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Table 2 EXAFS refinement results obtained for experimental FT(χ(k)*k3) of dehydrated VxOy/SBA-15, NH4VO3, and Mg2V2O7.

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

  Model VxOy-SBA-15 Mg2V2O7 NH4VO3
Type N R [Å] R [Å] σ22] R [Å] σ22] R [Å] σ22]
V-O 1 1.63 1.78 0.0075 1.74 0.0059 1.68 0.0012
V-O 1 1.70 1.78C 0.0075C 1.74C 0.0059C 1.68C 0.0012C
V-O 2 1.76 1.78C 0.0075C 1.74C 0.0059C 1.84 0.0012C
V-O 1 2.87 2.89 0.0017 2.77 0.0165 - -
V-V 1 3.36 3.30 0.0135 3.28 0.0134 3.47 0.0147
V-V 1 3.62 3.62 0.0135C 3.56 0.0134C 3.47C 0.0147C
V-Si 1 2.80 2.54 0.0121 - - - -
  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 parameters (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 (10.8 wt %), Mg2V2O7, and NH4VO3 (k range from 2.7-11.0 Å-1, R range 0.8-4.0 Å, Nind = 18, E0 = 0 eV in all cases, fit residual 3.6 (dehydrated VxOy/SBA-15) (Nfree = 9), 11.2 (Mg2V2O7) (Nfree = 7), 8.9 (NH4VO3) (Nfree = 5)) (Subscript C indicates parameters that were correlated in the refinement). Confidence limits and significance of fitting parameters are given in Table 3.