Characteristic Features of Raman Band Shifts of Scheelite-Type Molybdate Catalysts Exchanged with the18O Tracer via Redox Reactions

Abstract

The oxide oxygen of scheelite-type α-MnMoO4, β-CoMoO4, and α-Bi2Mo3O12were exchanged with the18O tracer using a reduction–oxidation method. The Raman band shifts in the spectra of these catalysts were examined. Normal coordination analysis and diatomic approximation reported in previous literature have shown that the Raman bands are correlated to the Mo–O stretchings of these molybdate catalysts. With α-MnMoO4, the band at 945 cm−1corresponding to the shortest Mo=O of a Mo tetrahedron was exchanged preferentially while with β-CoMoO4, which is isotypic with α-MnMoO4, all the oxygen of Mo tetrahedra were exchanged with an increase in the18O exchange. With α-Bi2Mo3O12which has two twin Mo tetrahedra, the bands at 865 and 845 cm−1were shifted preferentially. Oxygen insertion seems to take place more selectively on the Mo tetrahedron sites where Bi ions are present. A comparative study and discussions on the preferential exchange in these molybdates and reoxidation sites have been reported here.