Presence Of Overlayers Research Articles

Suppression of field emission from W(110) and W(111) by Cu overlayers

The total energy distribution and the Fowler-Nordheim preexponential factor have been measured for field emission from the (100) and (111) facets of tungsten in the absence and in the presence of overlayers of chemisorbed copper. It is found that at close to monolayer coverage on the (100) facet the adsorbate dramatically reduces the surface density of states at the Fermi energy, whereas on the (111) facet the surface density of states at the Fermi energy is little affected. Self-consistent, semirelativistic LMTO calculations of the k-resolved layer densities of states at the centre of the surface Brillouin zone have been carried out for a tungsten-vacuum interface with and without an ordered overlayer of copper. According to the calculations, a copper monolayer on W(100) strongly suppresses the surface density of states at the Fermi energy, whereas for a copper monolayer on W(111) no such suppression is predicted. The consistency of these results with the experimental data indicates the promise of the present method for calculating the electronic structures of metal-vacuum interfaces.

Effect of Zn substitutions in YBa2Cu3O6 +x phases on reactivity during the ammoxidation of toluene

Zn substitution in Y–Ba–Cu–O phases influences the catalytic ammoxidation of toluene. The activity for formation of both benzonitrile and CO2 at low and high O2 pressures, respectively, decreases strongly with increasing Zn content up to 4%. Above this substitution level, the activity, however, increases slightly. Characterisations of the surface by XPS and of the bulk by XRD analysis, indicate the presence of overlayers on the catalysts. In these overlayers, irrespective of the Zn content of catalysts, CuI states predominate at low O2 pressure and CuII at high pressure. These states are associated with selective and non-selective properties, respectively. The activity of the catalysts shows a general tendency to increase with surface copper concentrations as found from XPS studies. However, strong deviations from linearity indicate that the activity of the overlayer is greatly influenced by the bulk acting as a support.

Structure and Reactivity of RH in the SMSI State

AbstractHydrogenolysis of n-butane and CO hydrogenation were used to charac-terize the behavior of TiO2supported Rh in the SMSI state. The Rh was supported on a model support consisting of nonporous spherical particles of TiO2. The simple geometry of the support permits the metal crystallites to be imaged ‘edge-on’ in a transmission electron microscope. This allows detailed examination of the metal surface and the metal-oxide interface as a function of pretreatment. After high temperature reduction in H2, there is no significant change in the morphology of the Rh crystallites. However, the presence of amorphous overlayers 0.2–0.4 nm thick is clearly evident on the Rh surface. These overlayers can only be partially removed after oxidation at 473 K. The presence of these overlayers is well correlated with the drop in hydrogen chemisorption and altered reactivity. While the butane hydrogenolysis activity of Rh is considerably suppressed in the SMSI state, the CO hydrogenation activity is increased by a factor of 3. The H2uptake and the butane hydrogenolysis activity can be only partially restored after oxidation at 473 K. Treatment at 773 K in oxygen is necessary for restoring the pre-SMSI behavior.

Angular dependence of auger electron emission from Si and Cu(100) surfaces in the presence of overlayers

New experimental results are presented for the angular dependence of the emission of Si L 2, 3VV Auger electrons at 92 eV from Si(100) (2 × 1) and Si(100) (1 × 1)H surfaces, and of the emission of Cu M 2, 3VV Auger electrons at 63 eV from a Cu(100) (√2 ×√2)R45°O surface. Angular structure for the Si surfaces is weak compared to that from Cu and other metals, and the (2 × 1) structure shows weaker effects than the (1 × 1)H surface. These differences can at least partially be attributed to crystallographic effects. The only effect of oxygen adsorption on the Cu Auger emission from the Cu(100) surface seems to be to weaken the clean surface angular structure, which is probably attributable to elastic scattering in the overlayer. The results support the view that the angular dependence of substrate Auger emission does not provide a useful mechanism for investigating surface reconstruction and adsorption.