The surface properties of vanadium compounds by X-ray photoelectron spectroscopy

The electronic structures of vanadium carbides have been examined using X-ray photoelectron spectroscopy (XPS) before and after Ar sputtering. After Ar ion sputtering for 1 h, there was no significant variation observed in both the surface electronic structure and the amounts of core levels of V 2p, C 1s, and O 1s for vanadium carbides used in the present study. A species with a binding energy of 513.2 +/- 0.2 eV from vanadium carbides was newly observed, and was estimated to be V1.3+ using the plot of the V 2p(3/2) binding energies against the oxidation number. This value indicates that the absolute magnitude of charge transfer from V 3d states to C 2p states is 1.3 electrons per vanadium in vanadium carbides. Among the deconvoluted three carbon peaks, the carbidic carbon peak appeared at 282.4 +/- 0.2 eV and was considered to be due to the photoelectrons ejected from the carbon in vanadium carbide lattice. For all of 1-h sputtered vanadium carbides except for V8C7-4, the average ratio of C-c/Vdelta+ was nearly unity. Using NH3 decomposition reaction, the catalytic activity measurements exhibited that the vanadium carbide with the smallest C-c/Vdelta+ ratio had the highest activity, These results indicate that the most active catalyst was carbon-deficient at surface of the vanadium carbide used in this study. (C) 1999 Elsevier Science B.V. All rights reserved.