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

Abstract

In-situ Mössbauer spectroscopy was used to study Fe x Co 1− x MoO 4 molybdates and their mixtures with Bi 2(MoO 4) 3 in the 360–415°C range under a flow of a mixture C 3H 6/O 2/N 2 = 100/100/560 Torr. Ferrous molybdate FeMoO 4 was progressively oxidized to ferric molybdate and ferric oxide. Mixed cobalt and iron molybdates Fe x Co 1− x MoO 4 exhibited the following behaviour: (1) for all iron contents, the α phase spectrum diminished to the benefit of the β phase. The Fe 3+ sites of the solid solution underwent reduction. At high iron content, the Fe 3+ in Fe 2(MoO 4) 3 was detected and diminished by half under catalytic conditions; (2) when Bi 2(MoO 4) 3 was added to Fe x Co 1− x MoO 4, there was no reduction of Fe 3+ 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 Fe 2+ sites in the solid solution while Bi 2(MoO 4) 3 stabilizes Fe 3+ in the solid solution. It is proposed that Fe 2+-Fe 3+ pairs take an important place in the mechanism of propene mild oxidation.