Characteristic features of Raman band shifts of scheelite-type molybdate catalysts exchanged with the O-18 tracer via redox reactions

The oxide oxygen of scheelite-type alpha-MnMoO4, beta-CoMoO4, and alpha-Bi2Mo3O12 were exchanged with the O-18 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 alpha-MnMoO4, the band at 945 cm(-1) corresponding to the shortest Mo=O of a Mo tetrahedron was exchanged preferentially while with beta-CoMoO4, which is isotypic with alpha-MnMoO4, all the oxygen of Mo tetrahedra were exchanged with an increase in the O-18 exchange. With alpha-Bi2Mo3O12 which has two twin Mo tetrahedra, the bands at 865 and 845 cm(-1) were 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. (C) 1996 Academic Press, Inc.