In-situ Mössbauer spectroscopic study of iron site evolution in iron and cobalt molybdates catalysts in propene oxidation reaction conditions Original Research Article

In-situ Mössbauer spectroscopy was used to study FexCo1−xMoO4 molybdates and their mixtures with Bi2(MoO4)3 in the 360–415°C range under a flow of a mixture C3H6/O2/N2 = 100/100/560 Torr. Ferrous molybdate FeMoO4 was progressively oxidized to ferric molybdate and ferric oxide. Mixed cobalt and iron molybdates FexCo1−xMoO4 exhibited the following behaviour: (1) for all iron contents, the α phase spectrum diminished to the benefit of the β phase. The Fe3+ sites of the solid solution underwent reduction. At high iron content, the Fe3+ in Fe2(MoO4)3 was detected and diminished by half under catalytic conditions; (2) when Bi2(MoO4)3 was added to FexCo1−xMoO4, there was no reduction of Fe3+ sites in the solid solution. The α → β phase transition was not observed in the case of the low and medium iron contents. In the high iron content samples the α-phase spectrum decreased to the expense of the β phase but did not disappear totally after catalytic reaction. It is concluded that cobalt stabilizes Fe2+ sites in the solid solution while Bi2(MoO4)3 stabilizes Fe3+ in the solid solution. It is proposed that Fe2+-Fe3+ pairs take an important place in the mechanism of propene mild oxidation.