

Effect of hydrogen chemisorption on the photoionization threshold of isolated transition metal clusters

Large increases in the photoionization threshold energies of small V x , NB x , and Fe x clusters ( x = 3–25) induced by chemisorption of H 2 have been observed using photoionization time-of-flight mass spectrometry of a molecular beam. These shifts exhibit a definite dependence both on the number of atoms constituting the bare metal cluster and on the number of chemisorbed hydrogens, and are particularly large (≳ 0.8 eV) for multiple-H 2 chemisorption on small clusters. A simple frontier orbital model for the chemisorption process predicts the direction of adsorbate-induced shifts in cluster ionization threshold for both H 2 and NH 3 as adsorbates.

Effect of hydrogen chemisorption on the magnetism of nickel clusters

AbstractResults of self‐consistent field, local spin density, scattered wave calculations are reported for nickel clusters of 10, 13, and 14 atoms and these clusters interacting with one or two chemisorbed hydrogen atoms. The pure nickel clusters all have a reasonable average atomic magnetic moment (the average over all the clusters is 0.66µB) and the addition of hydrogen reduces this moment in each case. The reduction of magnetic moment is clearly larger on the nickel atoms that are nearest to hydrogen but there is also a noticeable change in the moments of the other atoms of the clusters. Three factors, of varying importance for the different clusters, contribute to the changes in the overall and local magnetic moments: (i) The extra electron brought in with the hydrogen goes into a down‐spin Ni d level, reducing the moment. (ii) The reelectron duced moment is accompained by a reduced exchange splitting and consequently some up‐spin d electrons, not directly involved in the bonding to hydrogen, are transferred to lower lying down‐spin d orbitals. (iii) For atoms close to the adsorbate, d character in the local density of states is pushed above the Fermi level through antibonding interactions with the hydrogen, further reducing the moments of these atoms.

The effect of hydrogen chemisorption on the conductivity of evaporated aluminum films

Very thin Al films (<10 nm) were prepared by thermal evaporation on electroded glass substrates in an ultrahigh vacuum system. TEM studies revealed that the films had a discontinuous structure consisting of imperfectly touching crystallites. Since hydrogen adsorption should not decrease electron specular reflection, entirely diffuse scattering of the conduction electrons at the film surface was assumed. At room temperature, hydrogen adsorption caused a decrease in film conductivity and equilibrium was reached rapidly. At temperatures above 80 °C, hydrogen adsorption caused an increase in film conductivity with a slow (16 min at 80 °C) attainment of equilibrium. The results indicate an activated dissociative process which occurs via a precursor state at the higher temperatures. At lower temperatures, an electronegative layer is formed: electrons are withdrawn from the Al film and the conductivity decreases. These results are in agreement with the observation that adsorbates often undergo both dissociative and nondissociated chemisorption on the same metal surface with the former being favored at higher temperatures.

The effect of hydrogen chemisorption on GaAs(100) and GaAs(1̄1̄1̄)

The interaction of hydrogen with the polar (100) and (1̄1̄1̄) surfaces of GaAs has been studied with LEED, angle-resolved photoemission and core level spectroscopy. It was found that the properties of the hydrogen-covered surface were independent of the composition of the initial surface. The core levels also showed an increase in the surface As concentration for initially Ga-rich surfaces. Angle-resolved photoemission results for GaAs(100) and GaAs(100):H are presented and the dispersion of a hydrogen-induced state is shown.

New low-energy-electron-diffraction fine structure on W(110) and effect of hydrogen chemisorption

New low-energy-electron-diffraction fine structure on W(110) and effect of hydrogen chemisorption

Effect of hydrogen chemisorption on the electrical conductivity of zinc oxide powder

Effect of hydrogen chemisorption on the electrical conductivity of zinc oxide powder

The effects of hydrogen and oxygen chemisorption on the ferromagnetic anisotropy of nickel-on-silica particles

After a consideration of the results so far obtained on the effects which oxygen and hydrogen chemisorption have on the ferromagnetism of nickel, it is shown that investigation of the effect of adsorption on the ferromagnetic anisotropy of nickel can be a considerable aid in forming an understanding of the chemisorptive bond on nickel. Then follows a brief discussion on the origin of ferromagnetic anisotropy and on the setup of experiments furnishing information about the effect of chemisorption on the ferromagnetic anisotropy. Experiments are presented in which the effect of hydrogen chemisorption on the magnetization of nickel-on-silica particles is determined by measurements in fields alternating with frequencies of 1 to 20 kc/sec and at temperatures from 77° to 373°K. It is demonstrated that the experimental results can be accounted for to a high degree. To permit a reliable comparison to be made between the effects of hydrogen and oxygen, the distribution of oxygen over the nickel particles in the samples must be equal to that of hydrogen. This has been achieved by deriving the oxygen from nitrous oxide. It appears that hydrogen chemisorption decreases the anisotropy of nickel particles up to about 50 A in diameter much more than does oxygen chemisorption. This suggests that the anisotropy is affected by a surface layer which is modified differently by hydrogen adsorption than by oxygen adsorption. Measurements on larger nickel particles (diameters about 70 A) demonstrate that, in a relative sense, hydrogen adsorption has a much smaller effect on the anisotropy of these particles. The cause of this difference, which is related to the size of the nickel particles, is discussed. A detailed analysis of the variation of the effect of oxygen adsorption with the frequency of the magnetizing field shows that oxygen adsorption decreases the anisotropy of larger nickel particles at low coverages only. At higher coverage, no further decrease in anisotropy is observed. Finally, an explanation is developed for the difference between the effects of hydrogen and oxygen adsorption on the anisotropy, as well as for the absence of a further decreasse in anisotropy energy on oxygen adsorption at higher coverages on larger nickel particles.

The effect of chemisorption on collective metal properties

From a discussion of the effects of chemisorption on electrical conductance, ferromagnetic moment and ferromagnetic anisotropy of metals, conclusions are drawn about the state of the metal surface atoms after chemisorption. Hydrogen chemisorption resembles alloying of the surface layer of the metal; the state of the metal surface atoms is not appreciably changed. Chemisorption of gases such as oxygen, carbon monoxide and nitrogen substantially weakens the binding of the surface atoms to the remainder of the metal. However, as appears from the effect of hydrogen chemisorption on the ferro-magnetic moment, the influence of hydrogen on very small nickel particles is much stronger, which is ascribed to the lower degree of coordination of the metal atoms in these particles. The decrease in ferromagnetic moment brought about by chemisorption points to a decrease in ferromagnetic coupling energy to a value smaller than the thermal energy, which converts the metal surface atoms generally into a paramagnetic state. However, there remains an interaction between the metal surface atoms and the remainder as appears from the effects of chemisorption on the ferromagnetic anisotropy. Effects of chemisorption on the anisotropy of small nickel particles, of evaporated metal films and on the spin wave resonance properties of evaporated films are discussed. Owing to the coupling of the metal subsurface atoms to the surface atoms that are paramagnetic after chemisorption, the anisotropy decreases on chemisorption. When, however, an anti-ferromagnetic surface phase of appreciable size is formed, as happens on oxidation, no decrease in anisotropy is found, since in that case the metal atoms have their atomic moments coupled to moments the orientation of which remains fixed.