Cooperation between redox couples at the surface of molybdates based catalysts used for the selective oxidation of propene

Abstract

FeMoTeO mixed oxides have been synthesized and tested as catalysts for the oxidation of propene into acrolein. In order to determine the role of their constituent metallic elements, these catalysts were characterized using bulk techniques such as TEM, XANES and Mössbauer spectroscopy, as well as surface techniques including XPS and LEIS. Our results confirmed that the active phase of these catalysts corresponds to an amorphous layer of molybdenum oxide containing Te and Fe, supported on the Fe2(MoO4)3 phase. They also show that the addition of tellurium significantly changes the redox dynamics of the catalytic surface, since it provides a fast re-oxidation route for molybdenum species. The study of MoTeO catalysts has shown that the presence of Te centers alone is not sufficient to fully re-oxidize the molybdenum. Although tellurium plays a role in the redox reaction mechanism, XPS on fresh and used FeMoTeO catalysts did not reveal any change in its oxidation state. However, the study of MoTeO samples in the early stage of their reduction by propene revealed the simultaneous formation of Te2+ and Te(0) species, indicating that the Te4+/Te2+ redox couple is involved in the redox cycle. Since Te(II) is unstable, it undergoes disproportionation, if not quickly re-oxidized, leading to metallic tellurium that is easily lost from the surface. The presence of iron provides an accessible route for the re-oxidation of tellurium, thereby restoring the Te(IV) oxidation. Since iron is unable to re-oxidize Mo(V) in the absence of tellurium, and although the usual role of tellurium is that of an α-hydrogen abstracting element, it acts as a redox bridge between iron and molybdenum. This behavior would account for the high efficiency of the FeTeMoO catalysts and shows that the three redox couples Fe3+/Fe2+, Te4+/Te2+ and Mo6+/Mo5+ cooperate in the re-oxidation step of the Mars and van Krevelen mechanism.