Surface-Analytical Studies of Supported Vanadium Oxide Monolayer Catalysts

• Published in: The journal of physical chemistry. B Vol. 108; no. 15; pp. 4823 - 4830
• Main Authors: Briand, Laura E, Tkachenko, Olga P, Guraya, Monica, Gao, Xingtao, Wachs, Israel E, Grünert, Wolfgang
• Format: Journal Article
• Language: English
• Published: American Chemical Society 15.04.2004
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp037675j

Supported vanadium oxide catalysts, consisting of surface vanadia species on Al2O3, ZrO2, CeO2, and Nb2O5 oxide supports, were investigated by X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) to elucidate the effect of calcination treatments as well as exposures to (nonmonochromatized) X-rays and He ions on the surface properties. It was found that calcination in air at 730 K of samples that had been previously calcined in air at 773 K and exposed to ambient atmosphere results in significant increases of the V intensity relative to the support signal both in XPS and ISS. This indicates that the surface vanadia species aggregate under the influence of moisture, but spread during calcination. The surface V(V) species were reduced to V(IV) upon extended exposure to X-rays of a nonmonochromatized source, which was accompanied by clustering as detected by ISS. Following a new methodology that avoids these effects by studying freshly calcined samples transferred without exposure to ambient atmosphere, without previous illumination by X-rays, and takes account of the abrasive effect of He ions by extrapolating the results of sputter series, it was found that in supported V2O5/ZrO2, V2O5/CeO2, and V2O5/Nb2O5 catalysts possessing a vanadia monolayer coverage or above, the supports are densely covered by two-dimensional surface vanadia species, and the underlying oxide support cations of Zr, Ce, or Nb are not exposed. For a supported V2O5/Al2O3 catalyst containing a monolayer surface coverage of vanadia, however, a slight exposure of the oxide support cation (Al3+) was noted, which may originate from the much higher surface area of this support (Al2O3 ≫ Nb2O5, ZrO2, and CeO2) resulting in a higher curvature of the surfaces covered by the supported vanadia species. The current XPS and ISS surface studies confirm that supported vanadium oxide catalysts consist of close-packed monolayers of surface vanadia species.