Raman Spectroscopy of Monolayer-Type Catalysts: Supported Molybdenum Oxides

Abstract

Oxides of the group VIb metals (Cr, Mo, W) and oxides of vanadium, rhenium, and niobium supported on a second high-surface-area metal oxide such as Al₂O₃, TiO₂, Si0₂, ZrO₂, and so forth are recognized as industrially important catalysts or catalyst precursors for various reactions [1–11], These materials frequently have been described as so-called monolayer catalysts based on a structural model which assumed spreading of the active oxide over the support surface. These catalysts have been investigated by a variety of techniques, conventional bulk sampling techniques as well as by surface-sensitive electron and ion spectroscopies, in an attempt to elucidate the nature of the catalyst surface species, and to study the coordination environment of the active metal center(s). Electronic spectroscopy gives rise to broad bands and the spectra are less informative than vibrational spectra. In addition, although techniques such as Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) are highly surface sensitive (with typical sampling depths of 1–2 nm), only indirect evidence concerning coordination environment can generally be obtained.