The structure of molybdenum oxide supported by silica SBA-15 has been studied by visible Raman spectroscopy, diffuse reflectance UV-Vis spectroscopy and X-ray absorption spectroscopy in the dehydrated state obtained after thermal treatment at elevated temperatures (≥350°C). No dependence of the molybdenum oxide structure on preparation procedure or loading has been observed within the range of loadings studied in detail (0.2 to 0.8 Mo/nm 2 ). X-ray absorption spectroscopy (XAS) reveals that the dehydrated state consists of a mixture of monomeric and connected molybdenum oxide centres. While the presence of crystalline MoO3 can be excluded by Raman spectroscopy, tetrahedrally and octahedrally coordinated MoO4 and MoO6 units are identified by XAS. The MoO6 units possess connectivity similar to that of MoO3 building blocks, whereas the MoO4 units are isolated or connected to other MoxOy units. These results are supported by UV-Vis spectra showing intensity at wavelengths (>300 nm) typical for dimeric and/or oligomeric species.

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The local structure of vanadium oxide supported on nanostructured SiO2 (VxOy/SBA-15) was investigated by in situ X-ray absorption spectroscopy (XAS). Because the number of potential parameters in XAS data analysis often exceeds the number of "independent" parameters, evaluating the reliability and significance of a particular fitting procedure is mandatory. The number of independent parameters (Nyquist) may not be sufficient. Hence, in addition to the number of independent parameters, a novel approach to evaluate the significance of structural fitting parameters in XAS data analysis is introduced. Three samples with different V loadings (i.e. 2.7 wt %, 5.4 wt %, and 10.8 wt %) were employed. Thermal treatment in air at 623 K resulted in characteristic structural changes of the V oxide species. Independent of the V loading, the local structure around V centers in dehydrated VxOy/SBA-15 corresponded to an ordered arrangement of adjacent V2O7 units. Moreover, the V2O7 units were found to persist under selective oxidation reaction conditions.

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In situ X-ray absorption spectroscopy (XAS) under reaction conditions of selective propene oxidation was employed to elucidate the local structure of as-prepared and activated molybdenum oxide supported on hollow vapor-grown carbon nanofibers (VGCNF). The local structure of asprepared MoxOy-VGCNF was very similar to that of hexagonal MoO3. During heat treatment in propene-and oxygen-containing atmosphere, asprepared MoxOy-VGCNF transforms into activated MoxOy-VGCNF above 623 K. The local structure around the Mo centers in activated MoxOy-VGCNF is similar to that of α-MoO3. Temperature-and time-dependent XAS measurements showed a rapid transformation from hex-MoO3 to α-MoO3 supported on VGCNF under reaction conditions. Subsequently, the resulting activated MoxOy-VGCNF catalyst exhibited a slowly increasing average oxidation state. The latter coincided with the formation of acrylic acid, which is hardly detectable during catalysis on regular, binary α-MoO3. Moreover, activated MoxOy-VGCNF is much more active in the selective oxidation of propene compared to α-MoO3. The correlation between catalytic selectivity and average oxidation state as a result of suitable reduction-oxidation kinetics corroborates the importance of structural complexity rather than chemical complexity.