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.