Polycrystalline Ni Surfaces Research Articles

Coadsorption of chlorine and oxygen on nickel: inhibitor effects

The adsorption of chlorine and the coadsorption of chlorine with oxygen on Ni(110) and a polycrystalline Ni surface have been studied at 300 K with XPS, UPS, AES and work function change measurements. Chlorine adsorbs dissociatively and remains chemisorbed on the surface in contrast to pure oxygen adsorption, which includes incorporation. Preadsorbed chlorine totally inhibits additional oxygen adsorption, while, vice versa, preadsorbed oxygen has hardly any effect on additional chlorine uptake. The results are discussed in terms of the ionic radii of chlorine and oxygen, the activation energy of penetration, and the heats of formation of nickel chloride and oxide, respectively.

Sputtering of Ni+ from polycrystalline Ni with adsorbed hydrogen or oxygen: A secondary ion mass spectrometry study

A study of the emission of the secondary ions of Ni+ during the secondary ion mass spectrometry (SIMS) analysis of a polycrystalline nickel surface (at room temperature) covered with up to a few monolayers of oxygen or hydrogen was undertaken. A comparison was made between the experimental data and the calculated predictions using the electron tunneling model (ETM). The results showed that the ETM can successfully predict the change in emission of Ni+ secondary ions from a polycrystalline Ni surface covered with up to a monolayer of hydrogen or oxygen. However, ETM predictions did not fit well with the behavior of the Ni+ secondary ions beyond one monolayer coverage of hydrogen or oxygen. In this paper a modification of ETM is described that successfully explains the emission of Ni+ secondary ions from a polycrystalline Ni surface for coverages beyond one monolayer of hydrogen or oxygen.

The coverage dependence of the heat of adsorption and the predicted wavelength of surface luminescence

To obtain the expression for the differential heat of adsorption, an adsorbed molecule or atom is approximated as a harmonic oscillator and its potential energy is allowed to be a function of coverage. Statistical thermodynamics is then used to obtain the expression for the differential heat and it is found to depend on molecular and surface properties. The value of the molecular properties are known and it is found that the surface properties can be evaluated from the electron energy loss spectrum, the equilibrium adsorption isotherms and the measured number of adsorption sites for a particular interphase. Once these have been obtained, a complete prediction (in the sense that there are no fitting parameters) of the differential heat of adsorption can be made. This heat has been evaluated for CO adsorbing on Ni(111) and on Ni(100) and for H 2 adsorbing on these surfaces as well. The results are found to give a complete explanation of the previously reported measurements of the differential heat of adsorption for each of these gases when either adsorbs on a polycrystalline surface that consists primarily of these two crystal planes. As part of the method, the minimum energy of an adsorbed molecule or atom is evaluated. This allows a new type of adsorption energy diagram to be drawn from which the possible transitions can be identified. One of these transitions on a polycrystalline Ni surface is predicted to be capable to leading to luminescence with a wavelength of between 3.3 to approximately 5.5 micrometer m, depending on coverage. This predicted wavelength agrees well with that observed experimentally by others.

Nitrogen adsorbed on clean and promoted Ni surfaces

The temperature dependence of the N(1s) spectrum of N 2 adsorbed on a polycrystalline Ni surface suggests the presence of two species. N 2 adsorbed on Ba- or Al-promoted Ni is more strongly bound to the surface as evidenced by the decrease in the intensity of the N(1s) signal due to the unscreened final state relative to the screened final state as well as an increased separation between the 5 σ and 4 σ levels; intra-molecular bonding on promoted surfaces is weaker as also found by EELS studies. Electron-withdrawing modifiers, on the other hand, decrease the strength of the metal-nitrogen bonding.

Nitrogen adsorbed on clean and promoted Ni surfaces

The temperature dependence of the N(1s) spectrum of N 2 adsorbed on a polycrystalline Ni surface suggests the presence of two species. N 2 adsorbed on Ba- or Al-promoted Ni is more strongly bound to the surface as evidenced by the decrease in the intensity of the N(1s) signal due to the unscreened final state relative to the screened final state as well as an increased separation between the 5σ and 4σ levels; intra-molecular bonding on promoted surfaces is weaker as also found by EELS studies. Electron-withdrawing modifiers, on the other hand, decrease the strength of the metal-nitrogen bonding.

The influence of chlorine on the catalytic decomposition of methanol on nickel foil: Transition from poison to promoter upon modification of deposit

The catalytic activity of a polycrystalline Ni surface with respect to the decomposition of CH3OH is completely blocked by chlorine adsorbed as Clads. CH3OH adsorbs molecularly but desorbs without any trace of a chemical reaction. After Ar sputtering at 600 K and further heating at 700 K, Clads is removed and the remaining chlorine is transformed into a nickel chloride. This comparatively small amount of nickel chloride is a promoter for the decomposition of CH3OH. The reaction occurs according to the dissociative mechanism; CH3Oads dissociates to form CH3ads and Oads, while COads, generated by dehydrogenation, dissociates to form Cads and Oads. Under equal conditions these reactions are not possible on a clean Ni surface.

