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Table 4 EXAFS refinement results obtained for different V loadings on VxOy/SBA-15 in the dehydrated state.

From: Structural characterization of vanadium oxide catalysts supported on nanostructured silica SBA-15 using X-ray absorption spectroscopy

  Model   10.8 wt %   5.4 wt %   2.7 wt %  
Type N R [Å] R [Å] σ22] R [Å] σ22] R [Å] σ22]
V-O 1 1.63 1.78 0.0075 1.77 0.0069 1.78 0.0075
V-O 1 1.70 1.78 0.0075 1.77 0.0069 1.78 0.0075
V-O 2 1.76 1.78 0.0075 1.77 0.0069 1.78 0.0075
V-V 1 3.36 3.30 0.0135 3.33 0.0114 3.37 0.0113
V-O 1 2.87 2.90 0.0017 2.87 0.0022 2.86 0.0015
V-V 1 3.62 3.62 0.0135 3.62 0.0114 3.65 0.0113
V-Si 1 2.80 2.54 0.0121 2.53 0.0139 2.54 0.0081
  1. Type and number (N) of atoms at distance R from the absorbing V atom in a model system assuming an ordered arrangement of V2O7 units (Figure 7) compared to experimental distances and XAFS disorder parameter (σ2). Parameters were obtained from the refinement of this model structure to the experimental V K edge XAFS FT(χ(k)*k3) of dehydrated VxOy/SBA-15 with different V loadings (i.e. 10.8 wt %, 5.4 wt %, 2.7 wt %) (Figure 9) (k range from 2.7-11.0 Å-1, R range 0.8-4.0 Å, Nind = 18, Nfree = 9, E0 = 0 eV in all cases, fit residual 3.6 (10.8 wt %), 7.0 (5.4 wt %), 7.8 (2.7 wt %)) (Subscript C indicates parameters that were correlated in the refinement). Confidence limits and significance of fitting parameters correspond to those given in for the 10.8 wt % sample.