V. Di Castro

XPS study of MnO oxidation

Abstract The oxidation of a MnO layer has been studied by XPS at 400°C. Pure Mn oxides have been measured for comparison and the Mn oxidation states have been identified by Mn2p binding energy, Mn2p satellite structure, Mn3s multiplet splitting and valence spectra. A progressive oxidation of MnO to Mn2O3 without intermediate formation of Mn3O4 has been observed. Comparison of core and valence spectra seems to indicate that a thin layer of Mn2O3 is initially formed on top of MnO. At higher O2 exposure the MnO is completely oxidized to Mn2O3.

“Copper chromite” Catalysts: XPS structure elucidation and correlation with catalytic activity

Abstract The oxidation state and composition at the surface of “copper chromite” catalysts at various stages of catalytic use and after reductive pretreatment have been studied by means of X-ray photoelectron spectroscopy (XPS). Catalytic activity for double-bond isomerization and for hydrogenation is found to be correlated with the oxidation state of copper at the surface. The XPS data support the hypothesis proposed previously that Cu(I) is the active species for the isomerization reaction, and Cu(O) for the hydrogenation of conjugated dienes.

XPS study of the growth and reactivity of FeMnO thin films

Abstract The formation of the Fe/MnO interface in the initial stages of growth (1–9 A) of an iron film has been studied by XPS at room temperature. The plot of the Fe 2p peak intensities as a function of the iron coverage is characteristic for a layer-by-layer growth, with a layer thickness of 2.7 A. The photoemission spectra suggest an initial oxidation of the deposited iron, followed by the growth of an upper metallic layer. Samples with different iron coverages have been exposed to oxygen: the systems at lower coverage undergo faster and more complete oxidation, while the sample at 9 A Fe coverage shows a behaviour similar to the one reported in the literature for bulk iron.

XPS characterization of the CuO/MnO2 catalyst

Abstract We followed by XPS the changes occuring at the surface of a CuO/MnO2 catalyst during two different reductive pretreatments with H2. The difference in catalytic activity for the hydrogenation reaction between the two pretreated catalysts has been correlated with the surface concentration and oxidation state of copper.

Photoemission study of two model catalysts using synchrotron radiation

Abstract Two model catalyst have been obtained by evaporation of copper on Al film and on Al 2 O 3 and studied by UV photoemission at room temperature. A strong interaction is observed between copper and aluminium, while copper interacts weakly with Al 2 O 3 and seems to form big clusters already at very low coverage. During the oxidation process, an aluminium oxide layer grows up on top of the CuAl interface, which remains unchanged while the copper evaporated on the Al 2 O 3 film is oxidized in the same way as massive copper.

A photoemission study of the Cu/MnO interface

Abstract The electronic structure of the Cu/MnO interface has been studied by UV photoelectron spectroscopy for copper coverages ranging from 1 to 12 A. The MnO substrate was prepared by exposing a freshly evaporated manganese layer to oxygen at room temperature. By monitoring the evolution of the Cu 3d level we present evidence that intermixing takes place at low copper coverages (1–2 A); this causes a shift of the Cu 3d peak towards higher binding energies with respect to the bulk metal. A bulklike copper phase, which does not affect the copper in the intermixed region, forms with increasing copper coverage.

Thin Cu films on aluminium: A photoemission investigation

The initial growth of a Cu film (up to 100 A) on polycrystalline aluminium at room temperature has been studied by photoemission spectroscopy using synchrotron radiation. For low covergages 1–8 A) copper diffusion in the aluminium substrate is observed, whilst at highert coverages copper clusters seem to be formed and increase in size on the surface. A further indication on copper clusters formation comes from the oxidation of a 12 A Cu/Al system which shows formation of an Al 2 O 3 layer on top of the unchanged Cu/Al interface.

Copper growth on Al2O3 and Al: An auger study

The initial growth of a Cu film on Al 2 O 3 and Al has been investigated at room temperature using Auger electron spectroscopy. Analysis of the Auger peak intensity as a function of Cu coverage and comparison of the experimental results with predictions of layer-by-layer and clustering growth models, suggest that the growth of copper takes place by clustering on Al 2 O 3 , whilst evaporated copper intermixes with the aluminium substrate.

Formation of a two-dimensional alloy/surface phase: Auger and STM study of Cu(Sn)(111)

The segregation of Sn in Cu and the formation of a Sn–Cu surface alloy has been studied by Auger electron spectroscopy (AES), reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). By annealing Cu(Sn) samples at T=500°C, we have found a Sn0.33Cu0.66 p(3×3)R30° reconstructed surface layer. By increasing T to 750°C, the tin concentration decreases up to 0.20, while the reconstruction is maintained. Atomic-resolution STM images of this phase show no rippling, but several steps. The surface lattice parameters are 0.47±0.02 and 0.51±0.02 nm, respectively, along and across the steps.

AES and photoemission studies of Cu growth on Cr2O3

Abstract The initial growth (1–20 A) of Cu on Cr 2 O 3 at room temperature has been investigated by Auger Spectroscopy and UV photoemission spectroscopy by synchrotron radiation. Analysis of the Auger peak intensities suggests that copper is growing in a layer-by-layer mode probably with the simultaneous formation of the second and third layers. The photoemission spectra show that deposited copper grows steadily as a single copper species which does not interact strongly with the substrate.