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

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, SbO3E trigonal pyramids similar to that in Sb2Mo10O31 whereas Sb-V forms, with two oxygens of the hexagonal window and two oxygens in the channel, SbO4 trigonal bipyramids like in TeMo5O16 (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](infinity) in the orthorhombic one. The results obtained allowed determining the formal stoichiometries of the two phases which are, respectively, (Sb2O)M6O18 and (SbO)(2)M20O56 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. (C) 2002 Elsevier Science B.V. All rights reserved.