The structure of catalytically active vanadium sites in V–Sb oxides: an EPR study of asynthesized and tungsten promoted working catalysts

Abstract

EPR investigation of complex V–Sb oxide catalysts used for the ammoxidation of propane to acrylonitrile (VSb y O x ( y=1, 2, 5), VSb 5WO x and 30 wt. % VSb 5WO x/Al 2O 3) indicated the presence of small amounts of isolated VO 2+ ions evidenced by characteristic hfs signals superimposed on broad singlets of interacting V 4+ centres. Almost equal g- and A-tensor parameters of the VO 2+ ions in the VSb y O x samples suggest their location in similar hosts, presumably in a XRD-amorphous V 5+O x -phase. Changes caused by tungsten doping indicate that W ions are incorporated in close neighbourhood to the vanadium ions. In samples with high antimony content additional V 4+–V 4+ pairs were detected. In situ measurements under reaction conditions (793 K, C 3H 8/N 2/O 2/NH 3) with a self-constructed in situ-EPR flow reactor revealed that not isolated VO 2+ centres but interacting V 4+ ions participate in the activation of propane while no changes of the vanadium signals occurred on addition of NH 3.