Study of the valence state and coordination of antimony in MoVSbO catalysts determined by XANES and EXAFS

TLDR: In this article, the coordination of antimony in the two phases, orthorhombic and hexagonal, of the MoVSbO mixed oxides catalysts has been investigated by EXAFS at the Sb K-edge at 77 K and its valence state by XANES at Sb L 1 -edge.

Abstract: The coordination of antimony in the two phases, orthorhombic and hexagonal, of the MoVSbO mixed oxides catalysts has been investigated by EXAFS at the Sb K-edge at 77 K and its valence state by XANES at the Sb L 1 -edge. While XANES revealed that both phases contained Sb III and Sb V , it also demonstrated that the phase M1 contained mainly Sb V while the phase M2 contained mainly Sb III . From data obtained with the two techniques, it can be proposed that in both phases, Sb III and Sb V are mostly located in the windows of hexagonal channels of the structures. Sb III forms, with two oxygens of the hexagonal window and one oxygen in the channel, SbO 3 E trigonal pyramids similar to that in Sb 2 Mo 10 O 31 whereas Sb V forms, with two oxygens of the hexagonal window and two oxygens in the channel, SbO 4 trigonal bipyramids like in TeMo 5 O 16 (orthorhombic form). In the channels antimony and oxygens are thus forming mainly dimers [Sb–O–Sb] in the hexagonal phase and infinite chains [Sb–O] ∞ in the orthorhombic one. The results obtained allowed determining the formal stoichiometries of the two phases which are, respectively, (Sb 2 O)M 6 O 18 and (SbO) 2 M 20 O 56 with (M = Mo, V) and to discuss the anionic and cationic sub-stoichiometries encountered. The comparison with the tellurium environment in the similar phases is discussed.