Adsorption of hydrogen on nickel single crystal surfaces

Abstract

Adsorption of hydrogen on Ni(111), (100), and (110) surfaces was studied by means of LEED; energy loss spectroscopy, flash desorption, and work function measurements, as well as by the technique of laser-induced thermal desorption. Noticeable variations of the LEED intensities for Ni(111) and (100) indicate the formation of disordered adsorbed layers, whereas with Ni(110) the formation of ``streaked'' diffraction patterns and (at high coverages) of a 1×2 structure were observed. Hydrogen adsorption causes a strong damping of the intensity of the surface plasmon excitation and the appearance of an electron loss peak around 15 eV. Flash desorption experiments revealed for Ni(111) and Ni(100) the existence of β1 and β2 states, the former being only filled after completion of the β2 state. Desorption from the β2 state follows a second-order rate law for Ni(111) and Ni(100), but is of first order with Ni(110). Maximum increases of the work function Δ φ by 0.195, 0.170, and 0.530 eV were observed with Ni(111), (100), and (110), respectively. Adsorption isotherms Δ φ=f(pH2) for Ni(111) and Ni(100) may be closely described up to medium coverages on the basis of a model that assumes second-order desorption and first-order adsorption kinetics. The validity of this model was further proved by measurements of the variation of coverage with time. The adsorption isotherms for Ni(110) have a unique shape indicating the occurrence of two-dimensional condensation due to attractive interactions in the adsorbed layer. Isosteric heats of adsorption at low coverages of 23 kcal/mole for Ni(111) and Ni(100) and of 21.5 kcal/mole for Ni(110) were evaluated. With Ni(110) the onset of condensation is characterized by an increase of Ead by about 2 kcal/mole. A steplike decrease of Ead at medium coverages accompanies the formation of the β1 states.

Effects of adsorbates on charge exchange in Li+ ion scattering from Ni(100)

“…19 The work function increases as hydrogen is adsorbed, but the magnitude of the change is relatively small. The increase of the work function suggests that the adsorbed hydrogen particles are negatively charged, 14 which has also been predicted by calculations 15,16 and shown experimentally. 20 The neutral fractions of the singly scattered Li particles were determined by dividing the integrated area of the ''Neutrals Only'' SSP by that of the ''Total Yield'' SSP.…”

“…Both of these adsorbate species attract electrons from the substrate, and it has been shown in prior work that they are negatively charged. [11][12][13][14][15][16] The neutralization probabilities of the Li projectiles are only minimally affected by hydrogen adsorption, which is consistent with the small work function change. Although adsorbed iodine is overall negatively charged, the surface work function is actually reduced.…”

Surface chemistry and catalysis on well-defined epitaxial iron-oxide layers

“…2.5). Sometimes K>1 and convex curves are observed [108][109][110][111]. Coverage dependent sticking coefficients allow to predict more realistic surface coverages at high gas pressures, when chemisorption via the highly populated precursor states becomes the rate limiting step.…”

Adsorption Site, Adsorption Energy, and Normal Vibration Frequency of H on Ni(100) via Total-Energy Calculations

“…The particular system we have chosen is P(lxl) H/Ni(100) because a variety of experiments that we can compare with: thermal desorption 10 electron energy loss spectroscopy l l , ion-channeling 12 and He scattering 13 have been performed on this system. We have performed calculations to determine the geometry of the adsorbed H, its binding energy, its vibration frequency and the barrier to surface diffusion.…”