Mechanisms of molybdenum substitution in vanadium antimonate

The formation of a solid solution containing the three elements V, Sb and Mo, which arc key-elements in the design of light alkane oxidation catalysts, has been studied by incorporating molybdenum into the pure VSbO4 compound as obtained in air at 700degreesC (V(0.28)(3+)V(0.64)(4+)square(0.16)Sb(0.92)(5+)O(4)). Monophasic compounds with a rutile-type structure have been obtained and characterized by X-ray diffraction, electron microscopy, Infrared Fourier transform, X-ray absorption and electron spin resonance spectroscopies. At low molybdenum content, Mo6+ substitute V4+ in the cationic-deficient structure. The charge balance is maintained by an increase of the cationic vacancy number. This leads to the formation of a solid solution corresponding to the formula V0.283+V0.64-3x4+Mo2x6+ square(0.16+x)Sb(0.92)(5+)O(4) with 0 < x < 0.09. At higher molybdenum content, Mo5+ are stabilized and substitute Sb5+ in the rutile structure. V(0.28)(3+)V(0.37)(4+)Mo(0.18)(6+)square(0.25)Mo(y)(5+)Sb(0.92)(5+)O(4) with 0 < y < 0.06. At higher molybdenum content the rutile phase is no longer stable and two new phases are formed: Sb2O4 and a new mixed vanadium molybdenum antimonate. (C) 2003 Elsevier Inc. All rights reserved.