Showing papers on "Chemisorption published in 1986"

Effect of incidence kinetic energy and surface coverage on the dissociative chemisorption of oxygen on W(110)

Abstract: The dissociative chemisorption of oxygen on W(110) has been studied using molecular beam techniques. Chemisorption probabilities have been measured as a function of incidence angle, θi, and kinetic energy, Ei, and of surface coverage and temperature. In addition, angular scattering distributions have been measured for a range of conditions and LEED has been used to examine surface structure. The initial (zero coverage limit) sticking probability is found to depend strongly on the incidence energy, scaling with En=Ei cos2 θi. This probability is ∼10% at En =0.1 eV, rising to essentially unity above En =0.4 eV. At half a monolayer coverage of atomic oxygen, the sticking probability is close to zero up to a threshold of ∼0.25 eV, above which it rises to over 50% by 1.3 eV. In most cases, the sticking probability is found to fall roughly linearly with increasing surface coverage. However, a less‐than‐linear fall‐off is observed for En ≥1 eV and for En ≤0.03 eV, the sticking probability actually rises with inc...

Dynamics of the activated dissociative chemisorption of N2 on W(110): a molecular beam study

Abstract: Molecular beam techniques have been used to study the dissociative chemisorption of nitrogen on W(110). Chemisorption probabilities have been measured as a function of incidence angle θi and kinetic energy Ei surface coverage and temperature. In addition, angular scattering distributions have been measured for a range of conditions and LEED has been used to examine surface structure. The initial (zero coverage limit) sticking probability is found to depend strongly on the incidence energy, scaling approximately with Ei, rather than with the velocity component normal to the surface. This probability is ≤3×10−3 for Ei≤30 kJ mol−1, and rises by more than a factor of 100 by ∼100 kJ mol−1, where it levels off at ∼0.35. It is argued that this behavior arises due to a strong chemical interaction prior to the barrier to dissociation. Angular scattering distributions revealed predominately quasispecular scattering with evidence as well for a diffuse component at low energies. The sticking probability falls steadily with increasing surface coverage and a saturation coverage of ∼0.25 atomic ML is observed for Ei∼10 kJ mol−1. At higher incidence kinetic energies, this saturation coverage increases to ∼0.5 ML at 200 kJ mol−1. LEED structures are also reported, corresponding to coverages of 0.25, 0.3, 0.5, and 0.52 ML. The 0.25 and 0.5 ML structures are identified as p(2×2) and c(4×2), respectively, for which structure models are proposed.

On the role of vibrational energy in the activated dissociative chemisorption of methane on tungsten and rhodium

Abstract: The vibrational energy dependence of the dissociative chemisorption probability of CH4 on W(110) is investigated with the use of a seeded supersonic molecular beam. By variation of the beam source temperature and seed gas mixture the degree of vibrational excitation of the incident CH4 molecules can be varied while the incident kinetic energy is held constant. The results are consistent with a model in which all vibrational modes are equally effective and vibrational energy and translational energy are approximately equivalent in promoting this highly activated process. Previous unsuccessful attempts to promote CH4 chemisorption on rhodium via vibrational excitation are consistent with our findings; we are also able to account for previous observations of enhancement of dissociative chemisorption in heated effusive beam experiments.

Carbon Monoxide Adsorption on a Platinum Electrode Studied by Polarization Modulated FT-IR Reflection-Absorption Spectroscopy. II. CO Adsorbed at a Potential in the Hydrogen Region and its Oxidation in Acids.

Abstract: : The carbon monoxide layer on a platinum electrode, which is adsorbed at 0.05 V relative to a normal hydrogen electrode (NHE) in 0.5l5 M sulfuric acid, and its oxidation to carbon dioxide at higher electrode potentials has been studied by both electrochemical and in-situ Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS). Polarization modulated FT-IRRAS was used to measured the vibrational spectra of adsorbed carbon monoxide as well as the evolved CO2 as a function of electrode potential. It is shown that the dominant surface species is linearly adsorbed CO, but that the bridge bonded species is oxidized first at about 0.20 V, giving rise to a decrease in the linear C-O stretching frequency of along with a broadening of the band. Oxidation of the linearly adsorbed CO begins at 0.35 V, producing a further, sharp decrease in the C-O stretching frequency as well as a considerable broadening of the band. It is concluded that the oxidation of the CO adlayer produced at 0.05 V occurs randomly throughout the adlayer, in contrast to oxidation at island edges, which is characteristic of CO adsorbed at 0.4 V. It is proposed that the difference in behavior of these two kinds of adsorbed CO is due to crystallographic modification of the platinum surface surface when the CO is adsorbed at 0.05 V in the hydrogen region which results in a higher density of bridge bonded CO.

