Structure of molybdenum oxide supported on silica SBA-15 studied ...

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<strong>Structure</strong> <strong>of</strong> <strong>molybdenum</strong> <strong>oxide</strong> <strong>supported</strong> on silica SBA-15 studied by Raman, UV–Vis and X-ray absorption spectroscopy J. P. Thielemann et al., Appl. Catal. A: General 399 (2011) 28-24 Table 2: Type, number (N), and XAFS disorder parameters (� 2 ) <strong>of</strong> atoms at distance R from the Mo atoms in MoxOy species in dehydrated MoxOy/SBA-15. Experimental parameters were obtained from a refinement <strong>of</strong> a hexagonal MoO3 model structure to the experimental Mo K edge XAFS �(k) <strong>of</strong> dehydrated MoxOy/SBA- 15 (Fig. 10) (k range from 3.6 to 14.4 Å -1 , R range from 0.9 to 4.0 Å, E0 = 8.0 eV, residual ~ 10.6, Nind = 23, Nfree = 13). Confidence limits in distances and � 2 parameters are indicated. Subscript C indicates parameters that were correlated in the refinement. Eventually, the local structure around the Mo centres in dehydrated Mo xO y/SBA-15 (5.5 wt% Mo) was determined by detailed EXAFS analysis. While seeking a suitable model system to simulate the experimental FT(�(k)∙k 3 ) <strong>of</strong> dehydrated Mo xO y/SBA-15 it became apparent, that one or two Mo-Mo distances were not sufficient. Despite <strong>of</strong> the reduced amplitude at higher shells (see Fig. 9), three Mo- Mo distances were required to account for the particularshape <strong>of</strong> the amplitude and imaginary part <strong>of</strong> the FT(�(k)∙k 3 ) <strong>of</strong> dehydrated Mo xO y/SBA-15 in the range between 2.5 and 3.5 Å. Therefore, theoretical phases and amplitudes calculated for various Mo-O and three Mo-Mo distances in reference hexagonal MoO 3 were used for EXAFS refinement. Theoretical and experimental Mo K edge FT(�(k)∙k 3 ) <strong>of</strong> dehydrated Mo xO y/SBA-15 are shown in Fig. 9. Structural parameters obtained from the XAFS refinement are given in Table 2. The results presented above demonstrate the potential <strong>of</strong> combining Raman-, UV-Vis and X-ay absorption spectroscopy to study the structure <strong>of</strong> silica SBA-15 <strong>supported</strong> highly dispersed <strong>molybdenum</strong> <strong>oxide</strong>. On the one hand, the Raman spectra reveal the absence <strong>of</strong> crystalline <strong>molybdenum</strong> <strong>oxide</strong> phases (e.g. �-MoO 3) with high sensitivity, whereas minority <strong>molybdenum</strong> <strong>oxide</strong> phases (
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