Raman band shifts of γ-Bi2MoO6 and α-Bi2Mo3O12 exchanged with 18O tracer at active sites for reoxidation

Abstract

Catalysts in which small amounts of crystalline γ-Bi₂MoO₆ and α-Bi₂Mo₃O₁₂ are present on supports have been prepared. The oxide oxygens of these samples were exchanged with ¹⁸O tracer and the Raman spectrum shifts of the γ and α phases were examined. With the γ phase, all bands were shifted to lower frequencies, with increasing ¹⁸O content, in the range 700–900 cm^–1. With the α phase, the tow bands at 860 and 845 cm^–1 were shifted preferentially to lower frequencies and other bands at ca. 900 cm^–1 were not shifted so much initially. Applying the correlation between Raman band position and Mo—O distance, the lattice oxygens, as oxidation sites, and the vacancies, as reoxidation sites, were discussed. With the α phase, the bands that were shifted correspond to the lattice oxygens of Mo tetrahedra with adjacent Bi ions, while the bands that were not shifted very much correspond to those situated next to Bi ion vacancies. In the catalytic oxidation of propene on the α phase, oxygen uptake also occurs at the terminal oxygen vacancies, corresponding to the bands at ca. 900 cm^–1.