Reactivity and Stability of Ultrathin VOx Films on Pt(111) in Catalytic Methanol Oxidation

The growth of ultrathin layers of VOx (< 12 monolayers) on Pt(111) and the activity of these layers in catalytic methanol oxidation at 10−4 mbar have been studied with low-energy electron diffraction, Auger electron spectroscopy, rate measurements, and with photoemission electron microscopy. Reactive deposition of V in O2 at 670 K obeys a Stranski–Krastanov growth mode with a (√3 × √3)R30° structure representing the limiting case for epitaxial growth of 3D-VOx. The activity of VOx/Pt(111) in catalytic methanol oxidation is very low and no redistribution dynamics is observed lifting the initial spatial homogeneity of the VOx layer. Under reaction conditions, part of the surface vanadium diffuses into the Pt subsurface region. Exposure to O2 causes part of the V to diffuse back to the surface, but only up to one monolayer of VOx can be stabilized in this way at 10−4 mbar.

Surface Compositions of Oxide Supported Bimetallic Catalysts: A Compared Study by High-Sensitivity Low Energy Ion Scattering Spectroscopy and X-Ray Photoemission Spectroscopy

It is well known that there is a critical relationship between the surface composition and catalytic performance for a bimetallic catalyst. However, in most cases, the surface composition is obviously different from that of the bulk. Moreover, the surface is normally reconstructed under reaction conditions. In this personal account, our recent progresses in determining the surface compositions of oxide supported bimetal catalysts by high-sensitivity low energy ion scattering spectroscopy (HS-LEIS) and X-ray photoemission spectroscopy (XPS) are summarized. Phase diagrams of the surface compositions under various conditions as a function of the bulk composition are established and compared. It is found that oxidation induces de-alloying and enrichment of PdO, CuO, SnO₂ on the surface, while H₂ reduction results in re-alloying. The addition of the second component not only modifies the nature of the active site, but also varies the dispersion of the active components. The support effects are discussed. The compared studies reveal that HS-LEIS can achieve a more reliable surface composition for oxide supported catalysts.

New Insights into the Role of Al2 O3 in the Promotion of CuZnAl Catalysts: A Model Study

The Cu/Al₂ O₃ /ZnO(0001)-Zn ternary model catalysts and their binary analogues were prepared and characterized. It was found that Al₂ O₃ grew on the ZnO(0001)-Zn surface by a layer-by-layer model, whereas Cu grew on the ZnO(0001)-Zn surface as two-dimensional clusters up to 0.2 monolayers (ML), and thereafter formed three-dimensional clusters. Because of the layer-by-layer growth of Al₂ O₃ on the ZnO(0001)-Zn, Cu/Al₂ O₃ can be considered without the effect of ZnO. Ternary model catalyst Cu/Al₂ O₃ /ZnO(0001)-Zn, which has all three parts on the surface, was prepared by deposition of Cu on the surface of Al₂ O₃ /ZnO(0001)-Zn. Low-energy ion scattering spectra showed that Cu preferred to locate at the Al₂ O₃ /ZnO interfaces. Compared with Cu/ZnO, the addition of Al₂ O₃ obviously suppressed the reduction of copper oxides and led to a higher concentration of Cu⁺ . The Cu clusters were found to be covered by thin ZnO_x overlayers after reduction of Cu/Al₂ O₃ /ZnO(0001)-Zn by using H₂ . Therefore, the high activity of industrial Cu/ZnO/Al₂ O₃ catalysts may origin from Cu⁺ -rich clusters at the Al₂ O₃ /ZnO interface that are covered by thin ZnO_x overlayers.