Evolution of defects in the bulk structure of MoO3 during the catalytic oxidation of propene

The evolution of characteristic defects in the bulk structure of MoO3 under propene oxidation conditions was investigated by in situ X-ray absorption spectroscopy and X-ray diffraction. Under the reaction conditions employed (273 K to 773 K, and propene to oxygen ratios from 1:1 to 1:5), orthorhombic MoO3 remains the only crystalline phase detected by XRD. The starting temperature for the reaction of propene and oxygen in the presence Of MoO3 coincides with the temperature at which reduction of MoO3 begins in He, H-2 and propene (approximate to 620 K). At temperatures below 720 K, and independent of the atmosphere used, partial reduction Of MoO3 is observed resulting in the formation of "Mo18O52" type defects in the layer structure of alpha-MoO3. At temperatures above 720 K, and under oxygen or in an oxidizing atmosphere, the "Mo18O52" type defects are reoxidized to MoO3. Evidently, the catalytically active molybdenum oxide phase develops under partial oxidation conditions at temperatures below 720 K, and does not possess the undisturbed ideal layer structure of orthorhombic alpha-MoO3. The results presented clearly show the necessity and the potential for bulk structural investigations of heterogeneous catalysts under the reaction conditions used. The bulk structure, and particularly the type and number of defects in the material ("real" structure) considerably affect the catalytic properties. Hence, in order to rationally design a most active heterogeneous catalyst, both the structure and reactions of the surface, and the structure, defects, and reactions of the bulk need to be known in detail, and carefully considered. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.