Comparison of Low-Energy Neutral Scattering (LENS) with Low-Energy Ion Scattering (LEIS) at clean and adsorbate covered Ni surfaces

Ne and He projectiles were scattered at clean and sulphur or oxygen-covered polycrystalline Ni surfaces. The analysis of backscattered neutrals results in an oxygen to nickel surface atom density of 0.7 for NiO and a sulphur to nickel density of 0.25 for a saturation coverage of sulphur on polycrystalline nickel, which is in good accord with expectation. Only differential scattering cross-sections, which can be calculated with sufficient reliability enter the data reduction. This is a definite advantage over ion scattering which is complicated by the widely unknown neutralization probabilities.

Comparison of low energy ion scattering (LEIS) with low energy neutral scattering (LENS) at a clean and sulphur covered polycrystalline Ni-surface

The analysis of backscattered Ne-neutrals from a sulphur covered polycrystalline nickel surface results in a sulphur to nickel surface atom density of 0.25, independent of the kinetic energy of the impinging ion. On the other hand, the ion spectra changed drastically, which can be explained by a local neutralization model.

Auger Electron Appearance Potential Spectrum of Ni

Electrons are accelerated onto a solid polycrystalline Ni surface. These primary electrons interact with the surface to produce one of two phenomena: x-ray flourescence, or Auger electron emission. Auger Electron Appearance Potential Spectroscopy (AEAPS) is a process by which the Auger component of the secondary electron flux is analysed to extract qualitative information about the electronic structure of the empty conduction band states. In AEAPS, the threshold behavior of the Auger transitions is examined by taking the first derivative of the secondary electron current with respect to the incident electron energy using the potential modulation technique. Width of the empty band, degree of localization of the conduction band wave functions, overlapping of the 3d band with the 4s, and satellite phenomena are among the important parameters ascertainable by AEAPS. The present AEAPS measurements of L3,2 levels of Ni are compared with the reported data from Soft X-ray Appear nce Potential Spectroscopy (SXAPS) of Ni. SXAPS is a complimentary process of AEAPS in that it examines the secondary x-ray fluorescence from the surface in much the same way as AEAPS examines the total electron yield. AEAPS, used alone, or in conjunction with SXAPS, is considered to be a powerful technique for the systematic investgation of series of metals, such as the transition, or lanthanide series.

Investigation of multi-component surface reactions by SIMS: The interaction between hydrogen and oxygen on polycrystalline nickel

Based on corresponding studies of H 2 and O 2 interaction with polycrystalline nickel, SIMS and TDMS investigations of O 2 and H 2 interaction with hydrogen, respectively oxygen covered polycrystalline Ni surfaces were carried out. O 2 exposure of a hydrogen covered surface only results in a removal of the hydrogen adsorption layer accompanied by the usual oxygen incorporation into the surface. H 2 exposure of nickel surfaces covered by various amounts of oxygen however results in the formation of OH groups on the surface with different binding energies. Heating of these hydroxide covered surfaces results in the disappearance of OH groups and simultaneous desorption of H 2O molecules.

Angular distributions of auger electron emission: Clean and gas-adsorbed polycrystalline Ni surfaces

The excitation angle (β) and emission angle (θ) dependences of the Ni M 2, 3VV (61 eV) and Ni L 3VV (850 eV) Auger emissions from clean polycrystalline Ni surfaces, and the S L 2, 3 M 2, 3M 2, 3 (150 eV) Auger emission from S-adsorbed poly-Ni surfaces have been investigated. In the case of Ni (61 eV) and S Auger emissions, the β-dependence shows the 1 cos β distribution, while a significant deviation from 1 cos β is observed for Ni (850 eV) Auger emission. The cos θ distribution and the intermediate between isotropic and cos θ distributions are observed for Ni (61 eV), and for Ni (850 eV) and S Auger emissions, respectively. Those results have been found to be in fairly good agreement with the calculations based on the simple continuum model without consideration of the diffraction effect and the inherent anisotropic emission.

Mean adsorption lifetimes of cesium chloride on nickel surfaces

A pulsed-molecular-beam method was applied to the study of the mean adsorption lifetimes of adsorbed CsCl on single crystal and polycrystalline nickel surfaces. The experiments were conducted in an ultra-high vacuum system free of hydrocarbon contaminates and with pressures less than 2 x 10/sup -8/ Torr. A molecular beam of CsCl was formed from species effusing from a double-oven effusion cell equipped with a conical orifice. A rotating chopper served to pulse the molecular beam; the CsCl molecular beam was on for 10.7 percent of the period of the cycle. The molecular beam struck a nickel target inclined at a 45/sup 0/ angle. Four targets were used in these experiments: Ni(111) surface, Ni(100) surface, clean polycrystalline Ni surface, and a contaminated polycrystalline Ni surface. The mean adsorption lifetimes were determined by measuring the rate of evaporation of CsCl from the surface by means of a surface ionization detector. The desorption has been found to follow first-order kinetics in cases of single crystal targets for which equilibrium surface coverage of the adsorbate is much less than 0.001 monolayer. The activation enthalpy for the desorption process, ..delta..H*, and the preexponential, tau/sub 0/, are dependent on surface structure and surface cleanliness, but theymore » are not dependent on surface coverage. In an attempt to interpret the present values of tau/sub 0/, a mobile adsorption model must be used.« less