Adsorbate ordering processes and infrared spectroscopy: An FT‐IRAS study of N2 chemisorbed on the Ni(110) surface

Abstract: A newly constructed apparatus for Fourier transform‐infrared reflection absorption spectroscopy (FT‐IRAS) studies has been used in a detailed study of the N–N stretching band of N2 weakly chemisorbed on the Ni (110) surface. The high resolution and the high signal‐to‐noise ratios of the spectra allow observation of subtle changes of the band shape which accompany changes in the overlayer structure. The results are discussed with respect to a detailed two‐dimensional phase diagram recently proposed for the N2/Ni (110) system. The IR data clearly reveal that for low coverages nonequilibrium adsorption occurs at 81 K, while equilibrium is attained at 125 K in agreement with the phase diagram. The present results indicate that the incommensurate overlayer structure which forms at the highest coverages is characterized by an intense sharp IR band at 2194 cm−1, a weak shoulder at 2204 cm−1, and a still weaker satellite peak at 2220 cm−1. The IR results for the incommensurate N2 overlayer are compared with model...

Adsorbate ionicity and surface-dipole-moment changes: Cluster-model studies of Cl/Cu(100) and F/Cu(100)

Abstract: A cluster-model study of Cl and F chemisorbed at a fourfold site of Cu(100) shows that the halogen ionicity is essential -1. The interaction and bonding arise, almost entirely, from the Coulomb attraction between the charged halogen and metal and from the polarization of the electrons associated with these units; covalent bonding makes a very small contribution. The polarization dramatically reduces the dipole moment from that given by the unpolarized ionic units. Thus, changes in the work function are not a measure of the adsorbate ionicity.

Dissociative chemisorption dynamics of H2 on Ni and Cu surfaces: Morphology and surface temperature effects

Abstract: A new representation of the potential energy surface (PES) for diatomic molecule–solid surface interactions is presented. It is based upon a delocalized effective medium form (i.e., homogeneous gas analogy) plus empirical two‐body terms and is transferable from face to face of a crystal. We have determined the parameters for H2–Cu and H2–Ni from experimental measurements of H–Cu and H–Ni adsorption energy, frequency, and height and from limited molecular beam scattering data. The PES for H2–Ni(100), Ni(110), Ni(111), and H2–Cu(110), Cu(100) are presented. A dynamical simulation using the classical trajectory‐generalized Langevin equation ‘‘ghost’’ atom formalism is implemented in order to calculate the dissociative sticking probability as a function of incident kinetic energy and surface temperature. The calculated probabilities are in general qualitative agreement with those measured experimentally, and are superior to those based upon the previous many‐body expansion approach [J. Chem. Phys. 84, 485 (19...

Observation of Molecular H 2 Chemisorption on a Nickel Surface

Abstract: We report the observation of molecular ${\mathrm{H}}_{2}$ chemisorption at the edge sites of a stepped Ni (100) surface that is covered with a dense layer of atomic hydrogen. No such state is observed on the flat Ni (100) surface. We suggest that this ${\mathrm{H}}_{2}$ chemisorption state can form stably as a result of residual unfilled $d$ states at these step sites, while the dissociative channel is blocked by the saturated atomic adsorbate layer.

Behavior of surface peroxo species in the photoreactions at titanium dioxide

Abstract: The electrochemical behavior of the surface species photogenerated at a polycrystalline titanium dioxide film electrode in alkaline solution was investigated by means of cyclic voltammetry. The obtained results are consistent with the same surface peroxo titanium species being formed when an illuminated TiO/sub 2/ electrode is subjected to an anodic bias or left in open circuit both in the presence and in the absence of oxygen. The decay of these species after the cutoff of illumination was observed to occur relatively slowly, about half of their initial amount being still detectable after 16 h of electrode immersion in sodium hydroxide solution. It was shown that the cathodic reduction of the surface peroxo species, photogenerated at the TiO/sub 2/ electrode, takes place at distinctly more negative potentials than that of dissolved molecular oxygen and that of both preadsorbed and dissolved hydrogen peroxide. The latter species (present mainly as HO/sub 2//sup -/ ions) were also observed to undergo rapid photooxidation at the TiO/sub 2/ photoanode, competing efficiently for positive holes with OH/sup -/ ions. These findings do not support the earlier postulated involvement of the hydrogen peroxide as an intermediate of the photoreaction leading to oxygen evolution. The pathways for the photooxidationmore » reactions at TiO/sub 2/ are briefly discussed in connection with the proposed mechanism of formation of the surface peroxo titanium species. Their role in controlling the surface electron-hole recombination, particularly at dispersed TiO/sub 2/ photocatalysts, is pointed out.« less