A study on BiFeMo oxide catalysts by means of 57Fe Mössbauer spectroscopy and electron microscopy

Abstract

BiFeMo oxide catalysts of various compositions have been investigated using X-ray diffraction, 57Fe Mössbauer measurements, and analytical electron microscopy. When a BiFeMo oxide catalyst with a composition of Bi/Fe/Mo = 3/1/2 is calcined at 523–773 K, two kinds of tetragonal Bi 3FeMo 2O 12 particles are generated, their average diameters being different from each other. Paramagnetic doublets in the Mössbauer spectra of these tetragonal particles are identical to those in an earlier report analyzed as monoclinic Bi 3FeMo 2O 12. In the case of a BiFeMo oxide catalyst of a composition, Bi/Fe/Mo = 1/1/1, calcined at 523–773 K, fine particles 15 nm in diameter are produced along with tetragonal Bi 3FeMo 2O 12 particles. The fine particles are found to be superparamagnetic α-Fe 2O 3 and/or small crystallites of tetragonal Bi 3FeMo 2O 12. During calcination at 823 K, the tetragonal Bi 3FeMo 2O 12 phase decomposed to a mixture of monoclinic Bi 3FeMo 2O 12, Bi 2O 3 MoO 3, Fe 2(MoO 4) 3, and ferromagnetic α-Fe 2O 3. Their particle sizes were from 15 to 50 nm in diameter. The Mössbauer spectrum of the BiFeMoO 6 phase reported by T. Notermann, G. W. Keulks, A. Skliarov, Yu. Maximov, L. Ya. Margolis and O. V. Krylov ( J. Catal. 39, 286 (1975)) was not observed but the pattern detected was interpreted to be the superposition of the spectra of the Bi 3FeMO 2O 12 and α-Fe 2O 3 phases. This was also confirmed by analytical electron microscopy.