Multiwavelength Raman Spectroscopic Characterization of Alumina-Supported Molybdenum Oxide Prepared by Vapor Deposition

Molybdenum oxide species on alumina have been prepared via gas phase deposition of bis(ethylbenzene) molybdenum with loadings in the range of 0.05–3.1 Mo atoms/nm². These materials have been characterized relative to analogous impregnated samples using UV–Visible diffuse reflectance spectroscopy, UV Raman spectroscopy, and visible Raman spectroscopy. The presence and distribution of monomolybdates, two- and three-dimensional polymolybdates, and Al₂(MoO₄)₃ depends on the Mo loading and synthesis technique employed, especially at low loadings. Alternately dosing bis(ethylbenzene) molybdenum and water generates uniform polymolybdates at all loadings. In contrast, pulsing the precursor into the reactor without intervening water leads to monomolybdates at low loadings and polymolybdates at high loadings. The ability to influence the distribution of surface molybdenum oxide species can facilitate the rational design of molybdenum based catalysts.