Supported vanadium oxide catalysts: Quantitative spectroscopy, preferential adsorption of V^4+/5+, and Al₂O₃ coating of zeolite Y

A series of supported vanadium oxide catalysts were prepared by the incipient wetness method as a function of the support composition (Al₂O₃, SiO₂, and USY), the metal oxide loading (0-1 wt %), and the impregnation salt (vanadyl sulfate and ammonium vanadate). These catalysts have been studied by combined DRS-ESR spectroscopies in order to quantify the amount of V⁴⁺ and V⁵⁺ and to unravel their coordination geometries. These spectroscopic fingerprints have been used to study the preferential adsorption of V^4+/5+ ions on SiO₂, Al₂O₃, and USY. Both V⁴⁺ and V⁵⁺ were preferentially adsorbed on Al₂O₃ and showed a much smaller preference for USY and SiO₂. The observed preference orders are discussed in relation with the properties of the support. In addition, a novel method is proposed to coat the external surface of USY with a thin film of Al₂O₃. The method is based on the deposition of USY with the so-called Keggin ion, [Al₁₃O₄(OH)₂₄(H₂O) ₁₂]⁷⁺, which is too big to enter the USY channels or pores. The obtained Al₂O₃/USY material showed a preferential adsorption of V⁴⁺ onto the Al₂O₃ film, suggesting that this method could be useful for vanadium passivation of FCC catalysts.